FQPUIARZY EXPLAINED 
 
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 UNIVERSITY 
 
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Cornell University Library 
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 Scientific ideas of to-day* PoPf;'^!,,^;r*= 
 
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Cornell University 
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 The original of this book is in 
 the Cornell University Library. 
 
 There are no known copyright restrictions in 
 the United States on the use of the text. 
 
 http://www.archive.org/details/cu31924012327767 
 
SCIENTIFIC IDEAS OF TO-DAY 
 
Regular Figures produced by Floating Magnets 
 
 In the first photograph the little upright magnetic needles may be seen projecting 
 downwards from the small corks. Left alone, the similar poles repel one another, but in 
 the other photographs ihe hand holds the opposite pole of a magnet above the centre of the 
 basin, whereupon ihe little magnets invariably foim defirite figures according to their 
 number. These experiments enable us to form a mental picture of the construction of the 
 atom. 
 
SCIENTIFIC IDEAS 
 OF TO-DAY 
 
 A POPULAR ACCOUNT OF THE NATURE 
 
 OF MATTER, ELECTRICITY, LIGHT, 
 
 HEAT, ^c. i^c. 
 
 IN NON-TECHNICAL LANGUAGE 
 
 By 
 CHARLES R. GIBSON 
 
 Author of " Electricity of To-Day," " The Romance of Modern 
 Electricity," "The Romance of Modern Photography," 
 
 With Forty-two Illustrations and Diagrams 
 
 LONDON 
 
 SEELEY AND CO. LIMITED 
 
 38 Great Russell Street 
 
 1909 
 
UNIFORM WITH THIS VOLUME 
 
 ELECTRICITY OF TO-DAY 
 
 By C. R. GIBSON 
 
 With many Illustrations. Extra Crown 8vo. 
 
 5s. net. 
 '* Clear, concise, and well arranged." — Scotsman. 
 
 ASTRONOMY OF TO-DAY 
 
 ' By C. G. DOLMAGE, LL.D., F.R.A.S. 
 
 With many Illustrations. Extra Crown 8vo. 
 5s. net. 
 
 3 1) 'iii'.i 
 
 X 
 
 '<<ii 
 
PREFACE 
 
 The more man's knowledge increases, the more do 
 his text-books of science become necessarily complex 
 and technical. To-day, few persons whose walk in 
 life lies in other directions have either the time or 
 the inclination to study technical treatises. And yet 
 our scientific ideas of to-day are so very different from 
 those of the preceding generation that many are de- 
 sirous of understanding them. There is no reason 
 why so many people should spend a lifetime upon 
 this planet and know nothing or little of the forces 
 by means of which the Ci*eator formed the world, and 
 maintains it in its present condition. Such questions 
 as, What are the atoms of matter made of ? What is 
 light ? What is electricity ? and so on, are surely of 
 interest to the tenants of this planet. 
 
 In the present volume the author has endeavoured 
 to explain the scientific ideas of to-day without using 
 language beyond the reach of any reader. His ex- 
 planations demand no previous knowledge of science 
 whatever, and no acquaintance with mathematics. The 
 Warningf which is appended to this preface, has been 
 suggested by incidents which have occurred in con- 
 nection with some of the author's former books. From 
 
 V 
 
Preface 
 
 questions put to him it has been evident, and indeed 
 the questioners have acknowledged the fact, that they 
 read the chapters irrespective of the order in which 
 they were placed in the books. 
 
 The author is indebted to Professor James Muir, 
 D.Sc, M.A., and Professor Magnus Maclean, M.A., 
 D.Sc, F.R.S.E., of the Glasgow and West of Scotland 
 Technical College, and to H, Stanley Allen, M.A., B.Sc, 
 Senior Lecturer in Physics at King's College, University 
 of London, for very kindly reading the proof-sheets. 
 The author is also indebted to the following gentle- 
 men in connection with the illustrations : — Professor 
 James Muir, Dr. R, M. Buchanan, Walter A. Scoble, 
 Charles Stewart, John M'Lennan, Glasgow ; Professor 
 E. E. Barnard, U.S.A. ; Edgar Senior, Edward 
 Chesterton, Rebman, Ltd. {Archives of the Roentgen 
 Ray), Arthur E, Smith, London ; also to Andrew A. 
 Muir, Glasgow, for very kindly drawing the diagrams 
 which appear in the text. 
 
 VI 
 
A WARNING 
 
 On picking up a book many readers are inclined to 
 glance through the Contents, select a chapter which 
 appears interesting from its title, and read that chapter 
 first. Needless to say, this is a very bad plan, and 
 especially so if the reader has no previous knowledge 
 of the subject; for in each chapter the author pre- 
 sumes that the preceding chapters have been read. 
 His aim is to make the whole text perfectly intelligible 
 to every reader ; but it would be quite impossible to 
 make each chapter complete in itself without a weary- 
 ing repetition of facts. By commencing at Chapter I. 
 the whole subject becomes very simple as one goes 
 along, each chapter leading on to the following one, 
 like easy stepping-stones across a wide river. 
 
 vii 
 
Contents 
 
 CHAPTER VII 
 
 PAGE 
 
 What is Magnetism ? 104 
 
 CHAPTER VIII 
 More about Electrons in Motion . .115 
 
 CHAPTER IX 
 What is Energy? 125 
 
 CHAPTER X 
 Waves in the ^ther 139 
 
 CHAPTER XI 
 What is Light? 153 
 
 CHAPTER XII 
 More about Light 171 
 
 CHAPTER XIII 
 The Explanation of Colour . . .192 
 
 CHAPTER XIV 
 Ideas obtained from the Spectrum . . 209 
 
 X 
 
Contents 
 
 CHAPTER XV 
 
 PAGE 
 
 The Birth of a Star 230 
 
 CHAPTER XVI 
 The Age of the Earth , . . , 243 
 
 CHAPTER XVII 
 Whence came Life ? 252 
 
 CHAPTER XVIII 
 More Electron Ideas 258 
 
 CHAPTER XIX 
 What are the X-rays ? . . , . 266 
 
 CHAPTER XX 
 
 How Radium was Discovered . . . 273 
 
 CHAPTER XXI 
 What are the Rays from Radium ? . . 285 
 
 CHAPTER XXII 
 
 Is the World going to Pieces ? . . . 294 
 
 xi 
 
Contents 
 
 CHAPTER XXIII 
 
 PAGE 
 
 The Cause of Radio-Activity . , . 305 
 
 CHAPTER XXIV 
 What is Gravitation? 310 
 
 CHAPTER XXV 
 Conclusion 317 
 
 APPENDIX I 
 
 The Ingredients of the World . . . 322 
 
 APPENDIX II 
 
 Historical Note on the Modern Theory 
 
 OF Light 328 
 
 APPENDIX III 
 Particulars of -^ther Waves . . - 332 
 
 APPENDIX IV 
 Counting the Invisible Electrons . . 335 
 
 Index 341 
 
 xii 
 
LIST OF ILLUSTRATIONS 
 
 Regular Figures Produced by Floating 
 
 Magnets Frontispiece 
 
 Testing a Theory facing p, i8 
 
 The Detail in a Speck of Matter 
 
 Invisible Objects 
 
 Electrical Attraction of Common Objects 
 
 Electrical Repulsion 
 
 Method of Producing a Stream of Elec- 
 trons 
 
 A Coil of Wire carrying an Electric 
 Current behaves like a Magnet . 
 
 An iETHER Disturbance around an Elec- 
 tron Current 
 
 Lines of Force around a Magnet . 
 
 Uprooting a Tree by a Man's Breath . 
 
 The Mechanical Pressure of Light 
 
 Bending a Beam of Light 
 
 Portrait taken by Invisible Rays . 
 
 A Powerful Magnetic Field .... 
 
 The Spectroscope in Use 
 
 Detecting a Small Quantity of Matter 
 
 (i) Dark Lines in the Solar Spectrum ; 
 (ii) Zeeman Effect 
 
 Spectra of Different Elements 
 
 xiii 
 
 26 
 
 38 
 54 
 66 
 
 78 
 
 94 
 
 no 
 122 
 
 134 
 150 
 172 
 186 
 200 
 218 
 228 
 
 240 
 256 
 
List of Illustrations and Diagrams 
 
 An X-Ray Photograph of a Cameo . .facing p. 268 
 Photographing through a Granite Boulder „ 274 
 
 Radioactive Effects „ 296 
 
 The Electroscope in Use „ 306 
 
 Proof that Water is Composed of Oxygen 
 
 and Hydrogen „ 3x4 
 
 DIAGRAMS IN THE TEXT 
 
 A, Internal Arrangement of a Steel Magnet . 107 
 
 B. Bending a Beam of Light 214 
 
 C How A Beam of Light is bent by a Prism . .215 
 
 A A Rontgen Ray Tube 270 
 
 £". The Range of ^Ether Waves 333 
 
 F. Experimenting with Electrons . , . .339 
 
 XIV 
 
SCIENTIFIC IDEAS OF 
 TO-DAY 
 
 CHAPTER I 
 
 INTRODUCTION 
 
 The professor and the student — The meaning of a theory — A 
 mistaken theory — Testing a theory — Laws of Nature 
 
 To make any scientific subject of real interest to the 
 general reader one must presume that the reader has 
 no previous knowledge whatever of the subject. 
 
 A learned friend has remarked to me that he 
 used to admire the way in which a professor, now 
 deceased, was wont to give his fellow-men credit 
 for knowing far more than they did know. I 
 admitted that this, no doubt, was a very generous 
 frame of mind, but I maintained that it was far 
 from being of assistance to the struggling student. 
 My friend told me that he could remember one 
 occasion, very many years ago, when he was in this 
 great man's class. The professor put a question to 
 a raw country student, and it was quite apparent to 
 the class that the student knew no more of the 
 
 ^5 
 
Scientific Ideas of To-day 
 
 subject than did the desk in front of him. Nothing 
 could have persuaded the learned professor of the 
 student's complete ignorance. The question was put 
 in another form, and yet in another and another, 
 till at last it only demanded yes or no as an answer, 
 and indeed it was clear which answer was wanted. 
 When the student answered correctly, the learned 
 professor almost lost his temper. " Man ! I knew 
 you knew it ! Why do you make me drag it out of 
 you bit by bit?" 
 
 My object in relating the foregoing incident is to 
 emphasise the fact that if one desires to treat a 
 subject "popularly," one must not presume any 
 previous knowledge of the subject on the part of 
 his readers. I trust, therefore, that any reader com- 
 ing across statements of facts with which he is well 
 acquainted, will bear with me, for the sake of those 
 to whom such statements may be of sei'vice. 
 
 It will be well to remember that all our present 
 knowledge has been gradually built up by man 
 himself. Man found himself upon this planet, left 
 to find out for himself the meaning of all that lay 
 around him. Our present object is not to trace how 
 civilised man came to make use of the forces of 
 nature, but to consider the ideas he formed concern- 
 ing things, and in particular the scientific ideas of 
 our own day. In other words, we are to deal with 
 theory rather than practice. 
 
 One sometimes finds people who consider theory 
 to be a very useless sort of thing — a sort of wild 
 
 i6 
 
Introduction 
 
 guess without which we should be none the poorer. 
 It must be clear that a theory is more than a mere 
 speculation. If I suggest that the moon is made of 
 green cheese, my speculation is not entitled to be 
 called a theory, I cannot bring forward any ob- 
 served facts to support my suggestion. 
 
 When the ancients observed the sun rising in the 
 east and setting in the west they very naturally 
 suggested that the sun was making a continual 
 journey round and round the earth. This theory 
 accounted for the rising and setting of the sun ; but 
 when the motions of the planets were carefully 
 observed, at a later date, it was found that the old 
 Ptolemaic theory could not satisfactorily explain these 
 planetary motions. But until the middle of the 
 sixteenth century man believed his habitation to be 
 securely fixed in space, while the sun, moon, and 
 stars all moved around him. 
 
 We have seen that the original theory was found 
 to be wrong, although it appeared at first to be 
 founded on observed facts. The mistake was that 
 all the facts had not been taken into account. 
 
 No doubt most of us have been deceived com- 
 pletely, at some time or other, as to whether we 
 ourselves or some other observed body was in 
 motion. The first sensation of going up in a 
 balloon, on a quiet day, is that the earth is falling 
 away from the balloon. A more common experience 
 is that of a train moving very quietly out of a rail- 
 way station in which another train is at rest. A 
 
 17 B 
 
Scientific Ideas of To-day 
 
 subject than did the desk in front of him. Nothing 
 could have persuaded the learned professor of the 
 student's complete ignorance. The question was put 
 in another form, and yet in another and another, 
 till at last it only demanded yes or no as an answer, 
 and indeed it was clear which answer was wanted. 
 When the student answered correctly, the learned 
 professor almost lost his temper. " Man ! I knew 
 you knew it ! Why do you make me drag it out of 
 you bit by bit ? " 
 
 My object in relating the foregoing incident is to 
 emphasise the fact that if one desires to treat a 
 subject " popularly/' one must not presume any 
 previous knowledge of the subject on the part of 
 his readers. I trust, therefore, that any reader com- 
 ing across statements of facts with which he is well 
 acquainted, will bear with me, for the sake of those 
 to whom such statements may be of sei-vice. 
 
 It will be well to remember that all our present 
 knowledge has been gradually built up by man 
 himself. Man found himself upon this planet, left 
 to find out for himself the meaning of all that lay 
 around him. Our present object is not to trace how 
 civilised man came to make use of the forces of 
 nature, but to consider the ideas he formed concern- 
 ing things, and in particular the scientific ideas of 
 our own day. In other words, we are to deal with 
 theory rather than practice. 
 
 One sometimes finds people who consider theoiy 
 
 to be a very useless sort of thing — a sort of wild 
 
 i6 
 
Introduction 
 
 guess without which we should be none the poorer. 
 It must be clear that a theory is more than a mere 
 speculation. If I suggest that the moon is made of 
 green cheese, my speculation is not entitled to be 
 called a theory. I cannot bring forward any ob- 
 served facts to support my suggestion. 
 
 When the ancients observed the sun rising in the 
 east and setting in the west they very naturally 
 suggested that the sun was making a continual 
 journey round and round the earth. This theory 
 accounted for the rising and setting of the sun; but 
 when the motions of the planets were carefully 
 obsei*ved, at a later date, it was found that the old 
 Ptolemaic theory could not satisfactorily explain these 
 planetary motions. But until the middle of the 
 sixteenth century man believed his habitation to be 
 securely fixed in space, while the sun, moon, and 
 stars all moved around him. 
 
 We have seen that the original theory was found 
 to be wrong, although it appeared at first to be 
 founded on observed facts. The mistake was that 
 all the facts had not been taken into account. 
 
 No doubt most of us have been deceived com- 
 pletely, at some time or other, as to whether we 
 ourselves or some other observed body was in 
 motion. The first sensation of going up in a 
 balloon, on a quiet day, is that the earth is falling 
 away from the balloon. A more common experience 
 is that of a train moving very quietly out of a rail- 
 way station in which another train is at rest. A 
 
 17 B 
 
Scientific Ideas of To-day 
 
 passenger sitting in the moving train and looking 
 at the stationary train is often deceived. He 
 sees the other train, or rather he beHeves he sees 
 the other train, moving away, and a httle later he is 
 greatly surprised to find that it is he himself who is 
 moving. Of course the illusion can only occur 
 when the train canying the observer moves very 
 gradually and very smoothly, A sudden jolt would 
 immediately dispel the illusion. No person really 
 doubts to-day that our earth is moving through 
 space, that it is spinning round like a top, and 
 at the same time making a continuous journey 
 around the sun. The only reason why man was 
 deceived so long was that our planet glides along 
 without any resistance being offered to its motion. 
 The railway passenger may discover the motion of 
 his train by a sudden jolt, but it is fortunate that 
 man has not experienced any similar jolting of the 
 planet. 
 
 It was a bold theory to bring the sun to a stand- 
 still and set the earth in motion. We know how 
 poor Galileo had to suffer imprisonment at the hands 
 of the Church for supporting this theory, which had 
 been proved seventy years previously by Copernicus, 
 However, astronomers were able to bring forward 
 so many observed facts to support this theory that 
 its truth was forced upon eveiy thinking person. It 
 took some time to convince people, for the earlier 
 theoi-y seemed the more natural, but there could be 
 no doubt that it must be the one way or the other. 
 
 i8 
 
Proving a Theory 
 
 The upper illustration was sent to me as a photograph ot a flash of lightning, 
 and the puzzle was how the flash had repeated itself five times on the photographic 
 plate. When it wa^ suggested to the photographer that this was nota photograph 
 of lightning but the lights from five street lamps, he could not accept this theory as he 
 felt confident that he had photographed a flash of lightning. Howe\er, he agreed to 
 test the matter by setting up his camera in the dark with ihi; shutter open as before, 
 and with a street lamp at some distance, whereupon he obtained the second photo- 
 graph. This proved clearly that the suggested theory was correct ; the writing 
 appearance being due to the movement of the camera in setting it up. 
 
Introduction 
 
 There is a good story told in this connection of 
 a well-known professor examining three raw students. 
 He asked the first, " Does the earth go round the sun, 
 or the sun go round the earth ? " 
 
 "The earth goes round the sun, sir." "You," said 
 the professor, quickly turning to the second student. 
 "Oh! the sun goes round the earth." "You," de- 
 manded the professor of the third student. "Oh! 
 it's sometimes the one way and sometimes the other." 
 This was the only alternative left for the poor fellow. 
 
 Our position then is this. We gather a number 
 of carefully observed facts together, and we then try 
 to explain them. We call this attempted explanation 
 a theory. We then look out for new facts and see if 
 our theory can explain these also. If it cannot we 
 must be willing to alter our theory or to construct 
 a new one (see illustration facing page i8). 
 
 We have another and important method of testing 
 our theories. We may be able to reason from a 
 theory that certain things should happen or should 
 exist if our theory be true ; we look out for these facts 
 and find if they really do exist. We must follow up 
 a theory to its logical conclusion and test our deduc- 
 tions by careful experiment. A theory then is a good 
 thing whether it proves to be correct or not ; it assists 
 in furthering and systematising the collection of facts. 
 Lord Bacon summed up the matter, three centuries 
 ago, in the following sentence, which is to be found in 
 his "Advancement of Learning" : "All true and faith- 
 ful natural philosophy hath a double scale or ladder, 
 
 19 
 
Scientific Ideas of To-day 
 
 an ascendant and descendant ; ascending from experi- 
 ments to the invention of causes, and descending from 
 causes to the invention of new experiments." 
 
 When we are quite satisfied that a theory is correct 
 — that it explains all the facts which have been ob- 
 served — we then raise the theory to a higher platform 
 and call it a Law of Nature. It is well to remember 
 that, with all other knowledge, these laws of nature 
 are of man's own making. It is amusing how some 
 people think that certain things happen and are bound 
 to happen because of these "laws of nature." As if 
 the universe were controlled by these laws which man 
 has constructed ! The laws of nature are only theories 
 which seem to be correct ; they are not facts, but 
 merely our views or ideas of facts. 
 
 20 
 
CHAPTER II 
 
 WHAT THINGS ARE MADE OF 
 
 Adding things together — The ingredients of the world — Two causes 
 of substances being rare — Chemical combination — Curious 
 partnerships — Invisible particles of matter— Atoms— Mole- 
 cules — Chemical affinity — Electrical attraction — Two different 
 electricities — The origin of the terms positive and negative — 
 The structure of matter — An analogy— Cohesion — Tempera- 
 ture — Solid, liquid, and gaseous 
 
 The inquiring mind is not satisfied to know that 
 certain things are made of a substance called glass 
 and other things of another substance called clay. 
 We want to know what these substances themselves 
 are made of. 
 
 Before we left school we had become quite accus- 
 tomed to the idea that most things are made by adding 
 other things together. We were interested to learn 
 that one kind of glass was made by boihng sand, 
 soda, and lime together, after the manner of making 
 toffee. We were, in our early days, amused to find 
 that paper could be made from any old rags. We 
 soon began to realise that man could only add things 
 together or subtract some things from other compound 
 things ; that there was only a certain definite amount 
 of matter in the world, and that this had existed from 
 the beginning, when "God created the heavens and 
 
 21 
 
Scientific Ideas of To-day 
 
 the earth." We soon realised that everything we see 
 upon the earth to-day existed in some form from the 
 beginning of time. Indeed, we became convinced 
 hke Solomon of old that " there is nothing new under 
 the sun." 
 
 We had not gone very far in our inquiries when 
 we learned that all compound substances are merely 
 different combinations of a certain number of simple 
 or elementary substances. While we have two or 
 three hundred thousand different compound sub- 
 stances, we find that they are all composed of two or 
 more of a limited number of simple elements or funda- 
 mental substances. 
 
 At present we know of some eighty elementary 
 substances, and the general reader is only conversant 
 with a comparatively small number of these. If one 
 examines a complete list of the elements, as given 
 in Appendix I., at p. 322, it will be found that 
 most people could not recognise the half of the 
 names. 
 
 A certain number of the elementary substances are 
 well known to us ; more especially the following 
 metals, which I have placed in the order of their 
 commercial monetary value : Plaiimim, Goldy Silver^ 
 Nickel, Mercury , Aluminium , Tin, Copper , Zinc, Lead, 
 Iron. Then we all feel somewhat familiar with the 
 gases — Oxygen, Hydrogen, Nitrogen, and Chlorine, 
 Leaving metals and gases, we come across the name 
 Carbon, which substance we know to occupy a very 
 important place in the universe, and even in our own 
 
 22 
 
what Things are Made of 
 
 bodies. We are composed chiefly of Carbon, Hy- 
 drogen, Oxygen, and Nitrogen, 
 
 Returning to the Hst of the ingredients of the world, 
 we find a number of other elements with which we 
 have some acquaintance, such as Phosphorus, Siilphtir, 
 Potassium, Sodmm, Arsenic, Antimony, Bromine, Cal- 
 citim, Cobalt, Iodine, Magnesitim, Selenium, Silicon, 
 and Uranium, To these we have to add Radium, 
 which remained a hidden treasure in nature until a 
 few years ago ; the discovery of which has proved of 
 immense importance to science, as we shall see later. 
 
 So far I have only mentioned thirty-one of the 
 known elementary substances. I doubt if the general 
 reader will recognise any of the remaining ingredients 
 of the world. Here are half-a-dozen of the most 
 curiously named — Yttrium, Xenon, Vanadium, Praseo- 
 dymium, Krypton, and Gadolinium, 
 
 Many of the elements never appear upon the lists 
 of chemicals for sale, and some have only been 
 obtained with great labour and care in the scientific 
 laboratory. We must remember, however, that a 
 substance may be worth much more than its weight 
 in gold from two totally different causes. The element 
 may only occur in very small quantities in the world, 
 or it may be that nature has so locked it up in 
 some compound substance that we require to expend 
 an enormous amount of energy to separate it. For 
 instance, you may buy a barrel of lime for a few 
 shillings, and we know that more than half of the 
 ingredients of the lime is composed of an elementary 
 
 23 
 
Scientific Ideas of To-day 
 
 substance called calcium. Suppose you said to the 
 seller that you would prefer to take only the calcium 
 which was contained in the barrel of lime. You were 
 aware that nearly three-quarters of the whole contents 
 were calcium, but you would be quite pleased with 
 half a barrel. If the seller were to carry through the 
 contract you would be rather surprised at his account. 
 Perhaps you had been willing to pay the total price 
 of the barrel of lime, or it may be that you were 
 expecting some reduction seeing you were only asking 
 for part of the contents of the barrel. Presuming 
 that you really had no previous knowledge of the 
 value of calcium, you would certainly think there 
 was some mistake in your account, for it would 
 not be less than fifty pounds, instead of the few 
 shillings you had expected to pay. It is strange, at 
 first sight, that any ingredient of common matter 
 should cost so much money, and yet be so plentiful 
 in nature. The comparatively high price of the 
 calcium is due to the expense of separating it from 
 its compounds. Some years ago the price of the metal 
 calcium was very much higher, as the means of pro- 
 ducing it then were still more expensive. 
 
 Thinking once more of the list of the fundamental 
 substances, one might suppose that if a man knew 
 the individual properties of each element, he should 
 be able to tell the properties of all the compounds 
 formed out of these. Far from it, for when these 
 elementary substances are in partnership with each 
 other they have entirely lost their individuality. It 
 
 24 
 
what Things are Made of 
 
 would seem quite natural to suppose that if we com- 
 bine two gases together we should merely make a 
 compound gas. It is true that we may easily make 
 a mixture of gases, but then that is just similar to 
 mixing sand and sugar together ; each retains its 
 own individuality. That is a totally different thing 
 from chemical combination. 
 
 When we were taught at school that ordinary water 
 was nothing more or less than a chemical combina- 
 tion of two gases, hydrogen and oxygen, we found 
 it very difficult to realise this fact ; it was certainly 
 not what we should have expected. Is this only a 
 theory, — that water is made of two gases, and nothing 
 else — or can we prove it ? The fact is very easily 
 demonstrated, for if we pass a current of electricity 
 through a vessel containing water, the water gradually 
 disappears, and if we adopt means of collecting the 
 gases rising from the water, we find that we have 
 nothing but hydrogen and oxgyen, (See illustration 
 facing page 314.) 
 
 You will notice how very completely these two 
 elementary substances lose their individual properties 
 when they enter into partnership with one another. 
 We know that hydrogen is a very inflammable gas, 
 but no sane person would attempt to set water on 
 fire. Then, again, many of us have boyhood recollec- 
 tions of some beautiful experiments with oxygen gas. 
 We found plenty of amusement in burning all sorts 
 of things in a bottle containing oxygen. We even 
 succeeded in burning pieces of steel watch-spring, 
 
 25. 
 
what Things are Made of 
 
 object he has been looking at is quite invisible to the 
 unaided vision — so small that it cannot be seen even 
 as a speck (see illustration facing p. 38), Here, 
 indeed, we have something very minute in size, and 
 yet these microbes are great giants compared with 
 the particles of which matter is composed. Why, 
 these very microbes each contain many thousand 
 millions of tiny particles. Surely we need not go 
 farther ; we can never hope to form an adequate 
 mind-picture of the size of these tiny bricks of nature. 
 We must simply picture all matter to be composed 
 of extremely minute particles, which we call atoms. 
 
 There will be just as many different kinds of atoms 
 as there are different elementary substances. There 
 is the atom of iron, the atom of gold, the atom of 
 hydrogen, the atom of oxygen, the atom of carbon, 
 and so on ; some eighty kinds of atoms being 
 known. We do not speak of an atom of water, for 
 the smallest particle of water which can exist as water 
 is made up of two atoms of hydrogen combined with 
 one atom of oxygen. This little combination of atoms 
 we call a molecule of water. It is clearly the smallest 
 particle of water which can possibly exist ; for if we 
 split it up it is no longer water, but becomes hydrogen 
 and oxygen gases. 
 
 A molecule is a group of atoms, but the atoms may 
 be all of one kind. We may speak of a molecule of 
 hydrogen, but then it is merely composed of two or 
 more atoms of hydrogen linked together. The mole- 
 cules of some compound substances are composed of 
 
 27 
 
Scientific Ideas of To-day 
 
 a large number of different atoms. For instance, a 
 single molecule of the compound known as alum 
 contains at least a hundred atoms, while the number 
 of atoms in each molecule of some other compound 
 substances is nearer a thousand. 
 
 We now picture the elementary atoms grouping 
 themselves into little congregations called molecules. 
 We picture the atoms grabbing hold of one another, 
 and we find that the different atoms have different 
 grabbing powers. For instance, when we combine 
 hydrogen and oxygen together, each oxygen atom is 
 able to grab two hydrogen atoms to itself. Therefore 
 when we break up water into its constituent gases, by 
 means of an electric current, we find that we get twice 
 the volume of hydrogen that we get of oxygen. The 
 copartnery agreement of the combination known as 
 "Water" reads that there shall be two members of 
 the Hydrogen family and one only of the Oxygen 
 family in the combination. 
 
 In our common table-salt there is a very simple 
 copartnery — one atom of sodium combining with one 
 atom of chlorine. Then again a single atom of gold 
 will grab three atoms of chlorine to form the chloride 
 of gold used in toning photographs. Other kinds of 
 atoms are capable of grabbing four atoms, while yet 
 some others have a greater appetite. The atoms of 
 some substances such as nitrogen and carbon have 
 various grabbing powers. Sometimes a nitrogen 
 atom will grab one atom, sometimes three, and at 
 other times five atoms. However, all we desire to 
 
 28 
 
what Things are Made of 
 
 notice at present is that the different elementary 
 atoms unite together in a great variety of ways, 
 and thus form the molecules of all compound sub- 
 stances. 
 
 We used to be taught at school that the atoms com- 
 bined together by a force called chemical affinity, but 
 as to the nature of that mysterious force we could 
 receive no enlightenment. It is only within compai-a- 
 tively recent years that we have recognised the fact 
 that chefnical affinity is nothing more or less than 
 electrical attraction between the different atoms. We 
 are all familiar with the phenomenon of electrical 
 attraction. Perhaps we have only seen it in the form 
 of an electrified rod attracting pith-balls or feathers, 
 but that the phenomenon may be observed with com- 
 mon every-day articles is shown in the photograph 
 facing page 54, Here we see an ordinary flower 
 vase, which has been well dried and briskly rubbed 
 with a silk handkerchief, attracting a bunch of feathers. 
 But we know that all bodies charged with electricity 
 do not attract each other. In the very earliest days of 
 experimental electricity it was observed that when a 
 glass rod was electrified, by rubbing it with a silk 
 cloth, the electrification was not the same as that of 
 a rod of sealing-wax similarly "excited." If one light 
 body was electrified by the excited glass rod, and 
 another light body was electrified by the sealing-wax, 
 those two light bodies did attract each other, but if 
 the two bodies were electrified from the same source 
 
 they invariably repelled one another. Two pith-balls 
 
 29 
 
Scientific Ideas of To-day 
 
 charged by the glass rod repelled each other, and if 
 both were electrified by the sealing-wax the same re- 
 pulsion took place. It was therefore quite evident that 
 similarly electrified bodies — or, in other words, bodies 
 charged with the same kind of electricity — would repel 
 each other (see illustration facing p. 66). It was also 
 apparent that the electrification from the glass rod 
 was not the same as that from the sealing-wax ; for 
 a body charged by the glass rod would not be re- 
 pelled, but would be attracted by a body charged 
 from the sealing-wax. 
 
 At first experimenters called the electricity pro- 
 duced by glass rods vitreous, and that from sealing- 
 wax resinous. When, however, it was suggested 
 by Benjamin Franklin that electi'icity was a single 
 mysterious fluid, he concluded that a body electrified 
 by a glass rod had an excess of the fluid, and so he 
 said it was positively electrified, or charged with 
 positive electricity. On the other hand, he supposed 
 that a body electrified by sealing-wax had a de- 
 ficiency of the electric fluid, and so he said that it 
 was negatively electrified — or, in other words, he said 
 that the sealing-wax produced negative electricity. 
 
 After a while people began to think that it was quite 
 ridiculous to speak of electricity as a fluid, but for 
 convenience' sake they retained the names positive and 
 negative electricities. To-day we have returned to 
 ideas not unlike Franklin's fluid theory ; but w^e shall 
 understand this better when we come to deal with the 
 present ideas concerning the structure of the atom. 
 
 30 
 
what Things are Made of 
 
 In the meantime, we shall be content to picture some 
 of Nature's atoms as being positively electrified, while 
 others are negatively electrified ; and we are familiar 
 with the idea of two oppositely electrified bodies attract- 
 ing each other. The hydrogen atom is electro-positive, 
 while the oxygen atom is electro-negative; these two 
 will therefore attract one another and become electri- 
 cally united, or, if we choose to call it so, we may say 
 that they are chemically united. We must be content 
 to accept this general statement until we are in a posi- 
 tion to see later how the atoms become electrically 
 charged. We shall see also how it is that atoms of 
 the one kind become electrically united. 
 
 So far we have formed quite a useful mental picture 
 of the structure of the molecules of matter. We see 
 the elementary atoms with their electrical charges 
 combining together, and thus forming the neutral 
 molecules. But even these molecules are far below 
 the range of the most powerful microscopes. We 
 think of the invisible microbe again, and try to realise 
 that it contains millions upon millions of individual 
 particles, or molecules, each of which contains several 
 atoms. We therefore picture a piece of solid iron as 
 being entirely composed of invisible atoms of iron. 
 
 The idea that a solid lump of matter can be en- 
 tirely composed of invisible things seems strange to 
 some people, but there is nothing mysterious in this. 
 Imagine yourself standing on some country-side 
 through which a very wide and dusty road passes. 
 The road after many windings goes up the face of 
 
 31 
 
Scientific Ideas of To-day 
 
 a far-distant hill, but as the white road is very wide 
 it is quite easy to follow its track on the distant hill- 
 side. You observe a man making his way along the 
 road towards the distant hill. As he proceeds on his 
 journey you observe that he appears to get smaller, 
 and by the time he reaches the far-distant hill he is 
 not visible even as a speck on the white road, which 
 we are supposing to have been made exceptionally 
 wide to suit our present purpose. The hill is so very 
 far away that even with a telescope you fail to see the 
 man. If you could never get any nearer to the man 
 he would remain invisible to you, but if a great army 
 of many millions of men appeared on the distant hill- 
 side you would then observe a dark patch. Here we 
 have the visible lump of solid matter composed of 
 particles which are quite invisible to us. 
 
 If we handle a piece of solid iron it is very apparent 
 that the invisible particles of which it is composed 
 must have a tremendous grip of one another. To this 
 force which binds the molecules together we have 
 given the descriptive title of cohesion, from the Latin 
 word cohcei'eo, meaning / stick. It is easy to demon- 
 strate the immense force with which the molecules 
 cling on to one another ; for if we take a bar of iron — 
 such as is used for making rivets, and measuring about 
 one square inch in section — we find that it re- 
 quires a pull equal to about twenty-five tons to draw the 
 molecules asunder at one place. Some steel wire will 
 stand a stress up to one hundred tons per square 
 inch. When we have succeeded in wrenching the 
 
 32 
 
What Things are Made of 
 
 molecules apart we need not place the severed parts 
 together in the hope that the molecules will cling 
 again to each other. It is apparent, therefore, that 
 the molecules require to be veiy, very close together 
 before they can attract one another. If we heat the 
 severed ends of the iron bar, we enable the molecules 
 to get within close range of each other again, and 
 we find that, when the bar has cooled, the little 
 particles have taken a powerful grip of one another 
 again. To form a clear idea of what happens in 
 this case we must try and picture the structure of 
 solid matter. 
 
 It almost goes without saying, that the molecules are 
 not like little solid bricks closely packed together. We 
 shall see later that we have definite experimental proof 
 that spaces do exist between the molecules. We must 
 picture all matter — even the most solid thing we can 
 think of — to be in reality porous ; steel, flint, marble, 
 glass, all are like sponges. 
 
 Further, we have long ago recognised the fact that 
 these little invisible particles can quiver or vibrate, 
 and that this vibration of the molecules is what we 
 popularly call its heat or temperature. We may set 
 the molecules of iron into a state of very rapid vibra- 
 tion by striking it with a heavy hammer. The iron 
 soon becomes so hot that we cannot touch it with 
 safety, and if we continue the hammering we soon 
 make the iron red-hot. Every body has some heat ; 
 if it has very little heat we say it is cold, but that is 
 merely by way of comparison. If the temperature of 
 
 33 C 
 
Scientific Ideas of To-day 
 
 the air in your sitting-room is 75 degrees Fahrenheit 
 (about 24 degrees Centigrade) you say it is unbearably 
 hot, but if your tea were served at the same tempera- 
 ture your remark would be that it was ridiculously 
 cold. A cold body could still be colder, therefore it 
 possesses some heat, and therefore its molecules are 
 in a state of quiver or vibration. Consequently we 
 picture the most dense solid as being composed of 
 isolated particles which are continually in motion but 
 never in actual contact. 
 
 Let us now consider the case of the severed bar of 
 iron. We heat the two ends either by hammering or 
 by placing them in a source of heat. In a fire we 
 have the molecules in a state of rapid vibration, and 
 these set the molecules of the iron into a similar condi- 
 tion. If we apply a very intense heat to the iron we 
 cause its molecules to make such long excursions that 
 they cannot attract each other so well as previously ; 
 they let go their firm hold upon each other, so that the 
 solid becomes a liquid mass. If we continue to apply 
 a very intense heat, the molecules completely lose their 
 hold upon each other, and the liquid mass becomes a 
 vapour or gas. But before the molecules have been 
 driven apart from their solid grip, wrought iron niust 
 be raised to a temperature of about 3000 degrees Fah- 
 renheit (about 1700 degrees Centigrade), and before 
 the little particles have been freed from their liquid 
 grip a further increase to 6000 degrees Fahrenheit 
 (3300 degrees Centigrade) must be attained. As soon 
 as the force (heat), which is driving the molecules 
 
 34 
 
what Things are Made of 
 
 apart, is withdrawn, the molecules once more get into 
 grips with each other returning from the gaseous con- 
 dition to the Hquid and then to the solid, provided 
 that is their natural condition at ordinary temperatures. 
 We take one more glance at the picture already 
 drawn of the construction of matter. We see all 
 bodies to be porous, and all composed of vibrating 
 molecules which are not in actual contact with one 
 another even in solids. We find the attractive force 
 of cohesion to be very much greater when the mole- 
 cules are closer together as in a solid, than when 
 farther apart as in a liquid. In a solid we picture 
 the molecules merely swinging to and fro like little 
 pendulums, whereas in a liquid we picture the mole- 
 cules, not only possessing this quivering motion, but 
 also being in some measure free to move about and 
 glide over each other. If we mix milk and tea to- 
 gether the molecules of the one liquid quickly pass 
 among the molecules of the other. That the mole- 
 cules of liquid will move about, of their own free will, 
 may be demonstrated by a simple experiment. If we 
 have a glass vessel partly filled with a solution of 
 copper sulphate — commonly known as blue-stone — we 
 may gently pour a quantity of water on to the top of 
 this blue liquid. At first we see the two liquids quite 
 separate, but gradually we find the molecules of the 
 copper sulphate making their way upwards, against the 
 force of gravity, and if left alone for a considerable 
 time, we observe from the colour that they have 
 permeated the whole water. 
 
 35 
 
Scientific Ideas of To-day 
 
 This phenomenon of diffusion is much more apparent 
 when the molecules have got completely beyond their 
 attractive distance of one another as in a gas. No 
 matter how small a quantity of gas is let free in a 
 glass vessel, the gaseous molecules quickly spread them- 
 selves throughout the whole space available. If the 
 stopcock of a gas-bracket is left open and gas allowed 
 to escape into a room, we soon become aware of the 
 presence of these molecules although we may be at 
 some distance from the gas-bracket. The molecules 
 do not take long to make their way among the mole- 
 cules of air and with it enter our noses and stimulate 
 the olfactory nerves which, acting upon the brain, 
 produce the sensation of smell. 
 
 So far we have dealt with three different conditions 
 of matter — solid, liquid, and gaseous — and in the 
 following chapter we shall consider what has been 
 called by some the fourth condition of matter. 
 
 36 
 
CHAPTER III 
 
 THE STUFF THAT ATOMS ARE MADE OF 
 
 The idea of atoms is very old — Have we proof tHat there are 
 particles smaller than atoms? — Weighing invisible objects — 
 How these smallest particles were discovered — Some interest- 
 ing electrical experiments — Suggestion of a fourth state of 
 matter — The electron — What we have found out about 
 electrons — They pass through a solid metal — The simple re- 
 lationship of energy, velocity, and mass — What the electrons 
 are — The size of an electron 
 
 Beginning with a lump of solid matter, we have seen 
 that it is built up of isolated particles called molectdeSj 
 and that these invisible molecules are composed of 
 smaller elementary atoms, which are electrically united 
 together to form the molecules. 
 
 Our next question is — What are the atoms made of ? 
 Without giving the subject much thought, some one 
 might reply that the atoms are made of gold, iron, 
 hydrogen, and so on, naming all the elementary sub- 
 stances known to the chemist. But that does not tell 
 us anything of the nature of the atoms ; these are 
 merely names which we have given to certain forms 
 of matter which we find we cannot break up into any 
 other substances in the way that the multitude of com- 
 pounds may be analysed, A certain bright-yellow, 
 and much-sought-after, metal we call goldj but the 
 
 37 
 
The Stuff that Atoms are Made of 
 
 by direct experiment that particles smaller than atoms 
 do exist. It may seem to the reader rather ridiculous 
 to say that we can definitely prove the existence of 
 such very small particles, when even the giant mole- 
 cules and atoms are hopelessly beyond the reach of 
 the most powerful microscope. His surprise will not 
 be lessened when he learns that we can measure and 
 weigh these ultra-microscopic particles — ^just as surely 
 as we can measure and weigh our own world and its 
 neighbouring planets. 
 
 Perhaps a rough analogy may be of some service 
 at the very outset. We cannot see a rifle bullet as it 
 flies through the air, but if we put an obstacle in its 
 path we soon become aware of its presence. We 
 could tell the velocity of a bullet without ever see- 
 ing it. The speed of projectiles has been determined 
 by means of an instrument called a chronograph. 
 This piece of apparatus is used largely in observatories 
 for recording the exact moment at which any observed 
 phenomenon occurs. The moment that the observer 
 sees a star cross a spider-web line in the eyepiece 
 of his telescope he presses an electric push, and 
 the chronograph, which may be situated at some 
 distance, records the exact time at which the contact 
 was made. In passing, it may be of interest to note 
 that the apparatus consists of a large cylinder driven 
 round by clockwork at a definite rate. A pen travels 
 slowly along the cylinder after the manner of the 
 trumpet on a phonograph. At the end of each 
 second the pen strikes a paper encircling the cylinder 
 
 39 
 
Scientific Ideas of To-day 
 
 and leaves a dot, thus spacing the paper off into 
 seconds. The pen is also under the control of the 
 distant observer ; when he presses the push the pen 
 makes an additional dot. Its exact position not only 
 indicates the particular second of time but the 
 thousandth part of the second during which the dot 
 was made. 
 
 In recording the speed of a flying projectile two 
 screens are erected at a measured distance from one 
 another, and as the flying bullet passes through these 
 it makes an electrical contact at each screen, and 
 causes the chronograph to record the exact time at 
 which it passed these two points. In this way the 
 velocity of the bullet is determined. No one will 
 suppose that we are going to find out about the 
 particles, of which atoms are made of, in this exact 
 fashion. I merely use this as a rough analogy of how 
 it is possible to gain exact information concerning an 
 invisible object. 
 
 First of all we wish to see how it was that these 
 invisible particles composing the atom were dis- 
 covered. The story is an interesting one. It had 
 been known for a long time that an electric spark 
 will pass more easily through a vessel or tube of 
 rarefied air than through the denser air at ordinary 
 atmospheric pressure. A very simple way of demon- 
 strating this is to attach an "electric egg" to an air- 
 pump, as shown in the photograph facing page 78. 
 The glass vessel is merely called an egg because of 
 its egg-like form. It is furnished with two brass rods, 
 
 40 
 
The StufF that Atoms are Made of 
 
 one of which is fixed to the base of the egg, while the 
 other slides through an air-tight passage at the top. 
 The whole vessel is air-tight, having an outlet at the 
 base which may be connected to the air-pump. By 
 arranging wires from the two brass rods to the ter- 
 minals of an induction coil, connected to a battery, 
 electric sparks can be made to pass between the two 
 little brass rods within the egg. We gradually separate 
 the rods until the sparking ceases, owing to the inter- 
 vening air-space offering too much resistance to the 
 discharge. 
 
 If we now pump out a little air we soon find the 
 sparking to recommence, showing that the thinner or 
 more rarefied air is a better conductor. As we go on 
 withdrawing the air we observe that the sparking 
 changes into a silent stream or thread of luminosity. 
 As the "vacuum" increases, the whole egg becomes 
 illuminated with a glow, then a little later the lum- 
 inosity breaks up into a number of narrow horizontal 
 discs or slices. We now find that the air has become 
 so rarefied that it has overstepped its good conducting 
 qualities, and we require to apply considerable elec- 
 trical pressure to cause a discharge to pass through this 
 high vacuum, 
 
 A number of interesting phenomena occur as the 
 exhaustion proceeds, and it may be remarked in 
 passing that an ordinary mechanical air-pump, such 
 as shown in the photograph, will not be able to 
 produce a good enough vacuum to obtain all 
 these phenomena, so that some other means, such 
 
 41 
 
Scientific Ideas of To-day 
 
 as a mercury air-pump, has to be adopted. How- 
 ever, what we wish to notice at present is that when 
 the exhaustion has reached a certain point all the 
 glow disappears from the interior of the globe, and all 
 would appear totally dark but for the fact that when 
 this high state of exhaustion is reached the walls of 
 the glass vessel begin to glow with a greenish phos- 
 phorescence. The colour of this phosphorescence 
 varies according to the ingredients of the glass. But 
 what is causing the glass to phosphoresce ? 
 
 Sir William Crookes, of London, who has done so 
 much of the pioneer work in this branch of science, 
 pictured a stream of radiant particles being shot like 
 bullets from the cathode terminal. These invisible 
 bullets strike the glass walls of the tube and cause 
 them to phosphoresce. If the air is not practically 
 all withdrawn, its remaining molecules suffer bombard- 
 ment and these become luminous, producing the glow 
 which at first filled the electric egg, and which may 
 be seen in the ordinary vacuum or "Geissler " tubes. 
 
 Crookes suggested that here we had matter in a 
 fourth state. In other words, we had only been fami- 
 liar with matter in three conditions — solid, liquid, and 
 gaseous. In the solid state we had the molecules of 
 matter holding on very firmly to one another. In the 
 liquid state they had in a great measure lost this hold 
 and were therefore farther apart and free to roam about 
 among themselves. In the gaseous state we had the 
 molecules still farther separated, all on the move, col- 
 liding against each other, and apparently repelling one 
 
 42 
 
The Stuff that Atoms are Made of 
 
 another. In the newly discovered fourth state, Crookes 
 suggested that they were "in a condition as far re- 
 moved from the state of a gas as a gas is from a liquid." 
 
 Crookes spoke of this state as radiant matter. It 
 was indeed a bold suggestion, but we shall see later 
 that his surmise has turned out to be correct. At the 
 time, however, this idea was not accepted ; the general 
 belief was that these flying particles were ordinary 
 material atoms. To-day the physicist has weighed 
 and measured the flying particles, and he has found 
 them to be very much smaller than the smallest 
 known atom, which is the hydrogen atom. 
 
 At the time of Sir William Crookes' discovery these 
 flying particles were called the cathode rays, because 
 they were shot off by the cathode electrode.^ Later 
 on Dr. Johnstone Stoney christened the particles 
 "electrons," while Professor J, J. Thomson, of Cam- 
 bridge, who has done so much in investigating the 
 structure of the atom, prefers to call them "cor- 
 puscles." Possibly the word electron will be more 
 distinctive to the general reader ; he cannot confuse it 
 with any other existing word. He is, therefore, better 
 able to keep the idea of the electron apart from ideas 
 of ordinary matter. We already have the word cor- 
 puscle signifying a minute animal cell, and although 
 
 ^ The cathode is the name given to the negative or leading-out electrode 
 of the tube. The positive or leading-in electrode is called the anode. 
 Some people find it difficult to remember which electrode is the anode 
 and which the cathode, but if one thinks of the electric current entering 
 and leaving, the words come in their alphabetical order — a before c — 
 anode (leading-in) and cathode (leading- out). 
 
 43 
 
Scientific Ideas of To-day 
 
 there cannot be any confusion between blood cor- 
 puscles and these flying particles in the vacuum tube, 
 yet the word corpuscle is more suggestive of ordinary 
 matter than the word electron is. I shall, therefore, 
 keep rigidly to the word electron. 
 
 Looking at a highly exhausted vacuum tube, through 
 which an electric discharge is passing, we do not see 
 the stream of flying electrons ; they are quite invisible. 
 We see only the glass phosphorescing under the bom- 
 bardment of these invisible bullets. By making the 
 cathode terminal of a saucer shape, we can focus the 
 shower of electrons to one spot on the glass. When 
 we do so we find that their path is always a straight 
 line. Here comes a fact which will seem very strange. 
 When a magnet is brought near the vacuum tube, we 
 see that the stream of electrons has been deflected from 
 its straight path, for they strike the glass at a lower 
 point. The more powerful the magnet the greater 
 will be the deflection of the electrons. All this will 
 seem strange to the reader who learned in childhood 
 that a toy magnet would attract pieces of iron and 
 steel but nothing else. Most of us have become fami- 
 liar with the fact that an electric current will be de- 
 flected by a magnet ; indeed, it is this force which 
 turns the wheels of our electric tramway cars and all 
 other electrically driven machinery. This stream of 
 electrons within the vacuum tube is similarly deflected 
 by a magnet, so it is apparent that the moving elec- 
 trons behave exactly like an electric current. 
 
 It is natural to suppose that these flying electrons 
 
 44 
 
The StufF that Atoms are Made of 
 
 are negatively electrified particles because they are shot 
 oif, or repelled, by the cathode or negative electrode. 
 This fact may be demonstrated in many ways^ per- 
 haps the simplest being to notice the direction in 
 which the electrons are deflected by a magnet. 
 
 It would be a very laborious task to count all the 
 sovereigns passing through the Bank of England. In- 
 deed the officials themselves do not trouble to count 
 out each thousand sovereigns. They simply weigh 
 out a certain quantity, and they know that they have 
 then a certain number of coins on the scales. The task 
 of actually counting the sovereigns in the Bank of 
 England would, however, sink into insignificance if 
 one were asked to count the number of invisible dust- 
 particles in the air of a room. A very clever experi- 
 menter, Aitken, of Falkirk (Scotland), devised means 
 of counting the number of dust-particles in the air. I 
 have given a description of Aitken's experiments in 
 Appendix No. IV,, at page 335, as these help us to 
 understand an even greater feat — the counting of elec- 
 trons. At present we are going to accept the statement 
 that it has been found possible to count the electrons, 
 and those who are willing to trouble with the details 
 relating how this seeming impossibility has been 
 accompHshed will find a general description in the 
 appendix referi'ed to. 
 
 From what follows, it will be seen that to be able 
 to count the electrons helps us to gain further infor- 
 mation about these invisible particles. For instance, 
 it is an easy matter to determine by experiment the 
 
 45 
 
Scientific Ideas of To-day 
 
 total amount of electrification carried by a quantity of 
 electrons, and being able to count how many electrons 
 there are, we can tell, by a simple division sum, the 
 amount of electricity carried by each electron. We 
 have seen already that the electron is negatively 
 charged, so we know further the amount of negative 
 electricity which it carries. 
 
 We shall be content to deal with general state- 
 ments here, leaving the details to be dealt with in 
 the Appendix. 
 
 Very elaborate experiments were carried out origin- 
 ally to determine the speed of the flying electrons in 
 a vacuum tube, and the calculated velocity turned out 
 to be enormous. It was found later that by subjecting 
 the stream of electrons to the deflecting power of a 
 known magnetic field, and also to the deflective in- 
 fluence of an electric field, the velocity of the particles 
 could be determined more easily. The results of 
 these experiments agreed with those got by the 
 more elaborate methods. 
 
 It was found that the speed of these flying electrons 
 varied under certain conditions. As the electrons are 
 shot off from the cathode terminal of the tube by 
 means of an electric discharge, it 'is a natural conse- 
 quence that their velocity will depend in part upon 
 the intensity of the electric discharge. Then it is easy 
 to realise that the velocity will also depend upon the 
 degree of vacuum existing in the tube ; the molecules 
 of air remaining in the tube will be in the way of the 
 flying particles and will impede them. If the exhaustion 
 
 46 
 
The Stuff that Atoms are Made of 
 
 is not very good, or " high," the speed of the electrons 
 may reach five thousand miles per second. This is 
 an enormous speed compared with that of a rifle 
 bullet which travels only about one-third of a mile 
 in a second. However, five thousand miles per 
 second is by no means the top speed of the electron. 
 Provided with a clear path in the form of a high 
 vacuum, and impelled by a considerable electric force, 
 the electrons will fly across the vacuum tube at a 
 speed of sixty thousand miles per second, or roughly 
 about one-third the velocity of light. It is difficult 
 to realise what such a speed means. We might think 
 of it as thirty times across the Atlantic in one second, 
 or from here to the moon in less than four seconds ; 
 but we must not imagine that it is possible to shoot 
 electrons across the Atlantic. We must provide them 
 with a clear space — a good vacuum — if they are to 
 attain these high velocities. 
 
 The foregoing suggests the question — Can electrons 
 be made to fly through the air ? It will be clear that 
 we cannot produce a stream of these electrons at all 
 unless we have a fairly good vacuum ; it was only 
 when we had withdrawn some air from the electric 
 egg that the sparking changed to a luminous thread 
 and ultimately to an invisible stream of cathode rays 
 — or in other words, flying electrons. We picture 
 the cathode terminal shooting the electrons off with 
 great velocity, but only to be stopped by the glass 
 walls of the tube. Could we not form a window of 
 some kind through which the electrons might continue 
 
 47 
 
Scientific Ideas of To-day 
 
 their flight into the open air ? I should not be the 
 least surprised to hear some one say that this would 
 be quite impossible, for any window that would let 
 the electrons escape would surely let the air into 
 the tube and the necessary vacuum would be gone. 
 The argument seems reasonable, but the facts prove 
 it to be wrong. Professor Lenard, of Germany, suc- 
 ceeded in making a vacuum tube with a window that 
 would not let the air in and yet would allow the fly- 
 ing electrons to escape. Judging from appearances 
 his window looks much more like a "shutter." It 
 was made of a thin sheet of solid aluminium metal. 
 When the flying electrons within the tube reached 
 this solid metal window they were not held up but 
 passed right through it. How did the professor know 
 that they had escaped ? They are invisible. Although 
 he could not see the flying particles he saw their track 
 in the open air, for as soon as they escaped through 
 the window they met with serious obstruction from 
 the surrounding air. The molecules of gases forming 
 the air were subjected to the bombardment of these 
 invisible bullets, and a faint phosphorescent glow 
 was produced, somewhat similar to that produced in 
 an ordinary geissler tube. The visible effect is small 
 and can only be seen in the dark, and only in the 
 immediate neighbourhood of the aluminium window. 
 If, on leaving the aluminium window they enter a 
 second vacuum tube, they will produce a very distinct 
 glow. However, their presence may be detected in 
 the open air by means of a phosphorescent screen. 
 
 48 
 
The Stuff that Atoms are Made of 
 
 The flight of the electrons in the open air is quickly 
 ended ; they cannot escape farther than about one 
 inch from the tube. What becomes of them ? Do 
 they fall down like spent rifle bullets ? As soon as 
 they have escaped they become attached to the 
 gaseous atoms of the air. In short, they are absorbed 
 by the atmosphere. 
 
 When these flying electrons escape into the air 
 they are called Lenard raySj after the brilliant experi- 
 menter, who so successfully plotted a way of escape 
 for the imprisoned particles. It must be understood, 
 however, that they are in reality the same cathode rays 
 or stream of electrons which we have within the tube. 
 As a matter of fact, Lenard himself believed that the 
 cathode stream was merely a stream of aether waves 
 or pulses. When Professor Schuster made some 
 calculations which gave clear proof that the cathode 
 stream and the Lenard rays were composed of particles 
 the idea seemed at first to be ridiculous. It was some 
 years before Lenard saw that Schuster was right. 
 
 When scientists were convinced that the cathode rays 
 were a stream of particles, Professor Lenard's experi- 
 ment had a great deal of significance attached to it. 
 The fact that these particles could pass through a 
 solid sheet of metal foil, through which the atoms of 
 the gases contained in the air could not pass, was 
 surprising, and seemed to indicate that these particles 
 were extremely small. The smallest atom is that of 
 hydrogen gas, the lightest substance known, and yet 
 that gas could not pass through the aluminium window, 
 
 49 D 
 
Scientific Ideas of To-day 
 
 We have seen that the velocity of flying electrons 
 has been determined, and it is of interest to know that 
 the mass of the electron, and also the energy exerted 
 by the flying particle, have also been determined, as 
 will be explained in Appendix IV, These three factors 
 lead us to a very interesting discovery. 
 
 It may be that some reader does not see clearly 
 the relationship of energy, velocity, and mass, but a 
 rough analogy should make the matter plain. It re- 
 quires a certain amount of energy to drive a nail into 
 a log of wood. If the carpenter selects a very light 
 hammer he must bring it down very rapidly upon the 
 nail. In this case we have a small mass, the hammer, 
 moving at a comparatively great velocity. If, on the 
 other hand, the carpenter uses a heavy hammer, he 
 finds that a comparatively slow velocity is sufficient to 
 drive the nail home. The same work may therefore 
 be done by a small mass moving at a high velocity as 
 by a greater mass moving at a lower velocity. I am 
 leaving out of account the loss of energy, which is not 
 the same in both cases. We have the three factors to 
 take into account — the amount of energy necessary, 
 the velocity, and the mass. It will be clear that if 
 any two of these factors are known the third may be 
 found by calculation. 
 
 In a former paragraph I mentioned the electric 
 charge of electrons. What bearing does this have 
 upon the subject ? It has a very definite meaning. 
 The mathematician has clearly proved that the inertia 
 of the flying electron is entirely due to its electrical 
 
 50 
 
The Stuft that Atoms are Made of 
 
 charge, and in fact that there is no electron apart from 
 the electric charge. This is indeed a strange thought, 
 and very difficult to grasp at first. An electron is no- 
 thing more or less than an electric charge in motion — a 
 unit or atom of negative electricity. 
 
 It is, of course, quite impossible to form any mental 
 picture of the actual size of an electron. To say that it 
 has about a thousand times less mass than a hydrogen 
 atom does not help us very much, for we have no 
 mental picture of the size of a hydrogen atom. To say 
 that it would require a regiment of one hundred thou- 
 sand electrons placed in a row to make up the diameter 
 of a molecule of ordinary matter, would only indicate 
 the relative sizes of these two ultra-microscopic objects. 
 Sir Oliver Lodge has suggested the following interest- 
 ing analogy to help us to realise the relative size of these 
 electrons to the atonis in which they exist. 
 
 Imagine a church one hundred and sixty feet long, 
 eighty feet broad, and forty feet high. The space con- 
 tained in this building is to represent an atom of 
 matter. Looking at this enormously magnified atom, 
 we should have great difficulty in seeing the electrons 
 contained in it. Each electron would be no larger 
 than the dot or full-stop at the end of this sentence, 
 and yet we shall see that these electrons are the stuff 
 that atoms are made of. 
 
 51 
 
CHAPTER IV 
 
 THE CONSTRUCTION OF THE ATOM 
 
 The electrons within the atom — Some analogies — A miniature 
 solar system — Different kinds of atoms — A picture of the 
 atom — Interesting experiments with floating magnets — Family 
 groups of atoms — Newlands* octaves — The periodic law — A 
 bold prophecy — Electro-positive and electro-negative — An 
 analogy — The atoms in partnership — Matter and electricity. 
 
 We wish to form some reasonable picture of the atom 
 composed of these infinitely small electrons. 
 
 It will be apparent that if the atom were built up of 
 electrons in the same way as a wall is composed of 
 bricks, a tremendous quantity of connecting cement 
 would be required to fill in the spaces between the 
 electrons. Picture the church in Sir Oliver Lodge's 
 analogy, as described at the close of the preceding 
 chapter. Try and imagine a few hundred small dots 
 spread throughout its whole interior. There would be 
 somewhere about a hundred feet of empty space be- 
 tween each tiny dot. But we are not to think of the 
 electrons as being fixtures in the atom in the sense that 
 currants are fixed in a cake. 
 
 In our school-days some of us used to play a game 
 in which the boys were divided into two parties. One 
 party took possession of some rising ground and tried 
 
 52 
 
The Construction of the Atom 
 
 to guard it against the attacks of the other party. Now 
 it is evident that unless we had enough boys to form 
 a complete protecting wall around the "castle," we 
 had to keep a good lookout for attacks at different 
 quarters. This was where the sport came in. Each 
 boy did his best to look after one particular section, 
 and it was only by rushing hither and thither that we 
 could hope to keep the enemy out. In other words, 
 one boy by keeping on the move was able to do the 
 work which otherwise should have been done by a 
 number of boys placed in fixed positions. If the de- 
 fending party were successful then the protected space 
 was as good as a solid square of boys. We picture the 
 electrons defending the atom by rushing about from 
 one point to another, with this difference, however, 
 that while the boys made excursions in all directions as 
 required, the electrons are moving in regular orbits. 
 
 Perhaps another analogy may help to make the 
 matter quite clear. Picture a child playing with a 
 hoop, and it is apparent that when the hoop is struck 
 by the child's stick, the hoop behaves just as though 
 it were a solid disc of equal weight ; the enclosed 
 space is protected by the surrounding hoop. Imagine 
 this hoop hung up in a horizontal position by some 
 invisible threads. We may strike it at any part of its 
 circumference and it is just as though we had a large 
 solid disc before us. Suppose, however, that instead 
 of a continuous hoop, we had a regiment of small 
 balls placed in a circle and leaving spaces between 
 each ball and its neighbours. You might strike be- 
 
 53 
 
Scientific Ideas of To-day 
 
 tween the balls and there is no idea of solidity there, 
 but let the circle of balls be set revolving at a high 
 speed, and if you then strike the circle your stick 
 will rebound just as though the circle were solid. It 
 will be apparent that the velocity must be great, but it 
 is not difficult to imagine the velocity being so great 
 that the balls might be placed at some considerable 
 distance from each other. If the velocity is increased 
 in a corresponding manner we may still have the 
 effect of a solid mass. This gives us a very rough 
 picture of the atom of to-day ; a congregation of 
 electrons revolving with enormous velocity in regular 
 orbits. We can see now how the far-scattered dots 
 are able to occupy the whole interior of the church, 
 in the analogy already referred to. 
 
 We see that the atom of to-day is in reality a minia- 
 ture solar system. We need not necessarily picture it 
 as a circle of electrons all in one plane ; the mathe- 
 matician merely prefers this arrangement because it 
 enables him to treat the subject best from a mathe- 
 matical point of view, and to make many interesting 
 deductions. However, we are not going to trouble 
 about mathematical problems, being content to accept 
 the results worked out by the authorities engaged upon 
 this subject. It will be sufficient for us to picture the 
 atom as a large congregation of electrons all moving 
 in regular orbits, one ring within another, and all 
 moving round at a very great velocity. We must 
 remember that all this energy is locked up within the 
 atom. We need not say any longer that anything is 
 
 54 
 
Electrical Attraction between Common Objects 
 
 If an ordinary flower vase be well dried and energetically rubbed with a silk hand- 
 kerchief, the vase will attract any light body, such as a feather, towards it. 
 
The Construction of the Atom 
 
 "as dead as a door-nail"; we picture every atom 
 composing the nail to be anything but dead in the 
 sense of being motionless. 
 
 But we have different kinds of atoms — those which 
 form gold, and those which form the dirt upon our 
 city streets. Are these atoms all made of the same 
 stuff — electrons ? We do believe so. Wherein then 
 does one kind of atom differ from another ? Simply 
 in the number and arrangement of its electrons. 
 
 So far we have pictured a multitude of electrons or 
 units of negative electricity congregated together to 
 form an atom. If this were all, we should have an 
 accumulated charge of negative electricity. Not only 
 that, but all these individual units of negative electricity 
 would repel one another, and our imaginary atom 
 would be dissipated. There imtst be an equal quantity 
 of positive electricity in the atom to produce an equi- 
 librium or balance. We cannot picture an equal 
 number of units of positive electricity, at least we 
 have never found such things to exist. We have 
 never found positive electricity apart from atoms of 
 matter, whereas we have become familiar with the 
 flying units of negative electricity in vacuum tubes. 
 Indeed we really know far more about these electrons 
 than we do about the atoms of matter. 
 
 As we cannot lay hands upon units of positive 
 electricity apart from the atom, we must form some 
 other reasonable picture of the positive electricity con- 
 tained within the atom. One picture suggested is that 
 of a uniform sphere of positive electricity throughout 
 
 55 
 
Scientific Ideas of To-day 
 
 which the electrons are distributed. The mathe- 
 matician is willing to accept this idea as a working 
 hypothesis, as he can thereby make reasonable deduc- 
 tions. The positive electricity attracts the electrons 
 to the centre of the sphere while the electrons repel 
 one another, and in so doing they tend towards 
 leaving the sphere altogether. In other words, the 
 electrons tend to fly away in all directions, but the 
 positive electricity pulls them back, and an equilibrium 
 is found 
 
 Not only has the mathematician been able to cal- 
 culate a great variety of arrangements of electrons 
 to produce possible atoms, but the experimentalists 
 have demonstrated a great variety of arrangements by 
 means of little floating magnets, or by small electrified 
 bodies floating on water. Such experiments with 
 different numbers of bodies show a great variety of 
 arrangement, different patterns or designs being 
 formed according to the number of bodies used in 
 the experiment. 
 
 It will be of interest to follow a few experiments of 
 this kind, and if one has any good means of equally 
 magnetising a number of steel needles it will be of 
 further interest to repeat the experiments. After the 
 needles have been magnetised, each is fixed in a small 
 cork, so that when the cork is floated on water the 
 needle will hang downwards in the water in a 
 vertical position. The needles are fixed so that 
 either all their north poles or all their south poles are 
 uppermost. If a number of such needles are thrown 
 
 56 
 
The Construction of the Atom 
 
 into a basin of water, to represent the electrons in 
 an atom, the needles will, of course, repel each other 
 and practically try to escape from the basin by 
 swimming to the edge, as shown in the first photo- 
 graph in the frontispiece. This is just what the 
 electrons would do in the atom, but for the con- 
 trolling charge of the opposite or positive electricity 
 drawing them to the centre. In our experiments we 
 therefore represent this controlling charge by placing 
 one pole of a magnet above the centre of the basin, 
 as shown in the second photograph. If we have 
 arranged the little needles with all their south poles 
 uppermost, we place the north pole of the controlling 
 magnet over the basin, as opposite poles attract one 
 another. 
 
 If three needles be thrown into the water they 
 arrange themselves so that they form the three corners 
 of a triangle. Four needles take up their positions 
 at the corners of a square, as shown in the second 
 photograph, while five needles similarly form a pen- 
 tagon or ^'five-sided square." When we throw in a 
 sixth needle we see a very interesting phenomenon. 
 The six needles do not attempt to form a hexagon or 
 *' six-sided square," but one needle goes to the centre 
 and the other five arrange themselves in a pentagon 
 as before. A seventh needle produces a phenomenon 
 of even greater interest ; one needle goes to the 
 centre, while the other six arrange themselves in a 
 ring at some distance from the central needle, 
 as seen in the third photograph. As we proceed 
 
 57 
 
Scientific Ideas of To-day 
 
 adding needle after needle many interesting changes 
 take place. 
 
 These, then, are the stable or equilibrium arrange- 
 ments, and they help us to form useful pictures of the 
 possible arrangements of electrons within the atom. 
 Elaborate experiments made in this way have verified 
 the arrangements suggested, by purely mathematical 
 calculations. 
 
 There is another point of great interest in connection 
 with the mathematician's stable configurations. He 
 finds that a great many of his different arrangements 
 of electrons are very similar to each other. For in- 
 stance, one of his possible atoms is constructed with 
 one electron at its centre and six others around it, as in 
 the third photograph, and as he proceeds tabulating all 
 the possible stable arrangements he finds another to 
 be just the same as the one mentioned, but with an 
 additional ring of eleven electrons outside of it, as 
 seen in the fourth photograph. Then further on 
 in his table he finds this larger arrangement with 
 still a further additional ring of fifteen electrons out- 
 side of it. Now if atoms are really constructed upon 
 this principle, we should expect to find some similarity 
 between the behaviours of certain different atoms in 
 nature. We should find certain groups of the atoms 
 having family likenesses, and therefore possessing 
 similar properties. We do actually find this in 
 nature, indeed this fact was recognised long before 
 any attempt had been made to dissect the atoms. 
 
 In our school-days we became aware how very 
 
 58 
 
The Construction of the Atom 
 
 similar potassium and sodium are to each other. 
 They are both very soft metals, which may be cut 
 quite easily with an ordinary penknife. They have 
 both a bright silver-like lustre when cut, but this soon 
 tarnishes or oxidises. They both possess the remark- 
 able property that when thrown upon a damp surface 
 they will blaze into flame. Potassium is the more 
 energetic in this respect ; it will even catch fire when 
 thrown into a basin of water, whereas sodium, under 
 the same circumstances, will also set up decomposition 
 of the water and evolve considerable heat, but will not 
 ignite. It does ignite, however, when merely placed 
 upon a damp surface. The chemist could tell us of 
 further properties which are common to both potassium 
 and sodium. 
 
 The chemist could also show us a third elementary 
 substance called lithhim, which is of the same silvery- 
 white appearance, and which is also a soft metal, 
 though not so soft as potassium and sodium. We 
 could not get lithium to catch fire on a damp surface, 
 but we should find that it possessed the same property 
 of decomposing water and evolving heat, though not 
 in sufficient degree to display the energetic combus- 
 tion that its two other relatives can. 
 
 Here we have a family group of three elementary 
 substances, and this is not an isolated case. We find 
 that all the other elements may be grouped into little 
 family parties in a similar fashion. The most interest- 
 ing point in this connection is that we do not require to 
 pick out the members of a family group by examining 
 
 59 
 
Scientific Ideas of To-day 
 
 their properties. If we know the atomic weights ^ of 
 the different elements we can divide them into their 
 respective families. 
 
 In 1863 John Newlands pointed out, in a letter to 
 the Chemical News, that if the elements were arranged 
 in the order of their atomic weights, beginning with 
 the highest and going down to the lowest — ^just like 
 the keys of a piano — those elements belonging to the 
 same family occurred at regular intervals in the scale. 
 Picturing the keyboard we select the note represent- 
 ing potassium, and we find that sodium is exactly an 
 octave above potassium, while an octave above that 
 again brings us to lithium. If we take the octaves 
 below potassium we find first of all the element 
 riihidiiuHj and then another octave lower ccesium. 
 Although the ordinary man is not familiar with these 
 substances, he learns from the chemist that they 
 possess a striking family resemblance to potassium, 
 sodium, and lithium. 
 
 The members of the other family groups were found 
 to ocpupy similar positions to each other. Later on 
 these octaves of Newlands were further elaborated by 
 the famous Russian chemist Mendeleeff, and also by 
 the great German chemist Meyer, and what is known 
 as the periodic law was established. 
 
 For our present purpose we need not trouble with 
 the full meaning of the periodic law. It means, in 
 
 1 The atomic weights are the proportions by weight in which the 
 various elementary substances unite together. The atom of hydrogen is 
 taken as the unit; hydrogen being the lightest of the elements. 
 
 60 
 
The Construction of the Atom 
 
 short, that if we know the weight of an atom of any 
 element, we may know its properties. It is of great 
 interest to note, however, that Mendeldeff placed such 
 confidence in this law, that he boldly prophesied the 
 existence of three other elements which had never been 
 discovered. He found three empty spaces in his 
 periodic table, which must be filled if the law was 
 perfect. He could tell what families these missing 
 elements should belong to, and so he boldly predicted 
 the chemical properties which those elements should 
 be found to possess when, at some future time, they 
 might be discovered. It is interesting to know that 
 Mendel^eff lived to see the discovei-y of the three miss- 
 ing elements, and to witness the fulfilment of his 
 predictions. One by one the missing substances were 
 brought to light, and each possessed the properties 
 predicted for it, or as our American cousins would 
 say, they exactly ''filled the bill." 
 
 This periodic law was established long before the 
 Cambridge physicists commenced their calculations as 
 to the possible arrangements of the electrons within 
 the different atoms. Professor J. J, Thomson now 
 suggests that the atomic weight of an element is pro- 
 portional to the number of electrons contained in its 
 atom. We must remember, however, that each con- 
 gregation of a certain number has a definite form of 
 arrangement. It is not at all analogous to having one 
 bag of sixty-seven marbles and another of sixty-eight. 
 We not only picture the electrons arranged in definite 
 
 designs of rings within rings, but we see a complete 
 
 6i 
 
Scientific Ideas of To-day 
 
 system of revolving planets ; a perpetual and regular 
 orbital motion. 
 
 The mathematician informs us that certain con- 
 figurations are not very stable, and, indeed, that some 
 are just on the verge of instability. For instance, one 
 arrangement has just sufficient electrons in the centre 
 to hold the outer ring together. If the outer ring 
 is disturbed by any external source, some of the 
 electrons in the outer ring may fail to return to 
 their former positions. The electrons are all flying 
 round at a great pace, so that any of them losing 
 hold of their system will fly off from it. These de- 
 tached electrons will immediately find a new home 
 in a neighbouring atom whose system is such that 
 it can readily accept them. We therefore picture a 
 continual interchange of a small number of detach- 
 able electrons between the atoms. Our simplest plan 
 may be to picture these detachable atoms as being 
 comets outside of the regular stable orbits. We shall 
 see later, however, that there are abnormal cases of 
 instability where the electrons fly ofF from the regular 
 orbits and are shot into the surrounding air with great 
 velocity, causing those wonderful phenomena asso- 
 ciated with radium in particular. Here we have the 
 atom really breaking up, which is quite a different 
 thing from the friendly exchange of detachable elec- 
 trons. 
 
 What difference does this friendly exchange of a 
 
 few detachable electrons make to the atom ? It means 
 
 that when an atom loses one or more electrons it no 
 
 62 
 
The Construction of the Atom 
 
 longer possesses a perfect electric balance. Some of 
 its negative charge has fled with the escaped electrons 
 while the positive electric sphere has remained con- 
 stant. The atom which has lost electrons consequently 
 becomes a positively charged body, for the positive 
 charge will preponderate over the reduced negative 
 charge; we call such atoms electro-positive, and the 
 more electrons the atom loses the more electro-positive 
 does it become. On the other hand, an atom which 
 has gained some electrons has necessarily added to 
 its negative charge, which then preponderates over its 
 constant positive sphere, so that such atoms become 
 electro-negative, and the more electrons gained the 
 more electro-negative does the atom become. We 
 should therefore find in nature some electro-positive 
 atoms and some electro-negative ones ; we should also 
 find different degrees of these conditions. These very 
 phenomena are well known to the chemist of to-day ; 
 he finds some atoms strongly electro-negative under 
 certain conditions, but less so under other conditions. 
 He finds similar degrees of variation in electro -positive 
 atoms. But we must remember that the interchange 
 of electrons with which we are dealing does not alter 
 the nature of the atom. A hydrogen atom is always 
 a hydrogen atom whether it happens to be more or 
 less electro-positive. If we could alter the number of 
 electrons in an atom to any degree we desired, we 
 could turn lead into gold, and mud into diamonds. 
 But we cannot perform such transmutations, for 
 neither the chemist nor the physicist can break down 
 
 63 
 
Scientific Ideas of To-day 
 
 the stable configurations of the atoms. We have evi- 
 dence, however, that Nature is a genuine alchemist, 
 and that she is carrying on actual transmutations of 
 which we were quite ignorant until the phenomenon 
 of radio-activity was discovered ; but, as already indi- 
 dicated, we shall consider this later. 
 
 In the preceding chapter we saw that chemical union 
 simply meant electrical union, and that an electro- 
 positive atom joined hands with an electro-negative 
 atom. We are better able to appreciate now the 
 reason why one strongly electro-negative atom of 
 oxygen can lay hold of two weaker electro-positive 
 atoms of hydrogen ; the negative charge of the oxygen 
 atom requires the positive charge of two hydrogen 
 atoms to produce an electrical equilibrium. The result 
 is a neutral molecule of water. 
 
 Another way of looking at the matter is to picture 
 the oxygen atom as being capable of accepting two 
 additional electrons when brought close to any atom 
 or atoms which are capable of losing such electrons, 
 A single hydrogen atom is only capable of losing 
 one electron, but two hydrogen atoms acting to- 
 gether can give the oxygen atom two electrons. 
 These three atoms therefore become electrically united, 
 or we may speak of it also as a chemical union. 
 
 Some reader may wonder why a mass of any ele- 
 ment does not exhibit an electric charge. If the 
 hydrogen atoms are electro-positive, why does the 
 gas in bulk not exhibit a positive charge ? When we 
 speak of the hydrogen atoms being electro-positive, 
 
 64 
 
The Construction of the Atom 
 
 what we really mean is, that they are capable of losing 
 an electron and thus becoming electro-positive. Left 
 alone, the hydrogen atoms are electrically neutral, 
 but as soon as they come within very close range of 
 oxygen atoms, two of the former each hand over an 
 electron to an oxygen atom, and thus upset the electric 
 equilibrium. It is this interchange of electrons which 
 produces the electrical charge of the atoms and causes 
 them to attract one another and form simple or com- 
 pound molecules. 
 
 It will be remembered that, in a former chapter, 
 when we were dealing with chemical union, there 
 appeared to be a difficulty if we considered the union 
 to be due to the attraction of opposite electric charges 
 in the atoms. Not only did electro-positive and electro- 
 negative atoms join hands — as in the case of an electro- 
 positive atom of sodium uniting with an electro-negative 
 atom of chlorine, to form that useful substance which 
 we call common table-salt — but an atom sometimes 
 acted as though it were electro-positive, and at other 
 times electro-negative. For instance, the compound 
 marsh-gas (fire-damp) is composed of one atom of 
 carbon and four atoms of hydrogen, both of which 
 are electro-positive to oxygen. We must therefore 
 understand that the terms electro-positive and electro- 
 negative are merely relative. Carbon may be electro- 
 positive to oxygen and yet electro-negative to hydrogen. 
 
 It is difficult to find a suitable analogy to illustrate 
 the foregoing fact. But perhaps it will be of some 
 assistance if we picture the atoms arranged in a scale 
 
 65 E 
 
Scientific Ideas of To-day 
 
 so that any atom may readily give up some of its 
 electrons to any other atom which happens to be 
 below it in the scale. We picture the atom which 
 is capable of accepting electrons to be electro- 
 negative, for it will then have surplus electrons ; the 
 atom losing electrons will represent the electro-positive 
 atom. We picture a certain kind of atom giving 
 electrons to another atom lower down in the scale, 
 and we say the former is electro-positive; but at the 
 same time we see that this same electro-positive 
 atom is able to accept electrons from another kind 
 of atom which is still higher in the scale, and in 
 this case it is no longer electro-positive, but becomes 
 electro-negative. 
 
 While the foregoing analogy may be of some assis- 
 tance, it is not complete. For instance, it does not 
 take into account the fact that two atoms of the same 
 element — say oxygen — unite together to form a molecule 
 of oxygen. Our analogy would suppose that because 
 both atoms are on the same level on the scale neither 
 could throw electrons at the other. The physicist can 
 adduce reasons, however, for supposing that when two 
 atoms of the same element come so near together that 
 the revolving electrons in the one atQ.m can exert force 
 upon the revolving electrons in the other, there is 
 an interchange of electrons which causes one of the 
 atoms to become electro-negative to the other. In 
 this way we may still picture two atoms of oxygen 
 as being electrically united to form a molecule of 
 
 oxygen. 
 
 66 
 
J 
 
 
 1 
 
 1 
 
 \ 1 
 
 Electrical Repulsion 
 
 Ifa tassel of ordinary sewing silk be briskly rubbed with a rubber tobacco pouch, 
 the silk threads will become similarly electrified and will therefore repel each other, 
 as seen in the lower photograph. The electrified threads will attract any unelectrified 
 body, so that those threads near the upright stand cling to it. 
 
The Construction of the Atom 
 
 Now we have formed a mental picture of the con- 
 struction of the atom. We see the electrons or units 
 of negative electricity incessantly whirling round in 
 regular orbits, and prevented from repelling each other 
 by the constant sphere of positive electricity within 
 which the revolving system of electrons is enclosed. 
 We picture certain stable configurations being due to 
 the number of electrons contained within the atom. 
 It is these different configurations which give us the 
 different propeiiies of atoms, or, in other words, con- 
 stitute the different elementary atoms. One configura- 
 tion we have called the atom of sodium. We can 
 never hope to see these atoms, even with the most 
 powerful microscopes, but when a vast multitude of 
 many millions of these atoms congregate together 
 we see a lump of matter which we call sodium. It 
 is a soft metal, with the peculiar property of catch- 
 ing fire when placed on a damp surface, as already 
 described. 
 
 Another configuration of electrons we call an atom 
 of chlorine ; it differs from the other in the number 
 of electrons and the resulting configuration, A vast 
 congregation of atoms of this kind produce a gas 
 which we recognise as chlorine. Its properties are 
 well known to all those who have studied chemistry. 
 But, marvellous to relate, when we have a great con- 
 gregation of these very same atoms — chlorine and 
 sodium — wedded together in couples, we have no 
 longer a gas and a metal, but a totally different sub- 
 stance which we use at the dinner-table to season our 
 
 67 
 
Scientific Ideas of To-day 
 
 food. One might feel inclined to say that salt is made 
 out of a gas and a metal, but that is really not a correct 
 idea. Salt is made out of two different kinds of atoms 
 — one of which goes to form a gas and the other a metal 
 — but these atoms themselves which go to form matter 
 are neither gas nor metal ; they are revolving systems 
 of electrons — pure electricity. 
 
 In short, we picture all matter, whether it be a 
 valuable diamond or a nauseating gas, to be composed 
 of atoms, and these atoms are nothing more or less 
 than little spheres of positive electricity within which 
 tiny units of negative electricity are constantly re- 
 volving in certain definite orbits, the one atom only 
 differing from another in the number and arrangement 
 of its negative units or electrons.^ 
 
 If this electron theory be true — then all matter is 
 made solely of electricity. A little fellow asked me if 
 there was any electricity in him, and he thought it a 
 huge joke when I said that he was made of electricity. 
 Of course, we must keep in mind that this is only a 
 theory, but the idea that our Earth is going round the 
 Sun was only a theory till we found so many facts 
 which fitted in with the theory that all thinking men 
 were willing to accept it as a fact. The electron theory 
 finds many facts to support it ; the electron has actu- 
 ally been separated from the atom, as in Crookes' 
 vacuum tubes, where we produce a real stream of pure 
 electrons. But we have not been able to separate the 
 
 * Whatever the sphere of positive electricity may turn out to be, there 
 is no doubt that it must vary also with the number of electrons. 
 
 68 
 
The Construction of the Atom 
 
 positive electricity in the same way, so the positive 
 sphere of electricity, in which the real electrons revolve, 
 is at present purely hypothetical. 
 
 There are other possible theories regarding the con- 
 struction of the atom, but the mathematics of this 
 theory which we have been considering have been 
 so beautifully worked out by Professor J. J, Thomson, 
 of Cambridge, that this theory seems to stand head 
 and shoulders above all others. No one will suppose, 
 however, that this theory, when completed, is to be 
 the final word of Science. Science must ever live and 
 progress from one step to another. 
 
 If matter is composed of negative electricity and 
 positive electricity, our next question will naturally be 
 —What is Electricity ? 
 
 69 
 
CHAPTER V 
 
 WHAT IS ELECTRICITY? 
 
 The nature of electricity — Franklin's original idea — Departure 
 from and return to Franklin's ideas — Positive and negative 
 electricities — A haphazard guess — What occurs when a glass 
 rod is rubbed with a piece of silk cloth — The meaning of 
 a discharge of electricity — What constitutes an electric 
 current — Volta's original discovery explained — How the 
 current passes along a wire — An analogy — The cause of 
 electrical resistance — How we increase electrical pressure 
 — Low-pressure currents — Insulators — Summary 
 
 Our thoughts regarding the nature of electricity are 
 very different from what they were only a few years 
 ago. The onlooker might say that our ideas in this 
 connection are retrograde, for it is a fact that our 
 present conceptions of electrical matters are not at 
 all unlike the very first ideas of Benjamin Franklin. 
 Franklin is perhaps best known to the general reader 
 by his historical experiments in drawing electricity 
 from thunder-clouds by means of flying-kites. But 
 Franklin was a very distinguished American statesman 
 as well as a great natural philosopher. 
 
 In the early days of electricity, about the year 1750, 
 Franklin suggested that electricity was a subtle fluid 
 pervading all matter. As time went on, the scientists 
 began to think that this was far too materiahstic an 
 idea of electricity. If one looks back over the literature 
 
 70 
 
What is Electricity? 
 
 concerning electricity, published between the date of 
 Franklin's theory and the advent of the present electron 
 theory, it is most apparent that writers considered elec- 
 tricity to be much more mysterious in nature than 
 Benjamin Franklin did. Indeed it is clear that the 
 writers ultimately endeavoured to fight shy of the word 
 electricity altogether, preferring to speak only of its 
 manifestations — the electric current, electrification, and 
 so on. To-day the electron theory brings us back to 
 much more materialistic ideas ; we feel quite familiar 
 already with the atom or tmit of negative electricity. 
 We might speak of atoms of electricity, but as the 
 word atom has such a definite idea of matter attached 
 to it, the word unit seems the better. Even the word 
 unit may convey too much of a purely mathematical 
 idea to some, so that we are quite pleased to find 
 that there has been a satisfactory christening, that the 
 unit of negative electricity is known as an " electron." 
 As regards the atom or unit of positive electricity we 
 are more ignorant. 
 
 In the light of present-day knowledge we see that 
 Franklin's one-fluid theory was really a very remarkable 
 forecast. Franklin said " the particles of this fluid 
 repel each other." That is exactly what our modern 
 electrons or negative particles do ; they repel each 
 other because like electricities are repellent. Franklin 
 said that the two different kinds of electrification 
 which had been observed — that produced by a glass 
 rod and that produced by sealing-wax — were simply 
 due to an excess and a deficiency of the fluid. It was 
 
 71 
 
Scientific Ideas of To-day 
 
 in this connection that the words positive and negative 
 electricity were introduced. A positively charged body 
 was supposed to possess a surplus of the fluid, while 
 a negatively charged body was supposed to have a 
 deficiency of the same fluid. Although the two 
 different kinds of electrification could be demonstrated 
 by simple experiments, there was nothing whatever to 
 indicate which body contained the excess and which had 
 the deficiency. There was nothing for it but to make 
 a haphazard guess as to which was which. Glass rods 
 were given the credit of possessing a surplus quantity 
 of the fluid particles when " excited," and were said 
 then to be positively electrified. We still speak of a 
 glass rod being positively electrified when it has been 
 rubbed with a piece of silk, but we do not suppose 
 that it contains a surplus of electrons ; we picture 
 exactly the reverse. We continue the old terminology 
 to save any confusion by a change, and it will be 
 observed from the preceding chapters that the intro- 
 duction of the idea of a constant positive sphere of 
 electricity in the atom prevents any real confusion. 
 The atoms of the glass rod have lost electrons during the 
 rubbing process, so that the constant positive spheres 
 preponderate. This idea enables us still to think of the 
 glass rod as being positively electrified. It is the nega- 
 tively charged body which has an excess of electrons, 
 but that too seems quite natural, because the addition 
 of small negative charges causes the total negative 
 charge to preponderate over the constant positive 
 
 charge. 
 
 72 
 
what is Electricity 
 
 ? 
 
 It may be of interest to form a graphic picture 
 of what happens when we rub a glass rod with a piece 
 of silk cloth. Electrons leave the glass rod and attach 
 themselves to the silk. The silk having gained a 
 multitude of little negative charges becomes negatively 
 electrified, while the atoms of the glass rod, having lost 
 these same negative units, show a preponderance of 
 their positive charges. But why was the flow of 
 electrons not the other way about ? Why did elec- 
 trons not go from the silk to the glass ? 
 
 Referring to the analogy suggested at the close of the 
 preceding chapter, we picture the atoms in the glass to 
 be higher in the scale and able to lose electrons to 
 the atoms in the silk ; this happens when the atoms 
 are brought as near to one another as possible in 
 the process of rubbing. But if we select some sub- 
 stance whose atoms are lower down in the scale 
 than the silk — say sealing-wax — then we can get the 
 silk to lose electrons to the seahng-wax. In this 
 case the silk becomes positively electrified, having lost 
 some electrons. 
 
 It will be noticed that the electrical condition of the 
 silk depends entirely upon which substance it is rubbed 
 against. Its position is relative to the others, but we 
 must not think from this that the terms positive and 
 negative electrifications are relative conditions. We 
 must firmly fix in our minds that if a body is positively 
 electrified it means that it has lost electrons, and if it is 
 negatively electrified it means that it has gained elec- 
 trons. These are definite conditions, and not degrees 
 
 73 
 
Scientific Ideas of To-day 
 
 of one condition. We have different degrees of 
 positive electrification and of negative electrification, 
 but the two conditions positive and negative are 
 exactly the reverse of each other ; the one being a 
 deficiency of electrons below the normal condition 
 of the substance, while the other condition is an 
 excess of electrons over the normal condition. We 
 have, therefore, a graphic picture of the silk cloth 
 gaining electrons when rubbed against the glass rod, 
 and, on the other hand, losing electrons when rubbed 
 against the sealing-wax, 
 
 I am well aware of the difficulty which the average 
 layman has with the terms positive and negative elec- 
 tricities. When dealing with the practical applications 
 of electricity in former works addressed to the general 
 reader, I have avoided the use of these terms, but it 
 will be clear that in dealing with the science of elec- 
 tricity it is an absolute necessity to speak of positive 
 and negative electricities, unless we fall back upon the 
 older terminology of vitreous and resinous electricities, 
 I believe the late Lord Kelvin would have been pleased 
 to see this older terminology adopted had it been 
 possible to make such a drastic change. The repeated 
 use of the terms positive and negative, however, dispels 
 any feelings of mystery, and I trust that with what has 
 already been said in this and the foregoing chapters 
 the meaning of the terms will remain perfectly clear. 
 
 It will be of interest to form a definite mental picture 
 of what we mean by a discharge of electricity. Perhaps 
 the simplest way will be to think of the discharge 
 
 74 
 
what is Electricity? 
 
 which takes place in a highly exhausted vacuum tube. 
 Electrons are shot off from the cathode or negatively 
 charged terminal. An electrical discharge is therefore 
 a discharge of electrons. It is always the body with the 
 excess of electrons, or in other words, the negatively 
 charged body, which discharges the electrons. The 
 discharge is really from the negative to the positive. 
 
 It will be remembered that the stream of flying 
 electrons in the Crookes' tube behaved exactly like a 
 conductor carrying an electric current. The stream 
 was deflected by an ordinary magnet, just in the 
 same way as a wire carrying an electric current is 
 deflected by a magnet. Are we to understand from 
 this that moving electrons constitute an electric 
 current ? That is our creed. We believe an electric 
 current to be nothing more or less than an electron 
 current. 
 
 We can prove by experiment that electrified spheres 
 in motion produce all the properties of an electric 
 current. We believe all electric currents to be electrons 
 in locomotion. What happens then when an electric 
 current flows along a copper wire ? We picture the 
 copper atoms very closely packed together, so close 
 that we cannot compress the metal perceptibly. It 
 will be evident that the nearer the atoms are to one 
 another the easier will it be for one atom to hand on 
 a detachable electron to its neighbouring atom. We 
 picture roaming electrons within the metal. If we can 
 only apply some external force to cause a flow of 
 electrons in one direction from atom to atom, we shall 
 
 75 
 
Scientific Ideas of To-day 
 
 have an electric current. We have several convenient 
 means of setting and maintaining the electrons in 
 motion. 
 
 More than a century has passed since Professor 
 Volta, of Pavia (Italy), discovered that when a piece 
 of zinc touches a piece of copper the zinc becomes 
 very slightly positively electrified and the copper cor- 
 respondingly negative. In the light of the electron 
 theory we should say that when the zinc and copper 
 come into contact some electrons escape from the zinc 
 atoms and find a home on the copper atoms. We 
 may imagine that there is a natural desire on the part 
 of the zinc atoms to give their spare electrons to the 
 copper atoms, but they cannot do so until the atoms 
 are brought comparatively near to one another by the 
 lumps of metal being placed in contact. The zinc 
 atoms cease giving up electrons as soon as the copper 
 atoms have accumulated sufficient electrons to balance 
 matters between them. 
 
 It may be remarked in passing that zinc is always 
 very willing to give up a large quantity of detachable 
 electrons. The late Professor Hertz, of Carlsruhe 
 (Germany), who laid the foundations upon which 
 wireless telegraphy has been worked out, discovered 
 and demonstrated the fact that zinc will part with 
 electrons with very slight encouragement to do so. 
 He took a plate of zinc and caused the light of an arc 
 lamp, or other source of ultra-violet light,* to fall upon 
 
 ^ Ultra-violet light simply means light which Is beyond the violet end 
 of the spectrum. Invisible rays which have an energetic chemical effect, 
 as will be explained later. 
 
 76 
 
what is Electricity? 
 
 it. The zinc plate showed signs of becoming positively 
 electrified. The plate was previously connected to a 
 sensitive electrometer which would indicate any change 
 of electrical condition in the zinc. The fact that the 
 plate now exhibited a positive charge proved that 
 electrons had escaped, these having been set free by 
 the bombardment of the ultra-violet light. It is an 
 interesting fact that when air is blown along the plate 
 the expelled electrons are carried away, attached to the 
 molecules of air, and a further expulsion of electrons 
 from the plate is more easily carried on until the zinc 
 plate shows a considerable charge of positive elec- 
 tricity. For the sake of those who know something of 
 electrical measurements, I may mention that this charge 
 sometimes reaches as high a pressure as thirty volts. 
 
 We have seen that a piece of zinc will give up its 
 spare electrons whenever it has an opportunity of 
 doing so, but in the cases which we have been con- 
 sidering, the atoms have remained fixed in their original 
 positions and have merely given off a spare electron 
 or two. Suppose that we give the atoms an oppor- 
 tunity of moving away from their anchorage, we shall 
 see how much more willing they are to give away 
 electrons. When a piece of zinc is placed in a 
 solution capable of dissolving it, some atoms are 
 freed from the solid metal and these atoms very 
 quickly part with electrons, indeed they seem only 
 too willing to leave their detachable electrons behind 
 them in the solid metal and to escape without them 
 into the solution. 
 
 77 
 
What is Electricity? 
 
 wire connecting the zinc to the copper ; we have 
 a continuous electric current in the wire. We may 
 make the wire long enough to reach an electric bell 
 in an adjoining room and then come back to the 
 copper, so that the electric current must pass through 
 the electric bell before it can get from the zinc to the 
 copper.^ 
 
 I have endeavoured to keep up the idea of the zinc 
 atoms giving electrons to the copper atoms, and we 
 may still do so if we picture the copper wire as being 
 an extension of the copper plate. We might bend the 
 copper plate so that its outer end touched the top of 
 the zinc plate, but this would not be so convenient 
 as making a connection with a piece of flexible wire. 
 This wire need not be of copper ; it might be gold, 
 silver, or iron. It is better, therefore, that we should 
 picture the atoms handing along the detachable elec- 
 trons. It may be of service to repeat an analogy 
 which I suggested in '' Electricity of To-day." 
 
 There is a game which I have seen children play, 
 and which may serve as an analogy of electrical con- 
 duction in metals. The children stand in a long row, 
 and at one end of the row is placed a heap of objects 
 — say, a large number of pennies. At a given signal 
 the children pass the coins along from one to the other 
 till they reach the other end of the row, where they are 
 deposited in a heap. No child is allowed to accept a 
 coin till he or she has passed on the previous one ; 
 
 ^ We cannot stop to consider the action of the electric current in the 
 bell; I have already explained the practical application in "Electricity 
 of To-day." 
 
 79 
 
Scientific Ideas of To-day 
 
 This behaviour on the part of the escaping atoms 
 completely alters the former condition of things. 
 When the zinc plate was merely put in contact with 
 a piece of copper the zinc atoms were able to give 
 off a few electrons to the neighbouring copper atoms. 
 But when the zinc is placed in a dissolving solution, 
 the atoms escaping from the zinc leave their detach- 
 able electrons behind them so that the zinc plate soon 
 accumulates a surplus of electrons, and therefore it will 
 be all the better able to give electrons to the copper 
 atoms. We picture a piece of solid copper placed 
 in the solution beside the zinc but not touching it. 
 The zinc is accumulating electrons, so we form a 
 bridge by which the surplus electrons can make their 
 way to the copper. This we do when we join the 
 outer end of the zinc by a piece of copper wire to the 
 outer end of the copper. We may imagine the zinc 
 atoms as having another opportunity of giving 
 electrons to the copper atoms, and as the zinc atoms 
 have accumulated a large quantity of detachable elec- 
 trons formerly belonging to those atoms which have 
 escaped into the liquid, the transfer of electrons 
 between the zinc and the copper is therefoi-e much 
 more energetic than was the case when zinc and 
 copper were merely placed in contact. It will be 
 apparent that as long as we keep up the chemical 
 action, or in other words, as long as more atoms keep 
 escaping into the Hquid and leaving spare electrons, 
 the remaining atoms are able to keep up the supply. 
 We have a constant stream of electrons along the 
 
 78 
 
a oT 
 
 u 3 
 
 ^ O rt 
 
What is Electricity? 
 
 wire connecting the zinc to the copper ; we have 
 a continuous electric current in the wire. We may 
 make the wire long enough to reach an electric bell 
 in an adjoining room and then come back to the 
 copper, so that the electric current must pass through 
 the electric bell before it can get from the zinc to the 
 copper.^ 
 
 I have endeavoured to keep up the idea of the zinc 
 atoms giving electrons to the copper atoms, and we 
 may still do so if we picture the copper wire as being 
 an extension of the copper plate. We might bend the 
 copper plate so that its outer end touched the top of 
 the zinc plate, but this would not be so convenient 
 as making a connection with a piece of flexible wire. 
 This wire need not be of copper ; it might be gold, 
 silver, or iron. It is better, therefore, that we should 
 picture the atoms handing along the detachable elec- 
 trons. It may be of service to repeat an analogy 
 which I suggested in " Electricity of To-day.*' 
 
 There is a game which I have seen children play, 
 and which may sei-ve as an analogy of electrical con- 
 duction in metals. The children stand in a long row, 
 and at one end of the row is placed a heap of objects 
 — say, a large number of pennies. At a given signal 
 the children pass the coins along from one to the other 
 till they reach the other end of the row, where they are 
 deposited in a heap. No child is allowed to accept a 
 coin till he or she has passed on the previous one ; 
 
 ^ We cannot stop to consider the action of the electric current in the 
 bell; I have already explained the practical application in "Electricity 
 of To-day." 
 
 79 
 
Scientific Ideas of To-day 
 
 the action has to be simultaneous along the line. A 
 second row of the same number of children stands 
 parallel to the first row, and these are also provided 
 with an exactly similar number of pennies. The game 
 is, of course, a battle royal between the two parallel 
 rows as to which row can transmit the whole of the 
 coins in this fashion from the one end of their line to 
 the other in the shortest space of time. Only one row 
 of children concerns us in our analogy, and we picture 
 the little ones as representing the atoms in a length of 
 metal wire. Each atom passes on an electron to its 
 neighbour, and accepts another electron from the 
 neighbour on its other side. For the sake of analogy 
 we start the game with each child having one coin in 
 his or her hand, so that the moment the signal is 
 given, representing the closing of the electric circuit, a 
 complete transfer commences simultaneously all along 
 the line. Instead of having a heap of coins at one 
 end, we might arrange the children in a circle and 
 give them one coin each, so that the coins would pass 
 round and round the circle. This is what we under- 
 stand by a complete electric circuit ; a battery or 
 dynamo acting as a pump in the circuit. We may 
 break the complete circuit, and then there can be no 
 passing on of electrons. 
 
 The first arrangement of the children's game, in 
 which we had the children standing in a row, is some- 
 what analogous to an earth circuit in electrical affairs. 
 The first child kept picking up coins, passing them 
 along, and the last child deposited the coins in a heap 
 
 80 
 
What is Electricity? 
 
 as they were received. We imagine, therefore, the 
 first atom at one end of the wire, which is dipping 
 into the Earth, to be helping itself to electrons one 
 at a time, passing them along, and the last atom at 
 the other end of the wire depositing these electrons 
 in the Earth. There is, of course, a battery or a 
 dynamo again acting as a pump, and we do not 
 have a simple row of atoms, but a myriad of atoms 
 acting simultaneously. 
 
 As the electrons pass along from one atom to 
 another they meet with some obstruction. Perhaps the 
 matter may be simplified by the following analogy. 
 In our schooldays we would sometimes form a circle 
 of boys on the cricket-field, and amuse ourselves by 
 passing the ball quickly from one to another round the 
 circle. It is obvious that the ball will meet with a 
 sudden obstruction at each forward step. It is this 
 kind of obstruction in the passage of the electrons that 
 we call electrical resistance. It is not difficult to picture 
 one set of cricketers much more expert in passing the 
 ball round the circle than another set might be, so 
 that the ball would really pass more easily round the 
 first circle. In the same way we find the atoms of 
 some metals are able to pass on the electrons much 
 better than the atoms of other metals are able to. We 
 speak, therefore, of good electrical conductors and of 
 poor conductors or insulators. All metals are really 
 good conductors, although some are much inferior in 
 this capacity to others. For instance, the electrons 
 meet with six times more resistance in passing along 
 
 8i F 
 
Scientific Ideas of To-day 
 
 an iron wire than they do in a copper wire of the same 
 size. If we wish an iron wire to carry the electron 
 current from one town to another, we must provide a 
 larger number of atoms to do the carrying, than we 
 should require if we used a copper wire. Hence an 
 iron telegraph wire is much thicker than a copper one. 
 The difference between the sizes of the two wires, when 
 suspended on telegraph poles, is very noticeable. The 
 general practice, until recently, was to use iron wires 
 for telegraph lines, whereas the telephone companies 
 have always used copper wires. As one walks along a 
 country road one can easily pick out the heavier iron 
 wires of the telegraph lines. 
 
 If we picture a battery acting as a pump and driving 
 the electrons along the wire joining the zinc plate to 
 the copper plate, it will be obvious that the longer we 
 make this connecting bridge the more resistance will 
 the electrons encounter. If we have a long telegraph 
 line acting as a bridge for the electrons, we must 
 apply greater pressure than can be obtained by a single 
 chemical cell such as we have been considering. We 
 may couple a number of such cells together, and in 
 this way increase the pressure. There are two ways of 
 coupling such cells together. We might connect the 
 zinc of one cell to the zinc of the next cell, and the 
 copper of that cell to the copper of the next cell, then 
 the zinc of the third cell to the zinc of the fourth cell, 
 and so on. But this would not increase the pressure. 
 The combined zincs could certainly provide a greater 
 quantity of electrons, and we could have a larger 
 
 82 
 
what is Electricity 
 
 ? 
 
 current, but the increased army of electrons would 
 have no greater stimulus to force their detachable 
 electrons away. Each zinc plate would simply add its 
 normal contribution to the combined flow. If, how- 
 ever, we connect the zinc of one cell to the copper of 
 the next cell, and the zinc of the second cell to the 
 copper in the third cell, and so on, zinc to copper, 
 zinc to copper, we shall have a totally different result. 
 We picture the first zinc passing on its accumulation 
 of electrons along the wire bridge to the copper in the 
 second cell. This copper passes on these electrons 
 through the liquid to the zinc standing in the same 
 cell, whereupon this zinc has not only its own accu- 
 mulation of electrons but as many more which have 
 been passed on to it from the first cell. The second 
 cell has therefore a greater pressure of electrons accu- 
 mulated on its zinc, and as we add cell to cell in this 
 fashion we go on increasing the pi-essure along the 
 connecting bridges. In the first case, in which we 
 say the cells are connected in parallel^ we have a low 
 pressure, and we therefore require a correspondingly 
 thick wire to carry a large current. A thinner wire 
 would carry the same current when we connect the 
 cells ill series J as in the second case. In the same 
 way we can do with a smaller bore of water-pipe if 
 we increase the pressure of the water. There is 
 another point of interest here. If we increase the 
 water-pressure to any great extent we must increase 
 the thickness of the metal pipe, or the water may force 
 an escape. In the same way we must increase the 
 
 83 
 
Scientific Ideas of To-day 
 
 insulation of a wire carrying a high-pressure electric 
 current. 
 
 In the case of a wire carrying" a high-presstuje 
 electric current, it so happens that the atmosphere 
 makes a very good insulator, but we have to see that 
 the supports of the wires are better insulators than are 
 necessary for wires carrying a low-pressure current. 
 Glass, vulcanite, and porcelain make good insulators ; 
 electrons meet with a tremendous resistance in trying 
 to pass through these substances. For all practical 
 purposes we consider that they completely block the 
 way of the electrons. If the electrons are forced 
 through them with an immense pressure, such as may 
 be obtained by a very large induction coil, the passage 
 of the electrons may rupture the glass. 
 
 At the Royal Institution (London) I have seen a 
 block of glass, three inches thick, which was fractured 
 by the electrical discharge of a large induction coil. 
 The glass was pierced right through, and it was no 
 mere pin-hole, but looked exactly as though some 
 machine-tool had been forced through and left a hole. 
 This is what takes place in quartz glass ; when 
 flint glass is used there is a complete fracture of the 
 block. I remember seeing Sir Oliver Lodge fracture 
 a thick glass tumbler, by an electrical discharge, in the 
 late Lord Kelvin's laboratory. 
 
 In the earlier part of this chapter we have con- 
 sidered the action of a chemical cell, in which the 
 elements were a plate of zinc and a plate of copper, 
 and while this is not a common form of cell at the 
 
 84 
 
What is Electricity? 
 
 present day, it gives us the general principle under- 
 lying the action of all batteries. A very common kind 
 of "battery/' in general use to-day, is a piece of zinc 
 and a piece of carbon immersed in a dilute solution 
 of sal-ammoniac. Our present purpose is not to deal 
 with the practical arrangements of different cells, but 
 merely to consider the scientific ideas concerning the 
 action of cells. 
 
 To sum up, we have seen that a charge of electricity 
 simply means an accumulation of electrons on one 
 body, and a corresponding deficiency on another 
 body. We may have wondered, at some time or 
 other, that when two bodies are rubbed together the 
 charge on the one body is always exactly equal and 
 opposite to the charge on the other body. The matter 
 is very simple ; the result cannot be otherwise. The 
 one body has lost a certain amount of electrons, and 
 the other body has gained the very same electrons. 
 A discharge of electricity is simply a discharge of 
 electrons from one body to another. 
 
 We have also seen that an electric current is merely 
 a current of electrons, and unfortunately we have 
 been in the habit of 'picturing this current flowing 
 in the opposite direction to that in which the electron 
 theory shows the real current to flow. The flow of 
 electrons is from the point at which there is an 
 accumulation, or in other words, from the negative 
 terminal to the point of deficiency, which is the 
 positive terminal. We have always considered the 
 current flowing from positive to negative, but our 
 
 85 
 
Scientific Ideas of To-day 
 
 mistake has been due to the early electricians attach- 
 ing the terms positive and negative to the wrong con- 
 ditions, as already explained. However, as long as 
 we keep in mind that it is a current of negative 
 electricity — or electrons — with which we are dealing, 
 there should be no cause for confusion. 
 
 If the electron theory be true, and as far as it takes 
 us it does seem to be true, then we have a fairly clear 
 idea of the meaning of an electric charge, and of an 
 electric current, but we have not answered the ques- 
 tion — What is Electricity ? Until we answer this 
 question we cannot say what the electron is ; at 
 present we merely know it to be a negative charge of 
 electricity. 
 
 It may be well to state clearly that no one knows 
 what electricity is, but we shall proceed to see upon 
 what other phenomena the electron theory sheds new 
 light. Before proceeding further, however, it may 
 simplify matters if we seek to become familiar with 
 the mysterious aether of space. 
 
 86 
 
CHAPTER VI 
 
 WHAT IS THE ^THER? 
 
 The spelling of the word aether — The existence of an all- pervading 
 medium — Wave-disturbances in water, air, and fether — 
 Diversity of waves in aether — Light and darkness — Heat does 
 not travel from the Sun — What travels ? — Transformations — 
 Invention of the idea of an aether — Its first reception — There 
 must be a medium — Demonstration of its existence — Not 
 ordinary matter — MendeMeff's idea. A possible theory of 
 the future — Analysing aether waves — X-rays — " Wireless" 
 waves — Velocity of aether waves — Constant velocities — Fields 
 in the aether 
 
 Whenever one picks up any book of modern science, 
 which treats of the physical side of things, one is sure 
 to find continual reference to the cether of space, A 
 very natural question arises as to what this aether is. 
 
 It is more common to spell this word etherj but as we 
 have a chemical combination of ordinary matter called 
 ether, it would seem wiser to spell the word cether when 
 it refers to the space-filling medium. It may be that 
 people fight shy of the latter spelling because of the 
 trouble in writing a diphthong, or it may be that there 
 is really no reasonable excuse for any one confusing this 
 space-pervading "something" with ordinary matter. 
 I do not suppose that any one is going to think of this 
 universal medium as being the same as that ether 
 which is made from alcohol, and is now so largely used 
 
 87 
 
Scientific Ideas of To-day 
 
 in place of chloroform. Nevertheless the word aether 
 seems to take us farther away from ideas of ordinary 
 matter. I shall, therefore, use the latter spelling, 
 although when one becomes familiar with the idea 
 of the all-pervading medium it matters very little which 
 spelling is used. It goes without saying that the 
 aether is not any form of ordinary matter, but let us 
 first of all try to realise the existence of the aether. 
 
 The idea of an all-pervading medium is not a wild 
 dream or mere speculation. If we study observed 
 facts we are forced to admit the reality of the aether. 
 The scientist is as sure of the existence of the aether 
 as he is of his own existence. It would be quite un- 
 reasonable, despite all the logic of John Stuart Mill, 
 to imagine one body acting upon another unless there 
 be some intervening medium between the two bodies. 
 To take a simple illustration, we may imagine two 
 men swimming about in a quiet pool of water. One 
 of the men might set up a series of waves in the water, 
 so that these would travel to his companion and 
 attract his attention. Observe that nothing really 
 passed from the one man to the other ; the intervening 
 medium was disturbed, and in this way the one body 
 acted upon the other, though at some distance from it. 
 The water did not travel from one man to the other ; 
 it was merely the wave-disturbance that travelled, 
 
 To take another illustration, we may imagine a 
 
 church-bell to be tolling in a distant steeple on a 
 
 quiet morning. The bell, though fixed at a certain 
 
 place, is acting upon the hearing apparatus of people 
 
 88 
 
What is the ^ther? 
 
 at a considerable distance. There is nothing passing 
 out from the bell to the distant audience. The bell is 
 merely disturbing the intervening medium — the air — 
 and in turn the vibrating air disturbs the ear-drums of 
 the listeners. Again it is merely the wave-disturbance 
 which has travelled. 
 
 One other illustration brings us to the point we 
 wish to realise. We imagine a dark, wild winter's 
 night, and we see a great lighthouse flashing out its 
 light to give warning to approaching steamers. The 
 lighthouse lamp acts upon the eyes of the far-distant 
 sailor. The fact is so commonplace that it seems 
 almost out of place to refer to it, but it is none the less 
 wonderful. Nothing has travelled across the inter- 
 vening space but a wave-disturbance in the interven- 
 ing medium. What intervening medium ? Certainly 
 not the air, for a gale of fifty miles an hour is driving 
 past the lighthouse, and yet the waves of light have not 
 been disturbed in any way. Sound waves in the air 
 would certainly have been affected by the flying 
 wind. There must be some other medium other 
 than the air, and it is this medium which has been 
 christened the cether. 
 
 We picture the lighthouse lamp disturbing the 
 surrounding aether, and setting up a series of waves 
 in it. These waves travel out to the distant sailor and 
 produce certain sensations in his brain by acting upon 
 his eyes. In every case we can imagine, one body 
 acts upon another distant body by disturbing some 
 intervening medium, 
 
 89 
 
Scientific Ideas of To-day 
 
 It is a strange fact that some agther waves produce 
 totally different results from others. The far-distant 
 Sun sets up certain asther waves which fall upon our 
 eyes and affect our sensory organ of vision. The Sun 
 also sets up another kind^ of aether wave which, 
 falling upon us, heats our bodies, and all other objects 
 upon which the waves fall. The aether transmits both 
 these disturbances at the one time ; light waves and 
 heat waves. 
 
 Not only has the cether this twofold duty to per- 
 form, but it must also carry the waves set up by a 
 wireless telegraph-transmitter. These electric waves 
 are very large disturbances in the aether, and it is 
 possible to communicate with ships at sea, provided 
 the ship has a wireless receiver which is sensitive to 
 these aether waves. All that we desire to notice at 
 present is the different results produced by aether 
 waves. 
 
 We must keep in mind the fact that light and heat 
 do not travel to us from the Sun, but merely asther 
 waves which produce these results. It is unfortunate 
 that these aether waves were ever called light and heat, 
 as this terminology certainly leads to confusion. We 
 have tried to mend matters a little by calling the cether 
 waves, which produce heat, radiant heat, but we have 
 made no attempt to rechristen the aether waves which 
 produce light ; we merely call them light. This leads 
 us into the use of some curious expressions ; for many 
 
 ^ We shall see later that all aether waves are of the same nature, but 
 differ in their wave-lengths. 
 
 90 
 
What is the JEthcr ? 
 
 of these light waves do not affect our eyes, so that we 
 speak of these particular waves as invisible light. On 
 the other hand, we naturally associate light with our 
 sensation of vision. 
 
 Suppose we have an ordinary photographic camera 
 set up in a studio, and we have the room brightly 
 lighted by an electric arc lamp. We have a specially 
 constructed screen or shutter in front of the lens, and 
 this screen cuts oiif all the ordinary or visible light. 
 When the visitor puts his head under the focusing 
 cloth to examine the picture, he of course sees 
 nothing upon the ground glass ; all is totally dark 
 within the camera. Nevertheless we are going to take 
 a photograph with this *' total darkness," A gentleman 
 sits in the orthodox manner to have his photograph 
 taken, and although there is absolutely no trace of 
 an image upon the focusing screen, we replace it with 
 an ordinary photographic plate. After a five minutes' 
 exposure the plate is developed, and a portrait is pro- 
 duced, the result being very good considering the 
 length of time the sitter had to remain in one position 
 (see illustration facing page i86). It is obvious that 
 some invisible light did enter the camera, and that 
 although this light makes no impression whatever 
 upon our sensory organ of vision, it does affect the 
 chemicals upon a photographic plate in the same way 
 as ordinary light. We speak of this invisible light as 
 ultra-violet light, signifying that it is beyond the violet 
 end of the spectrum when light is analysed by a glass 
 prism, 
 
 91 
 
Scientific Ideas of To-day 
 
 My present purpose is to point out that all light is 
 invisible, in the sense that it cannot be seen. Some 
 years ago there was a very interesting book published 
 under the title, "Light Visible and J n visible," but we 
 attach a special meaning to the adjectives. We simply 
 mean — Light which affects our eyes and light which 
 does not. If you can arrange to have a look at light 
 without allowing its aether waves to enter your eyes 
 you certainly find that it is invisible; it looks exactly 
 like darkness. It is true that if you are sitting in a 
 totally dark room and you allow a beam of sunlight to 
 enter through a slit in the window-shutter, you see the 
 track of the beam, but this is simply due to little dust 
 particles in the air reflecting the waves towards you. 
 If the air were absolutely free of dust particles you 
 would not see the track of the light across the room. 
 We can perform this experiment upon a very grand 
 scale. With the Sun as the source of light, and the 
 shadow of our Earth as the dark room, we can look 
 out on any cloudless night into the vastness of space. 
 This space is continuously filled with aether waves of 
 light sent out in all directions by the Sun, but we 
 do not see these waves. Some of them fall upon a 
 far-distant planet and then they are reflected towards 
 our Earth, and when the aether waves enter our eyes 
 we say we see the light of the planet. I think the mean- 
 ing of visible and invisible light will be quite clear ; 
 all aether waves are in themselves invisible because 
 the aether is invisible. We shall see later that only 
 
 a very small range of aether waves affect our eyes. 
 
 92 
 
What is the ^ther? 
 
 We shall remember that when we speak of light 
 travelling from the Sun to our Earth in about eight 
 minutes we do not mean that there is any actual 
 transfer of something from the one place to the other, 
 but merely a wave-disturbance in the intervening 
 medium. 
 
 Perhaps the matter will be clearer if we consider the 
 aether waves of radiant heat which the Sun sends out 
 and some of which our Earth entraps. We are so 
 accustomed to think of heat travelling along a body, 
 and from one body to another by means of some 
 material medium, that it is difficult at first to expel 
 this idea from our minds when thinking of the Sun 
 and the Earth. The space between the Sun and the 
 Earth is not heated ; there is merely an aether dis- 
 turbance. We picture the particles of which the Sun 
 is composed as being in a state of violent agitation or 
 vibration. These vibrating particles of matter disturb 
 the aether ; the action, as we shall see later, is through 
 some intermediate step, but that does not concern us 
 at present. We simply picture the vibrating molecules 
 of the Sun setting up a series of waves in the aether. 
 These waves travel outwards through the great aether 
 ocean, and some of the waves will, of course, fall upon 
 our little planet, which is really no more than a speck 
 in the universe. When these waves fall upon any 
 matter they immediately set its molecules into vibratory 
 motion, producing the condition we know as heat. 
 You will notice that there has been a real transforma- 
 tion. Vibrating particles of matter cause an aether 
 
 93 
 
Scientific Ideas of To-day 
 
 disturbance which, at some great distance, is again 
 transformed into the vibratory motion of other particles 
 of matter. 
 
 When we speak to a distant- friend by telephone, it is 
 obvious that no sound passes from the one town to the 
 other. The sound produced by the speaker controls 
 an electric current which passes out to the distant 
 station, and there it sets a metal diaphragm in motion, 
 which causes the surrounding air to vibrate and repro- 
 duce the distant controlling sound. Just as no sound 
 passes between the two distant places, no heat passes 
 between the Sun and the Earth, In both cases there is 
 a real transformation and reproduction. 
 
 It is only at the outset that the novice shakes his 
 head at any mention of the aether. His first inclina- 
 tion is to say that we might as well talk about " the 
 man in the moon." He may say that scientists have 
 merely invented the idea of the aether to help them out 
 of difficulties. The scientist admits the accusation. 
 The idea of the aether was suggested by a great Dutch 
 philosophei^, Huygens,^ more than two hundred years 
 ago, to explain the phenomena of light. Sir Isaac 
 Newton's more materialistic theory was much more 
 popular, and even when the original philosopher, 
 Dr. Thomas Young (London), adopted and worked out 
 this aether idea, he met with little encouragement from 
 scientific men. It is amusing to turn to an old number 
 
 ^ Long before this time sethers had been invented for the planets to 
 swim in, and indeed to help the ancient philosophers out of any apparent 
 difficulty, but the aether as we now accept it was definitely suggested by 
 Huygens at the close of the seventeenth century. 
 
 94 
 
A Coil of Wire carrying an Electric Current behaves like 
 
 A AlAClXET 
 
 In _ihe upper illustration the battery is not connected to the coil. When the 
 circuit is completed the large steel magnet swings round and takes up a position at 
 right angles to the face of the coil, as In the lower photograph. 
 
What is the ^ther? 
 
 of The Edinburgh Review (vol. v, p. 97, 1804), and 
 see how Young's ideas were attacked and ridiculed 
 (see Appendix II., page 328). It is very remarkable 
 indeed that so little credence was put upon the idea 
 of the aether that when Young published a pamphlet, 
 in answer to this attack in The Edinburgh Review, 
 only one single copy was bought by the public. 
 
 We cannot, therefore, blame the man of to-day, who 
 has not followed the trend of modern science, in find- 
 ing it difficult to accept a bald statement as to the 
 existence of the sether. He may say it is merely 
 a theory, which he can believe or not as he thinks 
 best. We quite agree with him, but at the same 
 time we would ask him if he believes that the Earth 
 goes round the Sun. This also is a theory. It is 
 true that astronomers can prove by observed facts 
 that the Earth mtist go round the Sun. In the same 
 way the scientist can prove from obsei-ved facts that 
 the sether must exist. 
 
 When any person of ordinary intelligence watches 
 an old-fashioned performance of marionettes, his mind 
 at once suggests that the puppets' arms and legs are 
 being pulled upwards by strings, or threads, or some- 
 thingf although he cannot see any such connections. 
 Common-sense tells him that some medium of com- 
 munication must exist. When we look at a magnet 
 drawing a sewing-needle towards it, or a large key, 
 as shown at page 200, the same common-sense tells us 
 that there must be some connecting medium. Indeed, 
 if any one really considers the matter seriously, he is 
 
 95 
 
Scientific Ideas of To-day 
 
 forced to accept the theory of an all-pervading sether, 
 A child soon learns in the nursery that if he wishes 
 his little wooden horse to follow him about in his 
 play he must have a piece of string, or some other 
 medium of connection, between himself and the toy. 
 Just as truly does the inquiring mind learn, in the 
 nursery of science, that any two pieces of matter must 
 have some intervening medium between them before 
 the one body can act on the other. There is certainly 
 no such thing as empty space. We may empty a glass 
 globe of air and dust and all matter, by means of a 
 mercury air-pump, but the glass globe is not empty; 
 it is still filled with aether. If we have an electric bell 
 inside the glass globe, we may set it ringing as ener- 
 getically as we like, but it can no longer affect our 
 ears; the medium of communication (the air) has 
 been withdrawn. But in the vessel, beside the bell, 
 we have a small electric glow-lamp, and we are making 
 the experiments in the dark. We could not tell 
 whether the bell was ringing or not, but as soon as 
 the current is turned on to the electric lamp we are 
 conscious that it is glowing. The lamp affects our 
 eyes, although the bell did not affect our ears. It 
 is clear that we have not succeeded in withdrawing 
 the medium through which the lamp acts; the glass 
 globe, though empty as far as ordinary matter is con- 
 cerned, is still filled with the aether, and this aether is 
 as real as the air we breathe. 
 
 That the aether pervades all space is quite apparent, 
 for not only does it transmit light from the Sun, but 
 
 96 
 
What is the ^ther? 
 
 across many billions of miles from the distant stars. 
 Our Earth, then, must be flying through the aether. 
 
 Picture a meteor approaching our Earth from inter- 
 stellar space. As soon as the meteor enters the upper 
 limits of our atmosphere the material of which the 
 meteor is composed becomes white-hot. This is due 
 to the enormous friction between the meteor and the 
 particles of air, and is remarkable because the particles 
 of air at such heights are comparatively few and far 
 between. But the meteor is travelling at a great 
 speed, probably not less than one thousand miles 
 per minute. Our Earth is travelling at practically the 
 same speed, through the aether, in its ceaseless journey 
 around the Sun. We cannot find that any resistance 
 whatever is offered by the aether through which we 
 are literally flying. If there is any resistance, it must 
 be so small that it has not appreciably affected our 
 planet during man's tenancy of it. 
 
 The great Russian chemist, Mendel^eff, who estab- 
 lished the periodic law, which we considered in an 
 earlier chapter, firmly believed that the aether is an 
 extremely thin gas. He supposed its particles to be 
 so very small that they could pass, with perfect ease, 
 between the atoms of matter, so that all matter was 
 perfectly porous to the aether. The physicists of to- 
 day are not inclined to accept this theory; it is too 
 materialistic, and yet it is not altogether inconceivable. 
 
 The youth at school is somewhat surprised when he 
 learns, for the first time, that gases may pass through 
 the solid walls of unglazed porcelain vessels, while the 
 
 97 G 
 
Scientific Ideas of To-day 
 
 same vessels will hold water without any of it escap- 
 ing. The student is more surprised when he learns, 
 from Lenard's experiments with cathode rays, that 
 electrons may pass with ease through a solid aluminium 
 shutter, through which it is impossible for any gas to 
 pass. We have only to go a step further and imagine 
 particles of aether which may pass through all sub- 
 stances with perfect ease. These sether particles may 
 be as much smaller than electrons, as electrons are 
 smaller than atoms, and, indeed, if that be their size, 
 Mendel^eff could find a place for them in his revised 
 table of the periodic law. It must be clearly under- 
 stood that this theory of Mendeldeff's is no more than 
 a suggestion, and that the idea does not find much 
 favour with the scientists of to-day. We have other 
 mechanical theories as to the actions of the aether, but 
 we have no other theory regarding the nature of the 
 aether. 
 
 We have been brought up to look upon the aether 
 as a mysterious "something" quite apart from ordi- 
 nary matter ; it is therefore difficult to credit the aether 
 with any granular construction, as suggested by the 
 great Russian chemist. At present we can only guess. 
 In any case we have the very interesting electron 
 theory which sets forth the following creed : — 
 
 The atom is composed of very small particles called 
 electrons, and the atom is practically a miniature solar 
 system. It may be that a future generation will 
 accept a creed which shall teach that the electron in 
 its turn is composed of small particles of cether, also 
 
 98 
 
What is the ^ther? 
 
 moving in regular orbits, within the electron, and if 
 so, what next ? This, however, is going beyond our 
 province, for these suggestions would not be acknow- 
 ledged as scientific ideas of to-day. 
 
 While the idea of the granular construction of the 
 aether is not generally accepted, there is practically a 
 unanimous agreement that the aether, whatever its 
 nature, is the primal substance of which all matter 
 is formed. 
 
 It is not so very long ago that the late Lord Salis- 
 bury remarked that the aether seemed to him to be 
 the nominative case of the verb to nndulatef for we 
 know practically nothing of the nature of the aether 
 but that it can vibrate or undulate. We need not 
 dwell any further upon speculations as to the nature 
 of the aether; we have plenty to interest us in con- 
 sidering the actions which take place in this all-per- 
 vading medium. 
 
 The aether is indeed marvellous in its capabilities 
 for transmitting all sorts of waves. The Sun sets up 
 certain waves in the aether, and we call these light 
 waves. If we analyse these waves by passing them 
 through a glass prism we find a great variety of wave- 
 lengths. Only a very small portion of this variety 
 affects our eyes, and causes the different colour sen- 
 sations. By placing a sensitive thermometer in the 
 dark space beyond the visible red end of the spectrum 
 we find that there are " invisible " waves present which 
 produce heat. At the other end of the spectrum, 
 beyond the patch of violet colour, we also " see 
 
 99 
 
Scientific Ideas of To-day 
 
 darkness." Here we find waves which will affect a 
 photographic plate, and exhibit other chemical actions. 
 Even if the aether could do no more than what has 
 been mentioned in this paragraph, it would be truly 
 marvellous, for it is transmitting all this great variety 
 of waves at one and the same time. 
 
 When one looks at an ordinary street light at night, 
 it is difficult at first to realise that the electric arc, or 
 the white-hot gas mantle, is setting up all this variety 
 of waves in the cether. 
 
 Even these do not exhaust the capabilities of the 
 aether, for when we set an X-ray tube to work, we 
 cause quite a different kind of disturbance in this 
 same medium. These Rontgen rays are different from 
 light waves ; the former can penetrate such substances 
 as wood and human flesh, both of which are opaque 
 to light. We shall return to a further consideration 
 of X-rays later ; at present we merely wish to notice 
 that they consist of a disturbance in the aether. 
 
 It is the same protean aether which carries the elec- 
 tric waves set up by a wireless telegraph transmitter, 
 which waves falling upon the distant receiver set it 
 in motion. It must also be by means of the aether 
 that one lump of matter attracts every other lump of 
 matter, but as to the nature of gravitation we have 
 really no idea even in this enlightened age. 
 
 There is one point which I find puzzles some 
 people — how it is that these aether waves can travel 
 millions of miles and yet keep up a constant speed 
 throughout their journey ? All the waves in the aether 
 
 I GO 
 
What is the ^ther? 
 
 travel at the rate of somewhere over eleven million 
 miles per minute. I find that the ordinary man is 
 more impressed with these figures than with the 
 orthodox 186,000 miles per second. When we come 
 to consider Light, we shall see how its speed of travel 
 has been determined ; in the meantime we merely 
 wish to see how the speed is constant, and does not 
 fall off as the distance increases. 
 
 As the speed of light is so great, the time it takes to 
 reach us — say, from some far-distant lighthouse — is 
 quite inappreciable. When, however, we consider the 
 passage of light waves from the Sun to the Earth, we 
 find that they take about eight minutes to cover the 
 ninety-two million miles separating us from the Sun. 
 We require to bestir our imaginations to think of light 
 waves taking thousands of years to reach us from 
 some of the distant stars, and yet this is really what 
 does happen. 
 
 The difficulty which some people have in picturing 
 a constant speed of travel over billions of miles is 
 really due to a misunderstanding. Possibly they think 
 of bullets and other pieces of matter being projected 
 with great velocity, but quickly losing speed until they 
 ultimately come to a state of rest. Try to picture a 
 sound wave travelling through the air. It is true that 
 the energy of the sound wave spreads out and dies 
 away at a distance, but during its whole journey it 
 has maintained the same speed. Wherein lies the 
 difference ? In the first case a piece of matter was 
 moved from one part of the Earth to another, and 
 
 lOI 
 
Scientific Ideas of To-day 
 
 as it went it had to combat two resistances — 
 collision with the molecules of air, and the attrac- 
 tive force of gravitation. In the second case there 
 was no transference of matter from the one place 
 to the other ; there was simply a series of waves 
 set up iri the air. When you set up waves in the 
 centre of a pool of water, you do not send water from 
 the centre to the shore of the pool. The Sun and 
 stars merely cause waves in the aether, hence the rate 
 of travel is constant. The energy of sound waves, or 
 water waves, dies away with the increase of distance 
 travelled, and so must the energy of the aether waves, 
 A blazing sun placed at a great distance only appears 
 as a faint star. It may be that its aether waves have 
 been so diminished in their long journey that they fail 
 to stimulate the nervous mechanism of our eyes, and 
 we only know of the existence of the far-distant star 
 because these greatly diminished asther waves are still 
 able to affect the chemicals upon a photographic 
 plate. 
 
 Sound waves are only constant in speed as long as 
 the medium through which they are being transmitted 
 is constant. A difference of temperature in the air 
 alters the rate of travel of waves in it. In the same 
 way the velocity of asther waves is constant as long as 
 they remain in the ocean of pure aether, such as we 
 have throughout interstellar space. When, however, 
 these waves leave the boundary of the pure aether ocean 
 and enter our atmosphere they do meet with some 
 resistance, and when they enter water their rate of 
 
 I02 
 
What is the iEther? 
 
 travel is appreciably lessened, while their progress is 
 completely barred by substances which are opaque. 
 
 In the earlier chapters we have tried to picture the 
 atomic structure of matter, and the electrons revolving 
 within the atoms, and now we seek to add to this 
 picture the great illimitable ocean of aether, surround- 
 ing and pervading all matter. It is in this surrounding 
 aether that magnetic and tlQcinc fields exist. We have 
 therefore to deal not only with electricity in matter, 
 but also in this surrounding medium. In this connec- 
 tion it will be of interest to see, first of all, what 
 magnetism is. 
 
 103 
 
CHAPTER VII 
 
 WHAT IS MAGNETISM ? 
 
 What produces a magnetic field? — A real aether disturbance — How 
 a piece of iron becomes a magnet — Molecular magnets — A 
 permanent magnet — Interesting facts concerning the mag- 
 netisation of iron ships — Causes of magnetisation— A kitchen 
 poker in the disturbed aether — How the iron molecule is 
 magnetic — The most efficient kind of magnet — Where the 
 energy comes from — Demonstration of the reality of the 
 aether disturbance — Some illustrations of Faraday's lines of 
 force 
 
 The popular idea of a magnet is simply that of a piece 
 of iron or steel which possesses a strange property 
 of attracting other pieces of common iron or steel 
 towards it. Most of us, however, have become 
 familiar with the fact that a coil of wire through which 
 an electric current is flowing also behaves exactly like 
 an ordinary magnet (see illustration facing page 94). 
 
 In an earlier chapter we have noted the magnetic 
 field produced in the cether around a magnet. Let 
 us inquire what it is that produces this magnetic field. 
 Taking, in the first place, the case of a magnetic field 
 surrounding a wire which is carrying an electric 
 current, we picture a flow of electrons within the 
 wire. Electrons are being handed along from atom 
 to atom. Is it possible that this simple locomotion 
 
 104 
 
what is Magnetism 
 
 ? 
 
 of electrons can so disturb the surrounding eether as to 
 produce a magnetic field ? The idea that electrically 
 charged bodies, moving with a steady motion at a 
 great velocity, would produce a magnetic field is not 
 a new one. This was an accepted fact before the 
 discovery of electrons. Therefore, when the electron 
 theory sets forth that a magnetic field is due to the 
 steady motion of electricity in a conductor we have 
 no difficulty in accepting this part of the creed. The 
 more electrons that are passing in a given time, the 
 greater is the disturbance in the surrounding aether. 
 Seeing that all magnetic fields are due to a steady 
 locomotion of electrons, we have no hesitation in say- 
 ing that there must be a steady flow of electrons within 
 a piece of magnetised iron. If this were not the case 
 we could not have the magnetic field surrounding it. 
 But we need not suppose that the electrons travel 
 round and round the lump of iron or steel, for we shall 
 see that the electrons revolving around their atoms 
 may, under certain conditions, produce the same result. 
 To help our imagination we may picture the atoms 
 with their revolving electrons to be something like 
 miniature Saturns with their revolving rings. In the 
 iron we see a myriad of little Saturns grouped together, 
 but with their rings lying in all directions, a perfect 
 higgledy-piggledy^ congregation. In such a condition 
 
 ^ While we picture the atoms to be in a higgledy-piggledy condition, 
 it should be understood that there is some method in their madness, 
 They really set themselves into little stable rings, or groups ; the result, 
 however, may be conveniently described as higgledy-piggledy or topsy- 
 turvy. 
 
 105 
 
Scientific Ideas of To-day 
 
 the steady motion of an electron around one atom 
 would set up an aether disturbance exactly opposed to 
 that set up by a neighbouring atom whose ring hap- 
 pened to be in an opposite position to that of the first 
 atom. All the atoms would be "at sixes and sevens/' 
 and the work done by one would be neutralised by 
 another. In this condition, a lump of matter would 
 show no magnetic field. If by any means we could 
 set all the atoms into such positions that their electron 
 orbits, or rings, were all in the same plane, then we 
 should get a result as illustrated in Fig. A. 
 
 Here we see six atoms with their electron orbits 
 placed in the same plane. We may imagine this to be 
 the section of a piece of magnetised iron. 
 
 It will be observed that there is the equivalent of a 
 flow of electrons around the magnet. The aether will 
 be disturbed in the same manner as it would be if the 
 electrons really went round and round the lump of 
 iron instead of each going round its own little orbit. 
 The condition of things is, therefore, just as though we 
 had a wire, forming a waist-belt around the lump of 
 iron, with an electron current flowing in the wire. We 
 should then have a magnetic field around this imaginary 
 wire. 
 
 Leaving the electron part, of the foregoing, out of 
 
 account, this theory of magnetism has been accepted 
 
 for two or three generations. We have been in the 
 
 habit of picturing each molecule of iron as a little 
 
 magnet, having a north and a south pole. In the 
 
 ordinary condition of iron we have supposed the little 
 
 io6 
 
what is Magnetism 
 
 Fig. a 
 
 htternal arrangement of a Steel Magnet 
 
 This diagram shows one end of a bar magnet, and the six small 
 circles represent some of the atoms of which the steel bar is com- 
 posed. The small arrows indicate the directions in which the 
 satellite electrons are revolving around the atoms. It will be 
 observed that the combined motion of the electrons is equivalent 
 to an electron current going round the body of the magnet, as 
 indicated by the large arrows. In the text we see that a steady 
 locomotion of electrons disturbs the surrounding aether, producing 
 what we know as a magnetic field around the steel bar. 
 
 107 
 
Scientific Ideas of To-day 
 
 magnets to be lying topsy-turvy, so that one neutralises 
 the action of the other, and there is no outward 
 magnetic effect. But when the iron is stroked by a 
 magnet, these little molecular magnets are forced to 
 turn round and place all their north poles in one 
 direction. The myriad of little molecular magnets 
 acting together can produce quite an appreciable 
 magnetic field in the surrounding aether. At the one 
 end of the iron we find all the molecular north poles 
 facing outwards, while at the other end we find all 
 the south poles facing outwards. Hence the magne- 
 tised piece of iron exhibits distinct north and south 
 poles. If we cut the magnet in two, we shall still have 
 a north pole at one end of each piece and a south pole 
 at the other. 
 
 Many facts have been brought forward to support 
 this molecular theory of magnetism. We find that when 
 the molecules of hard steel have been turned round, 
 under the influence of a magnet, they do not turn 
 back readily to their old positions. Hence we find 
 that the steel remains a " permanent " magnet. We 
 could disturb this molecular arrangement by hammer- 
 ing the steel, or by heating it to a red-heat. In the 
 former case we should find very soon that the magnet 
 was considerably weaker after a good hammering, 
 while in the second case we should find that we 
 had completely ''destroyed" the magnetism by heat, 
 the molecules having been enabled to return to their 
 former topsy-turvy condition. 
 
 When an iron ship is being built the magnetic poles 
 io8 
 
What is Magnetism 
 
 ? 
 
 of the Earth seek to turn the molecular magnets of the 
 metal so that they may lie with their magnetic poles 
 north and south. It is very remarkable how quickly 
 the hammering in of the rivets enables the molecular 
 magnets to obey the Earth's pull. Some interesting 
 experiments were made recently on the hull of a 
 cargo-steamer during the period of building. Owing 
 to a strike of the riveters the whole ship was com- 
 pletely plated and all bulkheads and decks built in, 
 while only about five per cent, of the rivets were fixed. 
 A careful note was taken of the ship's magnetism. We 
 are considering the ship as a piece of iron which the 
 Earth is seeking to magnetise. We shall suppose that 
 the amount of magnetism due to the ship was, at this 
 stage, only twenty-five per cent, of that due to the 
 Earth itself. Things remained as they were for a 
 month, till the riveters set to work once more. Then 
 the molecular magnets got a better chance of obeying 
 the Earth's pull. By the time the riveters had forty 
 per cent, of the rivets hammered in, the ship's magne- 
 tism had increased by more than thirty per cent., and 
 it went on increasing as the hammering continued. 
 
 We have considered two causes of magnetism. 
 We might call the first natural magnetisation, in which 
 iron is magnetised by the influence of the Earth's 
 magnetism. This is the cause of natural magnets or 
 lodestones found in the Earth, The second cause, 
 already dealt with, is the magnetisation due to stroking 
 the iron with a permanent magnet. We are aware of 
 other means of influencing these little molecular 
 
 109 
 
Scientific Ideas of To-day 
 
 magnets. We know that if we place a magnet in 
 the neighbourhood of a coil of wire in which an 
 electron current is flowing (as shown in the illustra- 
 tion facing page 94), the magnet at once turns round 
 and places itself at right angles to the face of the 
 coil. Look at this illustration and try to imagine 
 the magnet needle to be a greatly enlarged molecule 
 in a piece of iron which the coil of wire is supposed 
 to encircle. We see the magnified molecule turn 
 round when the current passes through the encircling 
 wire. It is not a difficult task to picture the whole 
 space within the coil to be filled with similar magnets, 
 all of which will obey the influence of the magnetic 
 field. In this way we may form a clear idea of what 
 takes place within a piece of iron when surrounded by 
 a wire carrying an electron current. 
 
 In the illustration facing this page we see what 
 happens when we place an ordinary kitchen poker 
 in the disturbed aether within the coil. The myriad 
 of molecular magnets within the iron poker are turned 
 round so that they act together, and this army of little 
 molecules is able to support a pair of scissors. 
 
 Every atom of matter — whatever name it may go 
 under — is made up of electrons moving in steady 
 orbits. We therefore find magnetic effects in every 
 substance, although these are in most cases very 
 small. The magnetic effects in nickel and cobalt 
 are quite appreciable though much less energetic 
 than in iron. Many alloys of copper, manganese, 
 and aluminium show very fair magnetic effects, but 
 
 no 
 
what is Magnetism? 
 
 iron stands head and shoulders above them all. 
 There must be some special feature in the con- 
 struction of the atom of iron which acts upon the 
 aether much more vigorously than other atoms do. 
 It has been suggested that one or more of the elec- 
 trons in the iron atom describe a much larger orbit 
 than is usual in atoms ; that they are somewhat 
 analogous to comets circling around the Earth, The 
 motion of these electrons is not under man's control ; 
 they are incessantly moving in the iron. Any piece 
 of iron contains the magnetic power, but, as we have 
 already seen, it is not apparent unless the multitude 
 of little magnetic fields are all acting together in one 
 plane ; unless all the little Saturns have their rings 
 lying in one direction. In this condition the iron is 
 magnetised. 
 
 If the magnetic force of iron is really a self-con- 
 tained force, one would expect that there should be a 
 definite limit to its power. Long ago this was found 
 to be the case ; it became apparent that magnetism 
 was not something which we were putting into the 
 iron, as when we charge a body with electricity. In 
 the case of a magnet we found that we soon reached 
 a point beyond which it was quite impossible to 
 increase the magnetism, and this was christened the 
 " saturation point." Just as in many other christen- 
 ings, the name was not very wisely chosen. The 
 word saturation at once conjures up ideas of soaking 
 something into the iron. However, in the light of 
 
 present-day knowledge, we understand that we have 
 
 III 
 
Scientific Ideas of To-day 
 
 reached the limit when we have succeeded in turning 
 the httle Saturns round into as good positions as it is 
 possible. We have as perfect a combination of their 
 little forces as we can obtain. 
 
 It is quite clear what kind of magnet will give us 
 the best result. We get a magnetic field from a coil 
 of wire in which an electron current is flowing, but 
 this is a comparatively weak field. This magnetic field, 
 however, can act upon the myriad of little magnetic 
 forces locked up in a piece of iron. Therefore our 
 best plan is to place a coil of wire around a piece of 
 iron, and keep an electron current flowing in the wire 
 by means of a battery, or other "electric pump." 
 
 It will be apparent that the foregoing arrangement 
 will give us the best kind of magnet which is possible. 
 As the particles of soft iron are more easily affected 
 by the current than those of hard steel are, we make 
 the cores of electro-magnets of ^oft iron. There is a 
 further advantage in this, for as soon as the control- 
 ling electron current in the wire is stopped, the myriad 
 of little magnetic forces in the iron return to their 
 higgledy-piggledy positions, and all trace of magnetism 
 has gone. We therefore have a magnet which will 
 attract and let go at will, I have already explained 
 the manifold applications of such magnets in the first 
 book of this series, " Electricity of To-day," 
 
 We must not run away with the idea that the soft 
 iron core of an electro-magnet merely concentrates 
 the magnetic field surrounding the coil. The weak 
 magnetic field of the coil calls into play the internal 
 
 112 
 
what is Magnetism 
 
 ? 
 
 forces of the soft iron. We may increase the magnetic 
 field around a coil of wire by increasing the flow of 
 electrons through the wire, but the magnetic energy 
 contained in a piece of iron is constant. The magnet 
 is only more or less powerful because its atomic 
 electron currents are brought into more or less 
 concerted action. 
 
 In iron and other magnetic bodies we suppose that 
 the orbits of the active electrons are large enough to 
 act upon each other across the space between the 
 atoms. In this respect iron is pre-eminent, w^hile 
 Dr. Heusler's alloys, to which I have already referred, 
 make humble seconds, and cobalt and nickel make 
 distant thirds. 
 
 In the illustration at page 200 I have endeavoured 
 to show that the magnetic field is a real aether dis- 
 turbance. You see a common iron poker placed at 
 some distance fi*om a large electro-magnet, and yet 
 the particles of the soft iron poker are affected by the 
 disturbed aether to such an extent that the poker is 
 able to attract a key against the force of gravity. The 
 intei'vening air has nothing to do with conveying the 
 power, for the experiment might be performed in a 
 vacuum. There is a real aether disturbance around 
 the large electro-magnet, and this sether disturbance 
 has a real effect upon the iron, causing the myriad of 
 little molecular magnets contained in the iron to fall 
 in line with one another and combine their forces. 
 
 In the second photograph we see a key being drawn 
 up towards the magnet by the sether disturbance. No 
 
 113 H 
 
Scientific Ideas of To-day 
 
 one will suppose that the finger has been magnetised ; 
 it is merely preventing the key from reaching the 
 magnet. If we were to fasten a string to the key and 
 tether it to the ground, we should see the key supported 
 in mid-air, for the string would serve the same purpose 
 as the finger does in the illustration. 
 
 Long before the birth of the electron theory, Faraday 
 pictured lines of force existing in the aether around a 
 magnet. In order to demonstrate the existence of 
 such lines, one may sprinkle iron filings upon a sheet 
 of paper, and when the pole of a magnet is placed 
 beneath the paper, the filings set themselves along 
 those lines of force. In the photograph facing page 122 
 I have reproduced some of the figures obtained in this 
 way by students in the Glasgow and West of Scotland. 
 Technical College, In order that the filings may be 
 retained in the positions which they take up, the paper 
 is coated with paraffin wax. After the figures are 
 formed, the paper is heated so that the filings become 
 attached to the paraffin wax when it cools. Here again 
 we have a demonstration of one set of molecules being 
 brought within molecular range of another set so that 
 they can attract one another with the force of cohesion. 
 
 In the present chapter we have been considering 
 the disturbance in the ^ther due to the steady motion 
 of electrons. The locomotion of electrons along a 
 wire produce a magnetic field around the wire. It 
 will be of interest to see what effect the starting and 
 stopping of these electrons may produce in the sur- 
 rounding aether. 
 
 114 
 
CHAPTER VIII 
 
 MORE ABOUT ELECTRONS IN MOTION 
 
 Matter is very lazy about moving — Perpetual motion on a grand 
 scale — Moving matter is unwilling to stop — The moving 
 electrons — Electrons affect other distant electrons through 
 the medium of the aether — An interesting telephonic trouble 
 The aether conveys the energy — The function of the telegraph 
 wire — An analogy of electric induction— Some "wireless" 
 experiments—Self-induction — Electrons affected by a moving 
 magnetic field — Faraday's great discovery — How the electric 
 current is produced by the dynamo 
 
 In our practical workaday life we soon realise the fact 
 that all matter is very lazy ; it requires a good push to 
 start it into motion. We see a man pulling veiy hard 
 in his endeavours to get a heavily laden barrow to 
 move, when some kindly disposed passer-by gives the 
 unwilling barrow an additional push and off it goes, 
 the man in charge being able to keep it going. 
 
 It is equally true that all matter is just as lazy or 
 unwilling to stop when it is once set in motion. Of 
 course the barrow-man would find it very difficult 
 to believe this, as he has to strain every muscle to 
 keep the barrow moving. That, however, is due to 
 the great friction between the wheels of the barrow 
 and the surface of the road. Let him get his barrow 
 on to the tramway rails, and he will find that about 
 
 115 
 
Scientific Ideas of To-day 
 
 one-half of his load has apparently disappeared. It is 
 obvious that the friction has been very greatly reduced ; 
 there is not the same resistance to the movement of the 
 wheels. Let him take the wheels off his barrow and 
 then try to pull it along ; he would find it quite 
 impossible to move it. The barrow-man would, 
 doubtless, be willing to admit that at least a very 
 great deal must depend upon the frictional resistance, 
 although he might not be able to endorse the state- 
 ment that matter is just as lazy to stop moving as it is 
 to start into motion. 
 
 The planets meet with no friction or resistance in 
 their long journeys around the Sun, and in their 
 continual movement we see true perpetual motion. 
 But I fear that even this grand demonstration by 
 Nature would not convince the hard-worked barrow- 
 man that but for the resistance offered by outside 
 sources his barrow would continue to move of its own 
 accord. 
 
 If we think of a bullet fired from a powerful gun, we 
 can realise that once on its way it is ceiiainly unwilling 
 to stop, and indeed unless any obstructing obstacle 
 offers considerable resistance the bullet will force its 
 way through the obstacle. Ultimately the bullet is 
 brought to a state of rest by the resistance of the air 
 and by gravitation pulling the bullet down to the 
 Earth. The truth is that it is only because we are 
 so accustomed to see all moving bodies brought to 
 rest, that we have difficulty in realising that this con- 
 dition is only brought about by the interference of 
 
 ii6 
 
More about Electrons in Motion 
 
 outside forces. If we consider the subject seriously 
 we shall soon realise that matter of itself is just as 
 lazy to stop moving as it is to start into motion. 
 This property of matter has been called its inertiaj 
 from the Latin word inertis, signifying inactive. 
 
 All that has just been said of ordinary matter is 
 also true of the invisible electrons. They possess this 
 property of inertia ; they are just as inert as matter. 
 The electrons, like the heavy barrow, require an addi- 
 tional expenditure of energy to set them into motion, 
 and when moving they will not stop until some outside 
 force is applied. When the flying electrons escaped 
 from the vacuum tube by means of an aluminium 
 window they were moving with a velocity of many 
 thousands of miles per second, and yet they were 
 pulled up within an inch by the resistance of the 
 particles of gases forming the air. The electrons 
 would not have stopped of their own accord. Just 
 as we see perpetual motion in the heavens, so do 
 we see, with our mind's eye, perpetual motion of 
 the electrons within the atom, where they meet with 
 no resistance. They are in motion, they have no 
 inclination to stop, and there is nothing to stop 
 them. 
 
 Let us consider what happens when we start and 
 stop an electron current in a wire. We find that if 
 there is another wire in the neighbourhood, and lying 
 parallel to the first wire, there is a disturbance of the 
 electrons in this wire. Each time that the current 
 is started and stopped in the first wire there is a 
 
 117 
 
Scientific Ideas of To-day 
 
 momentary current set up in the second wire. The 
 Telephone Companies found this a source of much 
 trouble at first. Two telephone wires running parallel 
 to one another on the same poles enabled a third 
 party to hear the conversation passing on the neigh- 
 bouring wire between other two subscribers. The 
 telephone engineers found it necessary to wire the 
 lines in a special manner, causing them to cross each 
 other from one side, of the poles to the other, so that 
 the lines were no longer parallel. I am referring 
 to the time when the telephones were worked over 
 single lines and an earth circuit. Now that complete 
 metallic circuits are used the trouble is not apparent 
 unless on very long lines. I remember hearing of 
 the following notable incident, which occurred some 
 twenty years ago : — 
 
 Some of the telephone subscribers in London com- 
 plained of clicking sounds in their telephones, and 
 these noises became annoying during a conversation. 
 It was found that these subscribers' lines passed along 
 a certain street under which some telegraph cables 
 were laid, and there could be no mistaking the cause 
 of the annoying clicks ; they were, undoubtedly, the 
 well-known Morse telegraph signals. The telephone 
 wires were on poles on the top of high buildings, 
 while the telegraph wires were buried in the earth, and 
 yet the electron current in the subterraneous wires was 
 undoubtedly causing electrons to move in the telephone 
 wires overhead. How could the electrons in the one 
 
 wire affect those in the other wire ? Only by disturb- 
 
 ii8 
 
More about Electrons in Motion 
 
 ing the intervening aether, which in turn disturbs the 
 electrons in the second wire. 
 
 It may be well to mention, at this point, that even 
 in the single case of an electric current flowing along 
 a wire, say, to a distant telegraph instrument, or to an 
 electric-bell, the energy is really transmitted through 
 the aether surrounding the wire. It is in the aether 
 that the electric and magnetic fields or lines of force 
 exist, and it is these two forces acting conjointly that 
 cause energy to be transmitted by the aether surround- 
 ing the wire. It is customary to say that the telegraph 
 wire merely acts as a guide to the aether disturbance, 
 but the wire is more than a mere guide. It is within 
 the wire that the electrons move, and thus disturb the 
 surrounding aether by the motion of their electric and 
 magnetic fields. 
 
 Returning to the consideration of two wires lying 
 parallel to one another, we find that when we start the 
 electrons in the first wire there is a momentary electron 
 current set up or induced in the second wire. This 
 disturbance, however, only takes place in the second 
 wire, at the moment the electrons are started and 
 stopped in the first wire. It is interesting to note the 
 direction of these momentary currents, and to simplify 
 matters we shall consider an analogy. 
 
 If a passenger is standing in a train or tramway car 
 which is at rest, when it suddenly starts forward the 
 passenger receives a sharp lurch backwards, or in the 
 opposite direction to the force producing the forward 
 motion of the vehicle. In a somewhat similar manner 
 
 119 
 
Scientific Ideas of To-day 
 
 the electrons in the second wire receive a sudden lurch 
 back in the opposite direction to the controHing cur- 
 rent in the first wire. Then, again, if a train or car, 
 travelling at a fair speed, is suddenly stopped, the pas- 
 senger standing in it is thrown forward in the direction 
 in which the vehicle had been moving. Just so do 
 the electrons in the second wire receive a forward 
 motion when the current in the first wire is stopped. 
 A passenger is more likely to meet with serious hurt 
 through the sudden stoppage rather than the sudden 
 starting of a train. In the former case the change of 
 motion is much greater. A train may be moving at 
 forty miles per hour when its motion is almost instan- 
 taneously brought to zero. But in starting from zero 
 the change is much more gradual ; it is impossible to 
 suddenly change the motion of a train from zero to 
 forty miles per hour. The same holds good in the 
 case of the electron. When the electrons are suddenly 
 stopped in the first wire the effect in the surrounding 
 aether is far greater than was the case when they were 
 started into motion. Hence of the two momentary 
 currents produced in the second wire, that due to the 
 stopping of the controlling current in the first wire is 
 by far the most important. So much is this the case 
 that we can often afford to neglect the momentary 
 current due to the starting. 
 
 As long as the electron current is flowing steadily in 
 the first wire there is a steady transmission of energy 
 in the aether surrounding it, but the electrons in the 
 second wire are not affected in any way. It is only at 
 
 I20 
 
More about Electrons in Motion 
 
 the moment of starting and stopping the electrons that 
 the electrons in the second wire are set momentarily 
 into motion. 
 
 It is natural to inquire the distance one wire may be 
 from another and yet cause the electrons in the second 
 wire to move. Sir William Preece, when acting as en- 
 gineer-in-chief to the British Post Office, succeeded in 
 setting up or inducing a current in this manner when 
 the two lengths of parallel wires were several miles apart. 
 This was one of the earliest methods of wireless tele- 
 graphy, but it was precluded from working over any 
 great distance ; for as the distance between the two 
 wires was increased, so had the length of the two 
 parallel wires to be correspondingly increased. This 
 difficulty could have been overcome if the long wires 
 would have acted in the same way when rolled up 
 into the form of two large coils. This, however, is 
 impossible, for then the whole aether disturbance is 
 concentrated, so that the disturbed aether reacts upon 
 other electrons in the same coil, and we have what 
 we describe as self-induction , Before the electrons in 
 one coil can act upon the second coil, the two coils 
 must be near one another. We have a practical appli- 
 cation of the action between two neighbouring coils in 
 the induction coil already mentioned, and with which 
 most people have become familiar in connection with 
 the production of X-rays, 
 
 What we wish to observe at present is that the 
 momentary currents in the second coil are caused by 
 setting up and withdrawing a magnetic field, or in 
 
 121 
 
Scientific Ideas of To-day 
 
 other words, by a moving magnetic field. In the case 
 which we have been considering, we have an electron 
 current quickly started and stopped in a coil of wire. 
 Each time the electrons start into motion there will be 
 a magnetic field formed around the wire, which will 
 be withdrawn as the electrons stop moving. We may 
 picture magnetic lines of force being suddenly thrown 
 outwards from the wire and then withdrawn. But 
 any moving magnetic field will serve the same purpose. 
 We may move a simple steel magnet in the neigh- 
 bourhood of a coil of wire, and we induce the same 
 momentary currents in the coil. We may let the 
 magnet remain at rest and move the coil of wire into 
 and out of the magnetic field, and the result is just the 
 same. This was Michael Faraday's great discovery. 
 When experimenting at the Royal Institution, London, 
 in the year 1831, he discovered that when he moved 
 a coil of wire between the poles of a magnet there 
 was an electric current induced in the coil. Faraday 
 pictured the moving coil cutting through the magnetic 
 lines of force, and as a result of this there was a momen- 
 tary electric current induced in the coil of wire. To- 
 day we form a more detailed picture. In imagination 
 we see electrons moving around the atoms of steel in 
 the so-called permanent magnet. These moving elec- 
 trons disturb the surrounding aether, producing that 
 state which we call a magnetic field. Then as the coil 
 of wire is quickly plunged into this disturbed aether, 
 we see a sudden activity among the electrons sur- 
 rounding the copper atoms in the wire. Electrons are 
 
 122 
 
^s^^s^^^F^^^:sr^^^^^'^p^''^~':^!M:r^-wss^ 
 
 
 
 
 '^^-'Mt****^^^!^,,,^ 
 
 ^^iai 
 
More about Electrons in Motion 
 
 hustled along from one atom to another, and these 
 moving electrons constitute what we call an electric 
 current. 
 
 It is obvious that it is quite immaterial whether we 
 move the coil of wire in the neighbourhood of the 
 magnet or move the magnet in the neighbourhood of 
 the coil. It is generally more convenient to move the 
 coil and let the magnet stay at rest. 
 
 It will be of interest to form a clear mental picture 
 of the manner in which the electrons behave in the 
 coil of wire as it enters and leaves the magnetic field. 
 Keeping before us the analogy of the passenger in the 
 jolting train, we picture the electrons in the moving 
 wire receiving a sudden lurch in one direction as they 
 enter the magnetic field, and then a sudden lurch in 
 the opposite direction as the wire is freed from the 
 magnetic field. We have seen the effect produced in 
 one wire when the electrons are suddenly started and 
 stopped in a neighbouring wire. In that case we found 
 that the effect produced by the more sudden stopping 
 of the electrons was much greater than that caused by 
 the more gradual starting. But in the present case the 
 conditions are quite different. The electrons pro- 
 ducing the magnetic field are in constant and regular 
 motion within the steel magnet, and it is the electrons 
 in the copper wire which are suddenly brought into the 
 magnetic field and then withdrawn. The suddenness 
 with which the electrons enter the magnetic field is 
 just the same as the suddenness with which they leave 
 the field, so that in this case the electrons lurch in one 
 
 123 
 
Scientific Ideas of To-day 
 
 direction just as much as they lurch in the other. If 
 the coil is kept rotating uniformly so that the wire 
 enters and leaves the magnetic field at a constant 
 speed, there will be a regular to-and-fro motion of the 
 electrons in the wire. This rapid swinging to and fro 
 of the electrons is what we call an alternating current 
 of electricity. 
 
 A dynamo is a simple machine in which a coil of 
 wire called the armature is quickly rotated between 
 the poles of a powerful magnet. In the armature of 
 all dynamos there is this to and fro or alternating 
 current, a surging to and fro of the electrons. As 
 explained when dealing with the practical side of the 
 subject in the first book of this series — " Electricity of 
 To-day " — we may lead out this to-and-fro current into 
 the electric mains, or we may, by means of a com- 
 mutator, cause this alternating current in the armature 
 to set up a direct or continuous current in the outer 
 mains. 
 
 The dynamo is the best means we have discovered 
 of setting the electrons in motion on a large scale. If 
 we only require a small electric current, it is more 
 convenient to set the electrons in motion by chemical 
 means, as in the ordinary battery, but for large currents 
 we depend upon the mechanical motion of a dynamo. 
 
 In the battery we see chemical energy transformed 
 into electrical energy, while in the dynamo we see 
 mechanical energy transformed into electrical energy. 
 It would be quite natural to inquire, at this point, 
 what energy really is. 
 
 124 
 
CHAPTER IX 
 
 WHAT IS ENERGY? 
 
 Energy in different forms — Definition of Energy — Transferring 
 energy — Energy as indestructible as matter — Meaning of 
 kinetic and potential energy — Heat energy — The vibration 
 energy of a muscle — Motion — Why the temperature of water 
 cannot exceed boiling-point — Transformation of energy — 
 Chemical energy and electrical energy — The conservation of 
 energy 
 
 The protean actor used to amuse our grandfathers, 
 and even now he appears occasionally at entertain- 
 ments. Sometimes he would represent four or five 
 different characters in the one act. He would be 
 before the audience as a mischievous boy, when he 
 would pretend to hear his grandfather coming. The 
 boy would run behind some article of furniture, 
 whereupon the door would open immediately and 
 in would come an old gentleman. Indeed so quickly 
 was the transformation made that one could hardly 
 believe that the two characters were represented by 
 the one actor. Energy is a grand protean actor ; it 
 appears in at least eight distinct forms, and its changes 
 from one form to another are instantaneous. 
 
 The usual definition of Energy is capacity for doing 
 work, and we say that work is done when force is 
 overcome through space. It will be observed that 
 
 125 
 
Scientific Ideas of To-day 
 
 the words energy and force have distinctive meanings 
 in science, although we often use the one word for 
 the other in ordinary speech. Force is any cause 
 which alters a body's state of rest or of uniform 
 motion in a straight line. 
 
 So far we might say that force is the power or ability 
 of setting bodies in motion, but this is not all. If a body 
 is already in motion we require to expend some force 
 to stop it. A man is wheeling a heavily loaded barrow 
 down a steep hill when he finds that it is getting beyond 
 his control. Some energetic onlooker rushes to his 
 assistance, and with their united forces they succeed 
 in stopping the would-be runaway barrow. 
 
 We recognise the foregoing statement as the first 
 law of motion, which was clearly stated by Sir Isaac 
 Newton more than two centuries ago. Although the 
 three laws of motion^ which were clearly set down by 
 Newton, are known as Newton's laws, it is of interest to 
 remember that these were really discovered by poor 
 old Galileo after he had suffered much at the hands 
 of the Inquisition. At the time he wrote his famous 
 dialogues on motion he was practically a prisoner, 
 having been ordered never to leave his house and to 
 receive no visitors. 
 
 In the illustration facing page 134, we see the 
 suggestion that a small force — the man's breath — 
 might supply energy enough to uproot a tree, but it 
 will be observed that the force is of no practical 
 use for that purpose, as it would require to be ap- 
 plied continuously for six hundred thousand years 
 
 126 
 
what is Energy 
 
 ? 
 
 before the necessary energy could be supplied in this 
 way. 
 
 When a football player is about to kick off, the ball 
 is placed at rest upon the ground, the player makes a 
 quick forward movement of his foot and imparts con- 
 siderable energy to the ball. A very good demon- 
 stration of one body transferring energy to another is 
 seen during a game of billiards. A quickly moving 
 ball strikes a stationary ball "full/' whereupon the 
 whole energy is apparently transferred to the second 
 ball, while the first is suddenly brought to rest. 
 
 We have no difficulty in realising the fact that 
 energy may be transferred from one body to another, 
 but this transference does not take place inde- 
 finitely without an apparent loss. Picture a long row 
 of billiard balls arranged in one straight line, and each 
 standing a little distance apart from its neighbour. 
 We imagine the first ball to strike the second one full 
 and to deliver up its energy to the second, which in 
 turn passes it on to the third, and so on along the 
 line. But as the energy is handed on to the more 
 distant balls we see a serious diminution in the amount 
 of energy exerted, and if the row is long enough the 
 whole energy is finally dissipated. We cannot say it 
 is destroyed, for we can no more destroy or create 
 energy than we can destroy or create the other entities, 
 matter and aether. We have become familiar with the 
 fact that at the creation of the world a certain definite 
 amount of matter was placed in this planet, and we 
 should become equally familiar with the idea that a 
 
 127 
 
Scientific Ideas of To-day 
 
 definite amount of energy was also placed in the world. 
 We cannot add to nor subtract from the total sum of 
 matter ; we can only transform it from one kind to 
 another. It is exactly the same with energy ; we can 
 neither add to nor subtract from the total sum ; we 
 can only transform it from one kind to another. 
 
 When the energy which was transferred to the first 
 billiard ball was passed on and on till it ultimately 
 disappeared, where did it go? It must still exist, as 
 it is indestructible ; it has gone into the forms of 
 sound and heat. We shall understand this matter 
 better if we consider, first of all, the different forms 
 which energy may take. 
 
 The form of energy which is most conspicuous is 
 the energy of moving matter. We must have definite 
 names for energy as it appears in its different forms. 
 We might call this first form the energy of motion. 
 The meaning will become clear if we take a single 
 example. A billiard ball when moving possesses energy 
 of motion ; it possesses the ability to cause other 
 billiard balls to move. This form of energy has a more 
 distinctive name given to it — kinetic energy. This 
 name is derived from the Greek verb kineo, I move. 
 The introductory branch of the science of mechanics 
 which deals with motions is called kinematics, and we 
 recognise the same idea of motion in the word kine- 
 matographj although it is more commonly spelt cine- 
 matograph. Here, then, we have one distinct /on;z of 
 energy — the energy of motion, or kinetic energy. 
 
 When we wish to impart energy of motion to an 
 128 
 
what is Energy? 
 
 arrow, we strain the bow and then suddenly release it, 
 when off flies the arrow possessed of considerable 
 kinetic energy. It was the bow which set the arrow 
 in motion, therefore the strained bow possessed 
 potential energy. We might call this the energy of 
 straifif but as the actual straining is not so apparent 
 in all cases, this form of energy has been given a 
 distinctive name. It has been called potential energy, 
 from the Latin potentia, meaning power. This title 
 does not appear to one at first as being very expressive. 
 Indeed, one might be inclined to think that a flying 
 arrow also possessed potential energy because it 
 possesses power, but that would be a wrong use of 
 the word. Potential energy is the name given to that 
 form of energy, not only represented by an actual 
 strain or deformation, but of any body placed in such 
 a position that when freed it can do work. When 
 we raise the weights of an old grandfather's clock we 
 have given the weights potential energy, although we 
 cannot so easily think of a strain in this case, as, for 
 instance, when we wind up the mainspring of a watch. 
 In the latter case the strain put upon the mainspring is 
 most apparent, and we say that it possesses potential 
 energy. It is obvious that potential energy may be 
 transformed into kinetic energy, or in other words, that 
 the energy of strain may be converted into the energy 
 of motion. Witness the strained bow and the arrow, 
 the raised weights falling when released, and so on, 
 
 I think it will simplify matters if we understand that 
 all forms of energy must come under one or other of 
 
 129 I 
 
Scientific Ideas of To-day 
 
 these two classes. Energy must either be kinetic 
 or potential ; the ability to do work can only be 
 possessed by a body already in motion, or by a body 
 held under a strain. We may speak of gravitational 
 energy, but that is merely a subdivision ; it is sup- 
 posed to be a case of strain in the aether. We pull 
 a stone away from the earth, and the earth pulls the 
 stone back again. 
 
 Consider the motion of any pendulum. While it is 
 hanging at rest it has no energy, but when we pull it 
 to one side we raise the weighted end of it against the 
 pull of gravity ; we expend some energy in doing so. 
 In this case we have given the pendulum energy of 
 strain, or potential energy, which, as soon as the 
 pendulum is freed, is transformed into energy of 
 motion, or kinetic energy. It will be observed that as 
 it falls it gradually loses its potential energy and gains 
 kinetic energy. It is carried past its position of rest, 
 and rises on the other side from which it started. As 
 it rises it gradually loses its energy of motion and 
 gains energy of strain, till at the topmost point of its 
 swing it has lost all its kinetic energy and has only 
 potential energy. Then it passes through the same 
 cycle of transformations again. The same laws of 
 motion hold good whether we are dealing with visible 
 lumps of matter or invisible molecules and atoms. 
 This to-and-fro motion of the pendulum describes the 
 vibrating atom. The atom possesses energy which is 
 constantly changing from kinetic to potential, just as 
 
 we saw in the pendulum. We have already seen that 
 
 130 
 
What is Energy? 
 
 the temperature of a body is due to the rate of vibra- 
 tion of its atoms. Hence we speak of the energy 
 possessed by the vibrating atoms zs heat energy. Heat 
 energy is simply to-and-fro motion on a very small 
 scale, but this is a distinctive form of energy. Let us 
 follow out the transformation to heat energy. 
 
 We hold a heavy hammer over a piece of iron. The 
 hammer while raised possessed potential energy, and 
 when freed it exhibits kinetic energy, which disappears 
 when the moving hammer strikes the iron. We find, 
 however, that the temperature of the iron has been 
 raised ; its atoms have increased energy. If we 
 multiply this effect by repeated blows, the increase 
 in heat energy becomes most apparent. But gradu- 
 ally the temperature settles down again to normal. 
 Where has the energy now gone ? We may say 
 that it has been dissipated or radiated away through 
 space, but undoubtedly it still exists, although we may 
 be quite unable to detect it even with the most 
 sensitive apparatus. 
 
 It is quite clear that when the hammer falls upon 
 a piece of iron and heats it, there has been merely a 
 change in the form of energy; the initial energy has not 
 been lost. This heat may be conducted away, in some 
 measure, to other bodies with which the iron is in 
 contact, but ultimately we find that the heat is radi- 
 ated away into space. It is here that we fail to follow 
 it ; we can simply state that it has been added to the 
 great reservoir of unavailable energy. We may picture 
 it as going to increase the temperature of our planet in 
 
 131 
 
Scientific Ideas of To-day 
 
 the same sense as one might picture the sea-level of 
 the world to be raised when we add a bucketful of 
 water to the ocean, 
 
 I wonder how many of us, in our youthful in- 
 quiries, were puzzled with the fact that water, at 
 atmospheric pressure, could never be hotter than 212 
 degrees Fahrenheit (100 degrees Centigrade) ? I have 
 clear recollections of puzzling over this fact, and 
 thinking that if we still kept adding heat after water 
 reached the boiling-point we should make it hotter. 
 The solution of such youthful difficulties is very 
 simple. When water steps over the line marked 
 "boiling-point" it is no longer hquid water, but 
 becomes steam. In other words, the molecules of 
 water can hold on to one another up to a certain 
 definite rate of vibration, but beyond that they can- 
 not. They remain in the liquid state up to boiling- 
 point, but above that they pass into the gaseous state. 
 It is only at the free surface of the water that the 
 molecules can escape, so the whole body of the water 
 remains at boiling-point, and the application of further 
 heat goes to free the molecules at the surface. Up to 
 boiling-point we see a simple transference of heat 
 energy from the source of heat — say, a fire — to the 
 molecules of the water. After that a certain amount 
 of energy seems to disappear; but we know that this 
 energy is expended in driving the molecules apart, so 
 that they no longer keep together in a liquid state, 
 but become separated into the gaseous condition of 
 
 matter. We know very well that this energy cannot 
 
 132 
 
what is Energy? 
 
 have disappeared in reality, and so we say that it has 
 been transformed into latent heat. 
 
 It is very doubtful if this term latent heat is a wise 
 one; it would seem to signify a dormant form of 
 energy, which really we cannot imagine. Surely the 
 energy must exist still as motion in some form or 
 other. We have the same misgivings in connection 
 with potential energy. The energy in these cases 
 cannot be asleep, as it were; there must surely be 
 motion, although it is beyond our ken. If one reads 
 the late Professor Tait's article on " Mechanics ** in 
 the Encyclopcedia Britannica, one sees that he was 
 impressed with the idea that motion must be locked 
 up in some mysterious fashion in the condition we 
 describe as potential energy. 
 
 It has been suggested that muscular energy is due 
 to vibratory motion of the muscles. Indeed, it was 
 pointed out by Dr. Wollaston, about a century ago, 
 that muscles did vibrate when maintained in a state of 
 tension. In the case of a muscular person supporting 
 a heavy weight the vibrations have been detected by 
 placing the observer's ear over the muscle, whereupon 
 a definite sound is heard, the vibrating muscle acting 
 in a modified manner as a tuning-fork. The physio- 
 logical considerations arising from such facts are very 
 interesting, but all that comes within our present pro- 
 vince is to notice that in a contracted muscle there is 
 a state of actual motion. 
 
 Where do we find no motion ? From what we 
 have seen in connection with the structure of the 
 
 133 
 
Scientific Ideas of To-day 
 
 atom, it is clear that the very existence of the atom 
 depends upon the constant and rapid motion of the 
 electrons within it. Think of the church analogy 
 to which I referred at the end of chapter iii., and 
 again at the beginning of chapter iv. There we see 
 a mere handful of far-scattered dots or "full-stops" 
 occupying the whole area of the church by keeping 
 constantly on the move. Withdraw the motion and 
 we can imagine the dots, instead of occupying the 
 whole interior of the church, being gathered together 
 in a small dust-pan. How much of the atom is really 
 motion ? Then again, if the electron, as has been 
 suggested, is simply the aether — or on Mendeleeff's 
 theory, the particles of the aether — in motion, how 
 much of the fundamental electrons is motion ? The 
 very fact that energy is uncreatable and indestructible 
 surely enables us to realise that it is a real thing. 
 
 We shall consider only one or two of the prominent 
 forms of energy. We cannot pass over chemical energy,* 
 it is sure to be of interest to us, as we are continually 
 seeing evidences of it. We know that many sub- 
 stances unite together in a very quiet and unobtrusive 
 way.^' We have such a case when the energy of light 
 waves falls upon the chemicals on a photographic 
 plate and, unnoticed, disturbs the arrangements of the 
 chemical atoms. But, on the other hand, we are 
 aware of very energetic chemical combinations taking 
 place within the cylinders of the modern petrol motor- 
 car. We know that all explosions are due to a sud- 
 den rush of the chemical atoms in changing from one 
 
 134 
 
what is Energy? 
 
 set of molecules to other gaseous forms which occupy 
 much more space. We have no difficulty in realising 
 that there is such a thing as chemical energy. We 
 should picture chemical energy as forcing the atoms 
 from one molecule to another. What, then, should 
 we think of electrical energy ? Remembering that an 
 electric current is the handing on of electrons from 
 atom to atom, we should say that electrical energy 
 forces the electrons away from the positive sphere of 
 the atom. 
 
 In passing, it will be of interest to note the close 
 connection between chemical energy and electrical 
 energy in the ordinary battery. It will be remembered, 
 from our consideration of electric batteries in chapter 
 v., that atoms escape from the zinc plate into the 
 liquid and there form new chemical combinations. 
 And, further, that these escaping atoms leave behind 
 them their detachable electrons, so that there is an 
 accumulation of electrons upon the zinc plate. We 
 saw that these accumulating electrons are handed on 
 from atom to atom in the wire which connects the 
 zinc plate or element to the carbon or other element of 
 the battery. This electrical energy continues as long 
 as the chemical changes proceed in the battery. We 
 therefore say that in the battery we have chemical 
 energy transformed into electrical energy, or in other 
 words, the motion of the atoms in the battery gives 
 rise to the motion of electrons along the wire. In an 
 electro-plating bath we have the very reverse of this. 
 We move the electrons along the wire circuit to one 
 
 135 
 
Scientific Ideas of To-day 
 
 of the electrodes in the liquid, and there we set up 
 chemical action ; atoms of silver or other metals con- 
 tained in the liquid are deposited on the surface of an 
 article acting as the leading-out electrode. 
 
 In the dynamo we see mechanical energy trans- 
 formed into electrical energy, which in turn may be 
 transformed into heat energy in the electric furnace, or 
 the electrical energy may be transmitted to a distance 
 and be converted once more into mechanical motion 
 by means of an electric motor. The kinetic energy 
 of a waterfall may be transformed into mechanical 
 energy by means of the old-fashioned mill-wheel, and 
 we might go on considering one series of transforma- 
 tions after another, but all such transformations are 
 so obvious that we need not stop to consider them 
 further. 
 
 We must remember that during all such transfor- 
 mations of energy there is some part of the initial 
 energy lost, but lost only so far as our inability to get 
 further work from it; no energy is destroyed. We 
 therefore speak of the conservation of energy, and we 
 call this one of the laws of Nature. We must re- 
 member, however, that these laws are of man's making, 
 and are merely theories which appear to us to be per- 
 fect, and on this account we have elevated them to a 
 higher platform than mere theories. Some day we 
 may find our laws of Nature require amendments to 
 be passed. 
 
 In the present chapter we have endeavoured to fol- 
 low the transformations and transmission of energy in 
 
 136 
 
what is Energy ? 
 
 matter ; not only in connection with visible matter, 
 but also with regard to the atoms and electrons con- 
 tained in matter. But energy is not confined to these 
 forms which we have been considering ; it takes upon 
 itself other forms outside of matter altogether, in the 
 aether of space. That all-important medium is capable 
 of transmitting energy over billions of miles, as we shall 
 see in the following chapter. 
 
 Our position to-day is that we can resolve the 
 Universe into matter — aether — and energy, but we 
 cannot tell exactly what any one of these three entities 
 is. We may explain matter away until we have 
 nothing but electricity left, as we have seen in earlier 
 chapters, but there still remains the question — What 
 is Electricity ? 
 
 In closing this chapter it may be of interest to 
 tabulate the principal forms of energy. In addition 
 to those forms mentioned in the following hst, some 
 might desire to add gravitational energy, cohesion 
 energy, mechanical energy, muscular energy, and so 
 on. These, however, may be placed under other 
 forms already stated in the table on following page. 
 
 137 
 
Scientific Ideas of To-day 
 
 THE PRINCIPAL FORMS OF ENERGY. 
 
 ICinetic Energy e,g, A flying bullet. 
 Potential Energy „ A raised clock-weight 
 
 Strain Energy 
 Chemical Energy 
 Light Energy 
 Heat Energy 
 Electrical Energy 
 Magnetic Energy 
 
 A tightened watch-spring. 
 
 Gunpowder. 
 
 Photography. 
 
 The Sun. 
 
 The electric current. 
 
 A magnet lifting a piece of iron. 
 
 138 
 
CHAPTER X 
 
 WAVES IN THE ^THER 
 
 Waves on the surface of water — Transverse vibrations — Velocity 
 of ^ther waves — What constitutes different "kinds" of aether 
 waves — Transmission of energy by sether waves — Energy con- 
 veyed from the Sun to our planet — Light exerts a mechanical 
 pressure — Why the tails of comets sometimes precede the 
 heads — Store of energy within the atom — Tremendous energy 
 of a rapidly moving body 
 
 As no one has seen, or ever will see, waves in the 
 cether, that medium being itself invisible, it is evident 
 that we must draw upon our imagination. Waves 
 upon the surface of the ocean are familiar objects to 
 all, but they do not provide us with a very good ex- 
 ample of wave motion for the following reason. We 
 often speak of some object having been washed ashore 
 by the waves, as though the waves had brought the 
 object from a distance. In reality it is the wind which 
 has been the driving force. We shall therefore find a 
 better example of wave motion in a still pond upon 
 the smooth surface of which we can cause ripples. 
 
 We drop a piece of wood into the centre of the 
 pond, and we see miniature waves spread outwards in 
 circles, one circle following another at regular in- 
 tervals. If the pond be a small one, the first wave 
 does not take very long to reach the shore, while the 
 
 139 
 
Scientific Ideas of To-day 
 
 others follow in a regular march, but the piece of wood 
 still remains at the centre of the pond. If we had 
 placed corks at different points on the surface, the 
 waves would not have carried a single cork any nearer 
 to the shore ; the corks would have merely " bobbed 
 up and down." To speak more scientifically, we should 
 say that the corks had vibrated in a transverse direc- 
 tion. By the word transverse we mean that they are 
 turned across the path of motion, the word being de- 
 rived from the Latin tranSf across, and versus, turned. 
 These are the kind of waves we have to deal with in 
 the cether, and we therefore speak of them as trans- 
 verse vibrations. 
 
 Looking at the ripples on the pond, it will be 
 observed that the wave motion is travelling from the 
 centre of the pond to the edge, while the vibrations 
 of the corks, and therefore of the particles of water 
 also, are up and down, or at right angles to the direc- 
 tion of the wave motion. All this is very simple, but 
 unless the novice keeps the analogy of the pond before 
 him he is apt to think that there is something mys- 
 terious in such a statement as — "light is transverse 
 vibrations in the aether/' The fact of the matter is 
 that one thinks of this kind of wave motion more 
 naturally than any other. 
 
 We have the to-and-fro motion of particles, like the 
 swaying of people in a crowd, or, better still, like the 
 movements of a long spiral spring. In this case we 
 have states of compression and rarefaction, and in a 
 long spiral spring one can see waves of motion passing 
 
 140 
 
Waves in the ^ther 
 
 along it from one end to the other. As the to-and- 
 fro vibrations are in the same direction as the wave 
 motion, we speak of the vibrations as being longitu- 
 dinal. Sound waves in the air, and in other substances, 
 are of this nature. However, as all the waves in the 
 aether are transverse vibrations it is only this one kind 
 of vibration which concerns us at present, and we shall 
 try, therefore, to keep before us the ripples upon the 
 surface of a smooth pond. 
 
 In an earlier chapter the different aether waves were 
 briefly mentioned. These were light waves, radiant 
 heat waves, and electric waves. For convenience sake 
 I spoke of these as different kinds of waves, meaning 
 that they had different properties, but we shall see that 
 they are really all of the one nature ; they are all trans- 
 verse vibrations, such as we see in a pond of water. 
 
 As all aether waves are of the same kind, and as they 
 all travel with the same velocity, it is obvious that they 
 can differ only in the rate at which they follow each 
 other. In other words, the only difference between 
 light waves and radiant heat waves lies in the distance 
 between the successive waves. Suppose we had a 
 float or plunger of some kind with a handle attached 
 to it, so that we could conveniently move it up and 
 down at the centre of our imaginary pond of still 
 water. If we were to move the plunger up and down 
 very slowly the waves would follow each other at some 
 considerable distance apart, whereas if the movements 
 of the plunger were rapid the waves or ripples would 
 follow close upon one another's heels. Far more 
 
 141 
 
Scientific Ideas of To-day 
 
 waves would arrive at the shore in one minute when 
 the plunger was moved rapidly. In order to compare 
 the different sets or trains of waves we could measure 
 the distance from the top or crest of one wave to the 
 crest of the following wave. Of course it would make 
 no difference if we measured the distance from the 
 depression or trough of one wave to the trough of the 
 next wave, or if indeed we measured the distance be- 
 tween any two corresponding points on neighbouring 
 waves. This is what we call the wave-length. It will 
 be observed that it has nothing whatever to do with 
 the length along the front or ridge of the wave. By 
 wave-length we simply mean the distance between two 
 successive waves. Possibly some readers would be 
 more inclined to call this the width or breadth of the 
 wave. 
 
 When we vibrated the plunger rapidly we produced 
 waves of short length. We see that there is a distinct 
 connection between the rate or frequency of vibration 
 and the length of the waves produced. The quicker 
 we vibrate the plunger, the shorter will be the resulting 
 waves. As the velocity of travel of all aether waves is 
 the same, the connection between the frequency and 
 the resulting wave-length is very simple. In one 
 second of time every aether wave will have travelled a 
 distance of 186,000 miles. Therefore if 1000 waves 
 are set up by the vibrator during one second, the 
 first wave will have travelled to a distance of 186,000 
 miles when the last wave is ready to start. In other 
 
 words, there will be 1000 waves equally spread over a 
 
 142 
 
Waves in the JEth^r 
 
 distance of 186,000 miles. We do not require pencil 
 and paper to calculate the wave-length in this case, for 
 if 1000 waves occupy a space of 186,000 miles, it is 
 obvious that each wave will occupy a space of 186 
 miles. We should say, therefore, that the wave-length 
 in this case was 186 miles. 
 
 Some of the aether waves used in wireless telegraphy 
 are measured in miles, while, on the other hand, 
 aether waves as short as one two-hundred-and-fifty- 
 thousandth part of an inch have been measured. It 
 is, of course, quite impossible to realise the smallness of 
 such dimensions, but we can appreciate the tremendous 
 range of different wave-lengths existing in the aether. 
 
 We have seen that the whole difference between any 
 one aether wave and another is in its length, or the 
 spacing between the waves, and, of course, there must 
 be a corresponding difference in the frequency or 
 number of vibrations per second. It is marvellous 
 that these aether waves which only differ in this way 
 possess such very different properties. 
 
 Commencing with the longest aether waves, we find 
 that these affect the detectors used in wireless tele- 
 graphy. We have seen that these electric waves may 
 be spaced miles apart, but other electric waves of the 
 same class have been produced as close together as six 
 waves within one inch. Even this is very long when 
 compared with the majority of aether waves. When 
 aether waves only measure a few thousandths of an inch 
 they produce heating effects, and we speak of them as 
 waves of radiant heat. As long as waves are longer 
 
 143 
 
Scientific Ideas of To-day 
 
 than one thirty-thousandth part of an inch we call them 
 dark-heat waves, for they do not affect our vision, but 
 as soon as they step over that boundary line they do 
 affect our eyes. They cause the sensation of red light 
 when there are about thirty-four thousand waves to the 
 inch. If the waves are still somewhat shorter — or closer 
 together — ^they produce the sensation of orange colour ; 
 still decreasing in wave-length they produce the sensa- 
 tion of yellow, then green, blue, indigo, and when they 
 have become so short that sixty thousand occupy one 
 inch they produce the colour sensation of violet.^ 
 After that they cease to affect our eyes altogether, and 
 we call them waves of ultra-violet light, signifying that 
 they are beyond the violet rays. 
 
 Although these same waves of ultra-violet light fail 
 to stimulate our sensory organs of vision, they very 
 actively aflFect the chemicals upon a photographic plate. 
 Because of their chemical properties these waves are 
 often called actinic waves. 
 
 These aether waves are all conveying energy. Re- 
 ferring one moment to the pond analogy, it is obvious 
 that if we expend some energy in giving the floating 
 plunger an up-and-down motion, there will be energy 
 transmitted by the resulting wave motion across the 
 surface of the pond. Any corks or other floating ob- 
 jects will imitate the up-and-down movements of the 
 plunger. We should say that the energy of the plunger 
 was transformed into wave motion in the water, that 
 energy was thus transmitted through the water to a 
 
 * More detailed particulars are given in Appendix III., at page 332. 
 
 144 
 
Waves in the -^ther 
 
 distance, where it was transformed once more into 
 kinetic energy, or energy of motion, in the moving 
 corks. In the same way we speak of the transmitter in 
 wireless telegraphy acting upon the aether ocean. The 
 transmitting apparatus transforms the energy of moving 
 electrons within it into wave-motion in the surrounding 
 ccther. This wave energy may be carried through the 
 aether broadcast 'across the Atlantic Ocean, and, strange 
 to say, an insignificant little detector on the far-distant 
 shore receives sufficient energy to cause some change 
 within it, and in this way signals are produced. 
 
 The transmission of heat energy from the Sun 
 through the aether to our planet is obvious to all. It 
 is interesting to note that it is possible to transform 
 this heat energy directly into mechanical motion. 
 We have a splendid demonstration in the large sun- 
 motors which are used in some of the farms in Cali- 
 fornia, 
 
 That the aether waves of ordinary light transmit 
 energy is .very obvious, for our sensory organs of 
 vision are stimulated by them, while the chemicals 
 upon a photographic plate are also affected. But the 
 fact that these aether waves of ordinary light exert an 
 actual mechanical pressure — just as the wind does — is 
 not so apparent. Indeed, it is only within recent years 
 that any experimental proof of this was obtained, for 
 the pressure is very small — extremely small as com- 
 pared with the pressure of the lightest breeze of wind, 
 or of any slight movement of the air. 
 
 Nearly forty years ago Clerk-Maxwell, one of those 
 145 K 
 
Scientific Ideas of To-day- 
 great mathematicians who must have been able to 
 dream in mathematics, stated that such a force — or 
 mechanical pressure — should exist in light, and he 
 calculated what the actual pressure should measure. 
 It is interesting to know that when experimental 
 proof of this force was devised, the actual pressure 
 was found to be of the order of magnitude that Clerk 
 Maxwell had calculated it should be, so long before 
 its discovery. 
 
 The demonstration was very simple. Very light 
 discs of platinum were suspended in a small glass globe 
 from which all the air had been exhausted. At this 
 time the best means of producing a high vacuum was 
 by means of the well-known mercury air-pump. A 
 small amount of mercury vapour remains in the ex- 
 hausted globe, so in order to get rid of this vapour 
 the globe was subjected to an intense cold and the 
 mercury vapour was frozen out. In this way the 
 vacuum was made as perfect as it was possible. This 
 was very important, for the demonstration would 
 mean nothing unless the vacuum was very high, as the 
 radiant heat waves would act upon any residual air 
 and cause the disc to move, just as one sees the little 
 vanes moving in the small radiometers so often ex- 
 hibited in opticians' windows. In this case it is the 
 heat waves which keep the small windmill revolving 
 by a continual bombardment of gaseous molecules. 
 The little windmill of the radiometer would not 
 revolve if placed in the high vacuum which was used 
 
 in this demonstration of the][mechanical push of light. 
 
 146 
 
Waves in the ^Slther 
 
 Having got rid of any possibility of a molecular 
 bombardment, the little suspended discs were exposed 
 to light, and there could be no doubt they were being 
 moved by the aether waves falling upon them. While 
 this pressure could be observed and measured under 
 these exceptional conditions, it will be understood that 
 the pressure is so slight that we have no knowledge 
 of it whatever in our every-day life ; the effect is quite 
 inappreciable upon any body placed in the great ocean 
 of air around us. 
 
 If you desire to find out in what direction the wind 
 is blowing when there is only an almost imperceptible 
 breeze blowing, you hold up some very light object in 
 the air. Why ? Because it will offer a very large 
 surface for the wind to act upon, while the pull that 
 gravity has upon it will be very small. It is not diffi- 
 cult to imagine a breeze so gentle that its effect upon 
 a whole bag of flour would be quite inappreciable, 
 but which would become apparent if the individual 
 particles of flour were falling as a fine rain through 
 the atmosphere. Picture for a moment a steamer 
 driving across the sea ; the pressure of the wind keeps 
 the smoke behind the steamer like a great tail. When 
 the steamer turns round and travels in the opposite direc- 
 tion it is not an uncommon thing to see this tail of 
 smoke preceding the steamer (see illustration facing 
 page 150). This is exactly what we witness in the 
 heavens in connection with comets. Away out in 
 setherial space we find comets making strange jour- 
 neys, sweeping round the Sun and then disappearing 
 
 147 
 
Scientific Ideas of To-day 
 
 once more on a long journey through space, perhaps 
 never to return. These comets have long tails, as 
 seen in the illustration, and while they are approach- 
 ing the Sun their tails behave in orthodox fashion 
 and trail behind the body of the comet. But when 
 the comet rounds the Sun and goes away from it, 
 a very curious sight is witnessed. The tail appears 
 to be "blown" on in front of the comet — just like 
 the steamer's smoke. This fact that the comet's tail 
 precedes its head when the comet is travelling away 
 from the Sun has been one of the mysteries of astro- 
 nomy. Of course, the Sun's gravitative force must be 
 attracting the particles of matter forming the comet's 
 tail, but it is apparent that some stronger force is 
 driving them away from the Sun. Gravity is pulling 
 the particles towards the Sun, and light is pushing 
 them away from it, and it is apparent that in this 
 particular case the light-push is greater than the pull 
 of gravity. How can we account for this ? 
 
 In the first place, we know that the particles of 
 matter forming the tail of the comet are extremely 
 small ; their size can be calculated accurately. The 
 Sun's gravitative pull upon these particles is corre- 
 spondingly small, but the extent of surface of these 
 particles is very great in proportion to their weight, 
 so that the pressure of light is correspondingly great. 
 We therefore find that Light is able to push these small 
 particles away from the Sun with greater force than 
 gravity can pull them towards it, and we therefore see 
 
 why a comet's tail always points away from the Sun. 
 
 148 
 
Waves in the ^ther 
 
 A few years ago a friend handed me an article 
 which had just been published by an eminent as- 
 tronomer. The subject of the paper was the comets, 
 and this action of the comet's tail was simply explained 
 as being " governed by the remarkable law that it must 
 be turned away from the Sun." My object in men- 
 tioning this incident is to emphasise the statement, 
 made in chapter i., that all the laws of nature are of 
 man's own making, and that therefore it is no explana- 
 tion to say that such and such a thing happens because 
 of one of these laws. To state that the comet's tail 
 is governed by a law that it must be turned away 
 from the Sun does not satisfy the reason, but the 
 theory of the mechanical pressure of light does. 
 
 No one can doubt that the aether transmits energy. 
 When energy leaves the Sun it is carried by the aether 
 for eight minutes before it arrives upon this planet. 
 We know that the idea of *' action at a distance" is now 
 quite defunct. No one can reasonably think of one 
 body acting upon another without the existence of an 
 intervening medium. If this exploded idea had been 
 true, there should have been no time at all required 
 for the Sun to act upon our planet. We shall see 
 later how this energy is transferred from matter to the 
 cether and again from the aether to matter. 
 
 Once possessed with the idea of the electrons 
 revolving within the atoms of matter at a prodigious 
 speed, we picture an immense store of energy within 
 the atom. In an earlier chapter we considered, in a 
 somewhat rough-and-ready manner, the relationship 
 
 149 
 
Scientific Ideas of To-day 
 
 of energy, velocity, and mass. In the present case the 
 electrons have not much mass to boast about, but 
 what they lack in bulk they make up for in velocity. 
 It is difficult for those not accustomed to science to 
 realise the true importance of velocity as a factor of 
 energy. 
 
 We have been referring repeatedly to the velocity of 
 light. Of course light is not a material thing, but try 
 and picture a small mass — such as the head of an 
 ordinary pin — travelling through space with the same 
 velocity as light. What amount of energy will the 
 flying pin-head possess ? 
 
 It is difficult to find any popular means of testing 
 the energy of this flying missile, but possibly at some 
 time or other we have seen machines for testing a 
 man's strength. I remember one particular form 
 which used to be in existence at country fairs. The 
 would-be strong man was asked to test his strength in 
 the following fashion. He had to strike an upright 
 lever with a heavy hammer, and in so doing he caused 
 an iron ring to be shot upwards on an upright pole. 
 The more energy the man brought into play the higher 
 up the pole did the ring go. I cannot recollect the 
 exact height of these poles, but they would not exceed 
 fifteen or twenty feet. Suppose our flying pin-head 
 entered as a competitor at this "try your strength" 
 demonstration. Judging from the size of the pin-head 
 it should make a very poor exhibition, but because of 
 its prodigious velocity it could certainly beat all comers. 
 Suppose the ring to weigh one pound, we can easily 
 
 ISO 
 
The Mechanical Pressure of Light 
 
 In the upper photograph we see a comet with its magnificent tail. As a comet 
 recedes from the sun its tail precedes its head, just as the steamer's smoke precedes it 
 when travelling with the wind. In the case of the comet we believe this to be due lo 
 the mechanical pressure of Hght, as explained at page 148. 
 
 [The above photograph of a comet was taken by Prof. E. E. Barnard, Yerkes 
 Observatory, U.S.A.] 
 
Waves in the ^ther 
 
 calculate the height to which the ring would rise, pro- 
 vided all the energy of the pin-head could be trans- 
 ferred to the ring ; we neglect the large proportion of 
 energy dissipated as heat. Even if we suppose that 
 the pull of gravity were constant at any distance from 
 the earth, we find that the ring would go to an enor- 
 mous distance. One mile might seem quite a decent 
 record, but, under such circumstances as I have in- 
 dicated, the ring would fly upwards for thousands of 
 miles. If we take the decreasing value of gravity 
 into account, we find that the ring sets off with so 
 much energy that it would be shot off the planet 
 altogether, never to return. Of course it would be 
 utterly impossible to give a pin-head the velocity 
 of light, but, by supposing an extreme case such as 
 this, the importance of the velocity factor is brought 
 out. 
 
 This analogy of the flying pin-head helps us to 
 picture the enormous energy which must be due to 
 the flying electrons contained within the atom. A 
 pin-head is an enormous giant compared to an elec- 
 tron, but the energy possessed by the flying pin-head 
 can stand a good deal of dividing. Besides, whatever 
 energy is contained in one atom, must be multiplied 
 enormously to give the total inter-atomic energy con- 
 tained in a small piece of matter. For instance, if we 
 desired to calculate how much internal energy was 
 contained in a small cube of solid copper, measuring 
 less than half an inch on each side, we should have to 
 multiply the internal energy of one atom by one quad- 
 
 151 
 
Scientific Ideas of To-day 
 
 rillion, for there would be that number of atoms 
 contained in the small piece of copper. 
 
 But surely all this about the internal energy of the 
 atom must be purely hypothetical, for it is locked up 
 within the atom and we cannot affect it in any way 
 for the purpose of measuring its value. That is our 
 position with the most of matter, but we have recently 
 found a few forms of matter in which this inter-atomic 
 energy is being unlocked by Nature. Some atoms are 
 breaking up and allowing the flying electrons to escape. 
 This, however, will be understood better when we 
 come to consider radio-active bodies, such as the 
 world-renowned element Radium. These radio-active 
 bodies are of so much interest that they demand at 
 least a chapter to themselves. 
 
 This chapter deals specially with energy in the 
 aether, and all waves in the aether are often classified 
 under the one heading Light, It will be of interest, 
 therefore, to consider at greater length the question — 
 What is Light ? 
 
 152 
 
CHAPTER XI 
 WHAT IS LIGHT? 
 
 Evidence against Newton's corpuscular theory of light — Electrons 
 and light — Proof in favour of the electro-magnetic theory of 
 light — Hertz detecting the electric waves — How the velocity 
 of light is measured — How the wave-lengths are measured — 
 Wasteful processes of producing artificial light — An ideal 
 method in Nature 
 
 Even in our childhood we appreciated the fun con- 
 tained in stories of fairies making collections of sun- 
 beams, catching them and keeping them in stoppered 
 bottles. Doubtless, before reading the earlier chapters 
 of this volume, there was not a reader who did not 
 know beforehand that light was simply waves in the 
 aether. Or if their thoughts on the matter were not 
 quite so definite, they knew at least that Light was 
 not a material thing. At the present time it would 
 be well-nigh impossible to ferret out any person who 
 believes in Newton's corpuscular theory of light. 
 
 I remember, in my schooldays, wondering how 
 Newton could call solemnly upon people to imagine 
 particles so small that they could be shot off by the 
 Sun and travel across ninety-three millions of miles, 
 between the Sun and this planet, in eight minutes ; 
 eleven million miles in one minute. Indeed, as a 
 
 153 
 
Scientific Ideas of To-day 
 
 schoolboy, it seemed to me rather a joke that such 
 a great man as Newton should seriously believe in 
 the possible existence of such particles. Now we find 
 that although Newton's corpuscular theory has been 
 abandoned completely, there do exist corpuscles or 
 electrons very similar to Newton's imaginary cor- 
 puscles. Further, we find that these small particles 
 are shot off from all very hot bodies; even by the 
 ordinary household lamp. Therefore the Sun is shoot- 
 ing off a continuous stream of these corpuscles or 
 electrons, and we have seen that these particles can 
 travel at a speed of sixty thousand miles per second 
 within a vacuum tube. Had Sir Isaac Newton known 
 these recently discovered facts he might have argued 
 that it was possible for these flying corpuscles to 
 attain a speed three times as great in the free vacuum 
 of interstellar space, and that they travelled from the 
 Sun to the Earth with a velocity of one hundred and 
 eighty-six thousand miles per second. Such an argu- 
 ment as this might have seemed reasonable, but it 
 could not have stood the test of modern experiments. 
 We have positive proof that these flying corpuscles do 
 not constitute lighty and this we shall see from what 
 follows. 
 
 We can experiment with these same flying electrons 
 in vacuum tubes ; it will be remembered that these 
 electrons have been found to be identical, no matter 
 from what source they are obtained. By experiment 
 we find that a stream of electrons does not possess 
 the properties of light. Light can be reflected, re- 
 
 154 
 
What is Light? 
 
 fracted, and polarised — these are its distinctive pro- 
 perties — but the flying particles do not possess these 
 properties. We cannot reflect, refract, or polarise 
 them. The meaning of polarising light will be made 
 clear in the next chapter. 
 
 Instead of being amused at Newton's corpuscular 
 idea, we are bound to marvel at his genius in fore- 
 seeing the possible existence of particles of seemingly 
 impossible dimensions, and capable of travelling at a 
 prodigious velocity. Although we find that we can- 
 not, by any artificial means, cause these electrons to 
 travel faster than sixty thousand miles per second, we 
 shall see when we come to consider the subject of 
 Radium that it discharges electrons from within itself 
 with a velocity of one hundred and twenty thousand 
 miles per second. This seems to be getting suspi- 
 ciously near the figures which represent the velocity of 
 light. Although we can prove that these flying elec- 
 trons do not constitute light, there is no doubt that 
 had these electron velocities been known at the be- 
 ginning of the eighteenth century they would have lent 
 substantial support to Newton's corpuscular theory. 
 In a later chapter we shall consider the effects caused 
 by these flying particles reaching us from the Sun ; 
 but in the meantime we note that it can be asserted 
 positively that these are not light. 
 
 We have no doubt whatever that light is a series 
 of aether waves, and from what we have already seen 
 in connection with moving electrons disturbing the 
 ^ther and causing magnetic and electric fields in 
 
 155 
 
Scientific Ideas of To-day 
 
 it, we are quite prepared to accept the fact that 
 the aether waves of light are caused by moving 
 electrons. 
 
 In the preceding chapter we saw that the aether waves 
 known as red light are so short, or, in other words, 
 follow each other so rapidly, that thirty-four thousand 
 of these particular aether waves occupy only the space 
 of one inch. It will be apparent that whatever the 
 object may be which is setting up such a rapid series 
 of waves, that disturbing object must be vibrating at 
 an enormous rate. When we were picturing the 
 plunger setting up waves at the centre of a still pond, 
 we saw that the faster the plunger moved up and 
 down the more waves were formed in any definite 
 distance or in any given time. Knowing the rate 
 of travel of these aether waves and the number of 
 waves per inch, a simple calculation shows us that 
 the rate at which an electron must be vibrating to 
 produce the waves of red light must be four hundred 
 billion times per second. Of course the figures are 
 beyond imagination, but all we can do is to picture 
 an electron flying round and round an atom of matter, 
 like a satellite around a planet, but making an enor- 
 ' mous number of revolutions in each second of time. 
 To say that one electron makes four hundred billion 
 revolutions per second, and another six hundred billion, 
 is only of use in enabling us to compare one velocity 
 with another. 
 
 It must be clearly understood that all atoms of 
 matter are made up of a number of electrons revolving 
 
 156 
 
What is Light 
 
 ? 
 
 in regular orbits, and that we cannot in any way 
 disturb these arrangements. But those free elec- 
 trons which circulate round the atoms as satellites 
 can be affected by outside forces. By applying heat 
 to a body we hasten the speed of these revolving 
 electrons. By cooling a body we cause these free 
 electrons to revolve more slowly. In a lump of iron 
 at ordinary temperature the motion of these electrons 
 is disturbed by molecular collisions, so that the result- 
 ing rates of revolution cause very long waves in the 
 aether. We call these heat waves, and every existing 
 thing is radiating heat in some degree. To return to 
 the very simple experiment of striking a piece of iron 
 by a heavy hammer, we are able, by repeated blows, 
 to cause the molecules to vibrate more rapidly, and 
 this enables these revolving electrons to quicken their 
 pace. Some of them soon reach a speed at which they 
 set up those rapidly succeeding aether waves, which 
 affect our eyes, and which it is usual to describe as 
 visible light. By means of delicate thermometric in- 
 struments we can show that some of the electrons 
 continue to revolve at the slower speeds and give rise 
 to the aether waves which we label dark-heat. When 
 a piece of iron has reached a white-heat, we can show 
 by means of the spectroscope that its electrons are 
 setting up the whole range of aether waves which make 
 up the visible spectrum — and in addition to the dark- 
 heat waves beyond the red of the spectrum we can 
 show that other aether waves of ultra-violet light are 
 also radiated from the glowing metal. It is clear, 
 
 157 
 
Scientific Ideas of To-day 
 
 therefore, that in the white hot metal we have electrons 
 flying around their atoms at speeds varying not only 
 from four to eight hundred billion per second, but 
 some much slower and others much faster. 
 
 We have seen that it is the revolving electrons which 
 provide the connecting-link between matter and the 
 aether. It is truly marvellous that such infinitesimally 
 small things as electrons — although distant ninety- 
 three million miles from us, in the Sun, can affect us 
 upon this planet. We can go still further and think of 
 revolving electrons attached to atoms in the far-distant 
 stars affecting us over a space of billions of miles. 
 
 All aether disturbances are caused by moving elec- 
 trons. While light and radiant heat are caused by 
 revolving electrons, the longest aether waves such as 
 are used in wireless telegraphy could not be set up 
 by electrons revolving in small orbits. They are pro- 
 duced by electrons surging to and fro in an electric 
 circuit. We should have no difficulty, however, in 
 appreciating the fact that all the different aether dis- 
 turbances are of the same kind and only differ as 
 regards their wave-lengths. 
 
 But some one might say that all this about light 
 being an electro-magnetic disturbance of the ^ther is 
 purely theoretical. A man, if he so desires, may say 
 that the moon is made of green cheese, but no one 
 will believe him, because he cannot bring forward any 
 observed facts to support his theory. What facts can 
 we bring forward to support this electro-magnetic 
 
 theory of light ? 
 
 158 
 
What is Light? 
 
 First and foremost, we can definitely prove that 
 light travels at the same speed as electro-magnetic 
 waves. Indeed, scientists were sure of this fact before 
 they could prove it experimentally. Long ago man 
 found out by observation that light travelled with a 
 velocity of 186,000 miles per second. How this 
 observation was made will be explained a little later. 
 But up to the year 1888 we could not measure the 
 velocity of electro-magnetic waves — indeed we could 
 only state that such things must exist. Nevertheless, 
 at that time, mathematicians were able to calculate 
 from electrical measurements what the velocity of 
 such waves would be if they could be found. The 
 result of these calculations was to bring out a velocity 
 exactly the same as the observed velocity of light. 
 Many people, who are not conversant with science, 
 shake their heads at the idea of a mathematical proof, 
 but if they were to make a serious study of the subject 
 they would not do so. 
 
 In the year 1888 a brilliant young professor in one 
 of the German universities found a means of detecting 
 and measuring electro-magnetic waves in the aether. 
 It was known at that time that an electrical dis- 
 charge, such as the sparking between two electrified 
 spheres, was of a to-and-fro, or oscillatory, character. 
 These oscillations would set up waves in the aether, 
 but no one could think of a means of detecting their 
 presence. Who should be able to invent an instru- 
 ment delicate enough to detect these seemingly un-get- 
 at-able waves ? 
 
 ^59 
 
Scientijfic Ideas of To-day 
 
 Dr. Heinrich Hertz, the distinguished professor 
 already referred to, gave the answer. No elaborate 
 apparatus was required, merely a simple loop of wire 
 with a tiny break in it. At one end of his room Hertz 
 had an induction coil arrangement producing electric 
 sparks, and therefore, according to the theories of 
 the mathematicians, producing electro-magnetic waves 
 in the surrounding aether. Hertz went about the room 
 with this simple loop of wire — something like a large 
 wrist-bangle — in his hand. He found that sparks 
 occurred at the tiny break in this loop of wire. It is 
 only fair to other experimenters to remark that so far 
 Hertz had discovered nothing new. Other scientists 
 had made very similar experiments ; notably Professor 
 Silvanus Thompson, of London, as may be seen on 
 referring to the Philosophical Magazine of September 
 1876. 
 
 At this date — a dozen years before the experiments 
 of Hertz — Professor Silvanus Thompson had shown 
 electric sparks occurring between two ordinary door- 
 keys when placed close together and taken to some 
 distance from the induction coil apparatus. But, when 
 speaking of Hertz's work, Professor Thompson has 
 said : " It never dawned upon me that these sparks 
 were the evidence of electric waves crossing the space. 
 That was Hertz's discovery. He did not go idly about 
 the room noticing the sparks, but explored the posi- 
 tions where the sparks were to be detected, and holding 
 his apparatus (the loop of wire) in the right position 
 
 to detect them." 
 
 160 
 
What is Light? 
 
 What we are interested in, at the moment, is the 
 measurement of these waves. When Heiiz found that 
 he could detect these invisible electric waves, he soon 
 adopted means of measuring them. He placed a great 
 sheet of metal against the wall of his room, and then 
 sent electric waves towards it so that the waves would 
 be reflected back upon themselves. It is well known 
 that when any kind of wave motion is reflected back 
 upon itself, the reflected waves interfere with the 
 original waves and cause what are known as station- 
 ary waves. Without going into details, we shall be 
 content to note that points of interference are pro- 
 duced at which one wave quite neutralises the other. 
 These are called nodal points, and it can be demon- 
 strated that the distance between any two nodal points 
 is exactly half one wave-length. We can therefore 
 understand that when Hertz found two distinct places 
 in his room at which his detector would not respond 
 to the electric sparker or wave producer, he knew 
 that he had found two nodal points such as just de- 
 scribed. He knew, therefore, that the distance between 
 these two places was exactly half the length of the 
 electric waves which were striking the metal sheet and 
 being reflected. By this means Hertz measured the 
 length of electric waves. 
 
 Hertz knew the rate of electric oscillations in his 
 
 wave-producing apparatus, so it was quite a simple 
 
 matter to calculate how fast the waves would travel 
 
 now that he knew how long the waves were. To make 
 
 the matter quite clear let us return, for a moment, to 
 
 i6i L 
 
Scientific Ideas of To-day 
 
 the pond analogy. By means of the plunger I might 
 be able to set up a series of waves at the rate of two 
 waves per second. I should know, therefore, that the 
 distance travelled by the wave motion in one second 
 would measure exactly the length of two waves. Some 
 one measures the waves which I am producing and 
 informs me that each wave measures exactly one inch, 
 and so I am able to say that the wave motion is travel- 
 ling a distance of two inches in one second. This, of 
 course, is a purely imaginary case, but it will serve to 
 illustrate how Hertz, knowing both the number of 
 waves produced per second and the length of a wave, 
 was able to calculate the rate of travel. He found that 
 the velocity was 186,000 miles per second, thus veri- 
 fying the mathematician's earlier calculations. In 
 this way Hertz proved that electric waves travelled 
 with the same velocity as waves of visible light. 
 
 The velocity of light had been determined more than 
 two centuries before Hertz measured the velocity of 
 electric waves. It would be difficult to explain the 
 exact method without mathematics, but the follow- 
 ing may give some idea. Nearly two hundred and 
 fifty years ago astronomers noticed an apparent 
 irregularity in the movements of one of Jupiters 
 satellites. Astronomers made time-tables showing 
 where the satellite would be at certain times, but 
 the satellite did not behave as it was expected to do. 
 At one period of the year it was fully a quarter of 
 an hour behind the scheduled time which it had 
 kept six months previously. Here w^as a puzzle for 
 
 162 
 
what is Light 
 
 ? 
 
 these astronomers. No one could suppose that the 
 
 satellite fell off in speed at one time of the year and 
 
 returned to its original speed six months later. Yet 
 
 the fact remained that when this satellite disappeared 
 
 behind its great planet, it was sometimes sixteen 
 
 minutes and thirty-six seconds late in reappearing. It 
 
 was well known that Jupiter was a very long way off 
 
 from us — nearly five hundred million miles — but no 
 
 matter how long light might take to travel from Jupiter 
 
 to our Earth, surely it must always take the same time. 
 
 It would do so provided the distance between Jupiter 
 
 and the Earth was always the same. But these 
 
 astronomers of the seventeenth century knew that this 
 
 distance was not always the same. While Jupiter 
 
 made one majestic march, in its far-distant orbit, 
 
 around the Sun, the Earth made nearly one dozen 
 
 complete circuits of its smaller orbit. Therefore at 
 
 one period of our year we should be nearer Jupiter 
 
 than we should be six months later. When we are at 
 
 the farthest side of our orbit away from Jupiter, its 
 
 light will have to travel the additional distance across 
 
 our orbit, which it will not have to do when we are at 
 
 the point of our orbit nearest to Jupiter. We all know 
 
 that we are about 93 million miles from the Sun, 
 
 so that the diameter of our orbit will be twice that 
 
 distance, or 186 million miles. We have seen that 
 
 these astronomers found by observation that Jupiter's 
 
 satellite was apparently sixteen minutes and thirty-six 
 
 seconds late, so it was quite clear to them that the light 
 
 from Jupiter took that time to cross the Earth's orbit. 
 
 163 
 
Scientific Ideas of To-day 
 
 This time is practically equal to looo seconds, and 
 
 during that time light has travelled i86 million miles, 
 
 so we need not trouble about pencil and paper to 
 
 calculate how far light will travel in one second. 
 
 We have simply to strike off the last three ciphers of 
 
 the 186,000,000 miles, and we find that light travels 
 
 186,000 miles per second. This is such an enormous 
 
 speed that light appears to travel instantaneously from 
 
 any one point upon our planet to another distant point 
 
 on it. 
 
 It is interesting to know that Galileo tried to measure 
 
 the velocity of light by covering and uncovering lamps 
 
 at a distance, but, as we should expect, no result could 
 
 be obtained. Nevertheless it has been found possible 
 
 to devise methods of measuring the velocity of light 
 
 by direct experiment. Without troubling about much 
 
 detail, it is of interest to note the principle of one such 
 
 experiment. An aperture is very suddenly opened and 
 
 closed so that a beam of light can shoot out through 
 
 the aperture, fall upon a mirror at a fixed distance, and 
 
 be reflected back again to the aperture, where it can 
 
 enter and be observed through an eye-piece. If light 
 
 were transmitted instantaneously, the reflected beam 
 
 could always re-enter the aperture, no matter how 
 
 rapidly it was closed. A simple means of opening 
 
 and closing the aperture very rapidly was devised. 
 
 Picture a disc with a row of small holes cut in it 
 
 around its outer edge ; a toothed wheel is what is 
 
 actually used. This disc is so arranged that the holes 
 
 pass across in front of the aperture in succession. If 
 
 164 
 
What is Light ? 
 
 the disc is revolved at a high speed we shall have the 
 aperture opened and closed with exceeding rapidity. 
 If the light takes any time at all to travel from the 
 aperture to the mirror and back again, we should find 
 that at a certain speed of rotation of the disc the light 
 wave would arrive back at the aperture at the moment 
 it was closed. No light would be observed to enter 
 the eye-piece when this was the case. Then if the 
 speed of rotation be increased till the reflected light 
 succeeds in entering the aperture by the hole which 
 follows the one from which it escaped, it will be clear 
 that it took the light just as long to travel from the 
 aperture to the mirror and back again as it took the 
 edge of the disc to travel the distance from one hole to 
 the following hole, A simple calculation from the 
 speed of rotation of the disc will give the time required 
 for this very small movement of the disc. We there- 
 fore know how long it took the light to travel the 
 measured distance between the aperture and the mirror. 
 This turns out to be exactly equal to a speed of 186,000 
 miles per second. Several experimenters have devised 
 other means of measuring the velocity of light, and all 
 the results fall within 185 and 186 thousand miles per 
 second. 
 
 Having satisfied ourselves that there is no guesswork 
 about the stated velocity of light, it will be of interest 
 to see how it is possible to measure the length of those 
 waves which are said to be only about one thirty- 
 thousandth part of an inch. One might think that 
 this could only be understood by those who can think 
 
 165 
 
Scientific Ideas of To-day 
 
 in pure mathematics, but fortunately this is not the 
 case. It will be remembered that Dr. Thomas Young, 
 the first Professor of Natural Philosophy in the Royal 
 Institution, London, was one of the pioneers of the 
 electro-magnetic theory of light. One of his famous 
 experiments was to show that two light waves could 
 so interfere with each other that they caused dark- 
 ness. Young took a very narrow beam of light of 
 one colour — say, red — so that all the sether waves 
 would be of the same length. He placed an obstruct- 
 ing screen in the path of this red beam, and only 
 allowed the light to pass through two small holes 
 placed very close together in the screen. Therefore 
 from the back of the screen two small beams of red 
 light proceed from two points very close together. The 
 light from these is made to fall upon a white screen, 
 and one would expect to find a patch of red colour 
 made up of the two beams of red light coming through 
 the two holes. However, Young found something 
 more than that. The image upon the screen was 
 made up of alternate red bands and dark bands, or in 
 other words, bands of darkness. When either of the 
 two small holes was closed the image on the screen 
 was merely a patch of solid red, but as long as light 
 passed through both holes these bands of darkness 
 were present. Young used the result of this experi- 
 ment as a proof of the wave theory of light. If 
 Newton's corpuscular theory were correct, then two 
 beams of luminous particles added together should 
 
 only give an enhanced luminosity. In other words, 
 
 i66 
 
What is Light? 
 
 if you add something to something the result cannot 
 be nothing. If, however, the two beams of light were 
 not composed of material particles, but were merely 
 wave motion in some medium, it was quite under- 
 standable that one wave might so interfere with another 
 and produce these bands of darkness at their meeting 
 point. 
 
 It was by means of this same simple experiment that 
 Young was able to measure the wave-length of orange- 
 red light. Picture a single train of waves passing through 
 No, I hole and striking the screen at a point directly 
 opposite it, while another train of waves passes through 
 No. 2 hole and strikes the same point on the screen, 
 which cannot, of course, be quite directly opposite the 
 second hole. It is clear that the waves passing through 
 No. aTiole will have a very slightly longer distance to 
 travel than those passing more directly from the first 
 hole. If these two waves meet on the first dark band 
 they interfere with each other, so the one wave must be 
 exactly half a wave-length behind the other. Therefore 
 the difference in the lengths of these two trains of 
 waves will be exactly one half wave-length. Young 
 found it possible to measure the very small difference 
 in these two distances, and found it to be only one 
 eighty-thousandth of an inch. This was, therefore, the 
 measurement of one half wave-length of red light, so 
 that the waves of red light measure one forty-thousandth 
 of an inch. The other colours of the spectrum could 
 be measured in the same way ; a detailed list of these 
 lengths will be found in Appendix III., at page 332. 
 
 167 
 
Scientific Ideas of To-day 
 
 We have pictured visible light as aether waves pro- 
 duced by electrons revolving around the atoms of 
 matter. Our chief method of producing these short 
 aether waves is by heating some substance to a high 
 temperature. But despite all we hear of economical 
 methods of artificial lighting, all our processes are 
 ridiculously wasteful. Imagine a man manufacturing 
 any useful substance, and for every ten pounds of 
 the substance which he is making he produces at the 
 same time ninety pounds of useless by-products, or 
 substances for which he is to get absolutely nothing. 
 No one has ever heard of such a wasteful manufactur- 
 ing process, and yet that is a true analogy of what 
 we do when we produce artificial light. Perhaps a 
 better analogy would be to think of an employer of 
 labour desiring to do a certain amount of useful work. 
 From experience he finds that he requires to employ 
 one hundred men to get the work done, but he is 
 aware that the work he wishes done could be accom- 
 plished by ten of the men if they only knew how to 
 do it. We burn a jet of gas in order to produce 
 artificial light. We are seeking to set up aether waves 
 of a certain length, but in doing so we produce only 
 three per cent, of these waves and ninety-seven per cent, 
 of waves we do not wish and could do very well with- 
 out, as they are simply dark-heat waves. The hotter 
 the body is which is setting up the aether waves the 
 better is the percentage of useful waves, but even with 
 electric arc lamps we can only attain an efficiency of 
 
 ten or fifteen per cent. 
 
 i68 
 
What is Light? 
 
 In our methods of artificial lighting we are, in some 
 measure, imitating the Sun, which produces thirty per 
 cent, of visible light waves. Nature, however, is not 
 wasteful ; the remaining seventy per cent, of invisible 
 aether waves set up by the Sun are required to supply 
 us with heat necessary to maintain life upon our 
 planet, and to produce chemical changes. If we 
 could only imitate Nature as we see her producing 
 light in the glow-worm, where practically the whole 
 of the aether disturbance is in the form of visible 
 light — no dark-heat waves being produced — we should 
 be able to cause illumination on a grand scale. Re- 
 ferring to the luminosity of the glow-worm. Sir Oliver 
 Lodge has remarked that if we could only obtain this 
 secret from Nature, "a boy turning a crank could 
 furnish sufficient energy to light an entire electric 
 circuit." 
 
 We have seen how Hertz produced, detected, and 
 measured aether waves by purely electrical means. It 
 has become common practice, in wireless telegraphy, 
 to set up these aether waves by causing the electrons 
 to surge to and fro in an electric circuit. We have 
 also realised that these waves only differ from visible 
 light in being much longer waves. We must, there- 
 fore, hasten the motion of the electrons in order to 
 produce the shorter waves of visible light ; but that is 
 our difficulty. The shortest aether waves we have been 
 able to produce by electric oscillations are about one- 
 sixth of an inch apart, whereas we must crowd no less 
 
 than thirty thousand waves into one inch before they will 
 
 169 
 
Scientific Ideas of To-day 
 
 affect our visionary apparatus. Nature accomplishes 
 this feat, but she does not employ a single to-and-fro 
 surging of the electrons ; she causes the electrons to 
 spin round their atoms hundreds of billions of times 
 per second. It is evident that we must find some 
 means of giving the electrons this violent rotary 
 motion before we can produce artificial light without 
 the enormous waste we have at present. 
 
 It may not be clear to all how the present enormous 
 waste arises in the production of light by means of 
 heating a body. When we heat a body we cause a 
 great disturbance among its molecules, and these in- 
 cessant collisions between them prevent the free revo- 
 lutions of the electrons around the atoms. Therefore 
 we have electrons travelling at all speeds — a large 
 proportion only attaining the speed at which dark- 
 heat waves are produced, while only a very small 
 percentage attain to the speed at which visible light 
 is produced. What we want is to cause all the elec- 
 trons to revolve at the high speed, 
 
 NoTB.— In dealing with the electro-magnetie theory of light I have not 
 endeavoured to treat the subject from the historical point of view, but as 
 this is of interest, I have given a short historical note in Appendix II. at 
 page 328. 
 
 170 
 
CHAPTER XII 
 
 MORE ABOUT LIGHT 
 
 The distinctive properties of light — The meaning of polarised 
 light — A wild analogy — How we can show when light is 
 polarised — Beautifully coloured lantern pictures from colour- 
 less slides — Experimental proof that heat waves and electric 
 waves possess the same distinctive properties as visible 
 light — How matter sets up aether waves — How aether waves 
 react on matter — How some bodies reflect, and others 
 absorb, light — A nonsense story — How tourmaline polarises 
 light 
 
 In the preceding chapter it was pointed out that there 
 is no doubt whatever concerning the statement that 
 dark-heat waves, and electric waves, are really invisible 
 light waves, and that the sole difference is in their 
 wave-length, or, in other words, in the distance 
 between the succeeding waves. 
 
 We are so accustomed to some of the properties 
 of ordinary light that we pass them by almost un- 
 noticed. We see light falling upon objects all around 
 us, and we do not stop to think that these objects are 
 only visible to us because they reflect some of the waves 
 falling upon them, and that these reflected aether waves 
 enter our eyes. Every person is well acquainted with 
 the fact that light can be reflected. A second property 
 
 of light which even the most unobservant of us cannot 
 
 171 
 
Scientific Ideas of To-day 
 
 have failed to notice for ourselves, is that light can be 
 bent out of its normally straight path. A straight stick 
 placed at an angle — partly in water and partly in the 
 air — looks exactly like a bent stick. This bending, or 
 refraction^ of light is shown very clearly in the photo- 
 graph facing this page. 
 
 A third property of light is that it can be polarised. 
 Although the name of this property seems rather 
 mysterious, and possibly leads many people to imagine 
 that the subject is a very difficult one, it is in reality 
 very simple. The waves of the sea can only vibrate 
 in an up-and-down or vertical direction, because they 
 occur upon a flat horizontal surface. The aether waves, 
 however, do not occur upon a surface, but are right 
 inside the great aether ocean, so that "up-and-down" 
 has no special significance for them. It is just as easy 
 for aether waves to vibrate at one angle as another. 
 
 For some purposes it is convenient to picture the 
 
 aether as a huge jelly. Picture an ordinary jelly such 
 
 as is served for dinner or supper, but we shall suppose 
 
 the cook has made us a very large and firm jelly for 
 
 experimental purposes. If we stick two long pins — 
 
 such as hat pins — into the jelly, placing one pin at 
 
 some distance from the other, we find that any 
 
 vibratory motion given to the one pin is imitated by 
 
 the second pin. The jelly conveys the energy from 
 
 the one pin to the other ; we have set up wave motion 
 
 within the jelly. It is immaterial whether the motion 
 
 is up-and-down or from right to left, indeed it may be 
 
 at any angle whatever. 
 
 172 
 
o 
 
 < 
 
 a 
 
 2 M-c 
 
More about Light 
 
 When ordinary light waves are produced by a heated 
 body, we picture the disturbance being caused by 
 electrons revolving around the atoms, and these will 
 be lying at every variety of angle, so that it would 
 be difficult to conceive all the aether waves vibrating 
 in one particular direction only. The polarising of 
 light simply means entrapping all the waves except 
 those that are vibrating in one particular direction. 
 The following analogy may help to make the matter 
 quite clear : — 
 
 Suppose some wild creature was approaching a high 
 wall in which the only gateway was a long vertical slot 
 from top to bottom, just wide enough to let the creature 
 go straight through. If this imaginary animal were 
 staggering about from one side of the road to the 
 other, being unable to stop this rapid side to side 
 motion, it is clear that when it reached the narrow 
 gateway the animal's progress would be barred com- 
 pletely. If, however, the imaginary beast's eccentricity 
 consisted in its continually jumping up-and-down, but 
 going forward in a straight line, the narrow vertical 
 gateway would offer no hindrance to its passage. If a 
 herd of such wild creatures were being driven towards 
 a wall with a number of high narrow gateways, it is 
 clear that only those animals which possessed the 
 vertical jumping motion would get through. There- 
 fore we should find on the other side of the wall a 
 smaller herd of animals, but all moving vertically. 
 
 In this wild sort of analogy the animals are supposed 
 to represent the wave motions of light. The obstruct- 
 
 173 
 
Scientific Ideas of To-day 
 
 ing wall with its vertical gateways represents a class of 
 substances, the most notable of which is the crystalline 
 gem tourmaline, A thin slice of this precious stone 
 will act towards the waves of light just as the wall with 
 its gateways acted towards the eccentric animals in our 
 analogy. We picture only those waves which have 
 a vertical motion getting through the tourmaline. 
 Therefore the light which does get through is vibrating 
 in one definite direction. We say that the light which 
 passes through the tourmaline is polarised. 
 
 All these statements about the polarising of light 
 may seem to be entirely imaginary. How can we tell 
 that this has really taken place ? We can see no 
 difference. 
 
 Returnirrg to the foregoing analogy for a moment, 
 we might imagine the wall to be turned on its side, 
 as it were, so that the gateway would lie in a horizontal 
 position. Or we may make the analogy more complete 
 if we imagine a high wall with a number of horizontal 
 slots for gateways. Under such circumstances it would 
 not be the animals with the vertical jumping motion 
 which could get through ; these would find their way 
 barred completely. But the animals with the stagger- 
 ing motion would be able to stagger through these 
 very wide horizontal slots or gateways. We have now 
 adopted means of stopping the passage of both classes 
 of eccentric animals. 
 
 If we, first of all, drive the ha-d, as before, through 
 
 the vertical gateways, we allow only those animals with 
 
 the vertical motion to get through. We drive these 
 
 174 
 
More about Light 
 
 jumping animals towards the second wall containing 
 the horizontal gateways, and none of them can get 
 through. The result is that we have no animals passing 
 through the second wall. It is just so with tourmaline 
 and light. We can produce vertically polarised light, 
 and by passing that through a second slice of tourma- 
 line, turned round on its side, as it were, or in other 
 words, turned through a quarter of a revolution, we can 
 block the way of these vertical waves and produce total 
 darkness. It is customary to call the first piece of 
 tourmaline the polariser and the second piece the 
 analyser, but they are exactly similar, and only require 
 different names to distinguish which piece of tourma- 
 line is being referred to. There are other methods of 
 polarising light, but all that we desire to note is that 
 this is a distinctive property of light. 
 
 It may be remarked in passing that very beautiful 
 colour effects may be produced upon the lantern sheet 
 by means of polarised light. If the polariser and 
 analyser are set so as to cut off all the light, we have a 
 dark screen. If we then place a thin slice of mica, 
 a well-known colourless substance, between the two 
 polarisers, one would expect to see nothing. But the 
 mica bends the waves out of their straight path, and 
 it bends some waves more than others, according to 
 their wave-length. Indeed it acts very much as a 
 glass prism does when producing the solar spectrum. 
 There is a spreading out of the different colours 
 contained in the light. The analyser when turned 
 round into one position blocks the way of certain 
 
Scientific Ideas of To-day 
 
 waves or colours, and when in another position it 
 cuts off the passage of other waves. In this way we 
 get different colours emanating from the lantern, 
 according to the position of the analyser. Other 
 substances act in the same way as mica, and the 
 colours transmitted depend upon the nature of the 
 substance, and also upon the thickness of the slice 
 through which the polarised light passes. 
 
 I have seen wonderfully coloured pictures produced 
 by perfectly colourless magic-lantern slides with the 
 aid of polarised light. The images upon the slides are 
 built up of various slices of different colourless sub- 
 stances. All are neatly pieced together, say, in the form 
 of a many-coloured parrot. The slide is colourless, 
 but when one sees the image upon the lantern screen 
 it is difficult to realise that the picture is not produced 
 by a painted or coloured slide. Additional interest is 
 given by rotating the analyser, whereupon the colours 
 completely change — the red tail of the parrot is now 
 blue, and so on. The colours keep changing, of course, 
 as the analyser allows the different wave-lengths to get 
 through. 
 
 From the foregoing we have become familiar with 
 the fact that light can be polarised. This property of 
 light does not come before our notice in our every- 
 day life, but there is another prominent property of 
 light which does so. We cannot fail to observe that 
 ordinary light falling upon a variety of objects is not 
 all reflected back. It is apparent to the least observant 
 
 among us that a white object reflects far more light 
 
 176 
 
More about Light 
 
 than a black object. What happens to the light which 
 is not reflected back ? It must be absorbed by the 
 object upon which it fell. This absorption of light 
 is a definite property, and one which is continually 
 coming under our notice. 
 
 We can sum up the different properties of light 
 under the titles — reflection, absorption, refraction, and 
 polarisation. Do all aether waves really possess these 
 same properties ? They must exhibit these same 
 phenomena if the statement is true that light, radiant 
 heat, and electric waves are identical, except in their 
 wave-length. We desire to see experimental proof of 
 this statement. 
 
 First of all we shall compare radiant heat with light. 
 We need scarcely trouble to call forward any witness 
 to prove that radiant heat can be reflected just as 
 visible light is. If one considers how it is that fruit 
 ripens quicker when growing on a wall than it would 
 do if merely on a tree, one is driven to the conclusion 
 that the heat is reflected upon the fruit, so that it has 
 these reflected heat waves added to the original heat 
 waves. 
 
 If we desire experimental proof of the reflection of 
 radiant heat waves we find many such experiments. 
 If we go, in imagination, to the Royal Institution early 
 in the nineteenth century, we find Sir Humphry Davy 
 demonstratiiig this very point. He has two large con- 
 cave mirrors of silvered metal, one of which is hung 
 up over the lecture-table with its mouth downwards, 
 while the second curved mirror lies on the table 
 
 177 M 
 
Scientific Ideas of To-day 
 
 mouth upwards. The one mirror is at the height of 
 an ordinary ceiling, and is so arranged that it can be 
 easily lowered to the table and raised again. A large 
 iron ball is made red-hot and then hung up by a hook 
 within this curved mirror, which is raised again. The 
 heat waves are now reflected down upon the second 
 mirror on the lecture-table, and after falling upon it 
 they are brought to a focus, or, in other words, they 
 are gathered together and meet at a point. If Sir 
 Humphry Davy placed his hand at this point he could 
 not hold it there for long, indeed if he placed any 
 inflammable substance at this point it was immediately 
 ignited by the heat. So much for the reflection of 
 radiant heat. We agree that, as far as this one pro- 
 perty is concerned, light waves and heat waves in the 
 aether are of the same nature. 
 
 The second property we shall consider is absorp- 
 tion. The absorption of radiant heat is so common 
 that it is difficult to think of any interesting experi- 
 ment. Suppose we have the ground well covered with 
 snow while the Sun is shining. We take two pieces of 
 cotton cloth of exactly similar make, but one piece has 
 been bleached white while the other has been dyed 
 black. If we place these two pieces of cloth upon the 
 surface of the snow so that the heat from the Sun may 
 fall equally on both, we shall find that the snow under- 
 neath the black cloth melts long before that under the 
 white cloth. It is obvious that the black cloth has 
 absorbed the heat waves, whereas the white cloth has 
 
 reflected them and thus protected the snow. We agree 
 
 178 
 
More about Light 
 
 that, as far as this second property is concerned, both 
 Hght and radiant heat behave ahke ; they can both be 
 absorbed by certain substances. 
 
 What about the next property — refraction ? We 
 shall have to make some laboratory experiment to 
 demonstrate this. We know that a prism of glass will 
 bend light out of its normally straight path, but a 
 prism of glass is opaque to the waves of radiant 
 heat. However, we can use a prism of rock-salt, 
 as this substance allows radiant heat waves to pass 
 through it. We first of all arrange a source of heat 
 to throw a beam of radiant heat straight at a sensitive 
 thermometer. The thermopile, or electric thermo- 
 couple, makes a very suitable thermometer for 
 detecting these waves of radiant heat. If we place 
 the thermometer a little to one side so that the heat 
 waves cannot reach it, it will register the normal 
 temperature of the room. If we place a prism of 
 rock-salt in the path of the heat waves we find that 
 we can bend them round so that they fall upon the 
 thermometer. We cannot see the waves, but we see 
 the temperature of the thermometer rising. We have 
 no difficulty in accepting the statement that, as far 
 as this third property of refraction is concerned, 
 both light and radiant heat are of the same nature. 
 Almost every schoolboy knows that a lens will re- 
 fract heat waves, bending them to a focus. Indeed, we 
 used to speak of ''burning glasses." I notice from 
 the daily papers that a boy has distinguished himself 
 by setting fire to the inside window blind of a shop, 
 
 179 
 
Scientific Ideas of To-day 
 
 the boy standing outside and concentrating the Sun's 
 heat waves upon the blind by means of a magnifying 
 glass. Fortunately the fire was got under before much 
 damage was done, but it might have been otherwise. 
 
 There only remains the property of polarisation, and 
 this may be demonstrated by means very similar to 
 those used in the polarisation of visible light, the 
 thermopile being used to detect the presence of the 
 heat waves. 
 
 What about the electric waves, such as are used in 
 wireless telegraphy ? Do these possess the same pro- 
 perties of reflection, refraction, and polarisation ? We 
 have a very convenient method of detecting these 
 electric waves. We may arrange that when they fall 
 upon a "wireless" receiver an electric bell is set in 
 motion. When we produce a torrent of electric sparks 
 we set up electric waves in the surrounding aether, and 
 these will spread out in all directions. If we place this 
 transmitter or sparking apparatus within a copper box 
 we shall entrap all the waves, but if we leave an open- 
 ing in one side of the box some waves will escape. 
 These will travel outwards in a straight line, but will 
 gradually spread out just as light does. If the wireless 
 receiver is within " the line of fire " the bell will ring. 
 But we place the receiver in another copper box which 
 also has an opening. We now place this box so that 
 its mouth is just out of reach of the electric waves 
 which shoot past it without entering. If we now hold 
 a sheet of metal in the path of the waves, we find that 
 by holding the sheet at a certain angle we can reflect 
 
 i8o 
 
More about Light 
 
 the electric waves so that they enter the receiver box 
 and cause the bell to ring. There is no doubt that 
 electric waves possess this property of being reflected. 
 
 Instead of using a sheet of metal to reflect the 
 electric waves, we may place a prism in their path, 
 and the waves in passing through the prism will be 
 refracted or bent round so that they enter the receiver 
 box. For this purpose we use a prism of paraffin wax, 
 as it is more transparent to the electric waves. 
 
 Several experimenters have devised different means 
 of showing that these electric waves are polarised ; 
 they are all vibrating in one direction. The simplest 
 experimental proof of this is that they will pass through 
 a special wire grid when it is held in one position, but 
 when it is turned round, so that the wires are at right 
 angles to the first position, the waves are blocked. The 
 reason of this is apparent from our former considera- 
 tion of polarised light. In the present case the waves 
 are already polarised, so that the grid takes the place 
 of the second piece of tourmaline, which is known as 
 the analyser. 
 
 There can be no doubt in our minds regarding the 
 statement that light, radiant heat, and electric waves 
 are all of the same nature ; we have definite experi- 
 mental proof that their properties are identical. It is 
 common practice to classify all these under the title 
 of '* Light," but this leads us to speak of visible and 
 invisible light, and this seems paradoxical. We have 
 associated the word light with our sensation of light, 
 so that it seems ridiculous to speak of dark light. We 
 
 i8i 
 
Scientific Ideas of To-day 
 
 should be quite justified in classifying the three classes 
 under the title of electric waves, but then that term is 
 convenient to describe those waves set up by a wireless 
 transmitter. Our best plan would be merely to call 
 them all cether waves, some of which set up heat within 
 the substances they fall upon, others affect our sensory 
 vision, while others produce electric effects. When we 
 come to consider the question of colour we shall see 
 the advantage in not calHng the aether waves light. 
 
 We are convinced that all aether waves may be 
 reflected, absorbed, refracted, and polarised, but how 
 do these things take place ? The experimental proofs 
 do not tell us the causes of the different phenomena. 
 
 Our thoughts naturally turn to the electrons, which 
 along with the aether are the most fundamental things 
 of which we have any knowledge. It is evident that 
 a very heavy burden falls upon those tiny charges of 
 negative electricity. We have seen that they are the 
 stuff that atoms are made of. Electric currents and 
 electric discharges are simply these tiny electrons in 
 motion. We have also seen how the movements of 
 these electrons give rise to magnetic fields, electric 
 waves, radiant heat, visible light, and every variety of 
 aether wave. 
 
 When electrons surge to and fro in a wire circuit 
 they set up very long waves in the surrounding aether. 
 When we have electrons surging to and fro in a wire 
 we say that we have an alternating current of elec- 
 tricity in the wire. We give it this name to distinguish 
 it from a direct or continuous current. In the latter 
 
 182 
 
More about Light 
 
 case the electrons are not surging to and fro, but are 
 moving steadily along in one definite direction ; we 
 pictured them being handed along the line from atom 
 to atom. When there is an alternating current in a 
 wire we picture a rapid to-and-fro motion of the 
 electrons, and we speak of having set up electric oscil- 
 lations in the wire. The most rapid rate of electric 
 oscillation that has been obtained was produced by 
 means of induction coils, and is calculated to have 
 been somewhere about five million vibrations per 
 second. This seems a very high rate, and yet it is a 
 long way from the four hundred billion vibrations per 
 second required to produce visible light. The electrons 
 giving rise to the aether waves which affect our vision 
 are not surging to and fro, but are revolving around 
 the atoms of matter. Those electrons producing the 
 ccther waves known as red light are going round 
 their respective atoms four hundred billion times in 
 every second. 
 
 Having become familiar with the idea of electrons 
 disturbing the all-prevading aether, and thus giving us 
 a definite connecting link between the aether and 
 matter, it will be of interest to see how these aether 
 waves react upon matter. It almost goes without 
 saying that, when an aether wave falls on matter, it 
 will be the electrons within the matter which are 
 affected. These tiny electrons have helped us out of 
 a big difficulty. Until we became aware of their 
 presence we could not understand how matter was 
 affected by aether waves falling upon it. Whenever 
 
 183 
 
Scientific Ideas of To-day 
 
 we discovered the existence of these tiny electrons all 
 became clear, for an electro-magnetic wave will surely 
 affect these small charges or atoms of electricity. 
 
 In the blazing Sun we picture electrons revolving 
 around myriads of atoms of matter at a great variety 
 of speeds. Why electrons go round some kinds of 
 atoms faster than they do round others, we shall see 
 later. These electrons in the far - distant Sun are 
 producing a great variety of different wave-lengths in 
 the aether. 
 
 First of all let us think of those long waves which 
 
 we call radiant heat. When they fall upon a piece 
 
 of matter on this planet they disturb the electrons 
 
 within the matter. Suppose the matter happens to be 
 
 a piece of metal. The electrons within the metal are 
 
 already in a state of commotion before the aether 
 
 waves fall upon them. But it is a disordered motion ; 
 
 some of the electrons are continually on the move 
 
 from one atom to another. It is not unlike a party 
 
 of very little children muddling through a square 
 
 dance. An electron whirls round one atom, suddenly 
 
 collides with another atom, whirls around it, and so 
 
 on it goes wandering through among the atoms ; there 
 
 is no regular periodic motion, simply a commotion. 
 
 The arrival of a series of aether waves, however, causes 
 
 a definite disturbance, and we can picture those long 
 
 aether waves very soon spending all their energy in 
 
 opposing the motions of these roaming electrons, 
 
 knocking them about from atom to atom. In this 
 
 general commotion the atoms and molecules are thrown 
 ^ 184 
 
More about Light 
 
 into a state of greater vibration, and we say that the 
 piece of metal has become heated. We see the radiant 
 heat from the Sun, after travelling millions of miles 
 in the aether ocean, being transformed into molecular 
 heat upon this planet. 
 
 What about those aether waves w^hich we call visible 
 light ? The Sun is also producing these, and when 
 they arrive upon this planet they are received in 
 different ways by different kinds of matter. In every 
 case the motions of the electrons within the substance 
 oppose the motions of the incoming waves. The 
 electrons always tend to move in the opposite direc- 
 tion to the impulse of the wave. We need not trouble 
 about the reason of this opposition, but any reader 
 desiring to inquire into the matter should turn back 
 to page 119, where we saw how electrons surging 
 to and fro in one wire affected the electrons in a 
 distant wire. 
 
 While we picture a general opposition, on the part 
 of all electrons, to the invading aether waves, we take 
 particular note that the really active opponents are 
 those electrons which are capable of vibrating at the 
 same speed as the incoming waves. We shall see the 
 reason for this in the following chapter ; all we wish 
 to note at present is that these electrons succeed in 
 stopping the incoming waves. But what happens to 
 the electrons ? The result depends upon whether the 
 electron is able to hold its ground or not. If it is 
 only loosely attached to its atom, the electron will be 
 driven away and knocked about from atom to atom 
 
 i8S 
 
More about Light 
 
 they reflect waves of all the wave-lengths from red 
 to violet we call them white. 
 
 We see that the meaning of reflection is something 
 totally different from our former ideas of it. We have 
 been accustomed to think of light being reflected from 
 a surface, very much as an india-rubber ball will be 
 sent back by an obstructing wall. Our ideas of to-day 
 are different ; we picture the incoming wave being 
 stopped and a new series of aether waves produced by 
 those electrons which succeed in stopping the waves. 
 The moment the bombarding waves cease, the same 
 moment do the electrons cease to revolve at the re- 
 quired speed to produce these regular vibrations giving 
 rise to visible light. There are a few exceptions to 
 this rule ; in some cases the electrons continue to 
 revolve for a lengthened period, and therefore emit 
 light after the bombarding waves have been with- 
 drawn. In such cases we say that the substance is 
 phosphorescent. Luminous paints when exposed to 
 daylight will continue to reflect light for some con- 
 siderable time after they are placed in a totally dark 
 room. 
 
 I can imagine some reader thinking that this new 
 idea of reflection seems quite unnecessary, and that he 
 would be content to continue thinking of light being 
 reflected by simply rebounding from a surface. He 
 could not explain phosphoresence upon this basis, and 
 what is more important he could not reasonably ex- 
 plain the phenomena of colour, as we shall see when 
 we come to consider that interesting subject. 
 
 187 
 
Scientific Ideas of To-day 
 
 No substance is capable of absorbing all the light 
 waves which fall upon it ; there are always a few 
 electrons at least able to hold their ground against 
 the incoming waves, and in so doing to send back or 
 reflect some light. Hence there is no substance — no 
 matter how black we may paint it — which cannot be 
 seen when light falls upon it, I have some recollec- 
 tions of an amusing "nonsense story/' telling of a 
 scientist who devised a pigment which could absorb 
 every light wave which fell upon it. This inventor 
 annoyed a fellow scientist by playing a practical joke 
 upon him. The inventor painted his friend's dog all 
 over with this pigment, so that the dog became in- 
 visible, and only its brass collar could be seen moving 
 about. The story went on to tell how the second 
 scientist, on finding out the secret, painted the inventor's 
 house all over with the same pigment while the in- 
 ventor was from home. When he returned home he 
 was veiy much alarmed to find that his house had 
 disappeared during his absence. Of course the story 
 is quite ridiculous, and even if the imaginary pigment 
 had been able to absorb every asther wave falling upon 
 it, the space occupied by the object would be seen as 
 a patch of dai'kness. 
 
 A consideration of the foregoing story may be help- 
 ful to clear up some other points. Suppose the author 
 of the story had been more of a scientist than his story 
 proves him to have been. He would have taken the 
 opposite extreme. He would have suggested that the 
 
 practical joker succeeded in making the body of the 
 
 i88 
 
More about Light 
 
 dog offer no obstruction to the aether waves, so that 
 the light could pass freely through the dog's body. 
 In other words, he would have succeeded in making 
 the dog's body perfectly transparent, and it would then 
 have been quite invisible. The author's difficulty 
 would have been to suggest how the practical joker 
 managed this. It is obvious that he could not apply a 
 pigment for this purpose. The plan he suggested was 
 simple, for the black pigment could absorb or stop 
 the aether waves at the surface. A perfectly trans- 
 parent pigment would accomplish nothing ; the light 
 would merely pass through it and be reflected by the 
 dog's body as before. He would have to imagine 
 some treatment that would affect the whole substance 
 of the dog's body. 
 
 In most substances the light waves merely penetrate 
 an extremely thin film of the surface, and there they 
 are either absorbed or reflected. When neither of 
 these results take place, the aether waves pass right 
 through the substance, and we say that it is trans- 
 parent to light. No substance is perfectly transparent ; 
 there are always a few electrons at least capable of 
 taking up the rotary motion necessary to send back 
 some of the aether waves. We know that some sub- 
 stances are wonderfully transparent. In the early 
 days of plate-glass, my grandfather built a residence 
 at some distance from his native town, and this 
 was the first house in the neighbourhood to have 
 plate-glass windows. When the house was finished, 
 an old gentleman called and was shown into one of 
 
 189 
 
Scientific Ideas of To-day 
 
 the sitting-rooms. When my grandfather went in a 
 little later he found the old gentleman with the collar 
 of his coat turned up and a plaid drawn around him. 
 He was under the impression that there was no 
 glass in the windows, and it being winter-time, 
 he feared getting cold. More than once I have 
 found it difficult to tell if there was a sheet of 
 glass between me and an object, but in such cases 
 the light has been a subdued one. Even the air is 
 not perfectly transparent. 
 
 In closing this chapter it may be of interest to sum 
 up the ideas concerning the behaviour of aether waves 
 when they fall upon a piece of matter. While we 
 have been considering chiefly those aether waves 
 which affect our vision, the statements relate also to 
 radiant heat and electric waves. 
 
 In the majority of substances the aether waves are 
 stopped by the electrons at the surface. If the 
 electrons are driven away from their atoms in the 
 act of stopping the waves, the waves are absorbed. 
 If the electrons remain attached to their atoms the 
 waves are reflected. In both cases the active elec- 
 trons are those which can vibrate at exactly the 
 same rate as the incoming aether waves. If a 
 substance has practically no electrons able to exactly 
 respond to the incoming waves, the waves are not 
 stopped, they pass through the substance. There is, 
 however, some opposition to the waves, and we find 
 that they are retarded, losing about one-third of 
 their original velocity, while in the medium. 
 
 190 
 
More about Light 
 
 In addition to the foregoing three distinct classes, 
 there will of course be many substances which act 
 in part like the one class and in part like another. 
 Some substances are semi-transparent, or we might 
 say semi-opaque. We use the word opaque for all 
 substances which do not let the waves pass through 
 them, whether they absorb or reflect the waves. It 
 is common knowledge that certain substances absorb 
 only part of the sether waves falling upon them and 
 reflect the remainder, and so on. 
 
 The cause of polarisation will be apparent. Such 
 substances as tourmaline possess electrons capable 
 of vibrating in one definite direction. Hence the 
 waves which get through such a substance are all 
 vibrating in one particular direction, as are the up- 
 and-down waves on the surface of the ocean. 
 
 What is of greater interest than all these phenomena 
 is the fact that some substances are capable of reflect- 
 ing only certain definite wave-lengths, thus producing 
 the phenomenon of colour. This subject is of so 
 much general interest that it demands a whole 
 chapter. 
 
 191 
 
CHAPTER XIII 
 
 THE EXPLANATION OF COLOUR 
 
 Why the subject of colour is apt to be confusing — Colour sensa- 
 tions — Ideas previous to Newton's discovery — Newton's ideas 
 — How substances absorb definite wave-lengths and reflect 
 others — The revolving electron — Astronomical analogy — How 
 one electron sets up longer waves than another — Tuning-fork 
 analogy — The waves forming the visible spectrum — Red 
 objects may appear black in certain lights — A perfect colour- 
 match and yet a very bad match — Some remarks on colour 
 vision — An erroneous idea of colour-blindness — Heat-indicat- 
 ing paints 
 
 It is surprising how so many people fail to grasp the 
 real meaning of colour. Until recently we had only 
 a partial explanation ; we knew that certain sub- 
 stances absorbed certain aether waves and reflected 
 others, thus causing the objects to appear coloured, 
 but the explanation of this selective property of 
 substances was not forthcoming. Why should one 
 particular substance always absorb certain definite 
 wave-lengths and not others ? With the discovery 
 of electrons there came a reasonable explanation of 
 colour. 
 
 To the ordinary person the subject of colour always 
 seems to be a troublesome one. There is really no 
 
 wonder that it is so, for even those who do under- 
 
 192 
 
The Explanation of Colour 
 
 stand the subject are content to speak of it in a most 
 confused manner. We fail to make a proper dis- 
 tinction between colour and light. We say that at 
 the close of the seventeenth century Sir Isaac Newton 
 discovered that ordinary white light was a mixture of 
 all the colours of the rainbow, and so we speak of 
 white light as a bundle of coloured rays. We think 
 of one substance absorbing certain colour rays and 
 reflecting others. 
 
 This manner of speaking is considered quite ortho- 
 dox, but I feel sure that it accounts for a great many 
 of the difficulties connected with the subject of colour. 
 We have really no right to call white light a bundle 
 or mixture of colour rays ; it is a stream of aether 
 waves of various lengths, and nothing more. Perhaps 
 an analogy will make the matter clear. On the battle- 
 field a flying bullet strikes a soldier and produces the 
 sensation of pain within him. The flying bullet and 
 the pain are two totally different things ; no one could 
 ever think of calling the flying bullet a pain, and yet 
 that is just the sort of thing we are doing in connec- 
 tion with light. In ordinary sunlight we have nothing 
 but aether waves of various lengths, and when these 
 fall upon our eyes they produce certain sensations of 
 colour. If they all enter the eye they produce a 
 certain sensation which we call "white," If we 
 screen off some of the waves and allow only waves 
 of certain definite lengths to enter the eye, then we 
 have a definite colour sensation according to the 
 
 lengths of the waves admitted. We have really no 
 
 193 N 
 
Scientific Ideas of To-day 
 
 right to speak of these aether waves as colours or 
 colour rays. The aether waves, like the flying 
 bullet, strike something and cause a sensation ; we 
 must clearly differentiate between the sensation and 
 that which gave rise to it. We can only speak of a 
 luminous body sending out colour rays in the same 
 way as a poet might speak of the enemy's cannon pour- 
 ing forth pain and death. The question of colour 
 should only concern us when we are studying the 
 senses ; the aether waves alone concern us when we 
 are dealing with what takes place outside of our- 
 selves. 
 
 Before Newton's time people believed all light to 
 be naturally white. When it was passed through a 
 piece of red glass they thought that the light was dyed 
 red by the glass. When white light fell upon a green 
 object they pictured the light to be made green by the 
 object, and so on. Of course, even Newton believed 
 light to be a material thing, composed of infinitely 
 small particles or corpuscles. It was a long fight 
 between Newton's corpuscular theory and the sugges- 
 tion that light was merely wave-motion in the aether. 
 
 Newton, however, upset all the existing ideas con- 
 cerning white light being a simple thing. By passing 
 a beam of ordinary sunlight through a glass prism he 
 was able to produce all the colours of the rainbow. 
 Instead of the beam of ordinary white light emerging 
 from the other side of the prism there came forth 
 distinct colours, spread out in the form of a band. 
 
 It was not supposed that the light had been 
 
 194 
 
The Explanation of Colour 
 
 coloured by the glass, as the glass was colourless. 
 There could be no doubt that the glass prism had 
 merely separated or sorted out the different constit- 
 uents of sunlight. This was, indeed, a great dis- 
 covery ; we are apt to lose sight of its importance. 
 How much knowledge has been gained by analysing 
 light in this manner will be shown when we come 
 to consider how the spectroscope has advanced our 
 scientific ideas. 
 
 It will be clear that we have two separate things to 
 consider in seeking a full explanation of colour. We 
 must deal with the asther waves themselves, and then 
 with the sensations resulting from the impact of these 
 waves on the retina of our eyes. 
 
 First of all, we wish to know how it is that substances 
 possess this selective property of absorbing certain 
 wave-lengths. We have seen, in a very general 
 way, from the preceding chapter how the electrons 
 behave under the impact of aether waves. There 
 is a general reaction — all the revolving electron 
 satellites within the substance oppose the incoming 
 waves, but the really active opponents are those 
 electrons which happen to be able to vibrate at exactly 
 the same rate as the incoming wave. But why should 
 one electron be able to vibrate or revolve at any one 
 particular speed better than another ? As all electrons 
 are identical — no matter from what source we with- 
 draw them — it is clear that the determining factor 
 does not lie within the electron itself. The atoms of 
 the different elements, however, are very different from 
 
 195 
 
Scientific Ideas of To-day 
 
 each other. We know, for instance, that a uranium 
 atom is nearly two hundred and forty times as heavy 
 as a hydrogen atom, and although gravitation has 
 nothing to do with the attraction between atom and 
 electron, we can imagine, from what we have seen 
 in the earlier chapters, that the varied constructions 
 of the atoms will be one of the determining factors 
 in the distance between the atom and its satellite. 
 There are other forces acting upon the electron in 
 addition to the attractive and repulsive forces within 
 the atom. There is the influence of surrounding 
 atoms ; indeed the forces which determine the position 
 of the natural or periodic orbit of a revolving electron 
 are extremely complex. For our present purpose it 
 is sufficient to know that each kind of atom, or in 
 other words, each elementary atom, has a definite 
 orbit which its electron will pursue, if free to do so, 
 when it is set in motion. 
 
 We picture some electrons revolving in close proxi- 
 mity to their atoms, while others revolve round their 
 atoms at a comparatively greater distance. In all 
 cases the actual distance can only be measured in 
 millionths of an inch. But we picture these electron 
 satellites to be revolving around their atoms at various 
 distances, just as we find the planets doing on a very 
 grand scale in the heavens. The planet Mercury 
 revolves around the Sun at a distance of about thirty- 
 six million miles, while Neptune makes an immensely 
 greater circuit at a distance not very far short of three 
 
 thousand million miles from the Sun. The orbits 
 
 196 
 
The Explanation of Colour 
 
 of the other known planets lie between these two 
 extremes. 
 
 But what does it matter whether an electron de- 
 scribes a small or a large orbit around an atom ? It 
 will make a very material difference, because the size 
 of its orbit — or in other words, the distance it is from 
 its atom — will determine the velocity with which it 
 will travel. We may understand this better if we look 
 once more at the movements of the planets around 
 the Sun, 
 
 There is one fact about the motions of the planets 
 which, I think, is very often overlooked by the general 
 reader. The farther away a planet is from the Sun, 
 the slower does the planet move. Of course the 
 farther away a planet is, the larger circle it has to 
 describe, Neptune takes one hundred and sixty-four 
 years to make one journey around the Sun, while our 
 Earth only takes one year. But that is not the point 
 I refer to. Our Earth is travelling through space at 
 a speed of a little more than eighteen miles per second, 
 while Neptune is going along at a pace of only about 
 three miles per second. In other words, our Earth is 
 going six times as fast as the outermost planet Neptune, 
 while the Sun's nearest neighbour, Mercury, is forging 
 ahead at a speed of twenty-nine miles per second 
 against our eighteen. It will be understood that I am 
 referring to these movements of the planets only by 
 way of analogy ; the forces which govern the velocities 
 of the planets are quite different from those which 
 govern the velocities of electrons. 
 
 197 
 
Scientific Ideas of To-day 
 
 From the foregoing it will be clear that electrons 
 which make a rapid journey around a small orbit 
 will set up short waves of great frequency in the 
 aether — such as the waves we call ultra-violet light. 
 Other electrons which revolve more slowly around 
 their larger orbits will produce long waves of lower 
 frequency — such as the waves we call radiant heat. 
 In orbits between these two extremes electrons will 
 revolve at speeds which produce all the variety of 
 waves which give rise to visible light, from the longer 
 waves producing the sensations of red to the shorter 
 ones producing the sensation of violet. 
 
 We are now in a position to undei'stand how some 
 substances absorb certain definite wave-lengths. We 
 see that the electrons have natural periodic motions 
 according to the kind of atom to which they act as 
 satellites. We may take it for granted that an electron 
 pays little attention to an incoming aether wave unless 
 the wave happens to be swinging to and fro at the 
 particular rate at which that electron would naturally 
 move. I think we shall do well to consider a well- 
 known lecture experiment by way of analogy. 
 
 If we have two sets of tuning-forks of varying 
 pitches mounted on sounding-boards, and if we place 
 the two groups at a little distance from each other we 
 find the following results. When we cause one par- 
 ticular fork in the first group to vibrate — by drawing 
 a violin bow across it — we find that if there is an 
 exactly similar fork in the second group that fork will 
 
 commence to vibrate also. The other forks which 
 
 198 
 
The Explanation of Colour 
 
 cannot vibrate in sympathy with the incoming air 
 waves remain practically silent. It is always of 
 interest, when performing this experiment, to stop the 
 first fork vibrating, whereupon the distant fork is heard 
 to continue sounding out the same note on its own 
 account, A fork vibrating at a certain number of 
 vibrations per second sets up air waves of the same 
 frequency, but these only affect another fork capable 
 of vibrating at the same rate. In the same way we 
 find that a revolving electron, in a luminous body, sets 
 up definite aether waves, and these only affect distant 
 electrons capable of revolving at the same rate. In 
 the case of the electrons we have already seen that the 
 motions oppose each other, and the energy of the 
 incoming wave is spent. But if the opposing electron 
 can remain attached to its atom, the electron will 
 behave like the tuning-fork in our analogy, and will 
 set up ^ther waves on its own account, and thus 
 radiate light. This is our present idea of the reflection 
 of light. 
 
 There is really nothing mysterious in the foregoing 
 idea of the electron stopping an aether wave and 
 setting up a similar wave. The tuning-fork behaves in 
 a similar manner towards air waves. When an air 
 wave strikes the silent fork the energy of the wave is 
 expended in setting the fork in motion. The incoming 
 air wave is stopped, but the fork having been set in 
 motion continues to set up other similar air waves on 
 its own account. Of course we must not press the 
 analogy too far, for in the case of the forks we are 
 
 199 
 
The Explanation of Colour 
 
 ducing waves and two octaves of ultra-violet light 
 possessing active chemical properties. 
 
 To return to the question of colour, we have to deal 
 only with the one single octave representing the visible 
 spectrum. The seven notes of this octave represent 
 the seven different wave-lengths which produce the 
 colours of the spectrum — Red, Orange, Yellow, Green, 
 Blue, Indigo, Violet. For our convenience we may 
 call the waves by the first letters of the colours which 
 they produce— R, O, Y, G, B, I, V.^ 
 
 We may picture a luminous body, such as the Sun, 
 to be a great myriad of atoms of different elements, 
 and we picture electrons revolving around each 
 atom. Among these revolving electrons there are 
 those setting up the seven wave-lengths with which 
 we are going to deal. These waves fall upon a piece 
 of substance upon this planet. If that piece of sub- 
 stance happens to contain a similar series of electrons, 
 then they wiU in turn send out similar aether waves, 
 and we say that the substance reflects white light. If, 
 however, the substance only contains electrons capable 
 of responding to R waves, it will only set up R waves. 
 By the word responding, as used here, I mean electrons 
 which can revolve at the same rate as the incoming 
 wave and remain attached to their atoms. If a sub- 
 stance only sets up R waves when the whole series 
 from R to V waves fall upon it, then only that part of 
 
 ^ We have been accustomed to think of seven colours, but it should be 
 understood that this number is merely arbitrary ; recent text-books omit 
 the Indigo. See footnote, p. 333. 
 
 201 
 
Scientific Ideas of To-day 
 
 our vision apparatus which is sensitive to R waves 
 will be affected, and we shall have the sensation of 
 red. For convenience sake we say that the object is 
 red, but we are well aware that the colour does not 
 reside in the object. All the other wave-lengths are 
 reflected or absorbed in the same way. 
 
 We cannot expect a substance to reflect a wave- 
 length which does not fall upon it. In the surface of 
 the cover of this book there are electrons capable of 
 responding to R waves. When white light falls upon 
 it we have R waves reflected to our eyes, and we say 
 that the cover is red. If we view the book by the 
 light of a mercury vapour lamp ^ we do not see it red, 
 because there are no R waves in this particular light 
 to stimulate these electrons. We see the cover practi- 
 cally black or dark brown, because the electrons in its 
 surface cannot respond to the waves falling upon them. 
 This is an extreme case, but we can find the same 
 thing in everyday life. 
 
 We shall suppose that in the evening a lady buys 
 a ribbon to match her hat. She is very pleased to 
 obtain such a good match to the colour, but in the 
 morning she has reason to regret her purchase ; the 
 ribbon and the hat are quite different tones of colour. 
 Then she remembers that she bought the ribbon by 
 
 1 The mercury vapour lamp is a long vacuum tube with a small 
 bath of mercury at each end, covering the ends of the wires, which are 
 sealed in the glass. An electrical discharge has to pass from the one 
 mercury bath to the other, and in so doing it vaporises some of the 
 mercury, and this discharge passing through the mercury vapour gives a 
 wonderfully powerful light, but of a most disagreeable and sickly colour, 
 the red-producing rays being absent. 
 
 202 
 
The Explanation of Colour 
 
 artificial light. The difficulty arises from the fact that 
 the artificial light did not contain all the variety of 
 wave-lengths contained in daylight. In the evening 
 the ribbon and the hat were tested only with certain 
 wave-lengths, and there were electrons in both objects 
 which behaved in exactly similar fashion under the 
 influence of these waves. In the morning, however, 
 the two objects were subjected to some other wave- 
 lengths which found responding electrons in the one 
 object and not in the other, and consequently the two 
 objects appeared to be quite different in colour. 
 
 It may be helpful to add a few remarks here about 
 our colour vision. Until recently the general idea was 
 that there were three nerve-endings in the human eye, 
 one of which was sensitive to what we have been 
 calling R waves, and when stimulated producing that 
 sensation which we term red ; another nei've-ending 
 was supposed to be sensitive to G waves, producing 
 the sensation of green ; while the third was sensitive 
 to V waves, and gave rise to the sensation of violet 
 colour. It is a curious fact, although there are seven 
 wave-lengths or colours in the solar spectrum, we 
 find it sufficient as far as our senses are concerned 
 to suppose only three individual sensations, all the 
 other colour sensations being merely combinations of 
 these three primary sensations. For instance, R waves 
 and G waves combined in certain proportions give us 
 the same colour sensation as the Y waves of the 
 spectrum do. In other words, when both the red and 
 green sensations are simultaneously stimulated we have 
 
 203 
 
Scientific Ideas of To-day 
 
 that sensation which we call yellow. If the same two 
 primary sensations are excited, in different degrees 
 from that which we have just supposed, we have the 
 sensation of orange colour. Then again, if the green 
 and violet sensations are simultaneously excited we 
 have the colour sensation of blue. All other colour 
 sensations are merely different combinations of these. 
 
 This theory of colour vision, which we have been 
 considering, was suggested by Dr. Thomas Young 
 (London) and Professor Helmholtz (Berlin), and has 
 therefore been known as the Young- Helmholtz theory. 
 The theory still holds good as far as the sensations are 
 concerned, but our present idea is that there are no 
 special nerve-endings as was supposed, for we fail to 
 find these in the retina of the eye. We believe that 
 the action of light in the eye is a purely chemical or 
 photographic one, although we fail to detect the 
 supposed chemical change in the human eye. In the 
 eye of the frog we do find a visible chemical action. 
 There is a chemical substance exuded over the retina, 
 and this has been named ptirpurine because of its 
 purple colour. The action of light upon this pur- 
 purine is to bleach it white. Probably there is some 
 equivalent chemical substance in the human eye which 
 does not happen to show any visible change. If we 
 imagine some substance having three different con- 
 stituents, one constituent having electrons which 
 respond to R waves, another with electrons respond- 
 ing to G waves, and the third sensitive to V waves, we 
 
 can afford to dispense with the three special nerve- 
 
 204 
 
The Explanation of Colour 
 
 endings. In some unexplained way these electrons, 
 when set in motion by the incoming aether waves 
 (light), may produce chemical changes, which in turn 
 produce the nerve impulses that give rise to those 
 colour sensations which we experience. 
 
 As we are not concerned so much with the physio- 
 logical side of the subject we need not trouble further 
 with colour sensation, but there is just one point I 
 should like to remark upon. I have found many 
 people hai^bouring an erroneous idea of colour- 
 blindness, and the reason of their error is not far 
 to seek. They have been told that a colour-blind 
 man sees dark-blue in place of red, and that pink 
 appears to him to be sky-blue. This seems to some 
 people to be rather a crossing of colour sensations 
 than a blindness, but that is not so. The statement 
 referred to is quite correct provided the red colour 
 is a compound colour such as crimson. A so-called 
 crimson object reflects two classes of aether waves 
 — those producing the sensation of red, and those 
 producing the, sensation of blue. If a man is colour- 
 blind to red, then only the blue sensation will be 
 stimulated. The same holds good with the compound 
 colour pink, which is practically a very pale crimson. 
 But the same man, if shown the solar spectrum upon 
 a screen and asked to point out the colours, will not 
 see blue at the red end of the spectrum. He will 
 describe the lower end of the spectrum as merely a 
 defect of light. 
 
 Considering only what takes place outside of our- 
 205 
 
Scientific Ideas of To-day 
 
 selves, we have quite a reasonable explanation of 
 colour. It is here that we see the benefit of the new 
 idea concerning the reflection of light. In the fore- 
 going we have supposed white light to be falling upon 
 the substance, so that its electrons were bombarded by 
 the whole range of aether waves constituting the visible 
 spectrum. But suppose that the luminous body is 
 only emitting one particular wave-length — say, R 
 waves — then no matter how varied the capabilities 
 of the electrons contained in the substance upon 
 which the light falls, it will only be those electrons 
 which can respond to R waves that will reflect light. 
 In other words, we say that when red light falls upon 
 any substance, that substance can reflect only red 
 light ; if the object contains no electrons capable of 
 responding to red light, then the object appears black. 
 In the same way, if R waves are not present in the 
 light falling upon the object, there cannot be any R 
 waves reproduced by that object. We have referred 
 to this fact already in connection with the mercury 
 vapour lamp. In its light we see no so-called red 
 objects. I have seen a dinner- table beautifully de- 
 corated with bright red flowers, the efi^ect being most 
 pleasing when viewed by ordinary electric light. 
 When the light was switched off and the room was 
 lighted by a mercury vapour lamp not a trace of 
 colour remained in the red flowers ; they were a most 
 funereal black. Not a single red object could be seen 
 in the room ; some were black, but others appeared 
 
 dark-blue for the reason mentioned in connection with 
 
 206 
 
The Explanation of Colour 
 
 colour-blindness. The substance of the flowers was 
 still capable of setting up R waves, but as no R waves 
 fell upon it the electrons lay dormant, if, indeed, we 
 can ever speak of an electron being at rest. The 
 moment the ordinary light was switched on again the 
 electrons responded to the R waves, and the flowers 
 appeared as red as ever. 
 
 A very simple demonstration of the same pheno- 
 menon may be made without any trouble. A mixture 
 of methylated spirits and common salt may take the 
 place of the mercury vapour lamp. When this mixture 
 is ignited there are practically no R waves emitted by 
 it, the light being entirely composed of Y waves. A 
 pure red appears perfectly black. The light of the 
 mercury vapour lamp is of more interest in one way ; 
 a crimson object appears blue, and we therefore see it 
 very much as a man who is colour-blind to red would 
 see the crimson object. 
 
 Picturing the electron revolving around its atom 
 and setting up aether waves of definite length (definite 
 colour), we can imagine readily that if the atom 
 itself were caused to vibrate at an abnormal speed, 
 the revolutions of the electron might be very materi- 
 ally affected. We have evidence of such occur- 
 rences. For instance, a certain chemical compound 
 — the double iodide of mercury and silver — appears 
 yellow at normal temperatures. If we hasten the 
 vibrations of its atoms by increasing the temperature 
 to about no degrees Fahrenheit (45 degrees Centi- 
 grade) the substance appears red. Another substance 
 
 207 
 
Scientific Ideas of To-day 
 
 appears scarlet at ordinary temperatures, but prac- 
 tically refuses to send out any aether waves of light 
 at a high temperature ; the substance then appears 
 black. On cooling, the substance again radiates R 
 waves. These substances are used as heat-indicating 
 paints, and some interesting experiments may be 
 made with them. For instance, if a sheet of paper 
 is coated over with the yellow paint, and then placed 
 in front of a fire or other source of heat, the paint 
 will very quickly change from pale yellow to red. 
 If any object is placed between the source of heat 
 and the painted screen, a shadowgraph of the ob- 
 ject is obtained, as the aether waves of radiant heat 
 do not get at those parts of the screen which are 
 sheltered by the intervening object. The demonstra- 
 tion would be more effective if a dark source of heat 
 were used, and the shadowgraph produced in the dark. 
 It would then be clear that the image was produced by 
 the long dark-heat waves which are beneath our range 
 of vision. 
 
 In the present chapter we have seen how the elec- 
 trons within a substance reflect or absorb aether waves 
 and then affect our colour sensations. We have not 
 seen how it is that a glass prism separates these aether 
 waves and spreads them out in the order of the spec- 
 trum. As already indicated, we have gained so much 
 knowledge by means of this method of analysing the 
 aether waves that it will be of interest to devote a 
 separate chapter to the ideas obtained from the 
 
 spectrum. 
 
 208 
 
CHAPTER XIV 
 
 IDEAS OBTAINED FROM THE SPECTRUM 
 
 How to produce a spectrum — The action of glass upon the aether 
 waves — Why the direction of the waves is altered — Analogy 
 of soldiers marching — How the different colours are separ- 
 ated — The part played by electrons in transparent sub- 
 stances — The construction of the spectroscope — Watching 
 the spectrum being gradually built up — How dark lines are 
 produced in the spectrum — What these tell us — Bright lines — 
 The chemistry of the Sun — Newton never noticed the dark 
 lines in the solar spectrum — The small quantity of matter 
 which the spectroscope can detect 
 
 The solar spectrum has been referred to repeatedly in 
 
 the foregoing chapters, and every person is conversant 
 
 in some measure with its appearance. Even those 
 
 who have never had an opportunity of looking through 
 
 a spectroscope have seen a spectrum at some time 
 
 or other upon the floors or walls of their dwellings. 
 
 These spectra may have been produced by a beam of 
 
 sunlight passing through a triangular piece of glass 
 
 suspended from a gasalier, or by the sunlight striking 
 
 a cut-glass bottle, or perhaps the bevelled edge of a 
 
 mirror. If any one has failed to observe these chance 
 
 spectra, he must at least have seen the solar spectrum 
 
 upon a gigantic scale in the rainbow when the Sun 
 
 shines on falling rain. Most of us have come across 
 
 209 O 
 
Scientific Ideas of To-day 
 
 coloured representations of the solar spectrum at some 
 time or other. 
 
 Small pocket spectroscopes may now be obtained 
 from opticians at a cost of about ten shillings, so 
 that the amateur may examine the spectra of different 
 elements for himself. If any one cares to take the 
 trouble to arrange that a beam of sunlight enters a 
 darkened room, then by holding any cheap glass 
 prism — one off an old gasalier will do — at some dis- 
 tance from the slot in the dark shutter, and at right 
 angles to the slot, one may form a beautiful spectrum 
 upon a sheet of white paper. That is what Sir Isaac 
 Newton did about two hundred and fifty years ago. 
 We wish to see how this separation of the different 
 eether waves is accomplished by the prism. 
 
 First of all we must note what happens when aether 
 
 waves fall upon any piece of ordinary glass, such as a 
 
 window pane. We shall deal with the aether waves 
 
 forming a beam of sunlight. We find that if these 
 
 aether waves come straight at the glass plate, or in 
 
 other words, if they strike the glass at right angles to 
 
 its surface, the waves go straight through and continue 
 
 in a straight line. But we saw in an earlier chapter 
 
 that aether waves are retarded while passing through 
 
 glass ; indeed their speed is reduced by some 60,000 
 
 miles per second. Wherever aether waves meet 
 
 with electrons the forward motion of the waves is 
 
 retarded. In interstellar space they meet with 
 
 practically no electrons, so that they maintain a 
 
 speed of 186,000 miles per second while they are 
 
 210 
 
Ideas Obtained from the Spectrum 
 
 crossing the billions of miles separating us from far- 
 distant stars. 
 
 This retardation of aether waves when passing 
 through glass does not produce any apparent phe- 
 nomenon when the waves go straight at the glass and 
 enter it at right angles to the surface. But picture a 
 beam of light striking the glass at an angle and see 
 what happens. The favourite analogy is to imagine a 
 line of soldiers as representing the wave front. Picture 
 the soldiers marching well in line and approaching a 
 patch of rough country ; they are not going straight at 
 it, but approaching it in a slanting direction, so that 
 the soldier on the extreme right will enter the rough 
 country first. His walk will be impeded, and we find 
 that his pace across the rough country is only two 
 miles per hour as against three miles in the open 
 country. As each soldier enters the rough country his 
 speed is similarly reduced, but it is apparent that the 
 soldiers on the extreme left of the line will have con- 
 tinued longer in the open country than the others. 
 Hence they will have maintained their full marching 
 pace of three miles per hour longer than those who 
 first entered the rough country. By the time the 
 soldiers on the extreme left enter the rough country 
 those at the other end of the line will have fallen 
 behind the original line of march, so that the direction 
 of the line of march is now altered. It is just as if the 
 soldiers had got the command of " Right Turn ! " 
 
 Once in the rough country they all march again in 
 even line, but notice that they are still marching in a 
 
 211 
 
Scientific Ideas of To-day 
 
 slanting direction, though not at quite such a sharp 
 angle as before (see diagram B, page 214). Looking at 
 the diagram, one can easily see that the soldiers on the 
 extreme right will be again the first to cross the boundary 
 line. They will be in the open country first, so that they 
 will get in advance of the others who are later in leaving 
 the rough country. What has taken place is exactly 
 the converse of what occurred when they entered the 
 rough country, so that the line of march has been 
 swung round into the same direction as it was in 
 originally. This is represented in the diagram B, In 
 the foregoing analogy we see the aether wave-front 
 striking the glass at an angle and being bent round on 
 entering the glass, while it regains its original direction 
 on leaving the glass. 
 
 We have been picturing a straight patch of rough 
 country with its two boundary lines parallel to one 
 another, as in first diagram. Suppose, however, that 
 the patch of rough country is of an irregular shape, 
 such as represented in the other diagram C, so that the 
 second boundary line is not parallel to the first, what 
 will happen ? It is evident that the man who was the 
 first to enter the rough country will be the last to 
 leave it, so that the line of march will be bent still 
 further round. It is just as though there had been 
 a second command of " Right Turn ! " This will be 
 apparent from the diagram, which not only repre- 
 sents the soldiers marching through a patch of rough 
 country, but equally well represents a beam of 
 light passing through a] glass prism. The aether 
 
 212 
 
Ideas Obtained from the Spectrum 
 
 wave is bent round both when entering and leaving 
 the glass. 
 
 If the beam of light which we pass through the 
 prism consists solely of R waves, we shall find that the 
 beam has not been bent veiy far from its original 
 direction. Suppose we mark the letter R at the place 
 upon the screen whereon those red-producing waves 
 fell If we then try a beam of G waves, leaving the 
 screen and prism just as before, we shall find that 
 these waves are bent further round, so that the patch of 
 " green light " falls further along the screen. Repeating 
 the same experiment with violet light, we find that 
 these waves are bent still further round, so that they 
 form a violet image at some distance beyond the green. 
 Had we tried orange and yellow lights we should have 
 found them to fall between the red and the green, 
 while blue and indigo would have taken their places 
 between the green and the violet. It is in this way 
 that the spectrum is formed. 
 
 We might extend the analogy of the soldiers, and 
 picture seven different companies all approaching the 
 rough country in one line ; they can all march equally 
 well in the open country. The men in No. i com- 
 pany do not find the rough countiy so great an 
 obstacle as the men in No. 2 company find it, so that 
 the line of march of No. i company is not altered to 
 the same extent as that of No. 2. These two com- 
 panies will be travelling in slightly different directions 
 from one another when they pass again into the open 
 country. No. 3 company is swung round still further, 
 
 213 
 
Scientific Ideas of To-day 
 
 This diagram represents a row of soldiers marching towards a 
 patch of rough country ; the dots indicate the successive stages in 
 their march. While passing through the rough country their rate 
 of march is considerably reduced. It will be observed that they 
 are approaching the boundary line at an angle, so that the soldier 
 on the extreme left of the page is the first to enter the rough 
 country. His pace is slackened before that of the others, so that 
 the line of march is altered. It is just as though the men had 
 
 Fig. B 
 
 Bending a beam of Light 
 
 received the command of " Right Turn !" Then as they leave the 
 rough country the soldier who was the first to enter is again the 
 first to leave, so that his pace is quickened before that of the others, 
 thus altering the line of march back to its original direction. It 
 is just as though the men received the command, " Left Turn ! " 
 
 This gives us a convenient analogy of the bending of a beam of 
 light in passing through a piece of glass or other transparent 
 medium, as explained in the text. Compare this diagram with the 
 
 nTinfncrrnnh farinp' nacre T72. 
 
Ideas Obtained from the Spectrum 
 
 This diagram represents the same row of soldiers as in Fig. B^ 
 but here they are marching through a triangular piece of rough 
 country. The soldier to the extreme left of page is longer in 
 difficulties than the others, so that the man on the extreme right 
 of page makes much better progress, and the line of march is very 
 considerably altered. In this case it is just as though the men 
 received the command, " Right Turn ! " both when entering and 
 leaving the rough country. 
 
 Fig. C 
 How a beam of Light is bent by a Prism 
 This serves as an analogy of the bending of a beam of light when 
 passing through a glass prism. In this diagram we are consider- 
 ing a beam of light of any one particular wave-length, such as the 
 waves which produce the sensation of red. The other aether waves 
 suffer a greater refraction. The shorter the wave, the more is it bent 
 from its original direction, so that the various wave-lengths con- 
 tained in white light will be spread out when passed through a 
 glass prism. In this way the coloured spectrum is produced, as 
 explained more fully in the text. 
 
 215 
 
Scientific Ideas of To-day 
 
 and so on with the others. If a halt is called, after the 
 companies have proceeded a little distance in the open 
 country, the companies will be apart from each other, or 
 spread out. In similar fashion we find the seven wave- 
 lengths in white light spread out by passing through a 
 glass prism, so that they form the well-known solar 
 spectrum. The patches of colour upon the screen are 
 really a multitude of images of the aperture through 
 which the light is passing. If the light is passed through 
 a circular hole, the images are circular discs of colour 
 overlapping each other. If the aperture is a straight 
 narrow slot, the image is composed of a myriad of 
 straight narrow bands or lines overlapping one another. 
 
 It will be of interest to see what causes the refraction 
 or bending of the aether waves, and why some are bent 
 more than others. So far we have been content to 
 know that the progress of sether waves is retarded by 
 the glass owing to the presence of electrons, and we 
 have pictured soldiers passing through a patch of rough 
 country by way of analogy. 
 
 We know that the conditions of the electrons in one 
 kind of substance vary from those in another, so we 
 are not surprised to find that some transparent sub- 
 stances have a greater refractive power than others. 
 Then we have observed already that the amount of 
 bending depends also upon the wave-length of the 
 aether wave itself ; R waves are least refracted, and V 
 waves suffer the greatest bending. 
 
 We have become familiar with the idea that only 
 
 those electrons which can respond to the rate of vibra- 
 
 216 
 
Ideas Obtained from the Spectrum 
 
 tion of the incoming wave take any veiy active part in 
 opposing the aether waves. We have seen that where 
 those sympathetic electrons exist, the aether waves can 
 only penetrate a thin molecular layer at the surface ; 
 there they are stopped. Whether they are absorbed or 
 reflected depends upon whether the electrons are driven 
 away from their atoms, or whether the electrons can hold 
 their ground and remain attached to their atoms. It 
 is apparent that in the case of glass and other trans- 
 parent substances neither of these things happen. The 
 aether waves are not absorbed, nor do the electrons 
 send back similar aether waves. It is most obvious 
 that the aether waves make their way right through 
 the glass. There can be no sympathetic or responding 
 electrons capable of stopping the waves, but the exist- 
 ing electrons, although incapable of revolving at the 
 rate of the vibrating waves, do offer some resistance or 
 opposition, and thus retard the progress of the waves. 
 We have seen how this affects the waves when they 
 enter or leave the glass at an angle. 
 
 With this picture of the revolving electrons before 
 us we can understand how some substance may be 
 transparent to one range of wave-lengths and not to 
 another. It will be remembered that in order to 
 demonstrate that the long dark-heat waves could be 
 refracted like ''visible" light, we had to use a prism 
 of rock-salt instead of glass. The glass prism is 
 practically opaque to these long heat waves, while the 
 rock-salt allows them to pass through just as the 
 glass allows the waves of "visible " light to pass. 
 
 217 
 
Scientific Ideas of To-day 
 
 In a stained-glass window we have a good demon- 
 stration of pieces of glass treated so that they will 
 absorb certain wave-lengths and allow other aether 
 waves to get through. If a piece of glass allows 
 only the R waves to pass through, we say in popular 
 language that the glass is coloured red. Which aether 
 waves will get through is determined by the capabilities 
 of the electrons contained in the substance. 
 
 It is strange how much difficulty some people have 
 in realising the simple facts of absorption and reflec- 
 tion. For instance, I have shown a well-educated man 
 the solar spectrum falling upon a sheet of white paper, 
 and asked him what would happen if we replaced the 
 white screen with a red one. His answer has been 
 that the red will not be seen, and that the other colours 
 of the spectrum will be mixed with red, the blue part 
 on the top of the red will give purple, and so on. 
 Another man has suggested that the red will not be 
 seen properly, but that the remainder of the spectrum 
 will be all right. It is clear that any one giving either 
 of these answers has not realised the meaning of re- 
 flection and absorption. The screen is '' red " because 
 its surface contains electrons capable of absorbing all 
 the aether waves except the red-producing waves which 
 it reflects. Therefore the whole of the aether waves 
 spread out in the spectrum will be absorbed with the 
 exception of the red-producing waves. We see no 
 spectrum upon the red screen ; only the small patch 
 of red. 
 
 From what follows we shall see that we have 
 218 
 
53 J 
 
 ^JV.-" 
 
 ^C 3 
 
 
Ideas Obtained from the Spectrum 
 
 obtained many interesting ideas from the spectrum. 
 
 In order to make observations conveniently, the glass 
 
 prism is mounted between two tubes, as is shown in 
 
 the accompanying illustration facing p. 218. One tube 
 
 is provided at one end with a vertical slot, through 
 
 which the light that is to be examined is passed. This 
 
 slot is usually adjustable, so that its width may be 
 
 altered at will. At the other end of this tube is a 
 
 lens, so that the beam of light from the slot emerges 
 
 through this lens as a pencil of parallel rays. This 
 
 tube with the slot and lens is called a collimator, but 
 
 it should really have been called a coliineator (Latin 
 
 col^ together, and linear a line). It is a curious thing 
 
 that the verb collimate ever got into our English 
 
 language. It was only by mistake that it did so ; 
 
 it was derived from a fancied Latin verb colliniare, 
 
 which was really a false reading of collineare. I 
 
 mention this in passing, as the name of the tube — 
 
 collimator — looks rather puzzling. The construction 
 
 of the tube is simplicity itself ; a slot at one end, 
 
 and a lens at the other. When the pencil of light 
 
 emerges from this tube it falls upon the glass prism, 
 
 striking it at an angle. In passing through the prism 
 
 the light is dispersed into its spectrum, and is bent 
 
 round so that it enters the second tube, which is 
 
 simply a short telescope for magnifying the image of 
 
 the spectrum. The complete apparatus is called a 
 
 spectroscope. Suitable means of measuring the amount 
 
 of bending of the rays may be added, in which case 
 
 the instrument is sometimes called a spectrometer. 
 
 219 
 
Scientific Ideas of To-day 
 
 It may be remarked in passing that the glass prism 
 is replaced sometimes by a grating. The dictionary 
 meaning of this word is a partition of parallel cross- 
 bars, and we are all familiar with street gratings and 
 the like. The grating referred to here is a series of 
 very fine parallel lines ruled on a glass plate. When 
 ordinary white light passes through this it is spread 
 out into its spectrum. There is one difference between 
 the action of a grating and that of a prism. The latter 
 spreads out the beam of light so that it forms a single 
 spectrum, whereas the grating forms a number of 
 spectra. A little white light gets straight through 
 between the lines, and forms a bright white image 
 at the centre of the screen. On either side of this 
 there are formed several spectra of diminishing bright- 
 ness. If the fine lines are ruled on a piece of poHshed 
 speculum metal, the light will be reflected in the form 
 of a spectrum. A grating of this kind has some 
 advantages over a prism, but we need not trouble 
 with further detail here. 
 
 If one looks at a very fine speculum grating one 
 does not see the lines, but the whole surface appears 
 to be rainbow-coloured. This property of sorting out 
 the different wave-lengths of light is not the sole right 
 of glass prisms and gratings. In these days of petrol 
 motor-cars, even the least observant of pedestrians 
 must see at times the most exquisite colours reflected 
 from the damp surface of the roadway upon which 
 some petrol has fallen. There we see a medley of 
 
 colours, and it is obvious that many of the different 
 
 220 
 
Ideas Obtained from the Spectrum 
 
 wave-lengths have been separated from the white Hght 
 falHng upon the oily surface. In this case the sorting 
 out is caused by the interference of reflected aether 
 waves, and the irregularity of the sorting out is due 
 to variations in the thickness of the film of oil lying 
 upon the damp surface. The same phenomenon is 
 seen upon large soap-bubbles, and this principle of 
 interference has been used by Professor Lippmann 
 to produce natural-colour photography. His process, 
 however, is practically a laboratory experiment. 
 
 The colour effect produced by the surface of mother- 
 of-pearl is due to very fine fines similar to the speculum 
 grating. It is a remarkable fact that if a sealing-wax 
 impression is taken of the surface of mother-of-pearl, 
 those very fine lines are sufficiently impressed upon 
 the wax to produce a similar colour result. 
 
 Let us go, in imagination, into a dark room and see 
 what we can learn by the aid of a spectroscope. We 
 have arranged a convenient method of heating a piece 
 of iron — say, by means of an electric current — and we 
 place the spectroscope in such a position that it will 
 receive any aether waves set up by the heated iron. 
 For some time we see nothing, either when we look 
 through the instrument or look directly at the place 
 we know the iron to occupy. 
 
 As soon as the iron begins to glow we look through 
 the instrument, and we see that section of the spectrum 
 which produces the sensation of red ; we see the red 
 patch and nothing more. We know from this that 
 there are electrons within the iron revolving at a speed 
 
 221 
 
Scientific Ideas of To-day 
 
 of four hundred billion revolutions per second. As 
 the temperature rises we observe that the piece of iron 
 glows more brightly. Looking through the spectro- 
 scope we see the orange part of the spectrum appear, 
 then comes the yellow, and gradually the green, blue, 
 indigo, and violet are added in turn. We have wit- 
 nessed the complete spectrum being gradually built 
 up as electrons were thrown into the different rates 
 of revolution. We are not to think that there are, 
 within the iron, electrons which would naturally 
 revolve at these various speeds if their atoms were 
 free from collision with one another. The reason 
 why we have such a variety of cether waves emitted 
 by the heated iron is that the electrons are being 
 forced to these paces by the commotion among the 
 atoms, due to their rapid vibration, and as the atoms 
 are crowded together the electrons are hampered, so 
 we find all sorts of rates of revolution. Every solid 
 body when raised to a white heat will behave in the 
 same way ; it will produce a complete spectrum. A 
 complete spectrum, such as this, tells us nothing of 
 the nature of the substance which is emitting the light. 
 We must have the atoms freed from one another to 
 such an extent that their electrons may revolve at their 
 natural periods. 
 
 If we melt the iron we free the atoms from their 
 solid grip, but if we examine the light from the molten 
 metal we still see a complete spectrum. If we have 
 any means of raising the temperature to 6000 degrees 
 
 Fahrenheit (3300 degrees Centigrade), we shall cause 
 
 222 
 
Ideas Obtained from the Spectrum 
 
 some of the atoms of the iron to escape into the air 
 in a gaseous condition, just as we have water vapour 
 rising from boiling water. If we direct the spectro- 
 scope towards the vapour, and cause white light from 
 some hotter source to pass through the iron vapour, 
 and then examine the resulting light, we see a very 
 curious phenomenon. We see the spech-um of white 
 light, but it has a series of fine dark lines cross- 
 ing it at intervals. It is apparent that we have lost 
 some of the aether waves which were contained 
 in the white light. We have not got the con- 
 tinuous spectrum which the white light should pro- 
 duce ; there are blanks here and there. We think 
 of the spectrum as being composed of a myriad of 
 images of the slot, all blended together to produce 
 one broad band, just like the coloured warp threads 
 of a rainbow ribbon. In the present case a number 
 of threads are absent here and there. 
 
 Looking through the spectroscope at the light which 
 has passed through the iron vapour, we have no diffi- 
 culty in suggesting where the missing aether waves 
 have gone. There is only one possible conclusion : 
 they have been absorbed by the iron vapour, or in 
 other words, they have been stopped by the electrons 
 attached to the iron atoms. The waves which suc- 
 ceeded in getting through to the spectroscope have 
 found no responsive electrons in the vapour. 
 
 Suppose we take a photograph of this discontinuous 
 spectrum. Of course we must take the photograph 
 through the spectroscope, and as the photograph will 
 
 223 
 
Scientific Ideas of To-day 
 
 show no colours, we take care to mark off the posi- 
 tions of the different coloured sections. We see so 
 many lines occur in the red section, so many in the 
 green, and so on. We find a great number of lines 
 throughout the spectrum. 
 
 We take other photographs of light passing through 
 the vapours from different elementary substances, and 
 when we compare them with each other we see that 
 they are quite different. We always get the same 
 lines from the one elementary substance. In the 
 light photographed through sodium vapour we find 
 only two black lines, and these occur in the yellow 
 part of the spectrum. These lines are so close to- 
 gether that in a simple spectroscope they appear as 
 a single line. Why should lines appear ? The lines 
 are simply images of the slot through which the light 
 is passing into the spectroscope. 
 
 From the foregoing we see that an element must 
 be in a gaseous state before we can produce its line 
 spectrum. We have seen that sodium vapour absorbs 
 two definite wave-lengths situated in the yellow part 
 of the spectrum. We know that the vapour must 
 contain electrons capable of revolving at speeds corre- 
 sponding to these particular waves. It therefore stands 
 to reason that if these electrons can be thrown into 
 their natural periodic revolutions they should emit 
 waves corresponding to these same rates of vibration. 
 That is exactly what we do find. If we burn a piece 
 of sodium in a bunsen flame and examine the flame 
 of the burning sodium, we find two bright yellow 
 
 224 
 
Ideas Obtained from the Spectrum 
 
 lines in exactly the same positions as the two dark 
 lines have already appeared in. 
 
 If we burn hydrogen gas, and examine the flame 
 by means of the spectroscope, we find three bright 
 lines. One of these is a very distinct line in the red 
 section, and another in the blue part, while the third 
 line is somewhat fainter, and is situated further along 
 in the blue towards the violet end of the spectrum. 
 Other fainter lines may be detected with more deli- 
 cate apparatus. With a small pocket spectroscope one 
 may distinguish these three lines very well. 
 
 We have another very convenient method of examin- 
 ing the spectra of gases. If we fill a glass tube with 
 hydrogen gas and then connect the tube to an air- 
 pump, we can withdraw most of the gas, leaving 
 what we call a vacuum. Although we speak of these 
 as vacuum tubes, we know that they must contain a 
 very small quantity of air or gas. A tube may be 
 exhausted till there remains less than one -millionth 
 part of the air that would fill the tube at ordinary 
 atmospheric pressure. In the present case the exhaus- 
 tion is not carried to such a high degree. We only 
 wish to separate the atoms sufficiently to give their 
 electrons freedom to revolve around the atoms at their 
 natural or periodic rate. Our next requirement is to 
 make this collection of free atoms self-luminous. We 
 know that we can make the contents of a vacuum tube 
 luminous by passing an electrical discharge through 
 the tube. When we connect the electrodes of the 
 tube to an induction coil or to an electrical machine, 
 
 225 p 
 
Scientific Ideas of To-day 
 
 we have quite the effect of an aurora within the tube. 
 The colour of the glow will vary according to the 
 kind of gas used in the tube. In the present case 
 we shall have a very pale-red light from the hydrogen 
 gas. We may examine this light through the spectro- 
 scope, and we see the well-known hydrogen lines. 
 Again the lines are bright — just as though the gas 
 were being burnt ; it is only when light passing 
 through a vapour is examined that we see dark lines, 
 indicating that these wave-lengths have been absorbed 
 by the gas. 
 
 The foregoing method of examining the spectra of 
 gaseous elements has been of very great advantage. 
 It has given us a means of obtaining the spectra of 
 rare gases, which we cannot obtain in large quantities. 
 It enables us also to produce the spectrum of oxygen, 
 and of other gases, which are non-inflammable. 
 
 As each elementary substance has its own definite 
 series of lines in the spectrum, we can tell from any 
 given spectrum what substances are producing it, no 
 matter how complicated it may appear. For instance, 
 if we photograph the spectrum of the Sun, we find 
 many thousands of lines scattered throughout the 
 spectrum. By carefully marking off those lines pro- 
 duced by hydrogen, those produced by iron, and so 
 on, we are able to tell exactly the different elements 
 contained in the Sun. We find no fewer than forty 
 different elements, and among these are hydrogen, 
 sodium, iron, copper, nickel, and zinc. These all 
 
 exist in gaseous form in the photosphere or outer 
 
 226 
 
Ideas Obtained from the Spectrum 
 
 atmosphere of the Sun. These vapours absorb certain 
 wave-lengths of the continuous spectrum produced by 
 the glowing Sun, and thus produce definite dark lines 
 in the spectrum. 
 
 We can learn a great deal about objects around us 
 by handling them and examining them ; we cannot 
 tell always what they are made of. Think of the Sun 
 more than ninety million miles beyond our reach, and 
 yet we can tell what it is made of. The chemistry of 
 the stars is due entirely to the spectroscope. 
 
 It is a curious fact that Sir Isaac Newton never 
 observed these dark lines in the solar spectrum. They 
 are present even in the spectrum formed by a common 
 glass prism, such as those used on old-fashioned 
 gasaliers. It has been suggested that Newton em- 
 ployed an assistant to examine the spectrum for him. 
 It might be asked how the assistant did not observe 
 the lines, I could quite imagine him passing over 
 these little black lines as due to some irregularities 
 in the glass prism. Of course, he could have tested 
 this by simply sliding the glass prism to one side or 
 the other and watching whether these lines moved 
 with the prism or remained in their definite places in 
 the spectrum. We must remember that two hundred 
 and fifty years ago men were not trained to those exact 
 methods of deductive experimenting to which we have 
 become accustomed in these later days. 
 
 One very interesting point in connection with the 
 
 spectroscope is the exceedingly small quantity of a 
 
 substance which it can detect. If we burn a few 
 
 227 
 
Scientific Ideas of To-day 
 
 grains of common salt in a bunsen-burner and examine 
 the light emitted, we see the sodium lines very dis- 
 tinctly even when using a pocket spectroscope. 
 
 It is a well-known fact that a single drop of blood 
 in a tea-cupful of water will show its characteristic 
 spectrum when white light passing through it is 
 examined. In this way it is possible to distinguish 
 between blood drawn from an artery and from a vein, 
 even although the quantity is so small. Arterial blood 
 is, of course, oxidised as it flows from the heart, the 
 blood having previously received oxygen from the 
 lungs. The blood returning by the veins is deoxidised, 
 having given up its oxygen to the body. In the 
 spectroscope there will be seen dark absorption lines 
 representing oxygen if the blood is arterial, while these 
 will be wanting if the blood is venous. We might 
 weave a Sherlock Holmes story around this fact. Some 
 beautiful lady is found dead under mysterious circum- 
 stances. Neither the doctors nor the police can offer 
 any explanation. A " Sherlock Holmes " is called in 
 and draws a single drop of blood from one of the 
 arteries. A spectroscopic examination of this tells 
 him that the lady has undoubtedly died of suffocation 
 from the fumes of burning charcoal, for in such cases 
 the whole of the blood in the body becomes deoxidised. 
 
 From the foregoing it is apparent that a very small 
 
 quantity of matter can be detected by the spectroscope, 
 
 but these are by no means extreme cases. To state 
 
 that a good spectroscope can detect one-millionth part 
 
 of a milligram will not convey much meaning to those 
 
 228 
 
Ideas Obtained from the Spectrum 
 
 unaccustomed to work with milligrams. With the aid 
 of the illustration facing page 228, we may grasp some 
 idea of the great sensitiveness of the spectroscope. 
 Here we see a very sensitive chemical balance, which 
 can easily detect the weight of a word written in 
 pencil. In the first illustration we see two pieces of 
 paper which exactly balance one another. We then 
 take one paper and write a single word upon it 
 with a lead pencil. In doing so we have rubbed a 
 very small portion off the point of the pencil on to 
 the surface of the paper. We see no difference in 
 the point of the pencil ; it is still good to write many 
 hundreds of words. But our balance is quite able 
 to detect the increased load, as is seen from the 
 second photograph. In this case we have detected 
 about four milligrams of matter, and this very sensitive 
 chemical balance can do even finer work. Here we 
 see the presence of a very small quantity of matter 
 detected by the balance. The spectroscope, however, 
 can detect one four-millionth part of this quantity of 
 matter. Think of the minute quantity of lead rubbed 
 off the pencil, and try to imagine this divided in four 
 million parts ; the spectroscope could detect one of 
 these infinitely small parts. 
 
 Our interest in the spectroscope does not end here ; 
 we shall see from the following chapter how this single 
 piece of apparatus has added to our knowledge valu- 
 able facts concerning the far-distant stars. 
 
 229 
 
CHAPTER XV 
 
 THE BIRTH OF A STAR 
 
 How we take a star's temperature — An element may have more 
 than one spectrum — Wireless telegraph analogy — The tem- 
 perature of the Sun — The birth of a star — Building up the 
 atoms — The atoms breaking down — Reservoir of latent 
 energy — How we know that some distant stars are approach- 
 ing us at great speeds — Railway whistle analogy — How the 
 stars send us wireless messages — Experimental proof that 
 light is due to electrons revolving around atoms — A marvel- 
 lous experiment. 
 
 How can we possibly tell the temperature of a star 
 distant many billions of miles from us ? Even had 
 we never heard before that such a thing was possible, 
 I think we might make a very good guess at how such 
 a thing could be done. We could at least suggest a 
 possible means of comparing the temperature of one 
 star with that of another, if we realised what took 
 place when we gradually heated a piece of iron and 
 examined the light emitted by it. Looking through the 
 spectroscope we saw, first of all, only the red part of the 
 spectrum. Then as the temperature of the iron was 
 increased, step by step there appeared the orange, 
 yellow, green, blue, indigo, and violet parts. We shall 
 know then that if a star produces only the red end of 
 the spectrum, that star is not so hot as one which 
 produces the red and the orange. The more of the 
 
 230 
 
The Birth of a Star 
 
 spectrum that appears, the hotter will be the star that 
 is producing it. This convenient thermometer does 
 not end with the violet part of the spectrum ; other 
 aether waves of higher frequency are produced. These 
 aether waves of ultra-violet light will affect a photo- 
 graphic plate, so by means of photography we can 
 extend the scale of our thermometer far beyond the 
 limit of the visible spectrum. When we find two stars 
 producing the same length of spectrum, we know that 
 these two stars are at similar temperatures. 
 
 Each element has its own distinctive line spectrum, 
 but we need not suppose that it is impossible for a 
 definite spectrum to show some variation. For a long 
 time it was believed to be quite impossible that the line 
 spectrum of an element should vary in any way what- 
 ever. But nearly half a century ago two eminent 
 Austrian scientists published a paper stating that some 
 of the elements could be made to produce totally 
 different spectra. Sir Norman Lockyer, who has done 
 so very much for this branch of science, demonstrated 
 in a very clear manner that the spectra of some 
 elements showed extraordinary variations when the 
 element was at different temperatures. The spectrum 
 produced by sodium when burned in a bunsen flame 
 is very much simpler than that produced by the same 
 element when placed in an electric arc, while a further 
 change is obtained by using an electric spark as the 
 source of illumination. Under these three conditions 
 sodium produces different wave-lengths in the sur- 
 rounding aether. The flame spectrum of iron shows 
 
 231 
 
Scientific Ideas of To-day 
 
 only a few lines, while its arc spectrum shows about 
 two thousand lines. It must be clearly understood 
 that the spectrum of an element is always constant 
 under the same conditions. A certain arrangement 
 of lines is known to us as being produced by sodium 
 at the temperature of a flame, while we recognise 
 another arrangement to be due to the same element 
 at the much higher temperature of the electric arc. 
 We see that the reading of stellar spectra is by no 
 means a simple thing. A certain arrangement of lines 
 not only indicates the element, but gives us some idea 
 of the temperature at which the element is. Here, 
 therefore, we have a further reading of our stellar 
 thermometer. There are other indications of tempera- 
 ture, but enough has been said to show how our 
 ideas of the temperatures of distant stars has been 
 acquired. 
 
 Scientists have been able to read a great deal of 
 valuable information from the spectral lines of the 
 Sun and stars. It is just as though the aether waves 
 were wireless telegraph messages despatched by the 
 revolving electrons in the far-distant stars, our spec- 
 troscopes being the telegraph receiving instruments. 
 With the aid of photography we get those stellar tele- 
 graph messages to record themselves, and the different 
 arrangements of spectral lines form the telegraphic code. 
 With this analogy in view we may look upon Sir 
 Norman Lockyer, and Sir William and Lady Huggins, 
 as the chief telegraphists. 
 
 Lockyer has shown that the spectral lines of iron 
 232 
 
The Birth of a Star 
 
 in the photosphere or luminous envelope of the Sun 
 are the same as those produced by iron at the tempera- 
 ture of the electric arc. This telegraph message informs 
 us that the temperature of the photosphere of the Sun 
 is about six thousand degrees by the Centigrade scale. 
 This telegraph message has corrected a very erroneous 
 idea which man had formed previously ; fifty years 
 ago we believed the temperature to be several million 
 degrees. 
 
 Without going into the details of the spectra, it 
 will be of interest to see what other telegraph messages 
 we have received upon this planet from the heavenly 
 bodies. We are not going to trouble about the code 
 signals, but we shall see what the telegraphists have 
 made out of the messages received. 
 
 We learn that the great Sun is cooling down very 
 gradually, and that many other stars are doing the 
 same. But we also learn that some stars are actually 
 getting hotter, and we estimate the hottest stars to be 
 about thirty thousand degrees Centigrade. 
 
 Our chief telegraphists have read the following 
 message for us, and whether or not they may have 
 made a perfectly correct interpretation of the code 
 signals, the message is of great interest, as it sug- 
 gests a very reasonable explanation of the birth of 
 a star. 
 
 First of all we have a great nebula, occupying a 
 space measured in millions of miles. This nebula is 
 composed of swarms of meteorites, which are fragments 
 of solid matter containing the elements as we know 
 
 233 
 
Scientific Ideas of To-day 
 
 them on this planet. These meteorites are cold bodies, 
 and we shall see in a later chapter whence they have 
 come. However, we can imagine these meteorites 
 colliding together as they tend to gravitate to the 
 centre of the mass. These collisions will produce 
 heat, so that the temperature of the mass will rise 
 gradually as it condenses. In the course of time the 
 temperature rises to such an extent that the whole 
 mass, now greatly reduced in bulk, becomes gaseous ; 
 this is the condition of the hottest stars. When this 
 condition has been reached there are no solid particles 
 remaining to cause further collisions and keep up the 
 temperature ; the star therefore commences to cool. 
 
 While the star is in its hottest condition we receive 
 wireless messages in the spectroscope, which are trans- 
 lated to mean that some of the elements have been 
 dissociated into simpler forms, owing to the enormous 
 heat, probably some twenty to thirty thousand degrees 
 Centigrade. To distinguish these dissociated elements, 
 they have had the prefix proto given to them as a 
 title. We speak of proto-hydrogen, proto-magnesium, 
 and other proto-elements as existing in very hot stars. 
 While in others not quite so hot we find proto-iron, 
 proto-copper, and so on. As the temperature decreases 
 these proto-elements disappear and the regular spectral 
 lines of the elements appear, just as we have them 
 on this planet. The colder the star the more elements 
 do we find in it. There can be no doubt that these 
 have been gradually built up, or condensed, in the 
 process of cooling ; it is undoubtedly a case of evolu- 
 
 234 
 
The Birth of a Star 
 
 tion. Comparing the messages from stars of different 
 temperatures, we find that only the Hghtest elements 
 exist in the hotter stars, and that the heavier elements 
 appear practically in order as the stars cool. 
 
 We have, no doubt, become familiar with the idea 
 that the atoms of all the elements are composed of 
 electrons. From the foregoing paragraph we see that, 
 at the very high temperatures which exist in some 
 stars, only a few electrons can club together to form 
 an atom, whereas at lower temperatures an increased 
 number of electrons congregate and form a heavier 
 atom. 
 
 It is natural to ask, at this point, what happens 
 when a star becomes so cold that it ceases to glow. 
 Let us say, when it has got into the condition of 
 this planet of which we have the good fortune to be 
 tenants. Here we have some eighty different elements, 
 the heaviest of all being uranium. What can happen 
 next ? Will this planet, and ultimately the whole 
 Universe radiate away its heat and then collapse into 
 one cold dead mass ? Until recently there did seem 
 no other reasonable conclusion. It will be remem- 
 bered, however, that in an earlier chapter we considered 
 the phrase "as dead as a door-nail." We know 
 nowadays that there is a tremendous internal activity 
 in every piece of so-called dead matter. Is it not 
 possible, then, that the atoms of matter may break up 
 again into other forms and ultimately free those rapidly 
 revolving electrons of which they are composed ? 
 There is no need for speculation upon this possibility ; 
 
 235 
 
Scientific Ideas of To-day 
 
 we have actual proof that this is taking place in 
 uranium and other heavy elements. The subject is of 
 so great interest that we shall deal with the breaking- 
 up of the atom in a separate chapter by itself. 
 
 Reading between the lines of the wireless messages 
 reaching this planet from the outer world, we picture 
 the Universe not as a machine wound up by the 
 Creator and allowed to run down to a state of rest, 
 but as an eternal change from electrons to proto- 
 elements to elements and back to electrons. 
 
 While the foregoing is intended to indicate the 
 scientific ideas of to-day concerning the Universe, it 
 must be kept in mind that there is a certain amount 
 of reading between the lines, or, in other words, of 
 speculative thought. When we read between the lines 
 of a friendly letter, we sometimes read correctly, and 
 unfortunately we sometimes read wrongly. It remains 
 for future generations of men to see how much of 
 our reading between the lines has been in the right 
 direction. 
 
 There is not the least doubt that many of the 
 theories we hold to-day must give place to newer 
 ideas. More modern theories will be added from 
 time to time. We should recognise the fact that 
 our present ideas are merely tentative, the best we 
 can suggest so far as we have been able to read the 
 secrets of nature. 
 
 Before leaving the subject of the spectroscope, there 
 
 is another kind of wireless message received from the 
 
 distant stars which it will be of interest to notice. 
 
 236 
 
The Birth of a Star 
 
 Sometimes when examining the spectra of stars a very 
 slight alteration of the lines is observed. The nature 
 of the alteration is that the spectral lines are not in 
 their normal positions in the spectrum. In some 
 cases the lines have moved slightly up the scale to- 
 wards the ultra-violet end, while in other cases the 
 lines are found slightly further down the scale than 
 the lines of the same element usually occupy. It is 
 evident that the rates of vibration in the first case have 
 been increased, and in the second case decreased. 
 The only reasonable translation of these messages is 
 that the star being examined in the first case is moving 
 towards the observer, and in the second case it is 
 receding. We have a very apt analogy for this in 
 every-day life. It is an analogy well known in 
 Physics, and is as follows — 
 
 At some time or other most of us have observed 
 how the pitch of a locomotive's whistle changes as 
 an express train approaches us and recedes from 
 us. Indeed, we might think that the locomotive 
 was using two whistles, if we did not know that its 
 whistle was sounding only one definite note. The 
 cause of this increase and decrease of pitch in the 
 whistle is not far to seek. The whistle is setting up 
 one definite rate of air vibration all the time ; but as 
 the train rushes towards us these arrive, one after the 
 other, quicker than they would if the engine were 
 standing in one position. Hence we hear a slightly 
 higher note. Imagine the whistle giving a definite 
 number of blows to the atmosphere in each second 
 
 237 
 
Scientific Ideas of To-day 
 
 of time. We then picture the first sound wave set 
 up by the whistle to be travelling towards us ; but 
 the engine darts forward as it gives the second blow. 
 It is just as though the engine made up very slightly 
 upon the first sound wave before it dealt the second 
 blow, so that the air waves follow each other closer 
 than they would otherwise do. They arrive in quicker 
 succession than they would do if the engine remained 
 standing while it dealt out the blows. More vibrations 
 arriving per second means a higher pitch. On the 
 other hand, when the engine is rushing away from 
 us, the vibrations or sound waves will be a little 
 farther apart from one another, the engine receding 
 at each blow. Fewer vibrations arriving per second 
 means a lower pitch. 
 
 With the aid of this analogy we can read the mean- 
 ing of the slightly altered spectrum. If we find that 
 the spectral lines have moved up the scale towards 
 the violet end of the spectrum, then we have no 
 hesitation in saying that the increase of "pitch" is 
 due to the star, which is emitting the aether waves, 
 rushing towards us. On the other hand, if the lines 
 are found nearer to the red end of the spectrum than 
 the normal position for such lines, then we know that 
 the star is receding from us. By very exact measure- 
 ments of the amount of displacement of the spectral 
 lines the rate of motion can be calculated. In this 
 way we know that Sirius (the Dog Star) is approaching 
 us with a velocity of over nine miles per second. 
 
 Fortunately it has a very long race to run ; our 
 
 238 
 
The Birth of a Star 
 
 planet will not be here to see the finish. Some 
 other stars have very much higher velocity in the 
 line of sight. In the same way the spectroscope 
 informs us that Capella is receding from us with a 
 velocity of fifteen miles per second, while others are 
 running away at double that speed. There is no 
 rough estimate, or guess-work, about these velocities; 
 with modern instruments and methods it is possible 
 to determine the actual velocity with an accuracy 
 of less than half a mile per second, even for the 
 most distant stars. 
 
 We have no doubt whatever that all these wireless 
 messages, received in the spectroscope from any possible 
 source, have been sent off by revolving electrons. 
 Indeed, we can easily demonstrate this fact in the 
 laboratory. As long as this idea of the cause of 
 aether waves was merely a theory built upon mathe- 
 matical calculations, the general body of people paid 
 little attention to it. About 1881 Professor H. A, 
 Lorentz, of Amsterdam, suggested the theory that the 
 aether waves of light were produced by means of 
 minute charged corpuscles revolving around atoms. 
 This was a reasonable theory, but no experimental 
 proof could be brought forward at that time to 
 support it. However, in 1 897, Professor Zeeman, 
 of Leyden, demonstrated by experiment in the 
 laboratory that these revolving particles did exist, 
 and that they did undoubtedly produce the aether 
 waves of light. Zeeman's experimental proof, which 
 is of the highest importance, is as follows, 
 
 239 
 
Scientific Ideas of To-day 
 
 We have become familiar with the idea that any 
 alteration in the speeds of the revolving electrons would 
 alter the wave-lengths of the aether waves produced 
 by theni. But how can we hope to act directly upon 
 those electrons to cause any alteration of speed ? 
 We know that electrons in steady motion constitute 
 an electric current, and we also know that electric 
 currents are susceptible to the influence of a magnetic 
 field. It was reasoning of this kind which led physicists 
 to try what effect a powerful magnetic field might 
 have upon a body emitting aether waves of light. It 
 was believed at first that any effect would be too 
 small to be perceivable, but once more the spectro- 
 scope came to the rescue. We have seen how the 
 spectroscope can detect very small variations of wave- 
 lengths in the asther. 
 
 Professor Zeeman placed a sodium flame between 
 the poles of a very powerful magnet, and arranged his 
 spectroscope so that he could examine the light emitted 
 by the flame. When the instrument was adjusted he 
 saw the well-known sodium lines. Then switching on 
 the current to the electro-magnet, each line was seen to 
 split up into two parallel lines (see illustration facing 
 this). Whenever the magnetic field was withdrawn 
 from the flame the spectral lines appeared single as 
 at first. What caused this curious phenomenon ? 
 
 It is clear that some of the aether waves were reduced 
 in speed, and therefore took up a slightly lower position 
 in the spectrum, while other waves were increased in 
 speed and produced a spectral line slightly further up 
 
 240 
 

 Q >^ 
 
 ^ S-^ 
 
 =1 ^--^ 
 
The Birth of a Star 
 
 the scale, thus producing two distinct lines in place 
 of a single one. This meant that the speed of some 
 electrons had been decreased and that of others in- 
 creased. This is exactly what we should expect. In 
 the great congregation of atoms in the sodium flame 
 there must be electrons with their orbits lying in all 
 planes, so that if one could see them one would observe 
 electrons revolving in all directions. Those going in 
 one direction will be accelerated by the magnetic field, 
 and those going in the opposite direction will be 
 retarded ; hence the change of the spectral lines. 
 
 There are many other points of interest in connection 
 with the Zeeman effect, but enough has been said to 
 sex've our present purpose. We see that there is direct 
 experimental proof that light is due to revolving elec- 
 trons. This is one of the most interesting experiments 
 I have ever had the pleasure of seeing. It is not an 
 elaborate experiment, but it requires the best modern 
 apparatus. More than one experimenter had looked 
 for this effect in vain, and Zeeman himself had made 
 an unsuccessful attempt. However, in 1897, ^^^^ 
 more perfect apparatus Zeeman was successful. It 
 is interesting to watch the spectral lines of the sodium 
 flame while a friend switches on the current to the 
 large electro-magnet. One sees the lines become two- 
 fold immediately, and their return to single lines 
 indicates the withdrawal of the magnetic field. 
 
 It is a marvellous experiment. Here we are con- 
 trolling directly those infinitely small electrons which 
 are circling around the invisible sodium atoms. We 
 
 241 Q 
 
Scientific Ideas of To-day 
 
 are operating upon things which are far below the 
 range of the most powerful microscope, and yet we 
 can read what is taking place, by recording the waves 
 produced in the aether and analysed by the spectro- 
 scope. 
 
 242 
 
CHAPTER XVI 
 
 THE AGE OF THE EARTH 
 
 No idea of a law of uniformity among the heavenly bodies — An 
 insect analogy — Youthful ideas of the world's age — Man the 
 same for thousands of years — The geologist's book of ancient 
 history — Lord Kelvin's estimate of the Earth's age — Does 
 radium play a part in maintaining the Earth's temperature ?— 
 The birth of the moon — The forming of the oceans — Calcu- 
 lating the age of the oceans — The birth of the solar system — 
 Are the electrons eternal ? — Interesting statement by Lord 
 Kelvin 
 
 Those wireless messages, received by the spectroscope 
 from the outer Universe, which we have been consider- 
 ing in the preceding chapter, do not bring us any direct 
 information regarding the age of the Universe, Indeed 
 it would be unwise to judge the comparative ages of 
 the stars by their temperatures, just as it would be 
 erroneous to judge the ages of men by their relative 
 heights. As man passes from infancy to manhood he 
 grows taller, but a youth measuring five feet is not neces- 
 sarily older than another measuring only four feet. H ow- 
 ever, when one is asked to guess a child's age, or more 
 particularly if one is asked to say which of two chil- 
 dren is the older, one usually depends very largely 
 upon the factor of height. And although we abandon 
 all idea of a law of uniformity, we are satisfied with a 
 
 243 
 
Scientific Ideas of To-day 
 
 general comparison of the stars by their relative tem- 
 peratures. 
 
 But all the stars are just as they have been since 
 man first made definite observations of them. No one 
 has ever seen a star change from one condition to 
 another. Imagine some insect, whose whole life was 
 summed up in a single day, being endowed with a 
 reasoning intelligence. As it looked on mankind it 
 would see living creatures of different sizes, and it 
 might reason that the smaller creatures gradually grew 
 to be larger creatures ; it could see a minimum and 
 a maximum, but it could witness no actual change 
 taking place during its short day, and so it could form 
 no opinion as to the rate at which men grew. It is 
 quite evident that we cannot form any idea of the age 
 of the Universe by direct observation, 
 
 Man, however, is the tenant of a planet which he 
 believes to have passed through all the different con- 
 ditions which he sees among the stars. Therefore his 
 most natural plan is to examine the interior of his 
 own planet, and try to read its history by means of 
 geology. 
 
 Possibly some of us have recollections of our own 
 
 early ideas of the age of the Earth. I can remember, 
 
 when I was a boy, looking at the date on the first page 
 
 of Genesis — 4004 B.C. — and calculating that the Earth 
 
 was therefore about six thousand years old. Of course, 
 
 our youthful idea of the Creation was that it occupied 
 
 seven days of twenty-four hours each, including the 
 
 day of rest. I can remember distinctly how I tried to 
 
 244 
 
The Age of the Earth 
 
 realise what those six thousand years meant, and it is 
 evident, from the analogy adopted, that the thinking 
 was done in church, I pictured twenty very old 
 women occupying a pew in front. Each woman was 
 exactly one hundred years of age. It was clear that 
 if those twenty imaginary women had appeared upon 
 this planet in succession, the second one appearing as 
 the first one died, and so on, they would make a com- 
 plete connecting link between the time of Christ and 
 the present day. In other words, the first old woman 
 would have lived about two thousand years ago, and I 
 had only to imagine three similar pews of old women 
 to take me back to the time of the Creation. It all 
 seemed quite reasonable and real at that time, and 
 made the imagined age of the Earth quite realisable. 
 
 The boy of to-day can only have such thoughts as 
 the foregoing at a very early age, but I was amused 
 with an incident which happened recently. I was 
 walking through a cemetery with a little fellow of 
 seven years, when he suddenly pulled me up in front 
 of an old-looking tombstone upon which the family 
 name of Adam was conspicuous. With anxious 
 curiosity the little fellow asked — " That isn't the grave 
 of old Adam of the Bible, is it ? " The boy-mind of 
 to-day, however, soon begins to inquire as to how old 
 the world really is. 
 
 Man cannot hope to dig very deep into the interior 
 of the Earth, but there are great mountain clefts, in 
 different parts of the world, and in these the different 
 strata of deposits may be seen. In this way man has 
 
 245 
 
Scientific Ideas of To-day 
 
 been able to look back over the pages of the Earth's 
 past history. 
 
 From excavations made in Egypt we see that at least 
 four thousand years ago there lived men and women 
 very similar to ourselves. From an amusing little 
 incident which was brought to light in one of the 
 more recent excavations one sees how very like the 
 youth of thousands of years ago must have been to the 
 youth of the present day, I heard one of the excavat- 
 ing party tell that they found upon a wall an informal 
 inscription, or scribble, which translated into English 
 reads — ''Julia, my Julia, is a little pig." It must be 
 presumed that the word pig was a term of endearment 
 in those days. I recollect another trivial incident re- 
 lated by the same authority. It was the inscription 
 upon a tombstone erected by a husband to the memoi7 
 of his deceased wife. The translation of this pathetic 
 inscription was — '* She had no fault but that she left 
 me," 
 
 We see therefore that during four thousand years 
 man has really changed very little. Indeed it is clear 
 that the time required for the evolution of man from 
 the simplest living organisms cannot be reckoned con- 
 veniently even in thousands of years. We are not 
 very surprised on this account to learn that the late 
 Lord Kelvin reckoned the age of the Earth as a 
 habitable planet to be about twenty million years. 
 His calculations are based upon the physical condition 
 of the Earth ; its internal temperature. From this he 
 calculates that it has taken twenty million years for 
 
 246 
 
The Age of the Earth 
 
 the Earth to cool down from a molten globe to its 
 present temperature. 
 
 Since the discovery of Radium, which continually 
 emits heat, suggestions have been made as to the 
 possibility of such radio-active substances assisting 
 in maintaining the Earth's heat for a longer period 
 than would otherwise be the case. A similar sugges- 
 tion has been made with reference to the life of the ' 
 Sun. It is evident that the late Lord Kelvin did not 
 count these suggestions as carrying any weight with 
 them. In a letter written by him in 1906, and pub- 
 lished at a later date in the British Weekly^ Lord Kelvin 
 made what is believed to be his latest statement upon 
 this subject. Speaking of the Earth and the Sun, he 
 says — "It seems almost infinitely improbable that 
 radium adds practically to their energy for the emission 
 of heat and light." It should be noted, however, that 
 some very eminent scientists of to-day think the sug- 
 gestion quite a reasonable one. 
 
 The amateur may judge the age of a full-grown 
 horse by its appearance or by its activity, but the 
 expert can tell its age from its teeth, up to a certain 
 age. We may count the age of a tree from its 
 possession of rings, and the age of some fishes may 
 be reckoned from certain markings on ,their scales. 
 There are several methods of calculating the age of 
 the Earth, but before considering these it may be 
 helpful to have a general statement of the scientific 
 ideas of to-day concerning the evolution of this planet 
 from a molten globe to its present condition. In the 
 
 247 
 
Scientific Ideas of To-day 
 
 days of long ago, when this planet was a molten mass, 
 it revolved upon its axis at an enormous speed, and 
 was surrounded by a dense atmosphere of water 
 vapour. We picture the tidal action of the Sun pro- 
 ducing enormous tidal waves in the outer envelope of 
 this molten globe. One such enormous wave rose to 
 so great a height that it broke away from the main 
 body, and in this we see the birth of our moon. 
 According to Sir George Darwin, this great event 
 happened some fifty-six million years ago. 
 
 As the planet cooled, the water vapour became 
 liquid, and oceans were formed in the hollows of the 
 Earth's surface, which had become irregular under 
 the enormous pressure of the water atmosphere, which 
 probably amounted to about five thousand pounds to 
 the square inch. The boiling waters of the oceans 
 would cool, and sedimentary deposits would be formed. 
 It is the presence of these deposits in the Earth's crust 
 which constitute the geologist's book of ancient 
 history. 
 
 Geologists were so impressed at first with the 
 enormous time required for the formation of these 
 accumulated deposits that they declared that the age 
 of the Earth could only be counted in "eternities." 
 Some geologists of to-day will not be content with 
 less than thousands of millions of years for the Earth 
 to become solid and reach its present condition. 
 
 It is interesting to note one method of determining 
 
 the time which has elapsed since the oceans were 
 
 formed upon the Earth. The oceans, having been 
 
 248 
 
The Age of the Earth 
 
 formed from the gaseous water atmosphere, were 
 originally fresh water, and only became salt very 
 gradually by the rivers carrying down sodium. Pro- 
 fessor Joly, of Dublin, has calculated the amount of 
 sodium contained in sea-water, and also the amount 
 carried down by all the rivers in one year. The latter 
 figures out to be about one hundred and sixty million 
 tons of sodium per annum, while the amount of 
 sodium contained in all the oceans is at least ninety 
 million times as much. Professor Joly therefore 
 assumes that it has taken ninety million years for 
 the oceans to reach their present degree of saltness. 
 
 It will be observed that Professor Joly's estimate is 
 in excess of Lord Kelvin's minimum of twenty million 
 years, but at one time Lord Kelvin's estimate ran as 
 high as four hundred million years; he ultimately 
 favoured the lower figure. Sir George Dai-win's 
 calculation of the age of the moon comes between 
 Lord Kelvin's minimum and Professor Joly's estimate. 
 So we see that the scientific men of to-day are by no 
 means agi*eed as to the time which has elapsed since 
 this planet of ours became a solid body, but all are 
 agreed that the time can only be measured con- 
 veniently in millions of years. If we admit that many 
 millions of years have been necessary for the cooling 
 down of this planet from a temperature of five thousand 
 degrees, what shall we say then of the time required 
 for it to cool down from thirty thousand degrees when 
 it was one of the hottest stars ? 
 
 One thing is clear, this planet had a beginning and 
 249 
 
Scientific Ideas of To-day 
 
 it must have an ending also. We think of the Earth 
 as having a definite hfe from the time when it and the 
 other members of the solar system broke away from 
 the great nebula which originally occupied the space 
 of the solar system. We can realise that all the 
 heavenly bodies had a beginning and will have an 
 ending, and that the very atoms of matter had also 
 a beginning and must have an ending, but what about 
 the electrons of which the atoms are composed ? Are 
 they eternal and never-changing ? May not the 
 electrons themselves have structures as intricate as 
 the atoms ? Here Mendeleeff's theory of aether par- 
 ticles might come in ; the electrons being pictured 
 as systems of revolving aether particles. It is no 
 wonder that man's brain reels before the infinitely 
 great things of the known Universe on the one hand, 
 and the infinitely small things of Nature on the other 
 hand. 
 
 We do not doubt the evolution of man, although 
 we may desire to modify Darwin's theory. Surely 
 we must accept also the theory of the evolution of 
 matter ? The old gulf between living bodies and not- 
 living matter is not so wide as it once was ; the real 
 difference may be somewhat analogous to that of an 
 electrified and an unelectrified body. But we believe 
 that life is something distinct from matter and energy ; 
 in the living body there is something which is not 
 present in the dead body. 
 
 To admit evolution does not mean that things 
 are as we find them because of some blind unliving 
 
 250 
 
The Age of the Earth 
 
 force. In this connection Lord Kelvin made some 
 very interesting statements in a public address a few 
 years ago. He said, " It is impossible to conceive 
 either the beginning or the continuance of life without 
 an overruling creative power. ... I feel profoundly 
 convinced that the argument of design has been greatly 
 too much lost sight of in recent zoological speculations. 
 Overpoweringly strong proofs of intelligent and bene- 
 volent design lie around us, , . . showing to us 
 through Nature the influence of a free will, and 
 teaching us that all living things depend on one 
 everlasting Creator and Ruler." 
 
 251 
 
CHAPTER XVII 
 
 WHENCE CAME LIFE? 
 
 The cycle of life— A curious rumour — Life dormant for thirty years 
 — Lord Kelvin on the origin of life upon this planet — A mis- 
 taken idea — Pasteur's great discovery — Life originated in the 
 sea — The construction of all Hving things — What protoplasm 
 is — Will life ever be originated in the laboratory ? 
 
 A VOLUME under the present title would not be com- 
 plete without some reference to the scientific ideas of 
 to-day concerning the origin of life. 
 
 I can imagine some of the old school being dis- 
 pleased that such a question — ''Whence came life?" 
 — should be raised at all ; it should be sufficient that 
 the Creator made man and all other living creatures. 
 However, as we have seen in the preceding chapter 
 that we believe in an evolution from electrons to 
 atoms, from one kind of atom to another, from simple 
 atoms to compound molecules, and ultimately in some 
 mysterious way to living matter, it is natural to inquire 
 into the origin of life. The true man of science does 
 not desire to drive the Creator from his Universe ; he 
 desires only to see the manner in which the Creator 
 has caused Nature to work out his designs. 
 
 If any scientific man were to maintain to-day that 
 
 the Sun was the originator of life, he would be put 
 
 252 
 
Whence came Life? 
 
 down as a quack, and rightly so. It is clear to all that 
 the Sun is absolutely necessary for the maintenance of 
 life upon this planet, but that is quite another matter. 
 
 The least observant among us must have been im- 
 pressed at some time or other with what we might 
 term the cycle of life. Commencing with the dry corn 
 seed falling into the earth, we see the plant grow and 
 bring forth grain, some of which when dried is kept to 
 be planted in the earth next season, and so on. Here 
 we might speak of an active life and an inactive life. 
 In the former condition the plant must continue to 
 breathe and to absorb moisture or it will die, but in 
 the inactive condition the dried seed may be kept for 
 several years and yet be transformed into a living plant 
 when placed in the earth. 
 
 Some years ago there was a rumour concerning a 
 seed which had been found within the wrappings of 
 an Egyptian mummy. The seed had been lying in an 
 inactive state for thousands of years, and it was re- 
 ported that when this ancient seed was planted it 
 showed signs of life and growth. However, this report 
 was contradicted later, and it is believed that there 
 was some error in saying that this particular seed did 
 germinate. Any farmer will tell you that corn seed 
 deteriorates, that he always uses last year's seed, and 
 indeed there seems no doubt that after a time a seed 
 will lose the life contained within it. 
 
 There is a remarkable case — well authenticated — in 
 connection with the spores, or seeds, of bacteria. These 
 spores behave in a similar way to dried corn seeds ; 
 
 253 
 
Scientific Ideas of To-day 
 
 they remain inactive until placed in a medium suitable 
 for growth. Some spores laid aside by Pasteur were, 
 after thirty years of inaction, placed in a suitable 
 medium, whereupon they developed into bacteria. It 
 is well known that some species of small worms may 
 be dried and kept for a long time in this inactive con- 
 dition — apparently dead — and yet become actively alive 
 when placed in water. 
 
 Even last year's corn seed appears as lifeless as a 
 chip of straw ; wherein lies the difference ? We can 
 analyse the corn seed into all the elementary sub- 
 stances which form it, and we see a wonderful design 
 in the arrangement of these elements, so that they are 
 ready to hand when they are called for by the heat 
 and moisture of the earth. We know how the seed 
 once planted shoots arms down into the ground to 
 absorb nourishment, and other arms up into the air 
 to receive the stimulus of the aether waves of light and 
 radiant heat. But we may examine the dried grain of 
 corn with all our modern methods, and yet we can 
 find no answer there as to the source of its life. 
 
 Granted that we once have life in any form upon 
 this planet our chief mystery vanishes, for it is most 
 apparent that life begets life. If it be true that there 
 can be no life without antecedent life, then how did 
 life originate upon this planet ! The late Lord Kelvin 
 believed that through all space and time life proceeds 
 from life and from nothing else. In an address which 
 this great thinker delivered to the British Association 
 about forty years ago he said, "The hypothesis that 
 
 254 
 
Whence came Life? 
 
 life originated on this Earth through moss-grown 
 fragments from the ruins of another world, may seem 
 wild and visionary. All I maintain is, that it is not 
 unscientific." 
 
 Long ago when people found living maggots on 
 putrefying flesh they jumped to the conclusion that 
 the insect's life originated in the decomposition of 
 the flesh. Simple experiments soon proved that the 
 maggots resulted from eggs laid in the flesh by flies. 
 The world-famed Pasteur, with the aid of a powerful 
 microscope, was able to show that putrefaction itself 
 was due to living organisms, which we describe as 
 microbes or bacteria. These bacteria increase at an 
 enormous rate in favourable circumstances, but they 
 follow the principle — life begets life. When there is 
 fear of infectious disease we sterilise our milk so that 
 any bacteria contained in it may be killed. In bring- 
 ing butcher-meat from our distant colonies we defy 
 the microbes of putrefaction by freezing them to death. 
 When the meat is removed from the freezing chamber, 
 no microbes can be originated within it, but living 
 microbes, carried to it by the air, may make fresh 
 attacks upon it. 
 
 Meat-jellies, or bouillons, are splendid breeding 
 grounds for bacteria, but if these substances are 
 thoroughly sterilised and hermetically sealed no 
 bacteria can possibly appear in them. A few years 
 ago we heard rumours that life had been originated in 
 sterilised bouillon by the action of radium, but this is 
 more than the experimenter himself really claimed. 
 
 255 
 
Scientific Ideas of To-day 
 
 He believed he had found a link between not-living 
 
 and living matter, but his experiments leave us no 
 
 nearer a solution of the problem before us — Whence 
 
 came life ? 
 
 There is a very definite idea that life originated in 
 
 the sea. It is certain that the elementary constituents 
 
 of sea-water and air are the same as those contained in 
 
 our own bodies, the best known of these being oxygen, 
 
 nitrogen, carbon, hydrogen, and sodium. Although 
 
 this indicates the place wherein life originated, it still 
 
 leaves us in the dark as to the origin of life. It is 
 
 interesting to note that in the first chapter of Genesis 
 
 we read, "Let the waters bring forth abundantly 
 
 the moving creatures that hath life, and fowl that 
 
 may fly above the earth in the open firmament of 
 
 heaven." 
 
 Man has found out a great deal about the nature of 
 
 life. The microscope has revealed the fact that all 
 
 living things are composed of very minute cells. Man 
 
 is composed of many trillions of such cells, but on the 
 
 other hand there are living things composed of one 
 
 cell only. But of what are those living cells made ? 
 
 They are made of a substance known as protoplasm^ 
 
 from the Greek proto, first, and plasso, 1 mould. This 
 
 substance is entirely structureless, and is composed 
 
 chiefly of carbon, oxygen, hydrogen, and nitrogen. 
 
 We have seen that these are the chief constituents of 
 
 our own bodies. We may picture the protoplasm 
 
 forming cells just as atoms form molecules ; we have 
 
 a variety of molecules, and we have a variety of cells. 
 
 256 
 

 H .y 
 
 rt o 
 U tn 
 
 Q -S 
 
 rt o 
 
 '■So 
 
 
 -^ 0) 1/1 
 
Whence came Life? 
 
 We see that a study of living matter, as far as we 
 can go, is really a study of chemical physics. 
 
 One thing has become clear from a study of the 
 simplest living organisms. They move and act only 
 because of outside influences ; they simply react. They 
 are affected by chemicals in their environment, by 
 vibrations in the air, or by waves in the surrounding 
 aether. The same truth holds good in man himself, 
 but we are such infinitely complex agglomerations of 
 cells that the resulting actions are difficult to trace. 
 
 However, what interests us at present is that to trace 
 the origin of life we may confine our attention to 
 protoplasm, as no serious thinker will doubt the plain 
 truths of evolution. 
 
 A few devout thinkers of the present day do not 
 think it wise to be so dogmatic as to say that it will 
 be for ever impossible to originate life in the labor- 
 atory. Suppose for a moment that we should accom- 
 plish this seeming impossibility, man will not have 
 become a creator, he will only have discovered the 
 methods of the great Creator. To discover the way in 
 which a machine is working is a totally different thing 
 from making the machine. At present man places 
 certain quantities of different elementary atoms to- 
 gether, heats them, and forms complex molecules, but 
 man has not created these. To create is to make 
 out of nothing. If the chemist or the biologist suc- 
 ceeds in making protoplasm artificially, our religious 
 convictions need be shaken in no way. 
 
 257 
 
CHAPTER XVIII 
 
 MORE ELECTRON IDEAS 
 
 A real connection between sun-spots and aurora and magnetic 
 storms — The connecting link — Why aurora are only seen 
 towards the poles of the Earth — How does the Earth come to 
 be negatively charged— Atmospheric electricity— Lightning— 
 What constitutes the 'Earth a magnet ?— Magnetic storms — 
 How a cold nebula may emit light — Why does the Earth's 
 electric charge not increase ? 
 
 It is curious that although sun-spots appear to us as 
 dark holes in the Sun's photosphere, they are in reality 
 as bright as the light from a lime-light lantern. The 
 lime-light, as it issues from the lantern, is so bright 
 that we cannot look directly at it, nor can we look 
 directly at the Sun except through a dark glass. If the 
 lime-light were placed in front of the Sun, and both 
 viewed through a dark glass, the lime-light would look 
 like a black spot. 
 
 The number of spots on the Sun varies from time 
 to time ; there are weeks together when none are 
 seen, and also a maximum when more are seen than 
 at any other time. The interval between two maxi- 
 mum periods is about eleven years. For a long time 
 back we have been told that these variations of the 
 sun-spots affect the magnetic condition of our Earth, 
 
 and also that the number of those beautiful auroras 
 
 258 
 
More Electron Ideas 
 
 seen in the heavens varies with the number of spots 
 on the Sun, Some have even endeavoured to prove 
 that there is a real connection between the recurring 
 eleven years of sun-spot maxima and the variation of 
 grain prices. However, we shall be content to deal 
 with the first two statements. 
 
 Observations have proved clearly that, when the 
 sun-spots are most numerous, auroras and magnetic 
 disturbances are also most numerous. And when 
 any exceptional disturbance occurs upon the Sun 
 we have corresponding effects upon this planet in 
 brilliant displays of aurora and energetic magnetic 
 storms, which latter are a source of considerable trouble 
 in the working of our telegraphs. 
 
 What interests us at present is to see wherein Hes the 
 connection between the sun-spots and those phenomena 
 on the Earth. Once more the obliging electron puts in 
 an appearance, if we can speak thus of what is invisible. 
 The Sun, Hke all other incandescent bodies, is allowing 
 electrons to escape from it. The dissociation of the 
 Sun is so great that we may picture continuous streams 
 of electrons being shot off into the surrounding vacuum, 
 and these will be greatest when there are great out- 
 bursts through large sun-spots. We therefore picture 
 a gigantic cathode stream of electrons issuing from 
 the Sun. We recollect that the cathode rays are in- 
 visible, but we know that when they pass through the 
 rarefied air of a so-called vacuum tube there is a beau- 
 tiful glow produced in the tube. We should expect, 
 therefore, that the gigantic cathode stream from the Sun 
 
 259 
 
Scientific Ideas of To-day 
 
 would cause the upper rarefied air of our atmosphere to 
 glow in a similar fashion. But we remember that in 
 the laboratory the cathode stream was deflected by a 
 magnet, and as the Earth is a huge magnet, we are 
 not surprised to find that the Sun's cathode rays are 
 deflected so that they do not enter the atmosphere at 
 the tropics, but gradually descend towards the poles of 
 the Earth, This is the reason why auroras occur con- 
 tinually at the poles of the Earth. Those occurring 
 towards the north pole are called aurora borealis, and 
 those seen within the region of the south pole are 
 known as aurora aiistralis. 
 
 We see that the Earth is a great sphere which is 
 being bombarded continually with electrons, and we 
 know that any body having an accumulation or surplus 
 of electrons is negatively charged. Hence we have 
 the solution of what must have been a puzzling pro- 
 blem to many of us in our boyhood ; we wondered 
 how the Earth should happen to be negatively elec- 
 trified. This gigantic sphere — the Earth — charged 
 with negative electricity gives us a very convenient 
 standard of electric pressure, just as the sea-level gives 
 us a convenient standard for height and depth. We 
 make the Earth our zero of pressure. 
 
 We may picture the Earth as a great reservoir of 
 
 electrons. If a body having a deficiency of electrons 
 
 (a positively electrified body) is put in connection 
 
 with the Earth, there will be a flow of electrons from 
 
 the reservoir to the body referred to until there is an 
 
 exact balance within its atoms between the electrons 
 
 260 
 
More Electron Ideas 
 
 and their enclosing spheres of positive electricity. On 
 the other hand, if a body has an excess of electrons (a 
 negatively electrified body), and if it is put in con- 
 nection with the Earth, the body referred to will 
 discharge its surplus electrons into the great reservoir 
 until there exists an equilibrium within its atoms. 
 Taking water as an analogy of the electrons, we know 
 that water will flow out of a vessel above sea-level 
 into the sea, whereas water will flow from the sea into 
 a vessel below sea-level. 
 
 But some one may say that the electrons, shot at the 
 Earth by the Sun, will be waylaid by our atmosphere. 
 So they will, and they will ionise the air, or in other 
 words, they will cause the electro-positive and the 
 electro-negative atoms forming some of the mole- 
 cules of atmospheric gases to split partnership. We 
 shall try to picture what will take place in ionised 
 air. Water vapour condenses more easily upon 
 the electro-negative atoms, clouds will be formed, 
 and when these ultimately fall in the form of 
 rain, they will bring down the entrapped electrons 
 with them, leaving the upper air positively electrified. 
 In this way we have a reasonable explanation of the 
 electrical conditions which we find in the atmo- 
 sphere. 
 
 With the same facts before us, we can see how 
 clouds may become at times highly charged with a 
 surplus of electrons, so that there is a lightning dis- 
 charge from one cloud to another, or between a cloud 
 
 and the great reservoir — the Earth. 
 
 261 
 
Scientific Ideas of To-day 
 
 Another question which must have arisen in the 
 minds of many thoughtful readers is, How did the 
 Earth become a magnet ? No one can doubt that 
 the Earth is a magnet ; its influence upon magnetic 
 needles is most apparent. As natural magnets, or 
 lodesiones, are found in the Earth, some one might 
 jump to the conclusion that the presence of these 
 would constitute the Earth a magnet. Upon further 
 consideration such an one would find that this was 
 not a reasonable conclusion. Lodestone is found 
 only in a limited number of places, and in no very 
 great quantities. When the inquirer learned that rail- 
 way lines and iron railings placed in certain positions 
 are found often to be magnetised by the influence of 
 the Earth, he would be willing to conclude that lode- 
 stone is nothing more than certain iron ores magne- 
 tised in the same manner. How, then, did the Earth 
 become a magnet ? 
 
 It is true that the Earth is a huge electrically charged 
 sphere, and that it is constantly I'otating at a rapid rate 
 upon its axis, and we have experimental proof that 
 under such circumstances a sphere would have a weak 
 magnetic field at its surface. Calculations, however, 
 show that this is certainly not the cause of the Earth's 
 magnetic field ; it is capable only of accounting for 
 the merest fraction of the force present. The chief 
 factor appears to be electron currents within the 
 Earth's crust. If we were asked what physical con- 
 dition could cause a movement of electrons within 
 
 the Earth, we should think at once of differences of 
 
 262 
 
More Electron Ideas 
 
 temperature in connection with thernio-electricity. It 
 is not necessary to think of a junction of two dis- 
 similar metals being subjected to heat to produce an 
 electron current. We know that even a difference 
 of temperature in the same piece of metal will cause 
 a locomotion of electrons. Without going into the 
 detail of the subject, it may be stated that there 
 are conditions which account for a real thermo- 
 electric current in the Earth's surface. At the 
 same time it must be admitted that although this 
 electron current is .the most reasonable explanation 
 of the Earth's magnetism, we have no direct proof 
 of this theory. 
 
 There is one thing I should like to point out 
 in connection with the foregoing. If the Earth's 
 magnetism is due to a temperature gradient in 
 the surface of the Earth, one would expect that 
 the magnetic field would vary during the day. It 
 is a well-known fact that such variations do 
 take place, commencing with a minimum in the 
 morning, rising to a maximum about mid-day, de- 
 creasing till the evening, and remaining constant 
 dui'ing the night. 
 
 There is no doubt that this electron current in 
 the Earth's surface would be appreciably affected 
 by any exceptional sti^eams of electrons approach- 
 ing the Earth from the Sun. Hence the connection 
 between magnetic storms and the outbursts of sun- 
 spots. 
 
 Certain wireless messages received in the spectro- 
 263 
 
Scientific Ideas of To-day 
 
 scope from vast nebulas^ have been interpreted by 
 some scientists to indicate that the nebulae are cold 
 bodies ; this message could not be understood. How 
 could a cold body emit light ? We see a cold rarefied 
 gas glowing in the well-known vacuum tubes when 
 streams of electrons pass through them. As the 
 Sun and all stars are sending streams of electrons out 
 in all directions into the surrounding vacuum, some 
 of these would be intercepted by a gaseous nebula. 
 Here again the versatile electron helps us out of a 
 difficulty. 
 
 In closing this chapter it occurs to me that a 
 difficulty may arise in the minds of some readers 
 in connection with the continuous bombardment 
 of the Earth by electrons. Surely the Earth's nega- 
 tive electrification will be continuously increasing ? 
 It will be of interest to see how this does not 
 happen. 
 
 We know that electrons will be discharged from a 
 negatively charged body — such as the Earth — to a 
 positively charged body — such as the Sun. This is 
 the reverse of what we have been considering. But 
 how can electrons be sent both ways ? Only by two 
 different forces. Electrons will go from the Earth 
 to the Sun by electric pressure, the difference of pres- 
 sure between these two bodies being about one billion 
 volts. But the electrons which come from the Sun 
 to the Earth do not move under electric pressure ; 
 
 ^ These nebulae are quite a different class from the nebulae we con- 
 sidered in an earlier chapter, and which were composed of meteorites. 
 
 264 
 
More Electron Ideas 
 
 they travel, as we saw in a previous chapter, under 
 the mechanical pressure of light. In this way a 
 proper balance is maintained, and a continual passage 
 of electrons kept up. We see, as it were, a circular 
 dance of electrons throughout the solar system. 
 
 265 
 
CHAPTER XIX 
 
 WHAT ARE THE X-RAYS? 
 
 Are the X-rays waves in the sether? — How the X-rays were dis- 
 covered — How they are produced — The new photography — 
 What interested the world — What the X-rays are not — Our 
 present ideas of their nature 
 
 We have become familiar with the idea of aether 
 waves, some of which affect our vision, others heat 
 our bodies, some affect the detector of a wireless 
 telegraph receiver, while others which do not affect 
 our vision will act upon the chemicals on an ordinary 
 photographic plate. All these are electro-magnetic 
 waves in the aether. Are the X-rays to be included 
 in the same category ? If so, we should be able to 
 reflect, refract, and polarise them, just as we found we 
 could do with all the aether waves already mentioned. 
 But we cannot do these things with X-rays. At first 
 some experimenters believed they had succeeded in 
 reflecting them, while others thought they had polar- 
 ised them with the aid of tourmalines, but in every 
 case it was found, upon further investigation, that these 
 supposed effects were due to some error. It is clear 
 that whatever the X-rays may be, they are not regular 
 trains of electro-magnetic waves, such as light is. 
 
 It will simplify matters if we consider the method 
 266 
 
what are the X-rays? 
 
 of producing X-rays, and it will bo oi mlcnvst tx> ^vV 
 also how these were discovered. Needless tv> s,iY» 
 they were not invented any more than eleeiueity 
 was invented ; the X-rays had been shootiui; torth 
 from vacuum tubes for nearly twenty years before man 
 was aw^re of their presence. 
 
 Professor Rontgen was experimenting in 1895, hke 
 a great many other physicists, with \^cuum tubes. He 
 intended following up the experiments of Lenard, who 
 had succeeded in tracing the cathode rays outside of 
 the vacuum tube. In the physical laboratoiy of the 
 University of Wiirzburg (Bavaria) Professor Rontgen 
 was well supplied with modern apparatus, and with 
 excellent means of producing very high vacua in tubes. 
 The Germans excel in this class of work, 
 
 Rontgen had enclosed a vacuum tube within a shield 
 of black cardboard, which prevented any light escaping 
 from the phosphorescing glass of the tube. Lenard 
 had used a fluorescent screen to trace the escaped 
 cathode rays, and Rontgen had a similar screen beside 
 him on this occasion. These screens 1 had been in 
 use for a generation in connection with ultra-violet 
 light. 
 
 ^ Some substances continue to emit light after the exciting force has 
 been withdrawn ; we call these phosphorescent substances. Sulphide of 
 zinc, which is used in luminous paints, is a phosphorescent substance. 
 Some other substances only emit light as long as they are being sub- 
 jected to the exciting force ; we call \!cit%^ fiuorescent substances. Barium 
 plati no -cyanide is a fluorescent substance, and its electrons are affected 
 by the invisible short ultra-violet rays, and also by the X-rays. The fine 
 chemical crystals fluoresce only as long as the invisible rays keep falling 
 upon them. 
 
 267 
 
Scientific Ideas of To-day 
 
 When Professor Rontgen passed an electrical dis- 
 charge through the hidden tube he observed that his 
 fluorescent screen, which was lying on the table, 
 became luminous. It was apparent that this lumin- 
 osity could not be excited by any ultra-violet waves, 
 for the black shield surrounding the tube was quite 
 opaque to ultra-violet light. Even an arc lamp, which 
 is very rich in ultra-violet rays, was completely shielded 
 by such a shield. When Rontgen was asked later 
 what he thought when he made this observation, he 
 said : " I did not think ; I investigated." 
 
 Rontgen found that these new rays had a remarkable 
 power of penetration. Many substances, such as wood 
 and leather, which were opaque to light, were fairly 
 transparent to these new rays. The denser a body was, 
 the greater was its resistance to the passage of the rays. 
 The one thing, which arrested the attention of the 
 general public, was the fact that the living skeleton 
 could be viewed upon the fluorescent screen. When 
 Professor Rontgen found that metal weights could be 
 seen through a wooden box, it would be a natural 
 thing to try how his hand would appear, if indeed 
 he did not observe the bones of his fingers when 
 placing objects behind the screen. 
 
 It may be well at this moment to consider the 
 method of producing the X-rays, and of using the 
 fluorescent screen, although to most of us these 
 things are quite familiar. The electric current from 
 an accumulator or from the main is passed through 
 an induction coil ; perhaps some may know it better 
 
 268 
 
what are the X-rays? 
 
 under the guise of a sparking coil. A special vacuum 
 tube is connected to the terminals of the coil, so that 
 a discharge will take place between the two electrodes 
 within the tube. In the accompanying diagram it will 
 be seen that the cathode is saucer-shaped, in order to 
 focus the cathode stream upon a metal target placed 
 about the centre of the tube. This target may or 
 may not form the other electrode of the tube. How- 
 ever, we do not mean to trouble with detail ; we 
 wish to see what it is that gives rise to the X-rays, 
 
 When the electric current is passed through the tube 
 the stream of electrons strikes the metal target, and in 
 doing so they produce a sharp, sudden splash in the 
 aether. These were spoken of, at first, as aether waves, 
 some thinking that they were longer than the infra- 
 red waves, while others believed they were shorter 
 than ultra-violet waves. The idea of to-day is that 
 the X-rays do not constitute a regular train of waves 
 at all, but that they are very thin pulses of intense 
 electric and magnetic force. 
 
 The little target within the tube is placed at an 
 angle, so that when it is bombarded by the electrons 
 the aether pulses or X-rays will be deflected out at the 
 side of the tube, as suggested by the diagram. 
 
 The fluorescent screen has the fine barium platino- 
 cyanide crystals on the one side of the screen, the 
 back of which is covered with a black cloth lining. 
 The back of the screen is turned towards the tube so 
 that the X-rays fall upon the black lining, which offers 
 practically no resistance to their passage. The rays 
 
 269 
 
Scientific Ideas of To-day 
 
 Fig. D 
 
 A Rontgen-ray Tube 
 
 This diagram represents a single form of X-ray tube. The stream 
 of electrons, or negative current, passes from the cathode (-) to 
 the anode ( + ). We picture the electrons being shot off from the 
 cathode with great energy, and as the cathode is saucer-shaped 
 the stream will be focused on to the target, which is seen lying at 
 an angle. When the electrons are suddenly stopped by the target 
 they produce a sort of splash or pulse in the surrounding asther, as 
 indicated by the dotted lines. This aether disturbance is what we 
 know as Rontgen-rays, the properties of which are explained in 
 the text. 
 
 270 
 
What are the X-rays? 
 
 penetrate the parchment of the screen, and cause the 
 chemical surface to fluoresce. If the hand is placed 
 flat against the back of the screen, the rays will pene- 
 trate the flesh much more easily than they can pene- 
 trate the bones, so that the bones are clearly seen upon 
 the screen. For our present purpose we need not 
 consider the great value that Rontgen's discovery has 
 proved to the surgeon. 
 
 Rontgen was not long in trying if these new rays 
 would affect a photographic plate, and the world was 
 soon talking about the new photography. The idea 
 of taking a photograph of the living skeleton upon an 
 ordinary photographic plate, and doing so in the dark 
 without even opening the dark slide or envelope en- 
 closing the sensitive plate, was indeed something to 
 talk about. In the illustration facing page 268 we see 
 an ordinary photograph of a cameo, and alongside of 
 this we see a radiograph taken by X-rays. This shows 
 how the X-rays have penetrated some parts of the 
 cameo more easily than other parts. 
 
 What concerns us at present is the scientific ideas 
 concerning the X-rays. Scientific men had become 
 familiar with the cathode rays or streams of electrons, 
 and with the Lenard rays, which are really the cathode 
 rays that have escaped through an aluminium window 
 in the tube. We have seen in an earlier chapter the 
 great value of Lenard's experiment to the scientific 
 man, but it would seem of no importance to the man- 
 in-the-street. Nor would the outsider have been in- 
 terested specially in the discovery of the X-rays but 
 
 271 
 
Scientific Ideas of To-day 
 
 for the eeriness of being able to see and photograph 
 the living skeleton. 
 
 It will be noticed that our knowledge of the nature 
 of the X-rays is chiefly of a negative character. We 
 know that they are not a stream of electrons like the 
 cathode and the Lenard rays, for the X-rays cannot 
 be deflected by a magnet. We know that they are 
 not aether waves of the nature of light, as X-rays 
 cannot be reflected, refracted, or polarised. 
 
 We see very clearly what the X-rays are not, and the 
 best that can be said as to their nature is that they are 
 thin shells or pulses of intense electric and magnetic 
 force. It may be that the scientific world of to-morrow 
 will not be satisfied with this statement. As a looker- 
 on I have wondered sometimes if the X-rays may not 
 prove to be the electrons breaking up, just as we find 
 the electrons to be produced by the atoms breaking 
 up. If Mendel6efl*s theory turn out to be true — that 
 the aether is composed of particles — and if those 
 particles form the electrons, may the X-rays not be 
 the escaping aether particles expelled from the^electron 
 in its collision with the target in the X-ray tube ? In 
 any case the greater the collision (Le, the more 
 abruptly the electrons are stopped), the more energy 
 is exhibited in the form of X-rays. 
 
 272 
 
CHAPTER XX 
 
 HOW RADIUM WAS DISCOVERED 
 
 The public's overestimate of radium— What led up to Madame 
 Curie's great discovery — A Russian experiment — Becquerers 
 discovery of uranium rays — Similarity of circumstances with 
 Daguerre's discovery — Are the uranium rays the same as X- 
 rays ? — The Curies' work — Public interest in radium — Physio- 
 logical effects — The^action of the spinthariscope — Heat evolved 
 by radium — Photographic effects —The Birth of radium 
 
 It seems but as yesterday that Radium was discovered, 
 and yet it is more than ten years since Madame 
 Curie, wife of the late Professor Curie, of Paris, 
 brought to Hght this element which had lain hidden 
 in the world for millions of years. 
 
 Although this great discoi'ery was made in 1898, it 
 was several years later that the general public became 
 interested in it. Rumours went abroad that this new 
 element was going to revolutionise the workaday 
 world. All our existing methods of obtaining energy 
 were to go to the wall, incurable diseases were to be 
 cured, and the very foundations of physical science 
 were to be demolished. This was quite enough to 
 arouse general interest, but it must be clearly under- 
 stood that the scientific world did not share in these 
 prophecies. Indeed we shall see that men of science 
 were familiar with radio-active bodies before the dis- 
 
 273 S 
 
Scientific Ideas of To-day 
 
 covery of radium, although its forerunners were much 
 less active. Sir Oliver Lodge put the matter very 
 clearly at the time in this way — "A bare fact is 
 nothing, or little, till it is clad in theory. Sometimes 
 a fact is born before its clothes are ready. Sometimes 
 a 'layette' has been provided before a fact is born. 
 Radium is in the latter predicament. No fact con- 
 cerning radium need stand out in the cold for lack 
 of theoretic shelter." 
 
 Men of science do not stumble over discoveries by 
 mere chance ; there is always a train of facts leading 
 up to each discovery. It will be of interest to see 
 what led up to the unearthing of radium. One would 
 not think that there was any connection between Sir 
 William Crookes' discovery of the cathode rays pro- 
 duced in vacuum tubes and the discovery of radium, 
 and yet there is a straight line of descent. Then 
 again we could trace the lineage of Crookes' discovery 
 away back to the first rubbing of a piece of amber in 
 ancient times. 
 
 We have seen already that Crookes' experiments 
 
 led to Rontgen's discovery of the X-rays. The fact 
 
 that these invisible rays would affect a photographic 
 
 plate led others to try if phosphorescent substances 
 
 might not emit similar invisible radiations. But what 
 
 connection is there between phosphorescent substances 
 
 and X-rays ? The X-rays cause the glass of the tube 
 
 in which they are produced to phosphoresce, and 
 
 they also excite phosphorescence in many gems and 
 
 chemical crystals. 
 
 274 
 
By permission of 
 
 Rfbrnaii Ltd., I^o>ido*t 
 [Aj-chives of the Roent^e)!. Ray) 
 
 Photographing through a Granite Boulder 
 
 This experiment illustrates the wonderful penetrating power of radium rays. Pieces 
 of lead wire were twisted into the form of the words seen in the upper illustration. 
 These letters were laid upon a thick, light-proof envelope enclosing a photographic 
 plate. On the top of these was laid a one-inch board, and on the top of that a six- 
 inch boulder. A small glass tube containing some radium salts was laid on the top 
 of the boulder. The rays penetrated the boulder, the wood, arid the light-p roof 
 envelope, but were entrapped by the lead wire, and produced the photograph shown 
 in the upper illustration. (Exposure three days.) 
 
How Radium was Discovered 
 
 We all have some knowledge of phosphorescent 
 substances. We know that luminous paints, which 
 contain sulphide of calcium or sulphide of zinc, will 
 shine in the dark for some time if previously exposed 
 to sunlight. One practical adaptation of these lum- 
 inous paints has been to put them on match-box cases, 
 so that they will shine in the dark and indicate their 
 whereabouts. Some of us may have boyhood re- 
 collections of rubbing a few drops of phosphoric oil 
 upon our faces and hands to personate a real live 
 ghost. 
 
 It occurred to a Russian scientist to try if the 
 phosphorescent calcium sulphide could possibly affect 
 a photographic plate through a thin sheet of aluminium, 
 in the same way as Rdntgen's X-rays had been able 
 to do. Although the metals generally are opaque to 
 X-rays, a thin sheet of aluminium is practically trans- 
 parent to them. This experimenter, whose name — 
 Niewenglowski — looks strange to us, made the follow- 
 ing simple experiment. He protected a photographic 
 plate under a sheet of aluminium, and on the top of 
 this he placed some of the phosphorescent substance 
 resting upon a small square of glass. He left this 
 arrangement alone in the dark for one day and one 
 night, and when he developed the plate he found that 
 he had secured an image of the little square of glass 
 upon which the phosphorescent substance had been 
 lying, and there was unmistakable evidence that in- 
 visible rays had penetrated the thin sheet of aluminium. 
 A closer examination showed that the penetrating rays 
 
 275 
 
Scientific Ideas of To-day 
 
 were not X-rays, for they had been refracted by the 
 glass plate, as was evident at the edges. These rays 
 consist of very penetrating rays of light. To the 
 general reader they are of interest only as a stepping- 
 stone between Rontgen's discovery and that of radium 
 by Madame Curie. 
 
 The very same idea of some possible connection 
 between X-rays and phosphorescence occurred inde- 
 pendently to Professor Becquerel, of Paris. He tried 
 the photographic effect of every phosphorescent sub- 
 stance he could think of, and during those experiments 
 he found that certain uranium salts were very active 
 in emitting radiations which affected a photographic 
 plate. The curious thing was that those uranium salts 
 scarcely deserved to be called phosphorescent at all. 
 While luminous paint will shine for many hours after 
 exposure to sunlight, these uranium salts will only 
 phosphoresce for a very small fraction of a second 
 after the light has been withdrawn. The ordinary man 
 would have put these out of court without ever giving 
 them a trial. But Becquerel desired to give them a 
 fair trial, so he arranged that they might have an 
 opportunity of acting upon a photographic plate while 
 the salts were continuously exposed to sunlight. He 
 secured a photographic plate within a light tight en- 
 velope, and he placed this in the sunlight with some 
 crystals of uranium salts spread on the top of the black 
 envelope. When he developed the photographic plate, 
 he found that invisible rays had penetrated to the plate 
 
 and had produced an image of the uranium crystals. 
 
 276 
 
How Radium was Discovered 
 
 Becquerel arranged a second experiment, and this 
 time he placed a metal cross between the uranium salt 
 and the dark envelope which enclosed the sensitive 
 plate. He intended exposing this to sunlight for some 
 hours, just as he had done on the former occasion ; 
 but, as luck would have it, the sunshine vanished at 
 the time it seemed most wanted. We shall see that 
 this was a good fairy in disguise, for Becquerel put 
 the experiment aside, leaving it arranged as described, 
 and he intended to make a complete exposure when 
 the Sun was more favourable. But for some reason 
 or other he developed the photographic plate after it 
 had lain aside without further exposure, and we may 
 judge of his surprise when he found an image of the 
 cross upon the plate. This could not have happened 
 during the very short exposure of the uranium to 
 sunlight. Could the photographic action have been 
 going on without the exciting influence of the Sun 
 upon the uranium salts ? This could be proved by a 
 repetition of the experiment without the aid of sun- 
 light at all. This done, the same result was obtained 
 in the dark. The invisible rays were not due to the 
 phosphorescent property of the substance at all. 
 Indeed there was no doubt that these invisible uranium 
 rays were something quite new. 
 
 I think it is worth while, in passing, to note how 
 parallel this case is with Daguerre's discovery of prac- 
 tical photography. Having prepared the surface of a 
 polished silver plate by exposing it to the vapour of 
 
 iodine, Dagucrrc set up his camera intending to expose 
 
 277 
 
Scientific Ideas of To-day 
 
 the plate for several hours in sunshine, in the hope of 
 securing a picture. Just when everj^hing was in readi- 
 ness the Sun vanished, so Daguerre put his polished 
 silver plate aside in his chemical cupboard, intending 
 to repeat the experiment again at the Sun's pleasure. 
 Imagine Daguerre's surprise when he went to take the 
 plate from the cupboard next morning ; he found a 
 "perfect picture" upon it. It was clear that the very 
 short exposure had impressed a hidden or latent image 
 upon the plate, and that the vapour of some of the 
 chemicals in the cupboard had produced the visible 
 image. By experiment Daguerre found that the active 
 agent had been the vapour of mercury, and in this way 
 was practical photography discovered. These dis- 
 coveries, Daguerre's and Becquerel's, which were both 
 made in Paris, have always seemed to me to be 
 parallel cases of so-called accidental discovery. 
 
 It was evident that the " Becquerel rays" emitted 
 by uranium salts were quite independent of outside 
 influences. To make assurance doubly sure, Becquerel 
 prepared uranium salts from a solution in the dark, 
 and they were found to act upon a photographic plate 
 without the salts ever seeing the light of day. As time 
 went on it was evident that the activity of uranium 
 salts was continuous ; they seemed to lose nothing by 
 giving out these Becquerel rays. But are these radia- 
 tions the same as X-rays ? 
 
 At first it did seem as though the Becquerel rays 
 
 were merely X-rays, but even had this turned out to 
 
 be the case, the discovery would have been a great one. 
 
 278 
 
How Radium was Discovered 
 
 Rontgen produced X-rays artificially in the laboratory ; 
 these rays were the result of definite electrical energy 
 supplied from a known source. To find a natural 
 substance continually producing X-rays, without the 
 supply of energy from without, would be of infinitely 
 greater interest from a scientific point of view. 
 
 Becquerel found that these uranium rays, which 
 we have named after him, would discharge an electri- 
 fied body just as X-rays will do. The rays of ura- 
 nium would penetrate the same substances as X-rays, 
 and the former could not be reflected, refracted, or 
 polarised, so that in all these respects they were alike 
 to X-rays. But we shall see that they are not X-rays. 
 However, we must not lose sight of the primary 
 importance of Becquerel's discovery ; a certain sub- 
 stance in its natural condition is continually producing 
 invisible radiations. 
 
 It was only natural that other experimenters should 
 make investigations to find out if any other substances 
 would act in the same way as uranium, which, by the 
 way, is the heaviest of all the elements, A veiy im- 
 portant line of investigation was begun by Professor 
 and Madame Curie, evidently with the idea of making 
 sure that the radio-activity was due to the uranium 
 itself and not to any impurities contained in it. When 
 experimenting with different specimens of pitchblende, 
 a natural mineral substance from which uranium is 
 obtained, the Curies found that some specimens of 
 the pitchblende were more radio-active than uranium 
 
 itself. This proved that the radio-active properties of 
 
 279 
 
Scientific Ideas of To-day 
 
 the pitchblende were not due really to the uranium 
 itself. It may be remariced that it was shown later 
 that pure uranium salts, freshly precipitated, were 
 not radio-active. Still later it was found that in 
 time these salts became radio-active, but we had 
 better take the stepping-stones as they come. 
 
 The Curies were determined to lay by the heels 
 the one thing which was producing the radio-active 
 phenomena. By well-known chemical processes they 
 sifted out the different ingredients of the pitchblende. 
 It is interesting to note that the Curies were positive 
 that the really active substance for which they were 
 seeking was not in the uranium itself, for they set to 
 work on a large scale with pitchblende "tailings," 
 from which the uranium had been extracted for com- 
 mercial purposes, such as the colouring of Bohemian 
 glass. 
 
 The Austrian Government placed many tons of these 
 tailings at the disposal of the Curies, who erected re- 
 fining works on the outskirts' of Paris. There was no 
 idea of obtaining radio-active elements on a commer- 
 cial scale ; it was evident already that whatever sub- 
 stance was producing the phenomena of radio-activity 
 it must exist only in a very small quantity. By pains- 
 taking chemical analysis the Curies separated three 
 different radio-active substances, but one of these 
 elements was found in greater quantity than the other 
 two, if one may use the word quantity at all in con- 
 nection with these radio-active substances. The whole 
 
 radio-active products from eight tons of pitchblende 
 
 280 
 
How Radium was Discovered 
 
 could be carried on a threepenny piece. The chief 
 radio-active product was christened Radium by 
 Madame Curie. 
 
 There was no comparison- whatever between the 
 radio-activity of uranium and that of radium ; the 
 latter being estimated to be from one to two million 
 times more active than uranium. This greatly increased 
 radio-activity gave men of science a much better chance 
 of finding out what the radiations really were. 
 
 One thing which seemed to interest the public was 
 the exorbitant prices asked for specimens of i-adium, 
 but the price is not to be wondered at when one con- 
 siders the great amount of labour in extracting the 
 radium. When the man-in-the-street heard that 
 radium cost three thousand times the price of gold, 
 he was naturally interested. However, he would be 
 somewhat disappointed when informed that there was 
 less radium in pitchblende than there is gold in 
 solution in sea-water. 
 
 Another point that interested the pubUc was the fact 
 that radium had a decided effect upon the human 
 body. Professor Becquerel discovered this to his own 
 discomfort. He had carried a small specimen of 
 radium in a case in his waistcoat ^pocket, while he 
 came over to London to deliver a lecture, and in about 
 a fortnight's time he found that the flesh under this 
 pocket was beginning to redden, then followed a pain- 
 ful sore which took many weeks to heal. Professor 
 Curie also observed that his hands were injured some 
 
 time after delivering a lecture at the Royal Institution 
 
 281 
 
Scientific Ideas of To-day 
 
 (London), during which he had been handUng some 
 radium. This is not altogether surprising when we 
 consider the well-known physiological effects produced 
 by X-rays. However, the man-in-the-street was carried 
 away with the idea that at last a panacea for all ills had 
 been discovered. 
 
 During the first few years of the present century 
 every one was interested in radium. What the metal 
 radium looks like we cannot say, as no one has seen 
 it yet ; we only know it in its compounds. We can 
 get its atoms in partnership with chlorine atoms, 
 forming radium chloride, or with atoms of bromine, 
 forming radium bromide. These radium salts have 
 veiy much the same appearance as common salt, 
 but they emit a very feeble light in the dark. The 
 luminous effects seen in the small spinthariscope, or 
 spark-viewers, sold by opticians, are due to the radia- 
 tions from I'adium bombarding a phosphorescent 
 screen. But how can opticians afford to sell, for a 
 few shillings, instruments containing so precious a 
 substance as radium ? This we shall understand by 
 considering the construction of the instruments. This 
 scientific toy, invented by Sir William Crookes, con- 
 sists of a short brass tube having a magnifying glass 
 at one end and a small phosphorescent screen at the 
 other end, while in front of this screen and close to 
 it is fixed a tiny piece of wire which has been dipped 
 in a solution of radium salts. The very minute 
 quantity of the salts adhering to the wire is sufficient 
 to produce a veiy active bombardment of the screen. 
 
 282 
 
How Radium was Discovered 
 
 The effect, which no doubt many readers have seen, 
 is somewhat like a turbulent luminous sea. It has 
 been described by some as a swamp full of fireflies, 
 or as scintillating stai'S on a clear night. Others, 
 when looking into the spinthariscope, say that they 
 see splashes of light at the centre of the screen and 
 sparks towards its edge. 
 
 Whenever one chooses to pick up a spinthariscope 
 and look into it, one sees the continual bombardment 
 proceeding. One might imagine those sparks as 
 saying — " Men may come and men may go, but we 
 go on for ever.*' But we shall see later that the 
 "for ever" can only be used with some poetical 
 license. 
 
 Before inquiring what these radiations of radium 
 consist of, it will be of intei'est to note another pecu- 
 liar property of this recently discovered element. 
 
 If any object in a room stands at a higher tem- 
 perature than the surrounding air, we know that the 
 body has been heated by some artificial means. If 
 we find it gradually cooling, we know that the source 
 of heat has been withdrawn, but if we find it main- 
 taining constantly a temperature above that of its 
 surroundings, we know that it is in connection with 
 s^ome source of heat. In other words, we know that 
 energy is being supplied to it. The household cat 
 feels nice and warm on a cold day, but we know 
 that the heat is maintained only at the expense of 
 chemical energy within the cat's body. If we were so 
 cruel as to starve the cat, and thus stop the chemical 
 
 283 
 
Scientific Ideas of To-day 
 
 actions, we should find the cat's temperature to decrease 
 gradually until the animal died, whereupon its tem- 
 perature would quickly reach the same as that of the 
 surrounding air. Radium appeared to contradict this 
 general rule ; it remains about two degrees hotter than 
 its surroundings. The heat, however, is due to the 
 expenditure of internal energy, as we shall see in the 
 following chapter. 
 
 We have witnessed the photographic effects of 
 uranium salts, and, as we should surmise, the salts 
 of radium are still more active in this direction. Some 
 very clear radiographs have been obtained by the 
 radiations of radium. 
 
 Any one who cares may see the spectrum of sodium 
 in a pocket spectroscope, as one has only to burn 
 a little common salt and view the flame. Few of us 
 need hope to see the spectrum of radium, as it is 
 too precious a substance to ti-eat often in this fashion. 
 The spectrum has been produced, and it is, of course, 
 distinct from that of any other known element. 
 
 I had written down ''The Birth of Radium" as 
 the title of this chapter, but I feared that a casual 
 glance might give one the impression that Radium 
 was born in 1898. However, we shall see some facts 
 regarding the birth of radium in the course of the 
 following chapters. 
 
 284 
 
CHAPTER XXI 
 
 WHAT ARE THE RAYS FROM RADIUM? 
 
 The pioneer investigators — An important discovery — Three dif- 
 ferent kinds of rays — The properties of each — Their nature — 
 A record velocity for material particles — The source of 
 radium's heat — Enormous expulsion of material particles from 
 radium — Infectious radio-activity — A gas given off by radium 
 — A striking experiment—Liquefying the invisible — Disap- 
 pearance of the emanation gas 
 
 In the preceding chapter we have become familiar with 
 the properties of radium, but it will be of special in- 
 terest to see of what the rays of radium really consist. 
 We have seen that the uranium rays appeared to be 
 very similar to the X-rays, but with the more active 
 radiations from radium it became much easier to test 
 the nature of the rays. 
 
 Up to this point we have been dealing with the work 
 of French scientists, almost exclusively, and to them 
 belongs the honour of this great discovery of radio- 
 activity. But now we come to the unravelling of the 
 mystery, and in this we are proud to think that our 
 own countrymen have taken a very active part. Pro- 
 fessor Rutherford and Mr. Soddy at Montreal, and Sir 
 William Ramsay, and again Mr. Soddy in London, 
 were among the pioneers who investigated the nature 
 of radio-activity, 
 
 285 
 
Scientific Ideas of To-day 
 
 At the outset a very important discovery was made 
 by Rutherford. He found that there were three dis- 
 tinctly different radiations given off simultaneously, 
 and he labelled these with the three first letters of the 
 Greek alphabet — Alpha (a), Beta (^), and Gamma (7). 
 He found the alpha rays had only a very small pene- 
 trating po\ver, and could be stopped by a sheet of 
 notepaper. And while the beta rays could penetrate 
 a thin sheet of aluminium, the gamma rays required a 
 fairly thick sheet of steel or lead to stop them. Look- 
 ing at this one property of penetration alone, we could 
 form a very fair idea as to the natui'e of these three 
 different kinds of rays. 
 
 To begin with the gamma rays, we should guess that 
 these must be Rontgen rays from their intensely pene- 
 trating power. Then, remembering Professor Lenard's 
 experiment with the aluminium window through which 
 the cathode particles escaped, we should say that the beta 
 rays were the same well-known electrons, as they suc- 
 ceeded also in penetrating a thin sheet of aluminium 
 and yet are stopped by a sheet of metal which the 
 Rontgen rays can penetrate. We have only the alpha 
 rays left for consideration, and we should guess that 
 these must be invisible atoms of matter, as they fail 
 even to penetrate a sheet of notepaper. 
 
 If we had arrived at the foregoing conclusions we 
 
 should find that we had no occasion to alter our ideas. 
 
 These are confirmed by other investigators, and there 
 
 remains little doubt regarding the nature of these three 
 
 radiations. 
 
 286 
 
what are the Rays from Radium? 
 
 If the gamma rays are really Rontgen rays, they 
 should affect a photographic plate after passing through 
 objects which are transparent to X-rays ; experiment 
 proves that they do. They should also discharge an 
 electrified body as the X-rays do, and this condition 
 they fulfil also. If the gamma rays are X-rays they 
 should not be deflected by any magnetic field, as we 
 have failed to deflect X-rays ; this test is also confir- 
 mative. Hence we feel quite confident that the gamma 
 rays emitted by radium are really the well-known 
 Rontgen rays. But our previous knowledge of X-rays 
 has been that they were caused by the sudden stoppage 
 of flying electrons. Theory informs us that X-rays 
 should be produced also by the sudden starting of 
 electrons ; our difficulty in practice is that we cannot 
 start them with sufficient suddenness to make a splash 
 in the aether. But if the beta rays are really electrons, 
 and if they start with sufficient suddenness, we can 
 account for the presence of X-rays. We shall see that 
 these two ''ifs" disappear, in the following paragraph 
 concerning the beta rays. 
 
 We have seen that the beta rays behave like flying 
 electrons in being able to pass through a sheet of 
 aluminium foil. Electrons are charges of negative 
 electricity, and we have seen in an earlier chapter that 
 they are deflected easily by a magnetic field. The 
 beta rays stand this test, and they also prove themselves 
 to be negatively charged particles. Their speed has 
 been calculated from their magnetic deflection, and it 
 is found that some of them are travelling at enormous 
 
 287 
 
Scientific Ideas of To-day 
 
 speeds ; about one hundred thousand miles per second. 
 We therefore feel justified in saying that these elec- 
 trons are expelled with sufficient suddenness to produce 
 the gamma or Rontgen rays. As the velocity of beta 
 rays is considerably greater than that of electrons 
 within a vacuum tube, we are not surprised to learn 
 that the beta rays can penetrate a greater thickness of 
 aluminium than the Lenard rays can. Many other 
 tests have been applied to the beta radiations, and 
 there is left no shadow of doubt as to their identity 
 with the electrons of which we have seen so much in 
 the preceding chapters. 
 
 How will it be possible to test the nature of the 
 alpha rays ? We have surmised that they are atoms 
 of matter as they are imprisoned by a sheet of note- 
 paper, and we are fortunate in finding that they are 
 deflected by a magnetic field. They are deflected in 
 the opposite direction from the electrons, and we 
 know from this fact that they must be oppositely 
 electrified, or, in other words, that they must carry a 
 charge of positive electricity. Their positive charge 
 is easily demonstrated by enclosing the radium in a 
 metal box, from which these alpha particles cannot 
 escape. We find that the inside surface of the box 
 becomes charged with positive electricity, while the 
 negative electrons have escaped through the box, 
 and may be traced outside. As already explained, 
 these electrons have a greater penetrative power 
 than the electrons within a vacuum tube. Refer- 
 ring again to the alpha particles, we shall see later 
 
 288 
 
what are the Rays from Radium? 
 
 that these are really atoms of a very light gas called 
 helium. 
 
 These alpha particles are expelled at a velocity of 
 twenty thousand miles per second, which beats all 
 previous records for particles of matter. Indeed there 
 is no comparison between this and the fastest moving 
 body we can think of. Of course the race is a very 
 short one, for they are soon picked up by the mole- 
 cules of the gaseous mixture which we call the 
 atmosphere. 
 
 The prodigious speed of those helium atoms is quite 
 sufficient to account for the energetic bombardment 
 witnessed in the spinthariscope, as described in the 
 preceding chapter. It must be remembered that these 
 atoms are miniature in the extreme. Look at the full 
 stop at the end of the preceding sentence and try to 
 imagine a regiment of atoms standing shoulder to 
 shoulder right across the diameter of that dot. There 
 would be no less than five million helium atoms re- 
 quired to form that tiny line. The picture is beyond 
 our powers of imagination. 
 
 The remarkable property, which radium possesses, of 
 keeping up a constant temperature greater than that of 
 the surrounding air, is accounted for by the alpha 
 particles, alias the helium atoms. Picture a gramme 
 of radium salts, which is about as much as you could 
 conveniently heap up on a threepenny piece. From 
 this quantity there are expelled no less than one 
 hundred thousand million helium atoms per second. 
 It is difficult to realise what this means, but if the 
 
 289 T 
 
Scientific Ideas of To-day 
 
 helium atoms expelled in one single second could be 
 divided equally among the population of the whole 
 world, we should each receive about fifty thousand. 
 At the end of the first minute each person in the world 
 would possess about three million helium atoms, and 
 the total possession of each person at the end of the 
 first day would amount to thousands of millions. 
 Think of all this vast amount of helium atoms being 
 expelled from less than half a teaspoonful of radium 
 salts in a single day. And yet this discharge of 
 material atoms goes on incessantly year in and year 
 out for many centuries. The energy earned by these 
 flying atoms accounts for ninety-nine per cent, of the 
 total energy emitted by radium. The temperature of 
 radium is due to some of those atoms bombarding the 
 radium itself in their attempts to escape into the air. 
 
 From the foregoing paragraph it will be seen that 
 the flying electrons (beta rays) and the Rontgen rays 
 (gamma rays) only represent about one per cent, of the 
 energy emitted by radium. But both of these radia- 
 tions will affect a photographic plate more actively 
 than the helium atoms (alpha rays). Both the beta 
 and gamma rays will discharge an electrified body, 
 and will cause a phosphorescent screen to become 
 luminous, but the splashes of light seen in the spin- 
 thariscope are due to the alpha rays or helium atoms. 
 
 At first, one is apt to get a little confused about the 
 
 three different kinds of radiation. I find that the best 
 
 way to fix their natures in one's mind is to take them 
 
 alphabetically — alpha, beta, gamma — ^and then think 
 
 290 
 
what are the Rays from Radium? 
 
 of them as becoming less and less materialistic. We 
 set off with atoms of matter, then electrons, and last of 
 all aether disturbances known as X-rays. This enables 
 one to remember the different properties of the three 
 radiations from radium. 
 
 The Curies soon observed that radio-activity was an 
 infectious thing. Any substance remaining in the 
 neighbourhood of radium became radio-active also, 
 but not permanently so. These acquired radio-active 
 effects last for several hours, and in some cases for 
 days. It was observed that as soon as the presence 
 of the radium was withdrawn, the infected substance 
 began to lose its acquired properties. It is interesting 
 to note that the observer himself became radio-active, 
 and his presence was sufficient to discharge electrified 
 bodies, and to effect his electrical measuring instru- 
 ments. He might try to wash his hands of this 
 acquired property, in a literal sense, but without 
 success. Professor Curie had reason to be thankful 
 that this acquired property was not permanent ; other- 
 wise his further experimenting, with delicate electrical 
 measuring apparatus, would have been impossible. 
 
 At first this acquired radio-activity could not be 
 understood, but further experiments shed an interest- 
 ing light upon this phenomenon. It was found that 
 if the radium salts were dissolved, or if they were 
 heated, their radio-activity was much more infectious. 
 A neighbouring body became radio-active even when 
 sheltered from all the three kinds of radiations already 
 mentioned. 
 
 291 
 
Scientific Ideas of To-day 
 
 A very simple experiment proved that the acquired 
 radio-activity was not due to the radiations of radium. 
 A solution of radium salts was placed in a glass bulb, 
 while some phosphorescent substance was placed in 
 a second bulb. The two glass bulbs were then con- 
 nected by means of a bent glass tube, so that none 
 of the rays could possibly get from the radium bulb 
 to the phosphorescent bulb, for none of the radiations 
 can turn a corner. A stopcock was placed in the 
 connecting tube, so that nothing could pass from 
 the one bulb to the other as long as it was closed. 
 When the apparatus was taken into the dark, nothing 
 was seen until the stopcock was opened, whereupon 
 the phosphorescent substance became luminous. This 
 indicated that some radio-active gas had travelled 
 from the radium salts to the second bulb. Ruther- 
 ford had found previously that a radio-active gas 
 was given off by another radio-active element called 
 thorium, but in that case the gas or emanation was very 
 short-lived, disappearing in a few minutes. In the 
 case of radium, it was found that the emanation re- 
 mained radio-active for some weeks.^ 
 
 It is interesting to be in the dark and to watch 
 this emanation gas being drawn through a very long 
 tube of phosphorescent glass. As the gas passes along 
 the tube, the glass becomes luminous. In this way 
 one witnesses the actual passage of the emanation 
 from the radium solution to a distant receiver. 
 Special interest is added if the receiving bulb, which 
 
 ' The emanation gas must not be confused with the rays from radium, 
 
 292 
 
What are the Rays from Radium? 
 
 is also phosphorescent, is placed in liquid air. It is 
 quite apparent that the emanation gas liquefies when 
 it reaches this intensely low temperature. We cannot 
 pour out the liquid emanation as we can pour out 
 liquid air. Indeed there is no liquid to be seen, as 
 the quantity is so very small. Nevertheless we know 
 that the emanation liquefies, for instead of the gas 
 filling the bulb, we see from its phosphorescent effect 
 that it accumulates at the bottom of the receiving 
 bulb. 
 
 It is this emanation which makes its way to bodies 
 placed in the neighbourhood of radium, and deposits 
 some volatile solid upon them, making them tem- 
 porarily radio-active. If the emanation is kept in a 
 sealed tube, its radio-activity disappears in a few 
 weeks. 
 
 There is a great deal of further interest connected 
 with the emanation and the radiations of radium, 
 but I fear to trouble the general reader with too 
 much detail. There are some points, however, that 
 will help us to answer the question which stands as 
 the title of the following chapter, *' Is the world going 
 to pieces ?" 
 
 293 
 
Is the World going to Pieces? 
 
 few weeks. It is true that he can evaporate the solu- 
 tion and recover his radium salts, but these have only 
 one-quarter of their previous radio-activity. Needless 
 to say, the owner of the radium would not be so seem- 
 ingly rash unless he were confident that the radium 
 would regain its lost properties as quickly as the 
 emanation, which he has abstracted, loses its activity. 
 
 Being convinced that the radium we treasure to-day 
 will cease to exist as radium in a few thousand years, 
 it is evident that the radium we now possess cannot 
 have existed many thousands of years. Roughly speak- 
 ing, we might say that the length of radium's life is 
 between two and three thousand years. From whence, 
 then, does it come ? 
 
 If the "man in the moon " dropped down upon this 
 planet, and we handed him a bright red apple, he 
 might think that it had existed always in the condition 
 in which he saw it. But when he found it beginning 
 to decay and waste away, he would probably con- 
 jecture that it was only a temporary form of matter. 
 If he had dropped down into one of our cities where 
 he could only see apples collected together in barrels, 
 their origin might remain a mystery to him. If, how- 
 ever, in his subsequent walks abroad he found those 
 apples upon certain trees and nowhere else, he would 
 feel confident that he had discovered the parent of the 
 apple. Where, then, do we find radium in Nature ? 
 
 Not only do we always find radium in those minerals 
 which are richest in uranium, but there is always a 
 definite relation between the quantity of uranium and 
 
 295 
 
Scientific Ideas of To-day 
 
 the quantity of radium in every kind of pitchblende. 
 There seems to be no doubt that uranium is the parent 
 of radium. 
 
 In tracing the genealogic tree downwards from 
 uranium, which is the heaviest element, we find some 
 interesting facts. We have already spoken of helium 
 as the offspring of radium, but here the analogy is not 
 perfect, for these helium atoms are given off also by 
 uranium on its own account, and after radium has 
 given birth to the emanation, we find this descendant 
 giving forth the same helium atoms also. Indeed we 
 find that these alpha particles, or helium atoms, are 
 being produced at no less than seven stages in the 
 genealogic tree. 
 
 If we desire to consider the subject in detail we find 
 that uranium is not the direct parent of radium, there 
 being two stages between uranium and radium, so 
 that the former is really the great-grandparent of the 
 latter. Radium is followed directly by the emanation, 
 and that again by at least eight other stages. The 
 pioneer investigators are inclined to think that the 
 ultimate product will turn out to be the well-known 
 element lead. 
 
 If the uranium atoms are breaking down and form- 
 ing both radium atoms and helium atoms, it stands to 
 reason that each of these atoms must be lighter than 
 the uranium atom. The atomic weight of uranium is 
 238, while that of radium is 225, and that of helium 
 only 4. In the same way we should expect the pro- 
 ducts of radium to be of less atomic weight than 
 
 296 
 
Is the World going to Pieces? 
 
 radium. Therefore if lead is a possible product it 
 must be lighter than radium, and so it is ; radium's 
 atomic weight being 225, and that of lead 207, 
 
 Without going into further detail, it must be quite 
 evident that some of the elements are going to pieces, 
 each product being "more beautifully less" than its 
 parent. 
 
 Consider where all the energy of radium comes from. 
 One cannot get energy from nothing, although the 
 perpetual-motion optimist is slow to realise this. I 
 have heard people say that Nature will give more than 
 she demands in return, and they have quoted the 
 ordinary lever as a witness. But a moment's con- 
 sideration should dispel any idea of steaHng energy 
 by means of a lever. It is true that a man may move 
 a heavy stone with the aid of a lever, while he could 
 not move it by applying all his energies directly to the 
 stone. But a man may also carry a cart-load of coals 
 to the top of a building by taking one hundredweight 
 at a time, although he could not lift the ton of coals in 
 one lot. Observe that the man with the lever has to 
 move the free end of it through a great distance to 
 produce a very small movement of the stone. The 
 theory of the conservation of energy tells us that 
 Nature will only deal on a strictly business-like basis. 
 We must give her an equivalent for that which she 
 gives us. 
 
 We have seen that radium is constantly radiating 
 away an extraordinary amount of energy. Where does 
 it obtain this seemingly endless store of energy ? Most 
 
 297 
 
Scientific Ideas of To-day 
 
 assuredly it ,is self-contained, and our ideas of to-day, 
 concerning the construction of the atom, indicate that 
 it is the internal energy of the atom which is being 
 drawn upon. The rapidly revolving electrons which 
 compose the atom are being freed from their long 
 abode; some electrons escaping free, while others 
 congregate together to form atoms of lighter atomic 
 weight. 
 
 While the idea that lead is the ultimate product of 
 radium is merely hypothetical, the idea that helium 
 is a product of radium has received the solid support 
 of experimental proof. It will be of interest to see 
 how this proof has been obtained. 
 
 One does not hear of helium except in connection 
 with science ; indeed we were not aware till recent 
 times that it existed on this planet. More than a 
 generation ago Sir Norman Lockyer was examining 
 the extensive wireless message furnished in the spectro- 
 scope by the Sun when he came upon a spectral line 
 which had not been deciphered. This spectral line 
 which attracted his attention was near one of the 
 sodium Hues in the yellow section, and Sir Norman 
 observed that this particular line did not belong to 
 any known spectrum. Here was an element, in the 
 Sun, not known on the Earth, so Lockyer christened 
 the element Helium, from the Greek word helios, the 
 sun. This new element was found in other stars also, 
 and it is interesting to note that it is found only in 
 the hottest stars. We should therefore expect helium 
 
 to be a very light element, as we believe the lightest 
 
 298 
 
Is the World going to Pieces? 
 
 elements to be the first in the order of evolution which 
 occurs as the stars cool. Of course Lockyer could 
 not tell its atomic weight as long as the element 
 remained at so great a distance, but when iti was 
 found later to exist upon this planet its atomic weight 
 was determined, and it was found to be the second 
 lightest element known, the record being held by 
 hydrogen. 
 
 Before the days of radium, Sir William Ramsay, of 
 London, was examining the spectra of gases contained 
 in a certain variety of pitchblende known as clevite, 
 when he discovered the same spectral line which 
 Sir Norman Lockyer had found in the Sun and stars 
 some twenty-five years earlier. Helium was unearthed 
 upon this planet and observed here for the first time 
 in 1895. The discovery is a remarkable one, because 
 the quantity of helium gas contained in pitchblende is 
 so very, very minute. In a former chapter I remarked 
 that one advantage of being able to produce spectra by 
 means of an electrical discharge was that we could 
 observe the spectral lines of very minute quantities of 
 gas. This was the method which enabled Sir William 
 Ramsay to detect helium. 
 
 After this discovei-y, physicists became familiar with 
 the spectrum of helium, which shows five distinct 
 lines well spread throughout the visible spectrum. It 
 may be remarked that helium is a most independent 
 element, for it will not enter into partnership with any 
 other element ; it is one of the few inert gases which 
 defies all attempts at chemical combination. In addi- 
 
 299 
 
Scientific Ideas of To-day 
 
 tion to this, helium held a record of its own ; until 
 quite recently it resisted the chemists' effoi'ts to liquefy 
 it. The very lowest temperature obtainable, to which 
 all other gases succumb, left helium defiant. 
 
 Our present interest in this refractory gas lies in the 
 suggestion, made by Professor Rutherford and Mr. 
 Soddy, that helium was a product of radio-activity. 
 Its presence in pitchblende would seem to indicate 
 this, but the subject is not left open to speculation. 
 Sir William Ramsay and Mr, Soddy were investigating 
 the spectrum of radium emanation, which was of a 
 temporary character. After some days they observed 
 a few bright lines appearing, and as those became 
 more and more distinct they proclaimed themselves to 
 be a wireless message from some atoms of helium. 
 These atoms were not in the tube when it was sealed, 
 and they could not pass through the glass, so that 
 they must have been born in the tube. Undoubtedly, 
 helium was a product of radium emanation ; these 
 pioneers had actually witnessed the birth of helium in 
 their laboratory. 
 
 In the light of the foregoing discovery, we can 
 
 understand how it is that helium is always to be 
 
 found in radio-active substances. There is not the 
 
 least doubt that real transmutations have taken place 
 
 within pitchblende. Uranium atoms have broken down 
 
 and formed radium atoms, while the radium atoms 
 
 have become unstable and given place to helium atoms. 
 
 I purposely omit the emanation atoms, as they have 
 
 so short a life. 
 
 300 
 
Is the World going to Pieces? 
 
 What would the alchemists of the Middle Ages say, 
 if they could return to their old habitation to-day 
 and learn that actual transmutations do take place in 
 Nature ? It is evident that the American swindlers 
 who professed recently that they had transmuted silver 
 into gold wei-e not scientists. As Nature steps from 
 one transmutation to another, it is always from the 
 heavier to the lighter atom ; the atomic weights of 
 uranium, radium, and lead being respectively 238, 
 225, and 207. These charlatan alchemists claimed to 
 have transmuted silver (107) into gold (197). 
 
 In considering the population of a country we have 
 to take into account the birth-rate, the death-rate, and 
 the average age. If we desire to take a census of 
 radio-active elements we must go upon a similar plan. 
 From the death-rate, or rate of disintegration, of ur- 
 anium, we find that it has a very much longer life 
 than radium ; probably somewhere about sixteen 
 million years. Hence uranium is much more plentiful 
 than radium, but what the latter lacks in length of 
 life it makes up for in activity. A shorter life, but a 
 merry one. The same holds good of radium and its 
 emanation. The latter has a very short life compared 
 to that of radium, and it is much more radio-active 
 than the radium from which it has been obtained. 
 
 It is quite evident that highly radio-active substances 
 
 must be rare. The novelist of the future need not 
 
 picture his hero becoming a millionaire through his 
 
 finding a mine of radium, from which he can draw 
 
 illimitable stores of energy. It is apparent that there 
 
 301 
 
Scientific Ideas of To-day 
 
 would not be much of the hero left to enjoy his 
 impossible find, for we know that even very small 
 quantities of radium will produce serious physiological 
 effects upon the human body. The late Professor 
 Curie is reported to have said that he would not enter 
 a room with a kilogram — about two and a quarter 
 pounds — of pure radium, because it would doubtless 
 destroy his eyesight, burn all the skin off his body, 
 and probably kill him. 
 
 We set out in this chapter with the analogy of a 
 conjuror producing objects from some hidden store, 
 and we agreed that sooner or later his stock must give 
 out. We see that the same holds good of radium 
 and other radio-active bodies, but that fact does not 
 warrant us to say that the world itself is going to 
 pieces. If we lost all the uranium, radium, and other 
 well-known radio-active bodies, our planet would not 
 be depleted by any means. The scientific world, 
 however, began to search for radio-active properties in 
 ordinary matter, for there was the possibility either 
 that some of those radio-active elements might be 
 distributed throughout Nature, or that ordinary matter 
 might be radio-active of itself. 
 
 Water from the mineral springs at Bath was found 
 
 to be radio-active. The air of caves and cellars was 
 
 found to possess this new property of matter in quite 
 
 an abnormal degree, and it has been shown that even 
 
 the ordinary atmosphere is very slightly radio-active. 
 
 A Cambridge scientist found that freshly fallen rain 
 
 was radio-active, and his method of demonstrating this 
 
 302 
 
Is the World going to Pieces? 
 
 was very simple. Taking some freshly fallen rain in 
 a small platinum vessel, he evaporated the water very 
 rapidly by heating it. When he tested this vessel, 
 containing an invisible residue, he found that it would 
 discharge an electroscope, and it was clear that this 
 was due to some radio-active property which dis- 
 appeared in a few hours. Ordinary tap water, or rain 
 which had fallen some time previously, when treated 
 in exactly the same way, showed no such signs of 
 radio-activity, although air bubbled through some tap 
 waters has been found to be radio-active. Many of 
 the ordinary metals, and also glass, have been found 
 to possess radio-active properties. These facts seem 
 to indicate that radio-activity is a universal property 
 of matter, but this question cannot be definitely settled 
 at present. 
 
 The man who embezzles money in very large sums 
 is much more easily detected than the man who steals 
 little by little over a long interval of time ; the latter 
 may, unfortunately, escape detection altogether. 
 
 We have seen that electrons are continuously 
 expelled from red-hot wires, candle flames, and 
 from all bodies in the act of burning. These 
 electrons must, like the conjuror's productions, come 
 from somewhere, but these electrons, no doubt, are 
 some of the detachable ones forced away from their 
 atoms. 
 
 It would appear that chemical reactions also 
 give rise to a real dissociation of matter on a 
 very small scale. Dr. Gustave Le Bon, of Belgium, 
 
 303 
 
Scientijfic Ideas of To-day 
 
 maintains that he has definitely proved this by experi- 
 ment. 
 
 It is quite possible that all matter is radio-active, 
 although we cannot detect the shortage ; indeed it does 
 seem probable that the world is very, very gradually 
 going to pieces. 
 
 304 
 
CHAPTER XXIII 
 
 THE CAUSE OF RADIO-ACTIVITY 
 
 Atoms of unstable configurations — How to express the life of a 
 quantity of radium — A change beyond our reach — The sensi- 
 tiveness of the spectroscope — Where the internal energy of the 
 atom came from originally 
 
 When we were considering the construction of the 
 atom, we saw how the electrons would form definite 
 configurations according to the number of electrons 
 contained in the atom. The illustrations forming the 
 frontispiece assisted us in forming a definite mental 
 picture of the atom. 
 
 Professor J. J. Thomson has shown that certain con- 
 figurations would be unstable and would be liable to 
 break down. The atoms of radio-active elements 
 come under this category. If all the atoms composing 
 a specimen of radium were to break down simul- 
 taneously there would be a sudden disappearance of 
 the radium. But if only one atom in every ten 
 thousand million breaks down in each second, the 
 total disintegration will occupy some time. And as 
 there are about one thousand million million million 
 atoms in every gramme (15^ grains) of radium, it is 
 evident that there is plenty of material available for a 
 very long display. If we divide the total atoms by the 
 
 305 u 
 
Scientific Ideas of To-day 
 
 number disintegrating po- second, we find that the 
 gramroe of indium should last about three thousand 
 years. This is oniy a very rough way of expressing 
 the facts, for, as the radium's volimie decreases, it will 
 not lose the same quantity- each year. The more it 
 disintegrates the more slG':vIy will the remainder break 
 do^ATi. I: is because of this law that it is more con- 
 venient to say that radium will disint^rate half its 
 atoms in about thirteen hundred years. 
 
 In accordance with the same law we find that 
 although the emanation of radium takes a few weeks 
 to completely break down, yet one-half of it has dis- 
 appeared at the end of the first four days. Dealing 
 with uranium ir. li^.e same way, we say that half its 
 quantity would disappear at the end of six hundred 
 million years. 
 
 It is interesting to note that these \'arious rates of 
 decay, or disintegration, are constant and can neither 
 be hastened or retarded by man. It would not be 
 wise to be dogmatic and say that man will never be 
 able to hasten the natural rate of disintegration of 
 radio-active bodies. Who could have believed, a 
 century* ago, that we could ever get particles smaller 
 than atoms to transmit enormous quantities of energy 
 from place to place ? And yet that is what really 
 happens when we send electric power along a stationary 
 wire. WTio could have beUeved that we should ever 
 get these in\asible particles to carry our speech to 
 distant towTis, and to bring us immediate intelligence 
 
 of what is happening in all parts of the civilised world ? 
 
 306 
 
The Electroscopk tn Use 
 
 An insulated metal rod with two pieces of gold leaf attached to its lower end is 
 protected by a glass jar. When any electrified body is brought near the metal disc 
 attached to the upper end of the rod, the gold leaves become electrically charged and 
 repel each other, as shown in iht; lower photograph. The presence of radium may be 
 delected by an electroscope which has been charged previously, as explained in the 
 text. 
 
The Cause of Radio-Activity 
 
 With regard to radio-active substances, all that we 
 do say is that to-day we cannot affect the radio-active 
 changes taking place in nature. We may heat the 
 substance to the highest temperature possible, or 
 subject it to the lowest temperature obtainable, but 
 the same constant rate of change goes on. 
 
 The illustration facing page 228 helps us to appre- 
 ciate a comparison between the delicacy of the 
 chemical balance and the spectroscope. We marvel 
 that the spectroscope can detect one-millionth part 
 of a milligram of matter. What shall we say when 
 we are told that the electroscope is a million times 
 more sensitive than the spectroscope ? (See illustra- 
 tration facing page 306.)' 
 
 In an earlier chapter we tried to picture one four- 
 millionth part of the lead which was rubbed off the 
 pencil point when writing the single word shown in 
 the illustration. And we found that the spectroscope 
 could detect such an infinitesimal speck of matter. 
 We learn now that even one millionth part of this 
 already infinitely small quantity could be detected by 
 the electroscope, if the matter was as radio-active as 
 radium. Such divisions of a speck of an invisible 
 speck of matter are far beyond our powers of imagina- 
 tion. What then shall we say of molecules, atoms, 
 and electrons, contained within such an invisible 
 speck ? 
 
 With the chemical balance we detect matter by the 
 pull which the Earth exerts upon it. With the spectro- 
 scope we detect matter by the aether waves which 
 
 307 
 
Scientific Ideas of To-day 
 
 its revolving electrons emit. With the electroscope 
 we detect radio-active matter by its power to ionise 
 the air and thus carry away the electric charge 
 previously given to the instrument. But for this very 
 delicate electric detector, we should never have been 
 aware of the presence of some of the radio-active 
 elements known to us to-day. 
 
 Undoubtedly, the cause of radio-activity is the dis- 
 integration of the atom. It is because the atoms are 
 going to pieces, forming lighter atoms, and in so 
 doing discharging electrons, that we get the well- 
 known phenomena of radio-activity. 
 
 We know that the outward display of energy is due 
 to the internal energy of the atom, but whence came 
 that energy ? The late Lord Kelvin said, in a letter 
 already referred to, that the energy of radium was 
 "no doubt due originally to the excessively high 
 temperatures which have been and are being produced 
 in the Universe." But does it not seem unnecessary 
 to single out radium in this respect ? There can be 
 little doubt that the internal energy of all atoms has 
 been derived originally from the temperatures prevail- 
 ing while the electrons congregated together to form 
 the atoms. Indeed stellar chemistry seems to indicate 
 clearly that the lighter elements were formed first in 
 the hottest stars, and that the heavier atoms only 
 appeared at lower temperatures. We know that 
 all the radio-active elements have the heaviest 
 atoms known. I can imagine some one saying 
 that this would suggest that the lightest atoms 
 
 308 
 
The Cause of Radio-Activity 
 
 will contain the greatest internal energy, but we 
 must remember that the lightest atoms contain the 
 fewest electrons. 
 
 It is true that we have no evidence of the internal 
 energy contained in, say, an atom of iron, but that is 
 because the energy is locked up within the atom and 
 is suffering no apparent change such as we see in the 
 radio-active atoms. It is only when there is a change 
 or transformation that we can get any idea of the 
 energy. 
 
 309 
 
CHAPTER XXIV 
 WHAT IS GRAVITATION? 
 
 Newton's connection with gravitation — The anecdote of the fall- 
 ing apple — Newton's idea— The mathematics of his theory 
 break down — A new measurement of the Earth alters matters 
 — A great discovery — Interesting anecdotes concerning New- 
 ton and his calculations — Some medium is necessary for 
 gravitation — Where the electrons may come in 
 
 While the most exacting reader would not insist on 
 every scientific subject being dealt with in a volume 
 of this kind, some might be disappointed if no special 
 notice were taken of such an important subject as the 
 nature of gravitation. 
 
 The name of Sir Isaac Newton is so very intimately 
 connected with the subject of gravitation that not a 
 few people harbour the eironeous idea that Newton 
 was the first to observe the force of gravitation ; in- 
 deed that he discovered it. Needless to say, this is 
 nonsense. Man could not fail to observe this force 
 which is so prominent in our every-day life, and in 
 Newton's time this force had its distinctive name, just 
 as we have it to-day. Other philosophers had made 
 an earnest study of gravitation before Newton's day, 
 but it was Newton who discovered the laws of gravita- 
 tion and applied them to the whole universe. 
 
 Previous to Newton others had suggested that the 
 310 
 
What is Gravitation? 
 
 Sun attracted the Earth and the other planets, but it 
 was Newton who proved that the attractive power was 
 the very same gravitational force which we see at work 
 all around us on the surface of this planet, 
 
 I remember when a boy being one of the members 
 of an informal debating society composed entirely of 
 boys. At one meeting a member read an essay on 
 '* Newton's discovery of gi-avitation/' in which the 
 apple falling from the tree played a prominent part. 
 When I learned later that Newton did not discover 
 gravitation I naturally let the story of the apple go 
 also. Indeed in quite recent years many authors treat 
 this anecdote as mythical, but it is interesting to note 
 that its truth is vouched for by no less an authority 
 than Voltaire, who had the incident confirmed by 
 Newton's niece, who had lived with the great scientist. 
 Indeed the apple tree stood for a century and a half, 
 and was known in the early years of the last century, 
 when it was blown down in 1820. 
 
 To appreciate the anecdote of the falling apple we 
 must remember that up till this time (1665) no one 
 had ever connected the force attracting the planets to 
 the Sun with the force known as gravitation. Gravi- 
 tation was looked upon as a local force operating at 
 the surface of the Earth. The idea that this force 
 might extend out into space for millions of miles 
 would have seemed highly absurd. Philosophers had 
 provided the planets with ethers in which they might 
 swim around the Sun. 
 
 Doubtless Newton had puzzled many times as to 
 311 
 
Scientific Ideas of To-day 
 
 the nature of the force which he had discovered to 
 exist between the Sun and the planets. Very probably 
 he was thinking of this problem when at the age of 
 twenty-three he was sitting in his garden. An apple 
 fell from a tree, but Newton had seen apples fall 
 before. However, it suddenly occurred to him 
 that it might be this very same force which attracted 
 the moon to us and kept her faUing round and 
 round the Earth. He lost no time in calculating 
 whether or not the force of gravitation could account 
 for the pull of the Earth upon the moon. He was 
 very sorely disappointed when his figures proved that 
 this force was not sufficient to account for the amount 
 of fall of the moon per second. Instead of working 
 out to sixteen feet per second, the answer came out 
 about fourteen feet per second. Newton was a great 
 mathematician, his calculations were correct, so he 
 had to dismiss the thought of gravitation being the 
 force at work. Indeed he never mentioned his idea 
 to any person at that time, but put his calculations 
 aside. 
 
 Sixteen years later Newton returned to this subject, 
 being convinced that his former idea must be 
 correct. He also heard of a new and very exact 
 measurement of the Earth having been obtained by 
 Picard, of Paris, which proved that the Earth was 
 considerably larger than all previous determinations 
 had made it out to be. This would naturally alter 
 Newton's former calculations. If the Earth were 
 larger, the attractive force would be greater, the moon 
 
 312 
 
what is Gravitation? 
 
 would fall more feet per second. Newton lost no 
 time in repeating his former calculations with this 
 new data, and he very quickly saw that the figures 
 were going to work out correctly this time. The full 
 meaning of the discovery dawned on him; he was 
 too excited to complete the calculations himself. His 
 original theory had been right after all. This one 
 man had discovered the Creator's plan in the universe ; 
 common gravitation governed the attraction of all the 
 heavenly bodies. We cannot over-estimate the great 
 importance of Newton's discoveiy. 
 
 Newton worked out all the mathematics of the 
 subject so very fully that he left no problem for future 
 generations except to discover the nature of the force. 
 Two centuries and more have gone past and the 
 problem is still unsolved. 
 
 There are many interesting anecdotes related by 
 Newton's friends which go to show how his mind 
 was absorbed by the mathematical problems connected 
 with the subject of gravitation. He would rise in the 
 morning, but before he was half-dressed he would 
 begin his calculations and remain occupied in this way 
 till well on in the day. He was quite oblivious of the 
 fact that he was omitting to take his meals. There is 
 a good story told by one of his friends, who called 
 to see Newton one day when he was busy in his 
 study. Dinner-time came but the great man still 
 worked on in his study. At last the friend sat down 
 and ate the dinner prepared for Newton, and when 
 the great philosopher appeared some time later he 
 
 313 
 
what is Gravitation? 
 
 fatal objections to this idea also. However, we feel 
 confident that the all-pei-vading aether is the medium, 
 although no one can suggest the nature of the strain 
 produced in it. We pull a stone away from the Earth 
 and the two bodies react upon each other. 
 
 Before the days of the electron theory of matter, it 
 was suggested that if matter were a rarefaction of 
 the aether, there would be a stress of the aether towards 
 such a partial vacuum. The stress between two such 
 partial cavities would be least in the space between 
 them, so that they would be forced towards one another. 
 If the electrons should turn out to be rarefied aether, 
 then this idea may be developed into a reasonable 
 theory. 
 
 Suppose for a moment that we could imagine the 
 revolving electrons producing a sort of aether vacuum 
 within the atom. The more electrons the greater the 
 vacuum, and therefore the greater would the stress 
 be, impelling the masses of matter to move together. 
 We require very sensitive apparatus to detect the 
 attraction between any two pieces of matter ; the 
 Earth's attraction upon them is so immensely greater. 
 Indeed, all gravitational force is very small ; we only 
 observe it because of the gigantic mass of the Earth. 
 Electrical attraction is millions of times more energetic 
 than gravitation. In the illustrations facing page 54, 
 we see how much more powerful electrical attraction 
 is than gravitation. 
 
 Whatever the nature of gravitation may turn out to 
 be, there is abundant evidence that it is constant. It is 
 
 315 
 
Scientific Ideas of To-day 
 
 in no way affected by any alteration we can make in 
 the conditions of atoms or their electrons. But then 
 we cannot affect the main body of revolving electrons 
 which compose the atom ; they are constant. We 
 may look forward, therefore, to a theory connecting 
 these revolving electrons with the aether, producing a 
 constant stress wherever matter exists. And although 
 a century and a half has passed since Newton's great 
 work was given to the world, we may still hope that 
 the nature of gravitation will be discovered. 
 
 316 
 
CHAPTER XXV 
 
 CONCLUSION 
 
 The studies of science — Complete change of ideas — Our great 
 ignorance — An amusing conversation — Our position to-day — 
 The electron's place in the universe 
 
 Our scientific ideas of to-day are certainly very differ- 
 ent from those of our grandfathers. In the foregoing 
 chapters we have seen something of the immense 
 stride which has been made during the last decade. 
 
 It is curious how very different we have found 
 things to be from what our ancestors imagined them. 
 They believed light and heat to be material things ; 
 we know definitely that these are merely modes of 
 motion in the all-pervading cether. They believed the 
 atoms of matter to be indestructible and eternal, but 
 with the discovery of radio-activity, we have direct 
 evidence that this is not the case. Not so long ago 
 electricity was looked upon as a mode of motion — 
 a form of energy — and now we know that it is a 
 real existing thing, concerning whose particles we 
 have gained a great deal of exact knowledge within the 
 last few years. Just as we think the scientific ideas 
 of our grandfathers to have been very crude and 
 primitive, so may our ideas of to-day appear to some 
 future generation, 
 
 317 
 
Scientific Ideas of To-day 
 
 We are quite conscious that there is a very great 
 deal concerning which we know little or nothing. 
 For instance, we have no idea of the nature of the 
 aether, life, or positive electricity. And in the pre- 
 ceding chapter we saw that the long-standing problem 
 of the nature of gravitation still remains unsolved. 
 These are only a few examples of our ignorance. 
 Fortunately we realise that we have much to learn. 
 
 The first step to knowledge is to know that we are 
 ignorant, — Cecil. 
 
 When the great French savant, Arago, was asked by 
 a distinguished lady a number of puzzling questions 
 he modestly answered, "Madame, I do not know." 
 It seemed strange to the lady that so learned a man 
 should be so ignorant, and when she asked him how 
 it was that he, who was in the forefront of science, 
 did not know these things, he simply replied — 
 " Madame, I do not know." 
 
 Occasionally we meet the man who knows every- 
 thing. To his intimate friend he boasts that he always 
 gives some answer to every question. Needless to 
 say, that man does not possess a true scientific spirit. 
 However, even this man is usually wilHng to acknow- 
 ledge that he does not know what electricity is. A 
 few years ago, when journeying by train, I overheard 
 an amusing conversation between two fellow passengers. 
 They had bpth been brought up in the country, but 
 one had evidently some connection in the city with 
 electrical work. His friend said — "Man, you don't 
 
 318 
 
Conclusion 
 
 know what electricity is/' and I was surprised to hear 
 the accused say that he 'Mid so." His explanation 
 was that electricity was made of sulphuric acid and 
 lead, from which statement it is evident that he had 
 some acquaintance with accumulators. 
 
 Our tme position to-day is this. We find invisible 
 electrons at work all around us. These tiny particles 
 of negative electricity form various configurations ; 
 these are the atoms of matter. The atom is a 
 miniature solar system of revolving electrons. On 
 the outskirts of this revolving system there are 
 satellite electrons which set up waves in the sur- 
 rounding a3ther of space ; these waves we call light 
 and heat. 
 
 Then there are detachable electrons which can pass 
 from one atom to another. A steady locomotion of 
 such electrons along a wire constitutes a continuous 
 electric current, while a surging to and fro of these 
 electrons is described as an alternating electric current. 
 If the to-and-fro motion is rapid enough these 
 electrons set up those "wireless" waves in the aether 
 by which we send intelligence to and from ships far 
 out at sea. 
 
 A sudden expulsion of those detachable electrons 
 from one body to another constitutes an electric dis- 
 charge. These electrons are shot off like bullets from 
 the one object to the other. 
 
 We see how a discharge of electrons from the 
 Sun to the Earth causes aurorae, provides nuclei for 
 the formation of clouds in the upper atmosphere, 
 
 319 
 
Scientific Ideas of To-day 
 
 and accounts for atmospheric electricity with its occa- 
 sional displays of lightning. 
 
 We see that owing to an accumulation of these 
 electrons the Earth has become a negatively charged 
 body. 
 
 We see how the steady motion of electrons (an 
 electric current) produces a certain disturbance in the 
 surrounding aether ; this we describe as a magnetic 
 field. We account for the Earth's magnetism by a 
 flow of electrons within the Earth's crust ; the motion 
 being due to a temperature gradient. 
 
 We see some atoms giving up one or more detach- 
 able electrons which are accepted by other atoms, 
 producing a disturbance in their electric balance, 
 and causing the atoms to ath^act one another and 
 become chemically united. In this way we account 
 for the production of all the variety of compound 
 substances known. 
 
 We find that electrons, from whatever source they 
 are obtained, are always identical. 
 
 Referring to this electron theory, the Right Hon. 
 A. J. Balfour, when president of the British Associa- 
 tion, said — 
 
 "All will, I think, admit that so bold an attempt 
 to unify physical nature, excites feelings of the most 
 acute intellectual gratification. The satisfaction it 
 gives is almost aesthetic in its intensity and quality. 
 We feel the same sort of pleasurable shock as when 
 from the crest of some melancholy pass we first see 
 
 320 
 
Conclusion 
 
 far below us the sudden glories of plain, river, and 
 mountain." 
 
 We see that there is a complete evolution from 
 , electrons to atoms and molecules and through radio- 
 activity back to electrons. We have seen that in 
 this universal evolution, the evolution of man has 
 occupied a very small part of the vast time required 
 for the change from not-living to living matter. The 
 atoms forming our bodies have been in existence since 
 the foundations of the world were laid, and when we 
 have left this planet they will persist in some other 
 forms — 
 
 *' Imperious Caesar, dead, and turned to clay. 
 Might stop a hole to keep the wind away." 
 
 Science only deals with the physical and material. 
 Conclusions such as that just quoted from Shake- 
 speare's Hamlet relate only to the material side of 
 man. True Science does not seek to deprive man 
 of his soul, or to drive the Creator from his Universe, 
 but it honestly endeavours to study His marvellous 
 works. 
 
 321 
 
APPENDIX I 
 
 THE INGREDIENTS OF THE WORLD 
 
 The following tables are based upon the particulars deter- 
 mined by the International Committee of 1906. The first 
 table is arranged alphabetically, as is customary; in the 
 second table I have arranged the elements according to their 
 atomic weights; while the third table places the ingredients 
 in the order of their discovery. 
 
 The Names of the Elements (alphabetically arranged) 
 
 Aluminium 
 
 Antimony 
 
 Argon 
 
 Arsenic 
 
 Barium 
 
 Beryllium {alias Glucinum) 
 
 Bismuth 
 
 Boron 
 
 Bromine 
 
 Cadmium 
 
 Cccsium 
 
 Calcium 
 
 Carbon 
 
 Cerium 
 
 Chlorine 
 
 Chromium 
 
 Cobalt 
 
 Columbium (alias Niobium) 
 
 Copper 
 
 Erbium 
 
 Fluorine 
 
 Gadolinium 
 
 Gallium 
 
 Germanium 
 
 Glucinum (alias Beryllium) 
 
 Gold 
 
 Helium 
 
 Hydrogen 
 
 Indium 
 
 Iodine 
 
 Iridium 
 
 Iron 
 
 Krypton 
 
 Lanthanum 
 
 Lead 
 
 Lithium 
 
 Magnesium 
 
 Manganese 
 
 Mercury 
 
 Molybdenum 
 
 322 
 
Appendix 
 
 Neodymium 
 Neon 
 
 Silver 
 Sodium 
 
 Nickel 
 
 Strontium 
 
 Niobium {alias Columbium) 
 
 Nitrogen 
 
 Osmium 
 
 Sulphur 
 
 Tantallum 
 
 Tellurium 
 
 Oxygen 
 Palladium 
 
 Terbium 
 Thallium 
 
 Phosphorus 
 
 Platinum 
 
 Thorium 
 Thulium 
 
 Potassium 
 
 Tin 
 
 Praseodymium 
 Radium 
 
 Titanium 
 Tungsten 
 
 Rhodium 
 
 Uranium 
 
 Rubidium 
 
 Vanadium 
 
 Ruthenium 
 
 Xenon 
 
 Samarium 
 
 Ytterbium 
 
 Scandium 
 
 Yttrium 
 
 Selenium 
 
 Zinc 
 
 Silicon 
 
 Zirconium 
 
 It will be observed that the elements which we find occur- 
 ing in radio-active changes — such as the emanation gas — are 
 not included in this list, for they are known to us by their 
 radio-active properties alone. 
 
 The Elements in Order of their Atomic Weights 
 
 Hydrogen 
 
 
 
 1.008 
 
 Helium 
 
 
 
 4.00 
 
 Lithium 
 
 
 
 7-03 
 
 Glucinum 
 
 
 
 9-1 
 
 Boron 
 
 
 
 II.O 
 
 Carbon 
 
 
 
 12.0 
 
 Nitrogen 
 
 
 
 . 14.04 
 
 Oxygen 
 Fluorine 
 
 
 
 16.0 
 19.0 
 
 Neon . 
 
 
 20.0 
 
 Sodium 
 
 
 
 23.05 
 
 Magnesium 
 
 
 
 24.36 
 
 Aluminium 
 
 
 
 27.1 
 
 Silicon . 
 
 
 
 28.4 
 
 Phosphorus 
 
 
 
 31.0 
 
 Sulphur 
 
 
 
 32.06 
 
 Chlorine 
 
 
 
 35-45 
 
 Potassium 
 Argon . 
 
 
 
 39.15 
 
 39-9 
 
 Calcium 
 
 
 
 40.1 
 
 Scandium 
 
 
 
 44.1 
 
 Titanium 
 
 
 
 48.1 
 
 323 
 
Scientific Ideas of To-day 
 
 Vanadium . 
 
 51.2 
 
 Iodine 
 
 126.85 
 
 Chromium . 
 
 52.1 
 
 Tellurium . 
 
 127.6 
 
 Manganese . 
 
 55.0 
 
 Xenon 
 
 128.0 
 
 Iron . . . . 
 
 55-9 
 
 Calcium 
 
 132.9 
 
 Nickel 
 
 58.7 
 
 Barium 
 
 137-4 
 
 Cobalt 
 
 59.0 
 
 Lanthanum 
 
 138.9 
 
 Copper 
 
 63.6 
 
 Praseodymium , 
 
 140.5 
 
 Zinc . 
 
 65.4 
 
 Cerium 
 
 140.25 
 
 Gallium 
 
 70.0 
 
 Neodymium 
 
 143-6 
 
 Germanium 
 
 72.5 
 
 Samarium . 
 
 150.0 
 
 Arsenic 
 
 75.0 
 
 Gadolinium 
 
 156.0 
 
 Selenium . 
 
 79.2 
 
 Terbium 
 
 160.0 
 
 Bromine 
 
 79.96 
 
 Erbium 
 
 166.0 
 
 Krypton 
 
 81.8 
 
 Thulium . 
 
 171.0 
 
 Rubidium . 
 
 , 854 
 
 Ytterbium . 
 
 173-0 
 
 Strontium . 
 
 87.6 
 
 Tantalum . 
 
 183.0 
 
 Yttrium 
 
 89.0 
 
 Tungsten . 
 
 184.0 
 
 Zirconium . 
 
 90.6 
 
 Osmium 
 
 191.0 
 
 Columbium 
 
 94.0 
 
 Iridium 
 
 193.0 
 
 Molybdenum 
 
 96.0 
 
 Platinum . 
 
 . 194.8 
 
 Ruthenium 
 
 101.7 
 
 Gold . 
 
 • 197-2 
 
 Rhodium . 
 
 103.0 
 
 Mercury 
 
 200.0 
 
 Palladium . 
 
 106.5 
 
 Thallium . 
 
 204. 1 
 
 Silver . 
 
 • 107.93 
 
 Lead . 
 
 206.9 
 
 Cadmium . 
 
 1 12.4 
 
 Bismuth . 
 
 . 208.5 
 
 Indium 
 
 1 14.0 
 
 Radium 
 
 225.0 
 
 Tin . . . 
 
 1 19.0 
 
 Thorium 
 
 . 232.5 
 
 Antimony . 
 
 120.2 
 
 1 Uranium . 
 
 . 238.0 
 
 The atomic weights given here are not with hydrogen as 
 unity, but with oxygen as 16; both systems are now com- 
 monly used, the latter having the advantage of bringing out 
 more of the elements with whole numbers for their atomic 
 weights. It will be observed that the three heaviest elements 
 are highly radio-active. 
 
 324 
 
Appendix 
 
 The Elements in Order of their Discovery 
 
 A.D. 
 
 Elements. 
 
 Discovered by 
 
 1450. 
 
 Antimony . 
 
 . Valentine {German alchemist). 
 
 1450. 
 
 Bismuth 
 
 . Valentine „ 
 
 1520. 
 
 Zinc 
 
 . Paracelsus (Swiss chemist). 
 
 1694. 
 
 Arsenic 
 
 . Schroder (German). 
 
 1733- 
 
 Cobalt . 
 
 . Brandt „ 
 
 1738. 
 
 Phosphorus 
 
 . Brandt ,, 
 
 1751. 
 
 Nickel . 
 
 Cronstadt (Russian), 
 
 1766. 
 
 Hydrogen . 
 
 . Cavendish (EnglisK), 
 
 1772. 
 
 Nitrogen 
 
 . Rutherford „ 
 
 1774- 
 
 Manganese . 
 
 . Gahn (Swedish). 
 
 1774. 
 
 Oxygen 
 
 . Priestley (English). 
 
 1780. 
 
 Uranium 
 
 . Klaproth (Gerjnan). 
 
 1781. 
 
 Tungsten 
 
 . d'Elihujar (Spanish). 
 
 1782. 
 
 Molybdenum 
 
 . Hjelm (Swedish). 
 
 1782. 
 
 Tellurium . 
 
 . Reichenstein (German). 
 
 I79S- 
 
 Titanium 
 
 . Klaproth „ 
 
 1797. 
 
 Chromium . 
 
 . Vauquelin (French). 
 
 1801. 
 
 Tantalum . 
 
 . Hatchett (English). 
 
 1801. 
 
 Cerium 
 
 . Berzelius and Hisinger (Swedish). 
 
 1801. 
 
 Vanadium . 
 
 . Del Rio (Spanish). 
 
 1803. 
 
 Osmium 
 
 . Tennant (English). 
 
 1803. 
 
 Palladium . 
 
 . Wollaston „ 
 
 1804. 
 
 Iridium 
 
 . Tennant „ 
 
 1804. 
 
 Rhodium 
 
 . Wollaston „ 
 
 1807. 
 
 Potassium . 
 
 . Davy 
 
 1807. 
 
 Sodium 
 
 . Davy „ 
 
 1808. 
 
 Barium 
 
 . Davy (English) and Berzelius 
 
 (Swedish). 
 
 1808. 
 
 Strontium . 
 
 . Davy (English). 
 
 1808. 
 
 Boron . 
 
 . Davy (English) and Gay-Lussac 
 (French). 
 
 1808. 
 
 Magnesium . 
 
 . Davy (English). 
 
 1808. 
 
 Calcium 
 
 . Davy {English) and Berzelius 
 (Swedish). 
 
 rSio. 
 
 Chlorine 
 
 . Davy (English). 
 
 1810. 
 
 Fluorine 
 
 . Ampere (French). 
 
 1811. 
 
 Iodine . 
 
 . Courtois (French). 
 
 1817. 
 
 Selenium 
 
 . Berzelius (Swedish)* 
 325 
 
Scientific Ideas of To-day 
 
 The Elements in Order of their Discovery — continued 
 
 A.D. 
 
 Elements. Discovered by 
 
 1817. 
 
 Lithium 
 
 . Arfvedson {Swedish). 
 
 1817. 
 
 Cadmium . 
 
 . Herman and Stromeyer(6^£r;«a«). 
 
 1823. 
 
 Silicon . 
 
 . Berzelius {Swedish). 
 
 1824. 
 
 Zirconium . 
 
 . Berzelius „ 
 
 1826. 
 
 Bromine 
 
 . Balard {French). 
 
 1827. 
 
 Beryllium 
 
 . Wohler {German). 
 
 1828. 
 
 Aluminium 
 
 . Wohler 
 
 1828. 
 
 Thorium 
 
 . Berzelius (Swedish). 
 
 1828. 
 
 Yttrium 
 
 . Wohler {German). 
 
 184I. 
 
 Lanthanum 
 
 . Mosander {Swedish). 
 
 1843. 
 
 Terbium 
 
 . Mosander „ 
 
 1843. 
 
 Erbium 
 
 . Mosander „ 
 
 1844- 
 
 Ruthenium 
 
 . Glaus {German). 
 
 1846. 
 
 Columbium 
 
 Rose {English). 
 
 i860. 
 
 Caesium 
 
 Bunsen and Kirchloff ((7^r;«^«). 
 
 1862. 
 
 Thallium 
 
 . Crookes {English). 
 
 1863. 
 
 Indium 
 
 . Reich and Richter {German). 
 
 1868. 
 
 Helium (in t 
 
 he Sun) . Lockyer {English), 
 
 1868. 
 
 Rubidium . 
 
 . Bunsen {GermarC). 
 
 1875. 
 
 Gallium 
 
 . Boisbaudran {French). 
 
 1878. 
 
 Ytterbium 
 
 . Marignac „ 
 
 1879. 
 
 Thulium 
 
 . Cleve {Swedish). 
 
 1879. 
 
 Scandium 
 
 . Nilson „ 
 
 1879. 
 
 Samarium 
 
 , Boisbaudran {French). 
 
 1885. 
 
 Praseodymii 
 
 im . . Welsbach {German). 
 
 1885. 
 
 Neodymium 
 
 . Welsbach 
 
 1886. 
 
 Gadolinium 
 
 . Marignac {French). 
 
 1886. 
 
 Germanium 
 
 . Winkler {German). 
 
 1894. 
 
 Argon . 
 
 . Rayleigh and Ramsay {English). 
 
 1895. 
 
 Helium (on '. 
 
 Earth) . Ramsay {English), 
 
 1897. 
 
 Krypton 
 
 . Ramsay and Travers {English), 
 
 1898. 
 
 Xenon . 
 
 . Ramsay {English). 
 
 1898. 
 
 Neon . 
 
 . Ramsay and Travers {English). 
 
 1898. 
 
 Radium 
 
 . Curie {French). 
 .__it_/?_jiii__ .___i_i__. . __i 
 
 One is not surprised to find the commonest elements — such 
 as iron, copper, gold, lead, and carbon — missing from the 
 
 326 
 
Appendix 
 
 foregoing list, as we have no record of the early discovery of 
 these. From the curious names given to many of the ele- 
 ments one is quite prepared to find that these were discovered 
 by foreigners. Of the six elements last discovered, no fewer 
 than five of them have Sir William Ramsay's name attached to 
 them, while the last of the list — Radium — is, as every one 
 know5, a French discovery. Some may be surprised to find 
 uranium appearing so early on the list. Although it was dis- 
 covered in 1780, its radio-active properties were not observed 
 till 1896. 
 
 327 
 
APPENDIX II 
 
 HISTORICAL NOTE OX THE MODERN THEORY 
 OF LIGHT 
 
 1804 
 
 For more than a hundred years man had believed light to 
 be composed of material corpuscles, as suggested by Sir Isaac 
 Newton towards the close of the seventeenth century. ""We 
 can understand how difficult it was for those living in the 
 opening years of the nineteenth century to accept the new 
 idea put forward by Professor Thomas Young, of the Royal 
 Institution (London). Dr. Young's idea was that light was 
 merely a wave motion in the aether, and he set forth his theory 
 in the Bakerian Lecture of 1804. The lecture was published 
 in full in the Philosophical Transactions of that year. 
 
 I have referred already to the criticism which appeared in 
 the Edinburgh Review of 1804 (vol. v. p. 97). There are 
 seven pages of scathing criticism, the reading of which makes 
 one wonder what the critic would say if he could return to 
 this planet and see that Dr. Young was entirely right. For 
 the sake of those who cannot refer conveniently to the old 
 Journal itself, I make a few extracts. 
 
 After referring to past rebukes which the Journal had found 
 it necessary to give Dr. Young, the critic says that this 
 Bakerian Lecture " contains more fancies, more blunders, more 
 unfounded h3?pothesis, more gratuitous fictions, ... all from 
 the fertile, yet fruitless, brain of the same eternal Dr. Young. 
 ... In our Second Number we exposed the absurdity of this 
 writer's * law of interference,' as it pleases him to call one of 
 
 328 
 
Appendix 
 
 the most incomprehensible suppositions that we remember to 
 have met with in the history of human hypothesis. . . . But, 
 in truth, Dr. Young is of a quick conception in hatching 
 hypotheses. Whether it be, that those noxious things are 
 naturally of rapid growth, like rank weeds, or that he is of a 
 mind peculiarly ' nimble and forgetive.' . . . Dr. Young then 
 proceeds to demolish, at one blow, the whole Newtonian 
 theory of light, and to show, from Sir Isaac's own admeasure- 
 ments, that his idea of particles moving in consequence of a 
 projectile force is altogether absurd. This process of rapid 
 demolition is somewhat curious; and we extract it as a specimen 
 of his way." Then follows a long quotation from the lecture, 
 the truth of every word of which is amply proved to-day, but 
 the critic labels it " a metaphysical absurdity." He closes his 
 review with the following words : " We now dismiss, for the 
 present, the feeble lucubrations of this author, in which we 
 have searched, without success, for some traces of learning, 
 acuteness, and ingenuity, that might compensate his evident 
 deficiency in the powers of solid thinking. . . . We came to 
 the examination with no other prejudice than the very allowable 
 prepossession against vague hypothesis by which all true lovers 
 of science have for above a century and a half been swayed." 
 
 In the criticism, from which those extracts are made, there 
 is an attempt to disprove Young's theory, but it is apparent 
 throughout that the critic's real difficulty is that he fails to 
 appreciate the reality of the sether. 
 
 It is of interest to note that wave theories had been suggested 
 before Newton's time, but these were not in favour and were 
 deliberately discarded by the great philosopher. Huygens' 
 name is well known in connection with wave theories. 
 
 1815 
 
 Fresnel (France) did a great deal to advance the wave theory 
 of light. Although he wrote to a friend on the 28th of 
 December 1814 asking that some books might be sent to him 
 
 329 
 
Scientific Ideas of To-day 
 
 as he did not know the meaning of the phrase " the polarisation 
 of light," yet by the end of the follo^sing year Fresnel was 
 among the best authorities upon this subject. 
 
 1845 
 
 Michael Faraday— that " prince of experimenters"— when 
 
 working in the Rcva. Ins::rjt:o!i ly-^ndon), in 1845, ^as able 
 to show a real connection between light and magnetism. He 
 rotated a beam of polarised light by means of a powerful 
 
 1864 
 
 CLz^.K-ilAx^TEii (Cambridge) worked out the mathematics 
 of the electro-ir^^etic theory of light, and proclaimed from 
 zis calc-i-a:ioz5 fta: loz^ger dutric waves, of the same nature 
 as ligni, — ust exist in the ccther. These waves were not 
 tract^d er^erlzirr.tally for about a quaner of a century. 
 
 1S76 
 
 Dr. ToHN KsRR (Gla^ow) succeeded in demonstrating 
 Ilia.: 1 beam cf p^:':^sei light was affected by falling upon 
 the relished pole cf z powerful electro-magnet, when it was 
 re^erte- tfaerdfrom. 
 
 1880 
 
 H. A. Lc^izNTZ (Ansterdaj::) suggested that light was due 
 
 to minute charged corpuscles revolving around the atoms of 
 maner. He worked cu: the mathematics of this theory, but 
 aldiough munj mer. rrled to find experimental proof they failed 
 at that time, 
 
 1888 
 
 Heikrich Hertz (Germany) detected and measured the 
 electric waves prophesied by Clerk-Maxwell some twenty-four 
 
 330 
 
Appendix 
 
 years earlier. Hertz showed that these waves possessed the 
 very same properties as light, but were of longer wave-length. 
 This discovery made by Hertz completed the connection 
 between electricity and light, and to-day no one will dispute 
 the truth of the electro-magnetic theory of light. 
 
 1896 
 
 Professor Zeeman (Holland) produced experimental proof 
 of Lorentz's theory. He showed that the revolving electrons 
 were influenced by a powerful magnetic field, their speeds of 
 revolution being altered, as explained at page 240. 
 
 331 
 
APPENDIX III 
 
 PARTICULARS OF ^THER WAVES 
 
 ^THER waves are all of the same nature; they differ in 
 wave-length. In the visible spectrum we have the following 
 variety : — 
 
 Waves producing the sensation of Red 
 
 Orange 
 
 Yellow 
 
 Green 
 
 Blue 
 
 Indigo 
 
 Violet! 
 
 about 34,000 waves to the inch, 
 
 ,, 37,000 
 
 M 42,000 
 
 M 48,000 
 
 ,, 51,000 
 
 ,, 61,000 
 
 n 64,000 
 
 As the speed of travel of all these waves is 186,000 miles 
 per second, the number of waves passing any point in one 
 second will be : — 
 
 Waves producing the sensation of Red about 400 billion per second. 
 
 Orange ,, 440 ,, 
 
 Yellow 
 
 Green 
 
 Blue 
 
 Indigo 
 
 Violet 
 
 500 
 570 
 600 
 700 
 750 
 
 These figures also give us the rates of revolution which the 
 electrons perform around their atoms to produce those sether 
 waves. 
 
 From the following diagram we see that those vision- 
 
 ^ It will be clearly understood that between the extreme red and the 
 extreme violet ends of the spectrum there exists every variety of wave- 
 length — from one thirty-two-thousandth to one sixty-four- thousandth of 
 an inch. 
 
 332 
 

 ;*-H» 
 
 
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 seve 
 
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 small 
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 corner 
 enty-se 
 
 tions, V 
 
 
 
 
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 rCi O « 
 
 333 
 
Scientific Ideas of To-day 
 
 produdng waves only occupy a very small portion of the total 
 range of sether waves. Here we have the idea of a very long 
 piano-keyboard comprising twenty-seven octaves. The lowest 
 note or rate of vibration shown here is about 50 million 
 vibrations per second, but we know that much longer waves 
 occur in some systems of wireless telegraphy. At the extreme 
 right of the keyboard we have the highest note or rate of 
 vibration, which is about 3000 billion vibrations per second, 
 and this is the most rapid rate of vibration of which we have 
 any knowlec^e. 
 
 334 
 
APPENDIX IV 
 
 COUNTING THE INVISIBLE ELECTRONS 
 
 Some readers may desire to know something more of the 
 methods adopted in counting the electrons and in estimating 
 their velocities, and yet they may not care to go into the 
 matter as fully as the text-books do. In chapter iii. we were 
 content to deal with general statements, leaving the details 
 to be filled in by those who should choose to do so from the 
 particulars given here. 
 
 We saw that it was only natural to suppose that the electrons 
 were negatively electrified particles because they were shot off 
 or repelled by the cathode or negative electrode. This point 
 can be put to the test. If we place a small metal vessel within 
 the vacuum tube so that the cathode particles or electrons will 
 enter it, we can test the kind of electrification, and more than 
 that — we can measure the amount of charge received from a 
 given cathode stream in a given time. Our chief interest lies 
 in the counting of the invisible electrons, which would seem 
 an utterly impossible task. We know that this seeming im- 
 possibility has been accomplished. 
 
 It will be of interest to commence with Aitken's experiments 
 in counting the invisible dust-particles contained in the air, as 
 the method employed by him for that purpose was a stepping- 
 stone towards this later remarkable achievement of counting 
 the electrons. 
 
 Aitken pointed out that his experiments were based upon 
 the fact that water vapour would condense upon the invisible 
 dust-particles in the atmosphere, whereas it would not con- 
 dense upon dust-free air. He carefully filtered all the dust 
 
 335 
 
Scientific Ideas of To-day 
 
 out of a volume of air by passing it through plugs of glass 
 wool. Taking a glass globe filled with the dust-free air, and a 
 similar globe filled with ordinary air, he admitted a little steam 
 into both vessels. A cloud formed immediately in the globe 
 containing the ordinary air, while no cloud or fog was pro- 
 duced in the dust-free air. 
 
 Before the steam was admitted one could not see any 
 difference between the contents of the two vessels ; they were 
 both filled with invisible air. That there was a real difference 
 became most apparent whenever the steam was admitted. 
 The water vapour condensed upon the invisible (Just-particles 
 in the vessel containing the ordinary air and remained sus- 
 pended in the form of a cloud. While the second vessel, 
 which contained no dust-particles to act as nuclei, remained 
 clear, the water condensing on the walls of the vessel. This 
 is what would happen in our every-day life but for the presence 
 of minute particles in the atmosphere ; we should have no 
 rain, but the walls of our houses and every other object would 
 be wet. 
 
 Aitken found that, by varying the quantities of dust particles 
 in the air contained in the glass globes, he could imitate a 
 London fog, a Scotch mist, or a shower of rain. If there was 
 a great quantity of dust particlfes in the air, then the water 
 vapour in condensing would divide itself over the whole of 
 these, and each particle having only a little water vapour 
 attached to it could still float in the air, and in this way they 
 formed a regular fog. Dense fogs are therefore most plentiful 
 in large cities. 
 
 It was found also by experiment that a smaller quantity of 
 dust in the air meant that each dust particle laid hold of a 
 larger amount of water vapour, and in this way a Scotch mist 
 was produced. In this case the dust particles were more 
 heavily laden with moisture than the fog-particles were, but 
 even those heavier mist-particles are able to float for some little 
 time. Further experiments proved that if the dust-particles 
 present were still fewer and if there was much moisture in the 
 
Appendix 
 
 air, a visible drop of water was formed around each dust- 
 particle, and these over-burdened particles fell in the form of 
 a shower of rain. 
 
 It occurred to Aitken that if he could only count the rain- 
 drops he could tell how many dust-particles there were in the 
 air. This he was able to do by causing the tiny rain-drops 
 formed in a cubic centimetre of air to fall on a small silvered 
 mirror. The mirror was divided into a large number of equal 
 parts, and the number of rain-drops contained in one of these 
 tiny squares was counted with the aid of a powerful magnify- 
 ing glass. 
 
 C. T. R. Wilson found by experiment that if perfectly dust- 
 free air, contained in a glass vessel, be ionised,^ water vapour 
 will condense and form a cloud. But we cannot hope to see 
 the individual drops of water as Aitken did with difficulty upon 
 his much larger dust-particles. First of all let us see how the 
 cloud is produced. 
 
 In some of Aitken's experiments he adopted the following 
 method of causing condensation upon dust-particles. An air- 
 pump was connected to the glass vessel containing the air to 
 be tested, also some water vapour. One stroke of the air- 
 pump withdrew some air and caused the remaining air to 
 expand suddenly. The expansion of the air caused its tempera- 
 ture to fall, and this in turn caused the water vapour to con- 
 dense upon the dust-particles and form a visible cloud. This 
 same method has been adopted in counting the electrons. 
 
 A glass globe containing some perfectly dust-free air and 
 
 ^ In preceding chapters we have considered several means of ionising 
 the air. An electrical discharge, the passage of Rontgen rays, or the 
 radiations from radium, will ionise the air. It will be remembered that 
 the meaning of ionising is that the positive and negative atoms compos- 
 ing the molecules of gases become separated, and give us positive and 
 negative ions or wanderers. Even the splash of water may cause some of 
 the molecules in the surrounding air to split up into positive and negative 
 ions ; these have been detected at the base of waterfalls. A burning lamp 
 and a red-hot metal wire produce ions in greater quantities. 
 
 337 Y 
 
Scientific Ideas of To-day 
 
 some water vapour is subjected to X-rays, whereupon the air 
 is ionised. There is no visible effect produced until some of 
 the air is suddenly withdrawn by an air-pump attached to the 
 globe. Thereupon a visible cloud is formed. 
 
 Long before the Cambridge physicists ever thought of 
 counting electrons, Sir George Stokes had shown how it was 
 possible to calculate the size of rain-drops in a cloud by 
 observing the rate at which the cloud fell. The mathematical 
 formulse devised by Sir George Stokes came in very con- 
 veniently in these later experiments with clouds formed upon 
 negative ions or atoms having a spare electron. The experi- 
 menter knew exactly how much water vapour would condense 
 at each stroke of the air-pump. He therefore knew the total 
 weight of water in the cloud, and by thus estimating the weight 
 of each drop he could tell how many drops were in the cloud, 
 and consequently how many ions having a detachable electron. 
 This counting of the electrons was a most remarkable achieve- 
 ment, and with this data the mathematicians have been able 
 to estimate the size and mass of an electron. Having previously 
 determined the ratio of the electric charge of the electron to 
 its mass, the value of the charge was now arrived at. 
 
 Another point of interest to the general reader will be the 
 method of determining the velocity of the electrons. As 
 mentioned in the text, this was determined by measuring the 
 amount of deflection due to an electrostatic field, and that 
 due to a magnetic field. The following diagram shows the 
 method of measuring the electrostatic deflection. The elec- 
 trons are shot off from the cathode C, and after passing 
 through the slots in the partitions P the beam of electrons 
 goes straight to the end of the tube, causing a spot of phos- 
 phorescence to appear on the glass. When the plates E E are 
 oppositely charged by being connected to a few cells of an 
 accumulator, the beam is deflected. The plate which is 
 negatively charged will repel the flying electrons, while the 
 positively charged plate will attract them. Hence, if the 
 upper plate be the negative one, the beam of electrons will be 
 
 338 
 
Appendix 
 
 This diagram represents the special vacuum tube arranged for 
 deflecting the flying electrons. The stream of electrons is shot 
 off from the cathode C, and passes through the slots in the anode 
 P, which is connected to earth, so that it shields the large tube 
 from the electric field existing between C and P. The stream of 
 flying electrons passes in a straight line to the end of the tube, 
 where it strikes the phosphorescent glass and produces a luminous 
 spot. It will be observed that this stream passes between two 
 metal plates E E, If these two plates are oppositely electrified by 
 
 Fig. F 
 
 Experimenti7tg with Electrons 
 
 being connected to a storage battery, the stream of electrons will 
 be deflected from their straight path, and they will strike the glass 
 at a different place. In the above diagram the upper plate is 
 supposed to be negatively electrified, so that it repels the negative 
 electrons, while the lower plate being positively electrified will 
 attract them. In this way the stream of flying electrons is de- 
 flected downwards, and the luminous spot will appear at a lower 
 point than it did originally. A paper scale is pasted on the 
 outside of the tube at S, so that the amount of deflection may be 
 noted. By means of this tube a great deal of interesting know- 
 ledge has been gained, as explained in the text. 
 
 339 
 
Scientific Ideas of To-day 
 
 deflected downwards, so that the luminous spot appears lower 
 down on the end of the tube. A paper scale is pasted on the 
 outside of the tube, so that the amount of deflection may be 
 noted. 
 
 The same beam of electrons may be subjected to the in- 
 fluence of a magnetic field, by passing it between the lateral 
 poles of a magnet. The one force may be so arranged that it 
 neutralises the other, and by the two measurements thus 
 obtained the velocity of the electrons is determined. The 
 actual velocities of electrons are dealt with in the text 
 (chapter iii.). This same apparatus provides the mathe- 
 maticians with the ratio of the charge to the mass, mentioned 
 on page 338. 
 
 A set of apparatus used in determining the foregoing 
 particulars may be seen in the Science section of the Victoria 
 and Albert Museum (London), to which Institution the 
 apparatus has been lent by Professor J. J. Thomson. 
 
 340 
 
INDEX 
 
 Absorption of heat, 178 
 Absorption of light, 177, 186, 195 
 Actinic waves, 144 
 Action at a distance, 149 
 iEther, 87, 94, 98 
 ^ther waves, 89, 139, 161, 167, 
 332 
 
 Aitken, 45» 335 
 
 Alchemy, 301 
 
 Alternating current, 124, 182 
 
 Anode, 43 
 
 Artificial light, 168 
 
 Atom, 27, 37, 52, 67, no 
 
 Atom a solar system, $4' m. ^9^ 
 
 Atomic stability, 62, 152, 305 
 
 Atomic weights, 60 
 
 Aurora, 260 
 
 Battery, electric, 85, 124 
 Becquerel, Prof., 276 
 Becquerel rays, 278 
 Bon, Gustave le, 303 
 
 Cathode, 43 
 
 Cathode rays, 43 
 
 Cells in parallel, 83 
 
 Cells in series, 83 
 
 Chemical affinity, 29 
 
 Chemical energy, 135 
 
 Chemical union, 64 
 
 Cobalt (magnetic), no 
 
 Cohesion, 32, 35 
 
 Colour, 192 
 
 Colour-blindness, 205 
 
 Conservation of energy, 136 
 
 Corpuscles, 43 
 
 Corpuscular theory of light, 153, 
 
 166 
 Current, alternating, 124, 182 
 Current, continuous, 182 
 Current, electric, 75 ,78,85, 104, 1 23 
 
 Crookes, Sir William, 42, 274, 282 
 Curies, 273, 279, 281, 291, 302 
 
 Daguerre, 277 
 Darwin, Sir George, 248 
 Davy, Sir Humphry, 177 
 Deflection of electrons by a magnet, 
 
 44. 75. 338, 340 
 Diffusion, ^6 
 
 Discharge of electricity, 74, 85 
 Dynamo, 124 
 
 Earth's age, 243 
 
 Earth's magnetism, 262 
 
 Earth's negative charge, 260, 264 
 
 Electrical attraction, 29 
 
 Electrical discharge, 74, 85 
 
 Electrical discharge in vacuo, 40, 
 
 74 
 Electrical energy, 135 
 Electric battery, 85, 124 
 Electric current, 75, 78^ 85, 104, 
 
 123 
 Electric pressure, 82 
 Electric resistance, 81 
 Electric waves, 143, 180 
 Electro-magnet, 112 
 Electro-magnetic theory of light, 
 
 158, 166, 170 
 Electro-negative, 31, 63, 65 
 Electro-positive, 31, 63, 65 
 Electrons, 43, 51, 55, 133, 158, 182, 
 
 197, 207, 240, 259, 319, 335 
 Electron theory, 68, 71, 98 
 Electroscope, 307 
 Elements, 22, 322-326 
 Emanation gas, 292, 293, 295, 296, 
 
 300 
 Energy, 50, 125, 128, 150 
 Energy, chemical, 135 
 Energy, conservation of, 136 
 
 341 
 
Index 
 
 Energy, electrical, 135 
 Energy, forms of, 138 
 Energy, heat, 131, 145 
 Energy, kinetic, 128 
 Ether, the, 87 
 Evolution, 234, 250 
 
 Faraday, Michael, ii4» 122, 
 
 330 
 Force, 126 
 
 Fluorescent screen, 267, 269 
 Franklin, Benjamin, 30, 71 
 Frequency of vibration, 142 
 Fresnel, 329 
 Friction, 115 
 
 Galileo, 18, 126, 164 
 Gaseous matter, 35 
 Glass prism, 194, 210, 215, 227 
 Grating, diffraction, 220 
 Gravitation, 130. 148, 310 
 
 Heat, molecular, 33* 93' ^57 
 
 Heat, absorption, 178 
 
 Heat energy, 131, I45 
 
 Heat, latent, 133 
 
 Heat, polarised, 180 
 
 Heat, radiant, 90, 143, 177, 184, 
 
 208 
 Heat, reflection, 177 
 Heat, refraction, 179 
 Heat, waves, 90, 93, 143, 157 
 Helium, 289, 296, 298, 300 
 Helmholtz, 204 
 
 Hertz, Prof., 76, I59' ^^^l. 33O 
 Heusler's alloys, no, 113 
 Huygens, 94, 329 
 Hydrogen, 25, 225, 226 
 
 Induction, 119, 121 
 
 Inertia, 117 
 
 Insulators, 81, 84 
 
 Internal energy of atom, 308 
 
 Invisible light, 91, 92, 99, 181 
 
 Ions, 261, ^37 
 
 JoLY, Prof., 249 
 
 Kelvin, 74. 246, 247, 249> 251 
 
 308 
 Kinetic energy, 128 
 
 Latent heat, 133 
 
 Laws of Nature, 20, 136, 149 
 
 Lenard rays, 48, 49. 9^> 286 
 
 Life, origin of, 252 
 
 Light, absorption of, 177, 186, 195 
 
 Light, artificial, 168 
 
 Light, corpuscular theory of, 153 
 
 Light, electro-magnetic theory of, 
 
 158, 166, 170 
 Light energy, 138 
 Light, invisible, 91, 92, 99, 181 
 Light, mechanical pressure of, 145 
 Light, nature of, IS 3. ^55 
 Light, polarised, 172, 174, 176, 191 
 Light, properties of, 155, 171 
 Light-push, 148 
 
 Light, reflection of, 171, 186, 199 
 Light, refraction of, 172, 210, 214 
 Light, ultra-violet, 76, 91, I44» 
 
 157 
 Light, velocity of, loi, 162 
 Light waves, 90, 99, 173 
 Lightning, 261 
 Line spectra, 231 
 Lines offeree, 114 
 Lippmann's colour photography, 2 2 1 
 Liquid matter, 34 
 Lithium, 59 
 Lockyer, Sir Norman, 231, 232, 
 
 298 
 Lodestone, 109, 262 
 Lodge, Sir Oliver, 51. 169, 274 
 Lorentz, Prof., 239, 330 
 
 Magnet, 104, 108 
 
 Magnet, natural, 109 
 
 Magnetic deflection, 45, 75, 338, 
 
 340 
 Magnetic field, 104, 112, 122 
 Magnets, floating, 56 
 Magnetic saturation, 1 11 
 Magnetic storms, 263 
 Magnetism of the Earth, log, 262 
 Mass, 50 
 
 Mass of electron, 51, 338 
 Maxwell, Clerk, 145, 330 
 Mechanical pressure of light, 145 
 Mendeleeff, 60, 97, 250, 272 
 Mercury vapour lamp, 202 
 Meteorites, 233 
 Molecule, 27 
 
 342 
 
Index 
 
 Nebula, 233, 264 
 
 Negative charge of electron, 45, 
 
 335 
 Negative electricity, 30, 55, 72, 74, 
 
 S6, 260 
 Newlands' octaves, 60 
 Newton, 153, 194, 210, 227, 310, 
 
 312 
 Nickel (magnetic), no 
 Niewenglowski, 275 
 Nodal points, 161 
 
 Oxygen, 25 
 
 Paints, heat-indicating, 208 
 Periodic law, 60 
 Permanent magnet, 108 
 Perpetual motion, 116, 117 
 Phosphorescence, 42, 187, 267, 
 
 274, 275^ 
 Photographing the spectrum, 223, 
 
 226 
 Pitchblende, 279, 280 
 Polarised heat, 180 
 Polarised light, 172, 174, 176, 191 
 Positive electricity, 30, 55, 72, 74, 
 
 86, 260 
 Positive electricity, sphere of, S5j 
 
 68 
 Potassium, 59 
 Potential energy, 129 
 Preece, Sir William, 121 
 Pressure, electric, 82) 
 Pressure of light, 145 
 Prism of glass, 194, 210, 215, 227 
 Prism of paraffin wax, 181 
 Prism of rock-salt, 179, 217 
 Proto-elements, 234 
 Protoplasm, 256 
 
 Radiant heat, 90, 143, 177, 184, 
 
 208 
 Radiant matter, 43 
 Radio-activity, acquired, 291, 292 
 Radium, 155, 273, 281, 2S4, 294, 
 
 305 
 Radium radiations, 285 
 Ramsay, Sir William, 285, 299, 300 
 Reflection of heat, 177 
 Reflection of light, 171, 186, 199 
 Refraction of heat, 179 
 
 Refraction of light, 172, 210, 214 
 Resistance, electrical, 81 
 Rontgen, Prof., 267 
 Rontgen rays. See X-rays 
 Rutherford, Prof., 285, 286, 300 
 
 Schuster, Prot., 49 
 
 Self-induction, 121 
 
 Size of electron, 51 
 
 Soddy, 285, 300 
 
 Sodium, 59, 224, 228, 240 
 
 Solar spectrum, 226 
 
 Solid matter, 34 
 
 Sound waves, 141, 237 
 
 Spectroscope, 210, 219, 223, 228, 
 
 307 
 Spectrum (discontinuous), 223 
 Spectrum, visible, 157, 200, 205, 
 
 209, 215, 218, 227 
 Spinthariscope, 282 
 Star, birth of a, 233 
 Stellar temperature, 230 
 Stokes, Sir George, ^^S 
 Strain, energy of, 129 
 Sunspots, 258 
 
 Temperature, 33, sS 
 Thompson, Prof. Silvanus, 160 
 Thomson, Prof. J. J., 43, 61, 69, 
 
 305 
 Thorium, 292 
 Tourmaline, 174, 191 
 Transformation of energy, 128, i ^6y 
 
 144 
 Transmission of energy, 119, 127, 
 
 137, 144, 145 
 Transmutation of elements, 301 
 Transparency, 189 
 Transverse waves, 140 
 
 Ultra-violet light, ^6j 91, 144, 
 
 157 
 Undulatory theory of light. See 
 
 Electro-magnetic 
 Uranium, 276, 281, 296, 301 
 
 Velocity, 50, 150 
 
 Velocity of zether waves, 159, 162 
 
 Velocity of electrons, 46, 154, 
 
 Velocity of light, loi, 162 
 
 343 
 
Index 
 
 Velocity of star in line of sight, 
 
 237 
 Visible spectrum. See Spectrum 
 Volta, Prof., 76 
 
 Wave frequency, 142 
 
 Wave-length, 142 
 
 Wave-motion, 88, 93, 102, 139, 
 140, 172 
 
 Wave theory of light. See Electro- 
 magnetic 
 
 Waves, actinic, 144 
 
 Waves, electric, 143, 180 
 
 Waves in the sether, 89, 139, 161, 
 
 167, 332 
 Waves, sound, 141, 237 
 Waves, transverse, 140 
 Wilson, C. T. R., 337 
 Wireless telegraphy, 143 
 
 X-RAYS, 266, 286 
 
 Young, Dr. Thomas, 166, 204, 328 
 
 Zeeman effect, 241 
 Zeeman, Prof., 239, 240, 331 
 
 THE END 
 
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 By ARCHIBALD WILLIAMS, B.A., F.R.G.S. 
 
 With Twenty-six Illustrations 
 
 "A mine of Information and stirring incident." — Scotsman. 
 
 "Mr. WiUiams is most catholic in his choice, taking his readers to soar in a balloon 
 with the luckless Andree, to wander in African forests and Australian deserts, to seek 
 for the North Pole with Nansen, and even to note such an up-to-date expedition aa 
 that of the 'Discovery' in the Antarctic Regions, to cite but the most prominent. Mr. 
 Williams has done this work most judiciously, ... a book which will delight 
 both young and old alike." — Graphic. 
 
 " The book unites strong natural attractiveness with valuable geographical informa- 
 tion to a degree probably unequalled by any other that might be offered as appropriate 
 for tiie purpose of a gift book or the recreative library."— ScAooi Govermnent Chronicle. 
 
 "It is a kind of epitome of the best travel books of our time, and is exceedingly 
 well done." — ^codemy. 
 
 SEELEY & CO., Ltd., 38 Great Russell Street 
 
THE ROMANCE OF INSECT LIFE 
 
 DESCBIBIKa THE CURIOUS *» INTEHESTING IK THE INSECT WOBI.D 
 
 By EDMUND SELOUS 
 
 AUTHOR or "THB ROUANCK or THB AVIHAL TTOltLD," ETO. 
 
 With Sixteen THustrationa. Extra Crown 8vo. 5». 
 
 "An entertaining volume, one more of a eeries which seeks with much succesB to 
 desonbe the wonders of nature and science in simple, attractive form." — Oraphic. 
 
 "Offers most interesting descriptions of the strange and curious inhabitants of the 
 insect world, sure to excite inquiry and to foster observation. There are ants white 
 and yellow, locusts and cicadas, bees and butterflies, spiders and beetles, scorpioDs 
 and cockroaches — and especially ants— with a really scientific investigation of their 
 woDderful habits not in dry detail, but in ^ee and charming exposition and narrative. 
 An admirable book to put in the hands of a boy or girl with a turn for natural science 
 —and whether or not. " — EducatioTial Tiirui. 
 
 "Both interesting and instructive. Such a work as this is genuinely educative. 
 There are numerous illustrations." — Liverpool Courier. 
 
 " With beautiful original drawings by Carton Moore Park and Lancelot Speed, and 
 effectively bound in dark blue cloth, blazoned with scarlet and gold."— Xody. 
 
 "Admirably written and handsomely produced. Mr. Selous's volume shows careful 
 research, and the illustrations of insects and the results of their powers are well 
 done."— ITorW. 
 
 THE ROMANCE OF MODERN 
 MECHANISM 
 
 INTEHESTING DESCRIPTIONS IN NON^TECHNICAL LANGUAGE OF WONDERFUl 
 
 MACHINERY, MECHANICAL DEVICES, &* MARVELLOUSLY DELICATE 
 
 SCIENTIFIC INSTRUMENTS 
 
 By ARCHIBALD WILLIAMS, B.A., F.R.G.S. 
 
 AUTHOR OF "THE ROMANOH OF MODERN EXPLORATION," ETO. 
 
 With Twenty-six Illustrations. Extra Crown 8vo. 6$, 
 
 " No boy will be able to resist the delights of this book, full to the brim of instnio- 
 tive and wonderful matter. "—Brvfwh Weekly. 
 
 "This book has kept your reviewer awake when he reasonably expected to be other- 
 wise engaged. We do not remember coming across a more fascinating volume, even to 
 a somewhat blaafi reader whose business it is to read all that comes in his way. The 
 marvels, miracles they should be called, of the modem workshop are here exploited by 
 Mr Williams for the benefit of readers who have not the opportunity of seeing these 
 wonders or the necessary mathematical knowledge to understand a scientific treatise 
 on their working. Only the simplest language is used and every effort is made, by 
 illustration or by analogy, to make sufiBciently clear to the non-scientific reader how 
 the particular bit of machinery works and what its work really is. Delicate instru- 
 ments, calculating machines, workshop machinery, portable tools, the pedrail, motors 
 ashore and afloat, fire engines, automatic maclunes, sculpturing machines— these are 
 a few of the chapters which crowd this splendid volume."— Educational Newi. 
 
 " It Is difficult to make descriptions of machinery and mechanism interesting, but 
 Mr, Williams has the enviable knack of doing so, and it is hardly possible to open this 
 book at any page without turning up something which you feel you must read ; and 
 then you cannot stop till you come to the end of the chapter. " — Electricity. 
 
 " This book is full of interest and instruction, and is a welcome addition to Messrs, 
 Seeley and Company's Romance Series."— L«d* Mercwry. 
 
 " A book of absorbing interest for the boy with a mechanical turn, and indeed for 
 the general reader. "—iWiwational Time$. 
 
 "An instructive and well-written volume."— Kobfeiw. 
 
 SEELEY df CO., Ltd., 38 Gaaxx Russell Street 
 
WORKS BY C. J. CORNISH 
 
 THE NATURALIST ON THE THAMES. With 
 
 Thirty-eight IlIuBtrations. Demy 8vo. 7b. 6d, 
 
 "Will giTe great delight to any loyer of natural history. ... It is a most 
 fascinating volume, and is illustrated in a manner worthy of the text,*'— Scotsman, 
 
 "From the first page to tlie last there is not a single page which the reader 
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 ANIMALS OF TO-DAY: Their Life and Conversation. 
 
 With Illustrations from Photographs by 0. KsiD, of Wishaw. Grown 
 8vo. Gs. 
 
 "Quite one of the brightest books of popular natural history which have 
 appeared in recent years is Mr. Cornish's fascinating studies of ' Animals of To- 
 Da.y."' —Leeds Mercuiy. 
 
 NIGHTS WITH AN OLD GUNNER, and other 
 
 Studies of Wild Life. With Sixteen Illustratione. 68. 
 
 "Cannot fail to be Interesting to any lover of wild nature. The Illustrations art 
 numeroua and excellent."— PaH Mall Gazette. 
 
 ANIMALS AT WORK AND PLAY : Their Emotions 
 
 and Activities, Illustrated from Photographs by Gambibr BolTON, 
 F.Z.S., and others, and from Drawings. Third Edition, 6s. 
 
 " Such a book as Mr. Cornish's shows how much there is to repay the Intelligent 
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 "Good as the former books were, we are disposed to think that this is even 
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 WILD ENGLAND OF TO-DAY, and the Wild Life 
 
 in It. Illustrated with Original Drawings by Lancelot Speed, and 
 from Photographs. Third Edition. 6a. 
 
 " Those of ns who are left in town In the dull days will seem, In reading these 
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 LIFE AT THE ZOO. Notes and Traditions of the 
 
 Regent's Park Gardens. Illustrated from Photographs by Gambibr 
 Bolton, F,Z.S. Fifth Edition. 6a. 
 
 "Every lover of animals will find abundance of attraction and entertainment Id 
 Mr. Cornish's delightful volume."— Tim^. 
 
 THE ISLE OF WIGHT. With Four Etchings and 
 
 many other Illustrations. Super Royal 8vo. 3s. 6d. net. 
 
 THE NEW FOREST. With Four Etchings and many 
 
 other Illustrations. Super Royal 8vo. Ss. 6d, net. 
 
 Also a Pocket Edition, 16mo, cloth, 2s. net ; leather, 3s. net. 
 
 SEELEY to* CO., Ltd., SB Great Russell Street 
 
EVENTS OF OUR OWN TIME. 
 
 A Series of Volumes on the most Important Events of the last Half- 
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 THE WAR IN THE CRIMEA. By General Sir Edward 
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 THE INDIAN MUTINY OF 1857. By Colonel Malleson, 
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 Seventh Edition. 
 
 THE AFGHAN WARS OF 1839-1842 and 1878-1880. By 
 Archibald Forbes. With Five Maps and Plans, and Four 
 Portraits on Copper. Third Edition. 
 
 THE REFOUNDING OF THE GERMAN EMPIRE. By 
 Colonel Malleson, C.S.I. With Five Maps and Plans, and 
 Four Portraits on Copper. 
 
 THE LIBERATION OF ITALY, 1816-1870. By the Countess 
 Martinengo Cbsaresco. With Portraits on Copper. Second 
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 OUR FLEET TO-BAY AND ITS DEVELOPMENT DURING 
 THE LAST HALF-CENTURY. By Rear-Admiral S, 
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 " Undoubtedly the best summary of modern African history that 
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 SEELEY &* CO- Ijtu,, S8 Great Russell Strebt 
 
POPULAR SCIENCR 
 
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 a Preface by Professor Boulger. With Sixteen Illustrations. 
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 THE WORLD'S FOUNDATIONS. Geology for Beginners. By 
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 AMONG THE STARS; or. Wonderful Things in the Sky. 
 By Agnes Giberne. With Coloured Illustrations. Seventh 
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 THE STORY OF THE HILLS : A Popular Account of Moun- 
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 **A charmingly written and beautifully illustrated account of the 
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 WIRELESS TELEGRAPHY. Popularly Explained. By Richard 
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 F.R.S., and many Illustrations. Sixth Edition, Revised. 
 Is. 6d. 
 
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 the various systems of wireless telegraphy. Capitally illustrated." — 
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 SKELEY & CO.^ Ltd.j 38 Great Russell Street 
 
BY THE REV A. J. CHURCH 
 
 STORIES FROM HOMER. With Coloured Illustrations. 
 
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 THE STORY OF THE ILIAD. With Coloured Illustra- 
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 THE STORY OF THE ODYSSEY. With Coloured 
 
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 STORIES FROM LIVY. With Coloured Illustrations. 
 
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 "A genuine classical treasure." — Scotsman. 
 
 ROMAN LIFE IN THE DAYS OF CICERO. With 
 
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 SEELEY & CO., Ltd., 38 Great Russell Street 
 
A Catalogue of Books on Art 
 History, and General Literature 
 Published by Seeley &' Co Ltd 
 38 Great Russell Street, London 
 
 Some of the Contents 
 
 Elzevir Library ..... 
 
 • 5 
 
 Events of Our Own Times Series 
 
 6 
 
 Miniature Library of Devotion . 
 
 10 
 
 Miniature Portfolio Monographs 
 
 10 
 
 Portfolio Monographs 
 
 12 
 
 Seeley's Illustrated Pocket Library 
 
 . 14 
 
 Seeley's Standard Library . 
 
 • 15 
 
 The Publishers will he -pleased to post their complete Catalogue 
 
 or their Illustrated Miniature Catalogue on receipt 
 
 of a post-card 
 
CATALOGUE OF BOOKS 
 
 Arranged alphabetically under the names of 
 Authors and Series 
 
 ABBOTT, Rev. E. A., D.D. 
 
 Hints on Home Teaching. Crown 8vo, 3s. 
 
 Ho-w to Parse. An English Grammar. Fcap. 8vo, 3s. 6(i. 
 
 How to Tell the Parts of Speech. An Introduction to English 
 
 Grammar. Fcap. 8vo, 2s. 
 
 How to Write Clearly. Rules and Exercises on English Composition. 
 
 Crown 8vo, is. 6d. 
 
 Latin Gate, The. A First Latin Translation Book. Crown 8vo, 3s. 6d. 
 
 Via Latina. A First Latin Grammar. Crown 8vo, 3s. 6d. 
 ABBOTT, Rev. E. A., and Sir J. R. SEELEY. 
 
 English Lessons for English People. Crown 8vo, 4s. 6d. 
 ADY, Mrs. See Cartwright, Julia. 
 A KEMPIS, THOMAS. 
 
 Of the Imitation of Christ. With Illuminated Frontispiece and Title 
 
 Page, and Illuminated Sub-Titles to each book. In white or blue cloth, with inset minia- 
 tures. Gilt top ; crown 8vo, 6s. nett ; also bound in same manner in real classic vellum. 
 Each copy in a box, los. 6d. nett. ^ 
 
 " It may well be questioned whether the great work of Thomas a Kempis has 
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 ANDERSON, Prof. W. 
 
 Japanese Wood Engravings. Coloured Illustrations. Super-royal 8vo, 
 sewed, 2s. 6d. nett ; half-Unen, 3s. 6d. nett ; also small 4to, cloth, 2s. nett ; lambskin, 3s. nett. 
 
 ARMSTRONG, Sir WALTER. 
 
 The Art of Velazquez. Illustrated. Super-royal 8vo, 3s- 6d. nett. 
 The Life of Velazquez. Illustrated. Super-royal 8vo, 3s. 6d. nett. 
 Velazquez. A Study of his Life and Art. With Eight Copper Plates and 
 many minor Illustrations. Super-royal 8vo, cloth, 9s. nett. 
 
 Thomas Gainsborough. Illustrated. Super-royal 8vo, half-linen, 35.^ 6d. 
 
 nett. Also new edition, small 4to, cloth, 2s, nett ; leather, 3s. nett and 5s, nett. 
 
 The Peel Collection and the Dutch School of Painting. With many 
 
 Illustrations in Photogravure and Half-tone. Super-royal 8vo, sewed, 5s. nett ; cloth, 
 7s. nett. 
 
 W. Q. Orchardson. Super-royal 8vo, sewed, 2s. 6d. ; half-linen, 3s. 6d. nett. 
 ATKINSON, J. BEAVINGTON. 
 
 Schools of Modern Art in Germany. Fourteen Etchings, and many 
 
 Woodcuts. Imperial 4to, ;^t, iis. 6d. 
 
 BEDFORD, Rev. W. K. R. 
 
 Malta and the Knights Hospitallers. Super-royal Svo, sewed, 2s. 6d. 
 
 nett ; half-linen, 3s. 6d. nett. 
 
 BENHAM, Rev. Canon D. D., F.S.A. 
 
 The Tower of London. With Four Plates in Colours and many other 
 
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 Mediaeval London. With a Frontispiece in Photogravure, Four Plates 
 in Colour, and many other Illustrations. Super-royal Svo, sewed, 55. nett ; cloth, gilt 
 top, 7s. nett. 
 
 Old St. Paul's Cathedral. With a Frontispiece in Photogravure, Four 
 Plates printed in Colour, and many other Illustrations. Super-royal Svo, sewed, 5s. nett, 
 or cloth, gilt top, 75. nett. 
 
 2 
 
Seeley ^ Co Limited 
 
 BICKERSTETH, Rev. E. 
 
 Family Prayers for Six Weeks. Crown 8vo, 3s. 6d. 
 
 A Companion to the Holy Communion. 32010, cloth, is. ; roan, 2s. 
 
 BINYON, LAURENCE. 
 
 Thomas Girtin : His Life and Works. An Essay. With Twenty-one 
 Reproductions in Autotype. Imperial 4to, £i, 2s. nett. 
 
 Dutch Etchers of the Seventeenth Century. Illustrated. Super-royal 
 
 Svo, sewed, 2s. fid. ; half-linen, 3s. fid. nett. 
 
 John Crome and John Sell Cotman. Illustrated. Super-royal Svo, 
 
 sewed, 3s. fid. nett. 
 
 BIRCH, G. H. 
 
 London on Thames in Bygone Days. With Four Plates printed in 
 
 Colour and many other Illustrations. Super-royal Svo, sewed, 5s. nett; cloth, gilt top, 
 7s. nett. 
 
 BRIDGES, Rev. C. 
 
 An Exposition of Psalm CXIX. Crown 8vo, 5s. 
 
 CAMERON, D. Y. 
 
 Six Etchings by D. Y. Cameron and William Strang. With Bio- 
 graphical Notes. Imperial 4to, 6s. nett. 
 
 CARTWRIGHT, JULIA. 
 
 Jules Bastien-Lepage. Super-royal Svo, sewed, 2s. 6d.; cloth, 3s. 6d. nett. 
 Sacharissa. Some Account of Dorothy Sidney, Countess of Sunderland, 
 her Family and Friends. With Five Portraits. Demy 8vo, 7s. fid. 
 Madame : Memoirs of Henrietta, daughter of Charles I. and Duchess of 
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 Raphael in Rome. Illustrated. Super-royal Svo, sewed, 2s. 6d. ; half- 
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 Raphael : A Study of his Life and Work. With Eight Copper Plates and 
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 CESARESCO, The Countess MARTINENGO. 
 
 The Liberation of Italy. With Portraits on Copper. Crown Svo, 5s. 
 
 CHORAL SERVICE-BOOK FOR PARISH CHURCHES, THE. 
 
 Compiled and Edited by J. W. Elliott, Organist and Choirmaster of St. Mark's, 
 Hamilton Terrace, London. With some Practical Counsels taken by permission from 
 " Notes on the Church Service," by Bishop Walsham How. 
 
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 B. ifimo, sewed, fid. ; cloth, 8d. 
 
 The following portions may be had separately : — 
 
 The Ferial and Festal Responses and the Litany. Arranged by 
 
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 The Communion Service, Kyrie, Credo, Sanctus, and Gloria in 
 
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 3 
 
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 CHURCH, A. H., F.R.S. 
 
 Josiah Wedgwood, Master Potter. With many IlIustrations.TSuper- 
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 The Chemistry of Paints and Painting. Third Edition. Crown 8vo, 6s. 
 CHURCH, Rev. A. J. 
 
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 Nicias, and the Sicilian Expedition. Crown 8vo, is. 6d. 
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 COLVIN, Sir AUCKLAND, K.C.S.I., K.CM.G. 
 
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 "Should take rank as the standard history of our labours in Egypt"— The 
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 CORNISH. C. J. 
 
 Animals at Work and Play : Their Activities and Emotions. With 
 
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 Animals of To-day: Their Life and Conversation. With Illustrations 
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 Life at the Zoo. Notes and Traditions of the Regent's Park Gardens. 
 Illustrated from Photographs by Gambier Bolton. Fifth Edition. Crown 8vo, 6s. 
 
 The Naturalist on the Thames. With many Illustrations. Demy 8vo, 
 
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 The New Forest and the Isle of Wight. With Eight Plates and 
 
 many other Illustrations. Super-royal 8vo, 7s. 6d. nett. 
 
 Nights with an Old Gunner, and other Studies of Wild Life. With 
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 Crown 8vo, 6s. 
 
 Wild England of To-day and the Wild Life in it. With Sixteen 
 
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 CORROYER, E. 
 
 Gothic Architecture. Edited by Sir Walter Armstrong. With over 
 
 Two Hundred Illustrations. . Crown Svo, 6s. 
 
 4 
 
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 CUST, LIONEL. 
 
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 Paintings and Drawings of Albert Durer. Illustrated. Super-royal 
 
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 Albrecht Durer. A Study of his Life and Work. With Eight Copper 
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 DALE, J. M. 
 
 The Clergyman's Legal Handbook and Churchwarden's Guide. 
 
 Seventh Edition, Revised and brought up to date byj. S. Risley. 7s. 6d. 
 
 DAVENPORT, CYRIL. 
 
 Cameos. With examples in Colour and many other Illustrations. Super- 
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 Royal English Bookbindings. With Coloured Plates and many other 
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 DAVIES, RANDALL, F.S.A. 
 
 English Society of the Eighteenth Century in Contemporary Art. 
 
 With Four Coloured and many other Illustrations. Super royal 8vo, sewed, 5s. nett ; 
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 DAWSON, Rev. E. C. 
 
 The Life of Bishop Hannington. Crown 8vo, paper boards, 2s. 6d. ; 
 
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 DE LA BERE, K. BAGHOT. 
 
 The New Poultry Guide for British Farmers and Others. Showing 
 
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 DESTR^E, O. G. 
 
 The Renaissance of Sculpture in Belgium. Illustrated. Super-royal 
 
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 DODGSON, CAMPBELL. 
 
 The Etchings of Rembrandt By P. G. Hamerton, with an Annotated 
 
 Catalogue of all the Etchings by Campbell Dodgson. With Fifty Facsimile Repro- 
 ductions of the most notable Etchings in Photogravure. Only 250 copies printed, 225 for 
 sale, numbered 1-225. Double crown folio (20x15 ins.), £5, 5s, nett, 
 
 EARDLEY, WILMOT, Rear-Admiral S. 
 
 Our Fleet To-day and its Development during the last Half Century. 
 
 With many Illustrations. Crown 8vo, 5s. 
 
 ELZEVIR LIBRARY, THE. 
 
 Selections from the choicest English Writers. Exquisitely Illustrated, 
 
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 Volume I, Fancy &^ Humour of Charles Lamb. 
 
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 " Decidedly natty and original in get-up."— The Saturday Review, 
 
 5 
 
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 The War in the Crimea. By General Sir E. Hamley, K.C.B. With 
 
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 Great Britain in Modern Africa. By Edgar Sanderson, M.A. 
 
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 FARNELL, G. S. 
 
 Greek Conditional and Relative Sentences. Crown 8vo, is. 6d. 
 
 FERRAR, NICHOLAS. 
 
 The Story Books of Little Gidding: Being the Rehgious Dialogues 
 
 recited in the Great Room at Little Gidding Hall, 1631-2. From the Original Manuscript 
 of Nicholas Fekrak. With an Introduction by E. Cruwys Sharland, and several 
 Illustrations. Crown 8vo, 6s. 
 
 FLETCHER, W. Y. 
 
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 Bookbinding in France. Coloured Plates. Super-ioyal, sewed, 2s. 6d. 
 
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 FORBES, ARCHIBALD. 
 
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