PC ■ I i I ! .^-H'iilll']" III It i}-!ii.iyL!y i Ipilfe ■"'a? all T li'fi^WT re _7tr: ,1 111 # 111™. 41 li'i^^ik,,"'! ir,fi If' all lift: :, uH :^w^ j-tf imm OlarttEU Uttiowattg SItbrarg 3ti|ata, New ^atk BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND THE GIFT OF HENRY W. SAGE 1691 Cornell University Library arV13654 On the nature of things. 3 1924 031 275 864 olin,anx Cornell University Library The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924031275864 ON THE NATURE OF THINGS HUGH WOODS, M.D., B.A., F.C.S. Author of " JBther : Its Nature and Place in the Universe." NEW YORK : WILLIAM WOOD AND COMPANY MDCCCCXVIII , • PRINTED IN KNGLAND BV JOHN WRIGHT AND SONS LTD., BRISTOL PREFACE The title of this work is the same as that of the poem of Lucretius which was written more than two thouscmd years ago. Since that time men have gained a much clearer insight into " the nature of things " ; but while this is so in regard to the more complicated arrangements and properties of things, it is much less true in reference to their more funda- mental nature. Science has reaired its stiuctures, as builders erect towers, on foundations more or less solid, but without closely considering the exact nature of the foundations^ — how they were formed and from what. Much has indeed been written in discussion of such problems as the existence or non-existence of matter, but it is much more profitable to consider the nature ■ — so far as we can ascertain it — of what we call matter. We are aU prettj' well agreed as to what is matter, but there appears to be little agreement as to any comprehensive definition of matter, because we are not agreed as to what constitute its essential characteristics. Our knowledge of matter is limited to such characteristics of it as are directly or indirectly revealed to us by our physiccil senses, and even if we try to imagine other characteristics than these, they are only fantastic results of adding together. iv PREFACE taking away, multiplying, and dividing the impres- sions made by matter upon our senses. Mathematical science itself is ultimately founded on what we learn from our physical senses, and it is mere waste of time to try to go beyond the limita- tions of our senses unless and until some new sense arises in us, if indeed that were possible. Conse- quently we must accept, axiomatically, general principles which have to be accepted unless all human knowledge is regarded as founded on mere illusions. We can profess no knowledge of anything without agreeing upon the reality of such things as are ex- pressed by the words matter, motion, space, and time, or at all events of the sensations which give rise to such notions. If the sensations exist without any real corresponding existence which gives rise to the sensations it matters little, because experience shows that calculations founded on the supposition that there do exist real causes for the sensation prove invariably accurate so far as human senses are concerned. We shall therefore assume the exist- ence of the fundamental facts expressed by the words — space, matter, motion, and time, and we shall strive as far as possible to show how the whole of physical science can be built up without assuming more than these, and modifications of them. It can be shown that light, for instance, is a motion of matter, but it is, nevertheless, only by our senses that we are informed of the actual existence of that PREFACE V peculiar motion of matter which we call light. The same may be said of heat, dcdriciiv, magnetism, and chemical action. Why the eye perceives light, and the ear perceives sound, we need not for our present purpose tri- to explain. We know that it is so, if we know anything. It is, indeed, difficult to improve upon the funda- mental principles so accurately laid down in the beautiful verses of Lucretius, and it is perhaps pre- sumptuous to hop>e by our halting prose to open the eyes, or convince the minds, of those who, after a lapse of more than two thousand ^ears — during which science has steadUy proved and proved again and again the correctness of the essential doctrines of Lucretius — ^still have not eyes to see or ears to hear or minds to understand his teachings. Hugh Woods. TONBRIDGE, Augttst Tst, 1918. CONTENTS CHAPTER PACK I. — Space : Matter : Motion : Time i II. — States of Matte ii - i6 III. — Chemical Atoms and Molecules and Chemical Combination - 34 IV. — Modes of Motion 54 V. — Heat 70 VI. — Light 99 VII. — Electricity and Magnetism - 116 VIII. — Sound and other Physical Phenomena 137 IX. — Astronomy and Gravitation - 162 X. — Life and Vital Processes - 180 XI. — The Senses and the Mind - - 197 Conclusion - 215 Index - 246 ON THE NATURE OF THINGS. CHAPTER I. SPACE: MATTER: MOTION: TIME. Tangere enim et tangi, nisi corpus, nulla potest res. (Lucretius.) For nothing except matter can experience sensation or affect lie senses. A LTHOUGH it is from our sensations that we ''*- ultimately derive such knowledge as we have of space, motion, matter, and time, yet it is desirable to define as clearly as we can what is the meaning we attach to these words, and how far they are involved one with another. Space is perhaps the most difficult to define other- wise than by mere reference to the sensations common to us all, or by its relation to motion and matter. Matter again reveals itself to our senses chiefly by its occupation of space and its capacity for motion. Time requires for its explanation reference to matter, motion, and space. The fundamental property of matter, without which it would rightly be regarded as ceasing to be matter, is its power of occupying space to the exclusion of all else. Motion is change of the part of space occupied by a portion of matter when it leaves the place it occupies and passes on to occupy another place. Space chiefly manifests itself by making it possible for matter to cease to exclude I 2 THE NATURE OP THINGS other matter from a particular position. Without space motion could not exist, and without motion space would not be perceptible by us. Space and motion provide us with measures of time, and without space and motion time would be difficult to define. Time has no existence to an unconscious person, because he does not perceive the succession of sensations (which could not exist without movement and space wherein matter could move) whereby he takes note of time. Time is therefore the interval during which matter moves from one position to another. If matter did not move, time would not be perceptible to us. It is in fact only possible for us to appreciate intervals of time which do not fall below a certain limit, a very short but measurable interval of time being imperceptible by our direct sensations. It might seem to be sufficient to assert axiom- atically the existence of space, matter, motion, and time, taking it for granted that we are all agreed as to what these words mean, and recognizing that definitions confuse, sometimes, rather than explain fundamental conceptions derived from our elementary sensations ; but in the case of matter doubts have been thrown on its actual existence, and attempts have been made to show that its existence is not based on elementary sensation, but on a misinterpre- tation actually derived from motion. One of the most popular scientific theories of modern times is that matter is really a form of motion, and it is therefore necessary to have a definite conception of the distinction between matter and motion in order to avoid confusion. Yet, if SPACE: MATTER: MOTION: TIME 8 those who regard matter as consisting merely of motion try to define motion as it presents itself to our senses, they find a difficulty in doing so without admitting the existence of matter apart from motion. They must define matter as insensible motion, and motion as sensible motion, and they must admit that similar aggregations of insensible motion cannot at the same time occupy the same portion of space, or we should certainly have one kind of matter capable of neutrahzing another kind, so that matter would disappear as such, just as motion does dis- appear. There can be no reasonable doubt that its capacity for exclusive occupation of space is the essential feature of matter as it reveals itself to our senses. The old discussion as to how many angels could stand on the point of a needle showed that the denial of material existence to angels was equivalent to saying that they did not occupy space so as to exclude one another from the space they might at the time be occupying. There is indeed nothing absolutely essential to a definition of matter except its power of exclusively occupying space. Whether or not matter is ever motionless, there can be no doubt that matter might be motionless without ceasing to be matter. The form of motion which could give to our senses the impression which is in fact given by matter, would have to be so utterly different in its nature from what we are in the habit of calling motion, that there would be no advantage in caUing it by the same name. Motion, as apart from moving matter, is not actually conceivable by us as an5rthing but an 4 THE NATURE OF THINGS abstract idea, incapable of making any impression on our bodily senses. An immaterial idea might perchance be imagined as moving from here to Mars, but even then we irresistibly imagine a mind pos- sessed of that idea, and a mind without a corporeal investment is, like a mathematical point, without magnitude, with the additional disadvantage that it has not even a definite position. The smallest possible particle of matter may be imagined to be of any magnitude down to the mathematical point, which has, according to the old mathematicians, position but not magnitude. It therefore represents the imaginary infinity of sub- division of matter. The division of matter into particles which are infinitesimally small is theoretically possible, but to reduce matter to points as defined above would be (when interpreted) to annihilate it. This, so far as we know, is not actually possible, and there is certainly extremely strong evidence to prove that it is impossible. Even mathematically considered there is no possibility of infinite sub- division of matter, because infinity can never be attained ; or we may say that when a particle has been infinitely subdivided, further subdivision is impossible. Consequently, matter can never be annihilated by any process of subdivision. This amounts to saying that there must be particles of matter which cannot be further subdivided, if it be the fact that matter is indestructible as matter, or in other words, that there must be ' chemical atoms.' Experimental evidence strongly supports this view, and if by magnitude we mean, as of course we do, the occupation of a finite extent of space, we may SPACE: MATTER: MOTION: TIME 5 enunciate the following proposition and regard it as proved, namely : the smallest particle of matter has magnitude. It might however mathematically be supposed that though matter cannot be reduced to points, yet it might be reduced to occupying only one dimension of space ; that it might, in other words, be reduced to Unes, which have length only, without breadth or thickness. If matter could be reduced to lines, two linear particles of matter might intersect one another without displacing any part of either, or they might be superposed upon one another indefinitely without ever occupying any space, and by no possible arrange- ment could they ever occupy any space exclusively, which amounts to saying that they could not consti- tute matter as we know it. Therefore, if matter can be reduced to Hues, it can never be reconstructed again into matter, and it has therefore ceased to be matter, or has become annihilated. We may then confidently assert that matter has always more than one dimension in space. Neither can matter ever be reduced to mere surfaces without thickness, for similar reasons. Consequenth', matter has always more than two dimensions in space. Lest it might be imagined that matter might conceivably attain more than the three dimensions usuall}' attributed to space, we may point out that since the smallest particle of matter must be of three dimensions at least, no addition of matter can extend the dimensions of the particle in only one dimension. In emy case we need not waste time on the subject of a fourth dimension, which we cannot even imagine. It may therefore be regarded as 6 THE NATURE OF THINGS indisputable that the smallest existent particle of matter occupies exclusively some definite portion of space in three dimensions, and until motion, as apart from a moving portion of matter, can be shown to exist, or at least can be intelligibly imagined to exist, we must absolutely decline to be fooled by abstruse mathematical arguments into believing that motion, which is nothing more or less than the change of position of matter, is itself matter, or that matter is nothing but motion. We will not attempt to say how matter came into existence, or whether or not it has always existed as we now know it. It must however be regarded as actually existing, in the only sense in which we can regard anything as existing, and we have good grounds for beUeving that it is indestructible by any process of subdivision. We may also safely conclude that it is incapable of being created by any process of addition from what is not already matter, since this is involved in the 'statement that it cannot by any process of sub- division be converted into anything which is not matter. When we come to consider motion, as we have defined it, the question is somewhat different. If motion merely consists in a portion of matter leaving the space it has occupied to occupy other space, it might appear at first sight that it can be annihilated by simple process of replacing the matter in its old position ; but the movement or motion has evidently not been destroyed, for there has simply been equal motion in opposite directions, which is not the same thing as if the matter had never been moved. If a sword is moved from a position outside a man's body SPACE: MATTER: MOTION: TIME 7 to a position within his body, and then removed to its original position, it needs no argument to show that it is not the same thing as if it had never been moved at all. Motion of a portion of matter in an infinite vacuvun (if such were possible) would indeed be the same thing to our comprehension as if it had never moved, and motion can only be perceived by us, or have any meaning to us, when matter moves in relation to other matter. Motion in a limited vacuum would however have some meaning to us, even if imper- ceptible, because it would mean an increase or diminution of the distance between portions of matter. Motion can no more be destroyed than matter, space, or time. If motion has been stopped, it will, always be found that it has been changed into an equivalent of energy, in one form or other, which might under suitable conditions restore the motion again as before. Similarly, there is every reason to believe that motion cannot be created out of nothing. The motion of a particle of matter in a vacuum seems to involve nothing sensible beyond the change of its position relatively to other matter, and there are good grounds for believing that such motion would continue unchanged indefinitely. Matter at rest in a vacuum will continue at rest, and matter at rest, so far as our experience goes, can only be moved by other matter moving so as to displace it from its position ; unless we suppose that motion can exist as apart from moving matter, which seems an inconceivable supposition. Motion in fact can only be produced by pre-existent motion. 8 THE NATURE OF THINGS Some people will no doubt at once object to this statement, and will assert that there is evidence to show that matter at rest can be moved without other matter moving so as to displace it from its position, and it must be admitted that there are instances in which at first sight it would appear as though this were so. The action of a magnet, when it produces motion of steel particles, seems Uke the production of motion without a transmitted displace- ment of matter, but the magnet must first be moved before it can move a steel particle at rest, or some- thing else must be moved which allows the magnet to produce a motion which it did not produce previ- ously. The idea that ' attraction ' can take place otherwise than through the intervention of matter is unsupported by anything known to us through our senses, if we remember that matter is often quite invisible, and that it does not always give rise to a sensation of resistance to motion. We have defined motion as a change of position of matter, and we know of no instance in nature in which it can be proved that any particle of matter changes its position without another particle of matter changing its position. Motion has in all cases been preceded by motion, and if one particle of matter moves, it moves because another particle of matter displaces it more or less from the position it occupies by moving into that position. In some cases, as when a magnet moves particles of steel at some distance from the magnet, we cannot directly perceive the motion of matter which displaces the particles of steel by passing into the space occupied by them, as for instance when they are moved in an SPACE: MATTER: MOTION: TIME 9 apparent vacuum ; but we can indirectly show that an apparent vacuum is filled with gaseous matter, commonly called " aether,' to which the magnet directly imparts movement which is in turn directly communicated to the steel. The reasons for believing that an apparent vacuum is in fact filled with ' aether ' gas will be discussed later on. Let us now consider the case of two particles of matter, completely occupying exactly equal amounts of space, one of which is moving and the other at rest. If the moving particle strikes the resting particle, experience shows that, in the absence of resistance from other matter, the moving particle produces motion in the particle at rest, and in a perfect vacuum the two particles will move on together in the same direction at a rate which is exactly half of that of the moving particle, because the motion has been divided equally between the two particles. This cannot be proved directly by experiment, because a perfect vacuum of appreciable size cannot at present be obtained ; but it is merely the recognized rule, that the velocity, or rate of movement, of a body decreases in the proportion of the mass of the body moved, if we define mass as being the amount of space completely occupied (to the exclusion of all other matter) by the body, and vice versa. The usual methods of measuring mass can be employed, because they afford the most convenient way of estimating what amount of space a particular particle of matter does occupy exclu- sively. Thus we may regard the quantity of matter which completely occupies one cubic centimetre of space as being of equal mass with a similar quantity 10 THE NATURE OF THINGS of any other kind of matter which would, when sufficiently compressed, completely occupy a cubic centimetre of space ; even though we are not able in fact to compress the matter so that no other matter enters into its interstices. The mass of a quantity of homogeneous matter occupying one centimetre of space (in the ordinary sense of occupation, which is obviously not complete occupation) becomes doubled if the bulk of the matter is reduced by one half before it is inserted in the centimetre of space ; and so on. We are therefore only assuming a limit of compression at which matter, of whatever kind it may be, fills the whole space (which it occupies to the complete exclusion of any other matter of any kind) when we define equal masses of matter as the quantities of matter of whatever kind, which would, if sufficiently compressed to exclude any other matter, occupy equal space. Accordingly, if matter completely occupying one cubic centimetre of space impinge upon a similar quantity of matter in a complete vacuum, when the first portion of matter is travelling with a velocity V and the second portion is at rest, then, calling the first portion of matter m and the second portion m', the resulting motion of m and m' together, or v', equals ;j^, or since w= m', equals |. The recog- nized laws of motion are therefore in no way altered by our assuming that the unit of mass is the unit of matter which completely occupies the unit of space, or is the unit of space completely filled by matter ; or in other words, the unit bulk of matter so con- densed as to completely exclude any other matter from the unit of space which it occupies. SPACE: MATTER: MOTION: TIME 11 Weight can, however, under the system we propose, be expressed by the product of space completely occupied by matter multiplied by the velocity of a body falling to the earth in a perfect vacuum. To conform with ejcisting standards, the unit of weight may accordingly be defined as the space completely occupied by a gramme of matter, compressed so as to exclude all other matter from the space occupied by it, multiplied by the velocity of a particle of matter falling to the earth in a perfect vacuum at a definite distance from the centre of the earth. Again, since the gramme (or other standard of weight) is the unit 'to which, in practice, reference must be made, we may say that a gramme of matter, weighed in a perfect vacuum, divided by the velocity due to gravity, gives the space which a gramme of matter can completely fiU. In other words, two grammes of matter are necessary to fiU completely twice the space which is completely filled by one gramme of matter. The unit of absolute weight is therefore, in this system, the unit of space com- pletely filled by matter, multiplied by the unit of velocity. Consequently all the established laws of physics can be accepted under this system without any alteration, so long as we take as the unit of mass the space absolutely and exclusively occupied by matter, without reference to the kind of matter,. or the question whether or not it can, in actual fact, be compressed to the extent necessary to exclude all other matter so completely that if the matter itself were suddenly annihilated the space occupied by it would become a perfect vacuum. The accepted units of motion and energy seem 12 THE NATURE OP THINGS to require no new definition beyond what is involved, 3s far as energy is concerned, by the definition of matter given above. We must, however, at the risk of needless repetition, make quite clear what we mean by complete occupation or filling of space. Experience proves that all material bodies with which we are acquainted have interstices in their structure, and probably there is no such thing as space completely filled with matter until we come to the elementary atom. Nevertheless we can imagine space completely filled with matter by considering the various possible gradations between space completely filled with matter and space per- fectly vacuous. Of the two, perhaps, space which is a complete vacuum is the most difficult to attain. Let us suppose a solid body, the matter contained in which actually occupies completely only one half of its bulk, the other half of the space apparently occupied by it being really vacuous, and another solid body in which matter fills three-fourths of its apparent bulk, with the remainder vacuous. It will be readily admitted that if the two bodies are floating in a Uquid or gaseous medium of uniform density, the weights will be in the ratio of | to f , or of 2 to 3, if the apparent bulks are equal and the material of which the sohd bodies are composed is the same, or if the bodies are completely immersed and stationary. If, however, by some means the vacuous portions could be filled completely with identical matter in each case, or filled proportionately with identical matter, there should be no variation in the ratio of their weights, and if the liquid or gas in which they floated at rest were of uniform density. SPACE: MATTER: MOTION: TIME is their weight would, by the well-recognized physical law, be the same, and therefore simply measured by their bulk. The weight of a body is the mass multiplied by the velocity due to gravity, and the result is the same, whether we consider the mass uniform for bodies of different size while gravity varies, or consider the mass to vary and the velocity to be uniform. If therefore we assume, for the sake of argument,, that the velocity (due to gravity) of a body, occupy- ing the volume of one cubic centimetre, varies, inversely according to the completeness with which it occupies the space with the matter it contains, while the mass remains uniform, it will be seen that the weight remains the same as before. We should therefore not be assuming anj^thing at variance with experience if we maintained that the difference between a cubic centimetre iilled with gold and that of a cubic centimetre of aluminium arose, not from a difference in the masses, but from a difference in the velocity due to gravity when acting on gold and when acting on aluminium. If gravity were merely an ' attraction ' there would be nothing absurd in this supposition, because, we know how a magnet, for instance, attracts steel vigorously while not attracting gold at all. We are not going to contend that gravity has anything of the nature of an occult, or even magnetic, attraction,, but we mention this possibility as showing that the usual notion of ' weight ' is a very indeterminate one. If we assume, as we appear entitled to assume, that gravity acts on all matter equally apart from 14 THE NATURE OF THINGS the physical peculiarities of the matter, the theory which we shall put forward as an explanation of gravity merely requires the assumption, which is generally admitted, that the weight of a body, borne along in a liquid or a gas, completely immersed in it, must be equal to the quantity of Hquid or gas displaced by it. The term ' density,' in spite of artificial definitions of it, indicates the quantity of matter included in a definite space. Thus gold is more dense than silver, because a cubic centimetre of it contains more matter than a cubic centimetre of silver, and if a gramme of silver were compressed so that a cubic centimetre of it contained the same amount of matter as a cubic centimetre of gold, the compressed silver would have the same density as gold, although the silver retained its general physical and chemical pecuharities as compared with gold. There seems not to be any doubt as to what we aU mean when we speak of space, motion, and time, so far as their actual nature is concerned, and although there may be important differences of opinion as to the explanation of the way by which our senses recognize space, motion, and time, we will not here delay to discuss them, because we are concerned with the nature of things, and not with the processes whereby we recognize that nature, except so far as is needed to be sure that the things of which we discuss the nature are definitely defined, so that we may not attach different meanings to our words, and so fail in agreeing, merely because we are speak- ing of different things under the same names. The discussion of the processes by which we SPACE: MATTER: MOTION: TIME 15 recognize the nature of things is usually regarded as within the domain of metaphysics, whilst the study of the actual nature of things is the domain of physics. The study of metaphysics quite properly •comes after (iu^i"") that of physics, because we ought first to make sure what we can ascertain about the nature of things before we try to learn how we ascertain their nature. 16 CHAPTER II. STATES OF MATTER. Quapropter locus est intactus, inane, vacansque Quod si non esset nulla ratione moveri Res possent. — (Lucretius.) Wherefore there must be unoccupied space, empty, a vacuum. If this were not so, nothing could by any means be moved. TV /f ATTER exists in three different states which ^^ have well-defined characteristics to distinguish them, although there are intermediate conditions which, to some extent, exhibit characteristics of two of these states at the same time. The three states to which we refer are the solid, the liquid, and the gaseous states. There are, however, two conditions of matter which are not always so readily perceptible, but which are of fundamental importance when we come to consider how matter changes its state so as to alter a large part of its sensible characters. Matter may be either in motion or at rest, or, at all events, the degree of motion may vary from almost complete rest to very rapid motion. Probably there is no such thing in the universe, within the scope of our observations, as complete rest of matter. All matter, to the furthest limits of space within our utmost powers of perception, appears to be moving in some way or other ; but there are various modes or kinds of motion, which differ from one STATES OP MATTER 17 another as much as do different kinds of matter, if we consider the effect they produce on our senses. The movement of the earth, in its orbit round the sim, differs as much from that of the palpitating particles of a hot plate as does hydrogen from gold, to compare dissimilar things together. In order to examine the effects of motion, in its various forms, on matter, it is perhaps best to consider first what are the characteristics of matter at rest. We have no means at present of reducing matter absolutely to rest, and therefore we can only draw our conclusions from the results of diminishing the motion and varying its characters. The absence- of heat may be taken as an indication of the cessa- tion of motion, because motion of masses of matter,, unless absolutely unresisted, invariably produces, heat, which is itself merely a mode of motion. A body at absolute zero of temperatiire would be in a condition of complete rest, at aU events so far as the relation of its particles to one another is concerned, or as far as motion in a medium with any capacity whatever for resistance is concerned. Absolute zero is perhaps not attainable, but we can approach near enough to it to obtain evidence that a diminution of motion occurs pari passu with the diminution of heat, as of course must be the case if heat itself is merely a kind of motion. It does not, however, follow that the temperature of a body is a measure of the amount of its motion, although it is true that if there is no heat at all, there is no motion other them motion in a perfect vacuum, which, so far as we know, does not exist unless in very minute dimensions. At the Scime time motion 2 18 THE NATURE OF THINGS would not be possible if there were not some un- occupied space, because matter cannot move into a position already fully occupied by matter. We have referred to the three obvious and well- recognized states of matter — ^the solid, liquid, and gaseous states ; but these should not be regarded as indicating fundamental differences in the condition of matter. They are themselves due to different conditions of matter as regards motion. Matter at absolute zero of temperature is, as we maintain, devoid pf motion, and the imparting to it of motion in the form of heat converts it into liquid from the soUd state ; and a further addition of such motion converts it into gas. Heat becomes to some extent latent in the course of these changes in the condition of matter, owing to the form of motion characteristic of heat being changed into other kinds of motion, or into potential energy — that is to say, motion checked by opposing motion and itself rendered ' latent.' On the other hand, heat ceases to be latent and is set free. The resulting condition of motion and mobility determines the state of the matter as regards solidity, etc. A solid at a temper- ature above absolute zero does not consist of particles at rest, for the essence of heat is a to-and-fro motion of the particles of the heated body. Nevertheless it is evident that the particles of a solid, such as a block of stone or metal, have their movements closely restricted, or the shapes and sizes of such bodies would fluctuate more than they do. This arises from the fact that the particles of a sohd body under natural conditions enclose small con- glomerations of gaseous particles, and even, in the STATES OP MATTER 19 case of the more porous solids, of liquid particles. These, by the variations of the range of their move- ments associated with those of the solid particles, and consequently of the pressure they exert, cause a restricted expansion or contraction of the bulk of a solid body in the manner familiar to us when a sohd body has its temperature raised or lowered to an extent, on the one hand, insufficient to set the particles free from each other, as when a solid is turned into a gas, or, on the other hand, so as to bring them entirely to rest. The conversion of a soUd into a hquid is intermediate between the mere raising of the temperature of a solid and its conver- sion into a gas. In order to imderstand better how the solid, liquid, and gaseous states^ of matter can be explained by mere effects of motion, we must remember that what is usually called a ".vacuum ' is not in reality space devoid of matter. There are excellent grounds for believing that an ordinary vacuum, or the inter- sidereal spaces outside the atmospheres of the eeirth or heavenly bodies, are filled with gas composed of extremely minute particles, the exact size and nature of which we need not here discuss. This gas is com- monly spoken of as ' aether,' and owing to the fact that there are between its particles extremely mi- nute spaces truly vacuous, it has physical characters differing in certain respects from those of ordinary gases, which may be regarded as consisting of coarser particles dissolved or distributed in the aether just as particles of aqueous vapour, or other gases, are dissolved or distributed in air. A chemical atom may be regarded as the largest 20 THE NATURE OP THINGS particle of a given element — such as hydrogen, mercury, or silver — ^wliich exists without intrusion into its substance of aether gas. If, as seems to be the case, the particles of aether are the smallest existent (we need not say possible) particles of matter, it is evident that all the surrounding aether, if in motion, must, by its continual bombardment or pressure, tend to keep an atom undivided, unless a particle of aether can effect an entrance into the substance of the atom so as to detach one part of it from another. In other words, the aether exerts pressure on all sides on the atom after the fashion of gases in general. In the case of a sohd mass the more closely adherent particles enclose between them small quantities of aether at a lower pressure than that of the free aether gas outside them, and this produces adhesion Hke that of two smooth plates with a fine layer of imprisoned air between them, or the two hemispheres conmionly known as Magde- burgh hemispheres, which are firmly fixed together owing to the outside air being at higher pressure than the enclosed air. The coarser particles of a solid also adhere owing to the similar condition of the imprisoned air or other gases as compared with the outside air. Regarded in this way, it is easy to see how rapid movement due to heating of the imprisoned aether, or air, wiU first of all render the cohesion looser, as in the case of a Uquid compared vnth a sohd, and finally let loose the imprisoned gas and separate the particles, as occurs when the solid becomes itself gaseous, the coarser material particles floating amid the particles of the air or aether in a state of solution. STATES OF MATTER 21 which after all differs from what is called suspension only by the particles being so small as to become invisible, instead of being sufficiently large to be visible and to destroy the uniformity of appearance of the gas, and to produce opacity such as is seen when particles of watery vapour collect together sufficiently to produce cloud or mist in air. In the case of elastic solids, such as indiarubber, the condition resembles in its nature that of a liquid, but with the difference that the solid particles tend to return to their previous positions when the pressure or strain applied to them is relaxed. A similar kind of difference is observed in the case of glutinous Uquids as compared with aqueous or spirituous Uquids. In the case of these differences we have only to bear in mind that the ultimate particles or atoms (ajid consequently molecules, i.e., chemical combinations of atoms) no doubt vary both in size (within certain hmits) and in shape. There are good grounds for believing that the atoms of the heavier elements are larger than those of the hghter elements, and the fact that there appears to be a limit of atomic weight beyond which heavier atoms break up into lighter atoms (as in the case of radium) supports this view. On the other hand, the size of the sether atoms, unless there exist atoms still smaller than them, seems to be necessarily the limit of minuteness, because particles as small as aether atoms can hardly be broken up by the impact, or intrusion into their substance, of other sether atoms. As the pressure, or combined momentum, of the particles of different substances, when in the free or gaseous condition, must tend 22 THE NATURE OP THINGS rapidly to equilibrium, it is evident that there will be fewer heavy particles than light ones in a given space when the pressure is the same. Moreover, elements of lower atomic weight will, if this supposi- tion be true, tend to be less ready to become liquid or solid than elements of higher atomic weight, since there will be a greater tendency for large atoms to coalesce than for small ones, while at the same time, obviously, atoms of some shapes will more readily clog one another's movements, and tend to become mutually adherent, than atoms of other shapes. As an illustration, we may suppose a pint of water, in which float particles of a cubic shape, and a pint in which float the same number of particles, similar in size but of a spherical shape. Obviously the cubes would adhere much more readily than the spheres. Thus carbon atoms, or molecules, though smaller than those of quicksilver, would, consistently with what has been said above, become solid much more readily than atoms of quicksilver, if it be true that carbon atoms are cubes while quicksilver atoms are spherical. Before, however, discussing the shapes of the atoms of the various chemical elements, we have to consider the nature of chemical combination, and the distinc- tion between the modes of adhesion of chemically combined particles, and those of the coarser particles which adhere together although not chemically combined. We shall, however, first of all discuss a httle more fully the nature of ' adhesion ' of matter, and defer the consideration of the nature of chemical combination to the next chapter. Quite apart from any theory of atoms, it cannot be STATES OF MATTER 23 denied that the more minutely we examine masses of matter, of whatever nature it may be — whether animal, vegetable, or mineral— the more evident does it become that there is no direct continuity of matter, but that matter is built up like the walls of a house, of separate particles which for some reason or other adhere more or less firmly together until it becomes gaseous, and even then, with ordinary gases, it is evident that there is still cohesion between the atoms which together form the small masses or molecules of which the gases consist. There can be no doubt, however, that, as Lucretius insists, there must be empty space somewhere, or motion would be impos- sible ; and, indeed, if the smallest ultimate particles of matter have any variety of shape, it follows mathematically that there must be small vacuous intervals between these particles or atoms, unless there be an infinite series of atoms, decreasing in size step by step, to fill up the interstices between particles which do not fit precisely together. This infinite subdivision of matter seems impos- sible, and inconceivable, as Lucretius maintains, and as we have already pointed out. If we begin with a gas in which the tiny soUd particles float about quite loosely, and compress it without permitting the escape of the sohd particles through the walls of the compressing enclosure, it first of all becomes Hquid, and then solid, and the soUd itself can usually be compressed still further, up to an iU-defined Umit, increasing in density, in the inverse ratio of the space occupied by the solid contents of the com- pressing enclosure. In the process of compression we must suppose that the sohd particles are squeezed 24 THE NATURE OP THINGS more and more either into spaces which were pre- viously empty, or into spaces which contained some- thing that, in the process of compression, is forced out more and more completely. If the spaces are quite empty it is easy to explain the adhesion by the pressure of the gas outside without any counteracting pressure between the particles, but this would not explain a steady increase in the firmness of the adhesion as the compression increases, — ^unless we suppose an imperfect vacuimi at first, and a more perfect one as the compression increases. It has been customary, however, to imagine a power of attraction which acts only at extremely minute distances, increasing in intensity as the minute distances between the particles become more and more minute, and this kind of explanation is of course a very elastic one, as we can imagine any variation in the attraction which suits the facts, and can even supplement it by introducing imaginary repulsions to modify the attractions, if necessary. This way of overcoming difficulties is like the de- vice of the playwright who introduces a ' deus ex machina ' when he cannot conveniently invent natural means whereby his hero can be extricated from the perils with which he is surrounded. If we supposed that solid particles could still be compressed after they completely filled the space containing them, it would mean a process leading on to annihilation unless at last some limit of compressi- bility were reached. Moreover, we should have to suppose a different limit of compressibihty for different substances, unless we could by mere pressure convert elements of low atomic weight into elements of higher atomic weight. STATES OF MATTER 25 On the supposition that the atoms of ordinary matter float in a gaseous medium composed of extremely minute particles, we have only to suppose that, after the compression reaches a certain point, the gaseous meditun to which we refer, namely, the aether, is squeezed out so that the aether remaining between the soUd particles becomes lower in pressure than that which is outside them, though not com- pletely expelled imtil the last limit of compressi- bility is being reached. We shall explain later on that this leads up to chemical combination, as it is called, between the atoms, and that if the whole of the aether could be expelled, we should have the firmest possible adhesion due to the whole power of the aether pressure acting outside the united atoms, with none intervening between them. This might ultimately mean that one atom might be formed out of two or more atoms and the atomic weight of the resulting substance changed, or, in other words, transmutation of elements attained. This would, however, require the complete filling up of any vacuum intervening between the atoms. It may be, indeed, that by extreme pressiure atoms of lower atomic weight could be changed into atoms of higher atomic weight up to a limit dependent on circumstances which need not here be discussed, and on the other hand that by suitable methods atoms of higher atomic weight might be broken up to some extent. Be this as it may, it is certain that there is under terrestrial conditions a very moderate range of possible atomic weights, and recent discoveries as to radium, etc., go to show that when a high atomic weight is reached atoms tend to break up. There is also evidently a hmit of subdivision of atoms. 26 THE NATURE OP THINGS Further discussion of these interesting questions must be deferred to a later chapter. Under any conditions at present available, however, compres- sion of a simple gas such as hydrogen does not produce anything more in this direction than the association of hydrogen atoms so as to form mole- cules such as exist under usual circumstances. The process by which hydrogen is turned into a liquid, and then into a sohd, is simply a forcing together of the particles or molecules of it so that at last they form a solid mass, in which the particles adhere firmly while the pressure is maintained, and from which they soon begin to fly off when the pressure is relaxed below a certain point. This differs in no essential respect from what occurs if we subject to compression a powder, the interstices of which are filled with air. The air is squeezed out and the solid particles, instead of being blown about like dust or fluctuating almost like a liquid, form a firm solid mass. As the particles are more and more crowded together, there comes a time when the motions of the particles, relatively to one another, are limited in range, and finally they would be stopped altogether. If their motion were stopped absolutely, the temperature of the mass of particles would be absolute zero, for without motion there is no heat. The various states of matter, then, simply depend on the relative mobility of its particles. The fact is that our senses, from which alone we can derive any knowledge of anything, are affected solely by motions of matter in one form or another, and, this being so, the properties of matter of which we can STATES OF MATTER 27 take cognizance must be derived from motions of matter in one form or another. Whether it be by sight, touch, hearing, sensation of warmth (or cold), smell, or other physical sense, that we judge of the state of matter, we are merely judging by the percep- tion of varied motions of matter, some of which are perceived by one sense, some by another, either directly or indirectly. We may, indeed, safely regard matter as the only thing capable of affecting our senses, and conse- quently as the only thing of which we have any power of perception, for we are ourselves composed of matter, and there is not the slightest evidence that we have any power of thought or perception except through the matter of which we are composed. External matter reveals itself to us solely by its motion, or by its power of influencing the motion of the material particles of which we consist, and the vitality which is necessary to make our material bodies capable of perception and thought obviously consists in motion, for if the movements of certain parts of our bodies are stopped, all thought and sensation cease. Although it is of course true that we cannot perceive the final cessation of thought and sensation in ourselves, yet we can perceive in ourselves the gradual departure of them, and can note the evidences of their complete absence in others, coincidently with the cessation of vital move- ments, when death has fully established itself. There are in fact the strongest possible grounds for maintaining that nothing but matter (as we have defined matter) can be perceived by us, or thought of by us, and when we imagine the properties of matter 28 THE NATURE OF THINGS removed one by one by a process of mental abstrac- tion, we have nothing left when the last of the properties of matter is supposed to be taken away. In short, that which has not any of the properties of matter has no existence as far as we are concerned. As we have already pointed out, even space, apart from matter, would only be infinite nonentity, or an infinite vacuum. Although then they are due to vari- ations in the degree and character of the motions of matter, yet it is convenient nevertheless to retain the division of the states of matter into solid, liquid, and gaseous, because these states are readily recognized as distinct by our senses, and much may be learnt by noting what it is that produces changes which are in the main so well defined and so important. At the same time it is necessary to bear in mind that the fundamental changes which give rise to these changes of state are changes in the quantity and quality of the motion present, including in the terra motion the latent as well as the active forms of motion. To avoid confusion, however, it is gener- ally convenient to use the term energy to include both active and latent motion, reserving the term motion, in accordance with popular usage, for active motion only, whether such motion be perceptible by our unaided senses, or only capable of detection by indirect methods. As we have already explained, absolute absence of motion cannot be shown to exist, and it could appar- ently only exist at absolute zero of temperature, the existence of which is unknown. Matter at absolute zero of temperature need not necessarily be soHd, because it is easy to imagine a gas with a STATES OF MATTER 29- temperature of absolute zero, or, in other words, a set of atoms at rest relatively to one another, but with vacuous spaces intervening. It may, however, be reasonably believed that this condition of matter does not exist ; because, if matter outside such gas, and in contact with it, were moving, it must neces- sarily impinge upon (strike against) the resting atoms, and set them in motion, with a consequent rise of temperature. Probably no part of the universe could be absolutely at rest unless the whole of it were so. A soUd may be defined as matter adhering together with no motion of its component particles relatively to one another, unless within very minute ranges. The extent of the to-and-fro motions of the com- ponent particles of a solid are indicated by the temperature of the body, which only reaches absolute zero when the particles come to complete rest relatively to one another. A sohd moving as a whole in a complete vacuum might conceivably be at absolute zero of temperature ; but this could not be so if it were moving among other matter not moving in absolute unison with it, because the coUisions which would occur would no doubt set up oscillatory motion, or, in other words, rise of tem- perature. A sohd does not necessarily occupy with matter the whole of the space within its outer surfaces. Indeed, if it did so, as Lucretius has pointed out, there could be no motion within^its surfaces. There must be vacuous spaces within a solid unless we suppose a palpitating expansion and contraction of the sohd mass as a whole when the temperature changes. This does not occur, because. 30 THE NATURE OP THINGS if it did, the difference in this respect between a large solid block and a small one would soon become apparent. When we come down to the ultimate particle of matter — ^the atom — ^there is within its surfaces no vacuum. It completely fills the space within its surfaces. Consequently temperature does not exist as such when we consider an atom alone — -per se. A molecule, or little mass of atoms, is, however, capable of changes of temperature, even if it consists of only two atoms. An atom may therefore be regarded as at zero temperature, always of the same size, and always of the same shape, unless broken to pieces. It seems absurd to speak of an atom — an indivisible particle — being broken up, but the fact is not that it might not be broken up, but that it cannot be broken up without so changing its nature that we should regard it as something other than it was before, and that there appears to be a limit beyond which atoms cannot in fact be broken up under any conditions with which we are acquainted. The aether atom appears to be of this ultimate minuteness, though it is of course open to question whether or not there is a diversity of size even among aether atoms. This is too large a subject to enter upon just now. An atom then is necessarily solid, and completely fixed in bulk and shape. The bulk, as we shall try to show later on, determines its atomic weight, which is the measure of the space completely filled by it, or, expressed in another way, of the aether displaced by it. The shape of an atom, we shall contend subsequently, gives it special chemical STATES OF MATTER 81 properties, the surfaces being the determining factor in what is known as ' valency.' Of this also we shall speak hereafter. We must now discuss a little more fully the char- acter of the motions which occur respectively in solids, liquids, and gases. There may conceivably be no internal movement at all in a solid, as in the case of a single atom, or there may be the to-and-fro movements characteristic of heat. The same applies to liquids and gases in so far as the atoms individu- ally are concerned, since the atoms are in fact solid, although they are aggregated so as to form liquids or gases. In the case of solids, however, the adhesion is such that the component particles cannot move freely on one another without the mass being broken up ; whereas in a liquid the particles can move relatively to one another without fracture, with very great freedom in every direction ; while move- ments such as those due to heat cause them to change their relative positions rapidly without destroying their adhesion. In the case of gases the adhesion is so small that the sUghtest motion of a particle (not an atom) causes it to rebound from the particle with which it comes in contact. In nearly every case, with the same atomic composition and arrangement, the liquid occupies more space than the solid, and the gas very much more space than the hquid. Where this is not so, as in the case of water when compared with ice, the shape of the particles seems capable of producing the apparent anomaly. The whole series of changes from solid to hquid, and from hquid to gas, is analogous to that which 32 THE NATURE OF THINGS occurs in the solution of a solid in a liquid or in a gas. Again, when the solid particles are suf&ciently numerous in a given space they coalesce, and in the case of gaseous solution a liquid or soUd is formed, and, in the case of a Uquid solution, a sohd. In fact, we may simply regard the changes as being due to greater or less crowding together of coarser particles of matter in aether gas as a solvent, .^ther gas, however, differs from other gases in that, whereas the interstices of ordinary gases are filled with aether gas, the interstices of aether gas are vacuous, no particles of matter existing small enough to pass between its particles without separating them, or so as to act as a solvent for them in the ordinary sense of the expression ' solution.' Regarding sohds, liquids, and gases (other than aether), as solutions in aether, and bearing in mind the freedom with which aether passes through any enclosure, condensation of gases, liquids, and solids simply means an increase of the number of particles of matter within a given space. When the aggre- gation is sufficiently close the particles cease to rebound from one another freely, and, except at an open surface from which a number of particles are thrown off in the process known as ' evaporation,' they adhere firmly as a whole while freely changing partners or neighbours on any slight impulse. In this — ^the liquid — state, the intervals are filled with asther and other gases, at a lower pressure than that of the asther and other gases outside, and the particles consequently adhere after the fashion of plates or ' bodies from between which air has been pumped out, or otherwise partly expelled. This bond of union STATES OF MATTER 33 is at first very elastic, and allows pretty free move- ment, but when the difference of pressure in the intervals between the particles or bodies becomes greater, or the particles more crowded, the movement becomes more and more restricted until, in dense materials such as metals, or rocks, there is only such movement as occurs in soUd masses without perma- nent change in the relative position of the particles. Very finely ground solids become more and more like liquids in their movements, so that it is easy to see that, if the solid particles are made small enough, the solid must become a Uquid in all its physical characters. On the other hand, it is easy to note how the conglomeration of gaseous particles produces hquid droplets, and finally an ordinary liquid ; while suitable compression of liquids or finely ground solids, or even of gases, gives a solid mass. Again, by means of heat, or by the air pump diminishing the pressure on a liquid, the liquid readily breaks up into a gas. The difference between gases, liquids, and sohds is therefore — however such difference be produced — ^merely a greater or less crowding together of the minute soUd particles present in all of them, with the resulting inclusion or expulsion, compression or relaxing, of the intervening gas. In the formation of sohds, when expansion accompajiies the soUdification, there is necessarily an increase in the interstitial spaces, taken together, and a lowering of the pressure in the interstices. 34 CHAPTER III. CHEMICAL ATOMS AND MOLECULES AND CHEMICAL COMBINATION. Sic ipsis in rebus item jam Material Intervalla, Viae, Connexus, Pondera, Plagse Concursus, Motus, Ordo, Positura, Figurae Cum permutantur, mutari Res quoque debent. — (Lucretius.) So also in material things themselves, when Intervals, Passages, Connections, Weights, Strokes, Collisions, Movement, Order, Position, Figures Are changed, the things themselves must also be changed. /^HEMICAL atoms are particles of matter which ^^ entirely fill the space occupied by them, so that no other matter and no empty space intrudes within their limits. They cannot be broken up without entirely changing their nature. Being individually incapable of any internal movement, they are incapable of changes of temperature individually, or of imparting heat to other bodies or of taking heat from them so far as transference of heat to the substance of individual atoms is concerned. An individual atom may, indeed, be regarded as having the temperature of absolute zero since there are no heat-motions within its substance. Atoms may evidently differ from one another in size and shape. They undoubtedly differ in weight ; but Uttle appears to be known as to their size or CHEMICAL ATOMS AND MOLECULES 35 shape. Yet the weight of an atom, if we are right, depends entirely on its size or bulk. Setting aside the imaginary mutual attraction of particles or masses of matter, and regarding weight as merely an expression of the motion or energy existing in matter or imparted to it by blows of other matter inflicted on it, or the pressure exerted on it by other matter, we can only interpret ' gravity ' as being due to the impulses or pressure exerted on ordinary matter by the gaseous matter — aether — ^which fills space, except such part of it as is actually vacuous or is cilready filled by grosser particles of matter. By the ordinary laws of pneumatics the weight of an atom floating in aether is equal to that of the quantity of aether which it displaces so long as the aether gas is at uniform pressure. Any alteration of aethereal pressure must therefore alter the gravity of the atoms floating in the aether, and completely immersed in it, as soon as equiUbrium is established, so far as absolute weight is concerned but not rela- tive weight. In other words, if the pressure of aether is changed in any portion of space, the relative weight of an atom of gold and of an atom of oxygen remain unchanged in that portion of space, though their absolute weights, as expressed in terms of energy or motion, are changed. The same state- ment applies to the weight of an atom of aether itself (for the structure of aether is without doubt atomic), as regards both its absolute weight and its weight relatively to the atoms of gold, oxygen, etc., immersed in it. The recognized atomic weights of atoms are their relative weights, and do not change unless the atoms 36 THE NATURE OF THINGS are broken up. The absolute weight of a cubic centimetre of aether gas in the neighbourhood of the earth might be increased either by increasing its motion towards the centre of the earth, while the number of its atoms in a given space remained unchanged, or by increasing the number of atoms in a given space while the motion of them remained unchanged. In the same way, the apparent weight of a given quantity of lead would be increased if it were constantly impelled by an increased impulse towards the centre of the earth, or if more lead were introduced into it without altering its bulk, unless a similar increase took place in everything round about, when the change, though real, would not be apparent. With aether, just as with lead, the increased motion of a cubic centimetre of it towards the centre of the earth would not increase its apparent weight if there was a similar increase in the motion all about it. Yet the absolute weight would have been increased though the increase might not be perceived. The space occupied by an individual atom remains constant, and the weight of the atom (its atomic weight) is equal to that of the aether displaced by it when floating freely in the aether in the gaseous state. Therefore, if the oxygen atom is sixteen times as large as the hydrogen atom, and conse- quently sixteen times as heavy, it follows that a given volume of oxygen, as compared with the same volume of hydrogen, must, at the same pressure, contain the same number of atoms, or otherwise their weights would not be proportional to the atomic weights of oxygen and hydrogen, as they are proved by experiment to be. CHEMICAL ATOMS AND MOLECULES 37 Avogadro's law, therefore, follows necessarily from ■the supposition that atoms have atomic weights proportional to their size, and to the quantity of aether gas which they displace, the latter being evidently determined by their size. If the number of atoms in a given volume is increased, a larger amount of sether is displaced within that space, and sooner or later they must begin to clog the free movement of one another, thereby producing, first the liquid state, and then solidity, or the solid condition may follow the gaseous state directly, when conditions do not favour the production of liquid. If we suppose the whole of a given space finally filled with atoms, so as to exclude the whole of the asther, then these atoms would be indistinguishable from one large atom with size, and therefore weight, equal to that of all the atoms added together ; but this degree of crowding together of atoms cannot be ■effected by the means available to us, or we should be able to change two or more elementary atoms into one atom of an element of higher atomic weight. Two or more atoms may however be so approximated to one another as to form a chemical molecule, or, if the atoms are dissimilar, a chemical compound ; but in these cases the whole of the intervening aether is not expelled, some still remaining between the coarser atoms, though at a lower pressure than that of the aether outside the molecules. If it be the case that atoms of higher atomic weight are larger than those of lower atomic weight, an equal increase in the number of atoms in a given space must produce Uquefaction or soUdification sooner in the case of the elements of higher atomic 38 THE NATURE OF THINGS weight than in the case of those of lower atomic weight. In the main, experiment shows this to be the case, but there are many exceptions which at first sight appear wholly irreconcilable with this view. If, however, it be admitted (and it is very unlikely to be otherwise) that the shapes of elemen- tary atoms differ as well as their size, then the effect of such variety of shape seems quite adequate to the explanation of the exceptions, and if such exceptions guide us to assume for elementary atoms shapes which explain satisfactorily their chemical character- istics, it may well be that these ' exceptions may prove the rule.' We cannot, if we accept this theory, resort to the devices of those who use the ideas of ' chains,' etc., and imagine that the same elementary atom may at one time have one valency, and at another time another, just as it may suit our convenience. Having accepted a certain shape as the real one, we must adhere to it rigidly throughout. In order to examine the theory which we put forward, not as a mere graphical method of assisting our calculations, but as a statement of what we believe to be actual fact, it will be convenient to consider the recognized ' chemical elements ' first, so as to keep the question as simple as possible. In the first place, then, excepting xenon and krypton, we note that no element which is gaseous at ordinary temperatures is of a high atomic weight. The highest atomic weight — that of argon — is under 40. There are only two elements liquid at ordinary temperatures, i.e., bromine, whose atomic weight is about 79' 96, and mercury, whose atomic weight is about 200. All the rest are solid. Among the solids. CHEMICAL ATOMS AND MOLECULES 39 the ■ solidity,' in the sense of complete occupation of space without interstices, increases pretty uniformly with the increase of atomic weight. We find, how- ever, soUds occurring with quite low atomic weights, and we must therefore examine with the utmost care to see how this can be so if the bulk of the atoms varies directly with the atomic weights, and if solid- ity depends on the atoms clogging the free motion of one another owing to their size, or shape, or the numbers of the atoms compressed into a given space, or to some combination of these factors. We must also consider why so few chemical elements are liquid, when hquids are so common among their compounds at ordinary temperatures. Lithium has an atomic weight about seven times that of hydrogen, and according to our view this means that an atom of Uthiiun has a bulk about seven times as great as that of an atom of hydrogen. Therefore in the free state it displaces seven times as many atoms of aether as does an atom of hydrogen. In order to understand why Hthium should be solid, while chlorine or argon continue gaseous with much higher atomic weights, we have only to suppose that a lithium particle is of a thin flaky character, or perhaps of a spinous shape, so that, like flakes of snow, the specific gravity of it is very low, although it forms a soUd mass. The specific gravity of a substance, it should be remembered, represents the number of air or water molecules displaced by the particles of the substance, just as the atomic weight represents the number of aether particles displaced by the atoms. The molecules of an element consist, as has already been explained, of atoms whose 40 THE NATURE OP THINGS surfaces adhere, like plates, owing to the lower pressure of the aether gas intervening between the fiat surfaces of the atoms as compared with the pressure of the aether outside, just as, in air, two surfaces, of larger dimensions than those of atoms, adhere because the pressure of the air between them is lower than that of the air outside. The same explanation applies to the chemical combination of atoms of different chemical elements. If this is the true explanation of the nature of chemical combination, the shape of the combining atoms will to a great degree determine the chemical properties of the combining substances. If, for instance, it is assumed that the shape of an atom of a chemical element is that of a hemisphere, it will obviously be ready to combine with one other atom only, — or, in chemical language, be monovalent. If the atom is shaped like a penny, or has two flat surfaces, it will be divalent. If the atom has three flat surfaces, it will be trivalent ; if four flat surfaces, tetravalent ; if five, pentavalent ; and so on. To return to lithium, the shape of its atoms, which suggests itself as best suiting its ascertained physical and chemical properties, is that of a needle with a flat base. The shape of the hydrogen atom, which is also monatomic, may for many reasons be regarded as probably hemispherical with a flat base. The bulk of the lithium atom, since its atomic weight is y'o^, will be about seven times as great as that of the hydrogen atom, and the shape of the resulting molecule will be graphically represented by a figure in which the bases of the hemisphere and the needle are in apposition, one end of the molecule being Uke CHEMICAL ATOMS AND MOLECULES 41 the sharper end of a needle, and the other rounded off like the surface of a hemisphere. The size of it will be about eight times that of the atom of hydrogen. There is then no flat surface remaining to which another atom, such as that of hydrogen or that of lithium, might attach itself. If the atoms of lithium are packed together so as to form small masses, they will from their shape, if the flat end is much thicker than the sharper end, leave consider- able spaces free for the entrance of small gaseous atoms or molecules, and the intervening channels will be longitudinal, and such as to allow, as we shall explain later on, fairly free conductivity for electric currents. It must not, however, be supposed that we wish to dogmatize as to the shapes of particular atoms. The subject is obviously a difficult one, requiring much study and research. At present we only desire to show that the shapes of atoms may be the deter- mining factor of chemical and physical characters of chemical elements and their compounds. If we regard oxygen atoms as being shaped like pence, the bulk of each atom being sixteen times that of an atom of hydrogen, we have obviously in oxygen a divalent element, and water (H^O) molecules will be Uke pence with a hemispherical atom of smaller size attached firmly to each flat surface of the atoms of oxygen, shaped like pennies. Molecules of such shape will obviously be readily mobile one on the other, so as to afford the characteristic properties of a Hquid. We must, however, explain how hydrogen peroxide (K^O^) is formed, and this is very easily done by supposing two penny-shaped atoms attached 42 THE NATURE OF THINGS by the flat surfaces on one side of each, and hydrogen atoms attached to the two surfaces left free. The peculiar property which carbon atoms possess of building up the almost innumerable compounds which form the subject of organic chemistry, serves as a severe test of the theory which we are advocat- ing, and the shape of the carbon atom which suggests itself as capable of meeting the difficulty is the pyramid. We may in passing note that the ancient Egyptians, whose knowledge of science was probably in many respects far in advance of what followed the destruction of their knowledge, seem to have had a special veneration for the p5rramid. If we suppose the carbon atom to be a pyramid, such as is formed by dividing a cube into two equal parts, it is easy to see that an exceedingly numerous set of compounds may be formed if we regard the atomic surfaces as forming the basis of union in chemical compounds. If hydrogen atoms (hemi- spherical and one-twelfth of the bulk of the carbon atoms) are attached to the surfaces of a carbon atom, the compound is, from what has already been said, sure to be either Uquid or gaseous, and probably gaseous, as the bulk of the molecule is only that of an oxygen atom, which, when joined with another oxygen atom, to form a molecule, is still gaseous. While a carbon atom unites readily with four hydrogen atoms, it unites with only two atoms of oxygen, to form carbon dioxide ; and assuming the shape of the oxygen atom to be like a penny, and that of the carbon atom pyramidal, the bulk of the oxygen atom being ten as compared with twelve, the bulk of the carbon atom, it will readily be understood CHEMICAL ATOMS AND MOLECULES 43 that the oxygen atoms will extend well beyond the edges of the surfaces of the carbon atom to which they are attached, so that other oxygen atoms will be prevented from coming into close apposition with the two remaining free surfaces of the carbon atom. Carbon has the peculiarity of forming practically innmnerable so-called ' organic ' compounds, its pyramidal shape allowing its atoms to be built up into solid structures of various shapes and sizes, in a way which would be impossible for hemispherical, cylindrical, or even cubic atoms ; and the compounds of carbon therefore afford excellent opportunities of testing the question whether in the main the filling up of space with increasing numbers of atoms tends to produce solidity. If, for instance, we take a series such as that of the ' paraffins,' the increase in the number of atoms in a given space should cause a gradual passage from gas to liquid, and from liquid to solid. The paraffin series shows this tendency very distinctly. In working out the shapes of the atoms of other chemical elements, carbon will also probably be the best from which to start, especially as in its case we have strong independent evidence as to the probable shape of its atoms, if atoms have any fixed and distinctive shapes at all. Carbon in the crystalline form is rhomboidal, and pyramidal atoms would readily build up a rhomboidal mass in compact form. The amorphous condition can easily be understood by considering the difference between a niunber of pyramids thrown together in disorder, and the same when built up together in a regular compact manner. The exist- ence of the two principal crystalline forms of 44 THE NATURE OF THINGS carbon, viz., graphite and diamond, helps to remove the doubt we might at first entertain, as to whether the carbon atom is a pyramid or a cube or rhomboid in shape. The possibility has of course to be borne in mind that, within certain limits and subject to uniformity of bulk, there might be more than one shape for the atoms of the same element ; but, as crystals consist of large numbers of atoms, it is well to be sure that the various crystalline forms of the same element cannot be built up of atoms of identical shape before we fall back on the assumption that atoms of the same element can have different shapes. If the atoms of one element had different shapes under different conditions, there would surely be associated with the different shapes differences in chemical properties, just as in the case of compounds which yield the same quantities of the same elements on analysis, and yet differ fundamentally in their properties. Like many of the questions which arise in this brief discussion of the Nature of Things, the subject of the shapes of atoms is far too large for us to do more here than touch upon it, and we will therefore pass on with the remark that the shapes of the elementary atoms probably do not vary. This is not unnatural, because in the course of unlimited time the shape would no doubt conform to the surrounding conditions. We will now proceed to consider a little more fully the way in which the shapes of atoms influence their physical and chemical properties. The elements which are liquid at ordinary temperatures are probably composed of atoms spherical in shape, or CHEMICAL ATOMS AND MOLECULES 45 with rounded surfaces. Mercury, which has a high atomic weight — 200 — and whose atom is therefore a large one, has properties strongly suggestive of a spherical shape of its atoms. If we assume the mercury atoms to be spherical, it is not surprising that the atoms of mercury do not adhere so as to form molecules consisting of more than single atoms, because two spheres of equal size do not readily stick together in the way in which we contend that atoms forming molecules do adhere. Mercury atoms do, however, adhere firmly enough to atoms of several other elements, as for instance those of oxygen; but we can easily imagine an atom of oxygen shaped like a penny adhering to a sphere of a bulk twelve times, and more, greater than its own. Bromine, another element which is Uquid at ordinary temperatures, does form molecules of two atoms, and therefore in its case the spherical shape is not indicated. A shape which suggests itself as the probable shape of a bromine atom is that of a needle with round cylindrical stem and a small flat base at one end, the other end being sharply pointed. Hydrobromic acid — ^HBr — would then be formed by the attachment of a hemispherical hydrogen atom — which has a bulk only about sir of that of a bromine atom — by its flat base to the flat end of the bromine needle, thus rounding off this end bluntly. The bromine molecule, formed by two bromine needles adhering together by their flat ends, would form a double-pointed needle with a length twice that of the bromine atom. A molecule thus constituted would be readily broken into two, and the easy setting free of the atoms would tend to increase the 46 THE NATURE OF THINGS chemical activity of the element, or in part to account for it. The cylindrical shape of the bromine molecule and the absence of any flat surfaces on it accounts for its liquidity notwithstanding its fairly high atomic weight or atomic bulk. We may here digress a little to remark that it is quite natural to suppose that atoms have in general either regular mathematical figures, or at all events definite crystal- line structure — so to speak — because in the endless inter-bombardment of atoms, and their conglomera- tions, those regular figures which offer the most compact resistance would no doubt be much more fitted for survival in the inter-atomic or inter- molecular struggle which is constantly going on. We will next consider what are the fundamental characteristics the possession of which in common accounts for the strong family likenesses which are exhibited between the members of the well-recog- ndzed chemical groups. Similarity of shape seems much the most probable characteristic which will explain satisfactorily the grouping which evidently exists. Let us consider first of all the grouping of the elements according to 'atomicity' or "valency." Monatomic or monovalent elements agree in having a flat surface, and only one, which serves as a means of adhesion with a flat surface on other atoms, whereby the atoms become, as it were, linked together. Divalent elements have two such flat surfaces, and so on. Hydrogen, lithium, fluorine, sodium, potassium, chlorine, bromine, silver, iodine, rubidium, and caesium are examples of monovalent elements, and their compounds exhibit a consequent resemblance to one another. The important differ- CHEMICAL ATOMS AND MOLECULES 47 ences between these elements may in the first place be accounted for by the differences between their atomic weights, but this is not sufficient. Within the monovalent group there are well-marked sub- groups. There is for instance a very close similarity in certain properties between fluorine, chlorine, bromine, and iodine, or again between hthium, sodium, and potassium. Evidently the similarity in valency does not fully explain the resemblance between members of one of these sub-groups or the difference between members of the two groups. Neither does the difference between the atomic weights, or between the sizes of the atoms, suffice to explain the resemblances or the differences. If, however, we suppose the atoms of the elements to have characteristic shapes of their own, it becomes possible to give a satisfactory explanation of the special characteristics of the sub-groups. In considering what shapes will best explain the physical and chemical properties of the members of sub-groups of monovalent elements, and of individual monovalent elements, we are limited by the consider- ation that each atom has only one flat or " combining ' surface, and that the size of each atom is measured by the atomic weight of the element of which it is an atom. We have already put forward the supposi- tion that the hydrogen atom has one flat surface, the remaining surface being rounded so that the atom is somewhat hemispherical in shape — ^Uke a spUt pea for instance. This may be taken as the type on which the monovalent atoms are formed ; but we may suppose the rounded surface drawn out from the flat surface until the shape is that of a 48 THE NATURE OP THINGS needle. The area of the base, or flat surface, may evidently vary relatively to the whole surface of the atom, and we may have forms varying from a short cylinder, with one end rounded and the other flat, to a long tapering needle. Such variations of shape of course involve variations in the inter-atomic connections, and the inter-molecular intervals, with consequent differences in chemical, electrical, and other physical properties. We have already spoken of the probable shape of the lithium atom. The atom of sodium and that of potassium are also probably needle-shaped. The crystalline forms of the elements themselves do not readily suggest this, but, as the smallest crystals consist of large masses of atoms, this is not a fatal objection by any means. The compounds of sodium and potassium with hydrogen give needle-shaped crystals, and for reasons which will not here be discussed the indications afforded by the shape of the crystals of these compounds are not without significance. When we come to fluorine, chlorine, bromine, and iodine, and the other monovalent elements, we must remember that the base or flat surface may be square or triangular, or of some other angular shape, and pointed needle-shaped atoms may have a corresponding angular shape throughout, like that of many crystals. The variation of the shape of needle-like atoms, from the round needle to the six, twelve, or other sided needle, with the consequent variation of the interspaces when a number of the atoms or molecules are packed together in bundles, would evidently affect the density of solid masses of the elements as well as their other physical and CHEMICAL ATOMS AND MOLECULES 49 chemical properties. The variations in the proper- ties of the same element may be accounted for by the different positions in which the molecules of the elements can be firmly set, according to their shape, without supposing the atoms themselves to have more than one figure. At the same time it is of course possible, or at least quite conceivable, though improbable, that the atoms of the same element, while all of exactly the same bulk, and similar in type, might yet to some extent vary in figure, and such variation in figure might account for what are called ' allotropic ' forms. We cannot, however, here enter into a discussion of this very wide subject. Recurring to the consideration of individual ele- ments, we have reason, in the case of oxygen, to think that its atoms are shaped like pence, and we. may evidently have a series of elements formed in this type, extending into cylinders with two flat ends, but, as well as the round cylinders, we may have shapes built upon the type of a square instead of a circular penny, or there may be more than four sides meeting one another angularly. This would lead us to expect to find two or more parallel series of divalent elements, and we find magnesium, calcium, strontium, and barium forming a well- marked series, with copper and other divalent ele- ments forming a less well-marked and smaller series. Mercury has individual peculiarities which require special consideration separately. The same thing applies to the monovalent elements, among which we find a pair of groups, one containing hydrogen, fluorine, chlorine, bromine, and iodine, and the other hthium, sodium, potassium, and silver. The trivalent 4 50 THE NATURE OF THINGS do not appear likely to include liquids, because the three-sided shape is by no means conducive to free movement of the atoms or molecules one on the other. It fonns an essentially solid group, and like the pentavalent and hexavalent groups, it includes but few different elements. The tetravalent elements have probably shapes of the type of the pyramid as regards the atoms, and six-sided as regards the molecules. These will evidently vary according to the equality, or otherwise, of the four flat surfaces, the forms with equal sides being evidently capable of forming very solid structures, and very varied compounds, with other elements. Carbon has prob- ably four-sided atoms, with at least three of the sides equal ; while silicon, sulphur, and tellurium not im- probably have atoms with the sides of the pyramids and of the base of different sizes. There are perhaps three main groups of them : carbon representing the one group ; silicon, sulphur, selenium, and tellurium the second ; and aluminium, iron, cobalt, nickel, tin, lead, and platinum the third. The pentavalent elements may probably be divided into two groups : those with a base and four sides forming angular cones, and those with five flat sides and no definite base. To the former group probably belong arsenic and antimony, nitrogen and phosphorus belonging to the other. The irregular shape of nitrogen and phosphorus would thus account for their explosive compounds, and, in the case of nitrogen, its shape, combined with the small size of its combining surfaces, would explain its so-called inertness. Hexavalent elements are few in number, including chromium and manganese, which have CHEMICAL ATOMS AND MOLECULES 51 very distinctive chemical characteristics. Sulphur is capable of being regarded as either tetravalent or hexavalent, and, if we remember that a molecule of a tetravalent element with two atomic surfaces joined together, and so apparently obliterated, becomes practically hexavalent, this is not to be wondered at. The fact that no element can well be regarded as truly heptavalent, or of still higher valency, strongly supports the view that it is the flat surfaces that form the basis of combination, rather than that there are connecting links or bands such as are commonly imagined in explain- ing chemical constitution ; although, indeed, their actual existence can hardly be said to be seriously assumed as having any reality. A solid body with more than six sides would, unless large, have small surfaces, and atoms with more than six sides would be Ukely to have not more than six surfaces available for combination, the remaining surfaces being covered by the overlapping edges of the atoms combined with them. Again, with hexa- valent atoms which have only six flat surfaces, it is evident that when combined with relatively large atoms, some of the surfaces may be thrown out of action by the attached atoms overlapping the surfaces to which they are attached, and so prevent- ing atoms from becoming attached to the overlapped surfaces after the close fashion required for chemical combination. Thus a hexavalent atom may with some atoms become tetravalent, or even divalent. Experience leads us to the conclusion that this kind of thing does actually occur. On the other hand, if this theory of the nature of 62 THE NATURE OF THINGS chemical combination is true, a monovalent atom cannot increase its valency, nor can a divalent one, or one of higher valency. A divalent atom, again, is not likely to become monovalent. A trivalent element might however quite conceivably have, in addition to its three main combining surfaces, two other flat surfaces much smaller in area, and it would, therefore, not be surprising if trivalent atoms some- times appear to be pentavalent ; but in tliis case, if the atom appears to be pentavalent, it is to be expected that two of the live atoms combining with it will be small atoms relatively ; that is to say, atoms of low atomic weight. We will not here attempt even to enumerate the many and various inferences which may be drawn from the supposition that the combining powers of atoms depend on the flat surfaces which they possess : the reader wiU be able to work these out for him- self, and to follow them out in the more complex conditions of compounds, the valency of which will similarly depend on their free flat surfaces, which may be either simple or compound, consisting either of the surfaces of single atoms, or of surfaces compounded of the surfaces of two or more atoms combined together, so that some of their flat surfaces are con- tinuous. We may, however, mention the compound ammonium, which acts almost as though it were an element, its molecule combining Uke an elementary atom in most respects. Here we have the pentavalent atom of nitrogen combined with four atoms of the monovalent hydrogen. Evidently there is one surface still unoccupied, and it is probably a surface not quite similar to the four other flat surfaces (as may easily CHEMICAL ATOMS AND MOLECULES 53 be the case in a five-sided atom) . By the intervention of an oxygen atom, a fifth atom of hydrogen readily attaches itself to the compound, forming NH4OH. If we now examine a little more closely the position we are assuming — the atom of nitrogen is in bulk fourteen times that of the hydrogen atom — and as the fifth side is probably smaller than the other four sides, it is not likely in any case to be much larger than the flat surface of an oxygen atom. Having regard to the probable shape and size of the four hydrogen atoms, and of the oxygen atom with a hydrogen atom fixed to one of its two flat surfaces, the compound NH4OH is pretty sure to consist of non-adherent molecules, and to be either gaseous or liquid — probably gaseous. If, however, we substi- tute a chlorine atom for the OH group, the chlorine atom being, as we have suggested, of a shape Uke a needle, then the compound, ammonium chloride, the resulting molecules of which are by their shape pretty sure to clog one another's movements, may be expected to form a solid at ordinary temperatures, especially as the bulk of the chlorine atom is rela- tively rather large. Whether we are right or wrong in our guesses at the shapes of particular atoms, it will be seen that if chemical combination is due — as we contend it is — ^to the close juxtaposition of the flat surfaces of atoms whose bulk is measured by their atomic weights, then the shapes of the atoms will suffice to explain the main chemical properties of the various chemical elements. 54 CHAPTER IV. MODES OF MOTION. Prima moventur enim per se Primordia rerum Inde ea quee parvo sunt Corpora conciliatu Et qucisi proxima sunt ad vireis Principiorum Ictibus illorum caecis impulsa cientur : Ipsaque quae porro paullo majora lacessunt. Sic a Princlpiis ascendit motus, et exit PauUatim ad sensus, ut moveantur Ilia quoque in solis quae lumine cernere quimus ; Nee quibus id faciant plagis apparet aperte. — {Lucretius.) For in the first place the elementary atoms move of them- selves, And through them the particles which are formed by a small congregation of atoms Which are, as it were, next to the force exerted by the elementary atoms. And are driven along impelled by the blind strokes of the latter. These in turn drive along particles a little larger than them- selves. Thus from the elementary atoms the motion ascends and passes Gradually to the senses, so that those particles are set in motion Which we can perceive moving in the rays of the sun. Nor is it quite clear by what interchange of movement this is effected. TVyTOTIGN, like matter, can neither be made nor ■'■*-'■ destroyed, but as matter can be changed so that it seems to disappear, so may motion be altered so that it no longer appears as perceptible motion. At first sight, moreover, it appears as though equal MODES OP MOTION 66 amounts of motion acting in opposite directions destroyed one another, but it is easy to show that while actual change of position of the matter which had been moving may no longer occur, yet at the same time the matter is being steadily impelled to the same extent as before. Allowance must be made for an3' part of the motion which has been converted into other forms of motion such as heat, etc. To avoid confusion, the term ' energy ' is used to include both those cases in which actual change of position, or motion in the common sense of the word, occurs, and those in which no apparent change of position is observed. It must, however, be remem- bered that no portion whatever of the motion has been destroyed. Motion, as we have said, has no existence as apart from motion of matter, and it may therefore be regarded quite correctly as a state or condition of matter. All matter appears to be possessed of energy, even though it seems under certain conditions to approach indefinitely near to a state of apparent absolute rest. It is improbable indeed that matter does in fact exist anywhere in the universe in an absolutely fixed position in space. In discussing the various modes of motion, it is best to begin, as Lucretius suggests, with the elementary particles or atoms of matter. If, as some maintain, chemical atoms are themselves composed of still more minute particles, or are in fact not true atoms, we ought to begin with the most minute particles which can be shown to exist ; but we are not prepared to accept the view that the atoms of chemical elements might be further subdivided without converting or ' transmuting ' them into 56 THE NATURE OF THINGS different elements. In any case, the ultimate particles — primordia rerum — are, we maintain, such that they completely occupy a definite amount of space, being neither divisible without complete change of their nature, nor capable of any contraction or expansion. They cannot, therefore, experience any internal motion, but can only move relatively to other matter external to themselves. It is conceivable that the earth, with all the tur- moil of motion of its constituent parts, might move in empty space without any interchange of energy between it and the sun and the other planets and the stars, but it is evident that this is not so in fact, for the rays from them undoubtedly produce motion on the earth, and, as motion cannot be created anew, this motion must be transferred from them to the earth. Evidently then there is all the time moving matter between them and the earth ; for motion, as we have already explained, has no existence, even in the imagination, as apart from moving matter. We may conclude, therefore, that the atoms, larger than those of aether, floating about in the intersidereal gas — the aether — (which by its power of transmitting the motion which gives rise to light is proved to fill space as far as the universe extends), naturally tend to become agglomerated, because it is only when an atom meets an atom of its own size or larger than itself that its motion is counterbalanced or overcome. The consequent formation of spherical or spheroidal celestial bodies (including of course the earth itself) can be best explained as due to cyclonic disturbances in the aether gas ; but the way in which this occurs will be explained in another place, because if we MODES OP MOTION 57 turn aside to explain all such matters as they arise in the course of our aipiment, there will be no end to our digressions. When once these conglomerations of atoms have been formed, as we see that they have been formed, there are large masses floating in the boundless ocean of aether, which beats upon them as the sea beats upon the land, imparting motion to them, and in turn receiving motion from them. Within these large masses motion continues with many modifications, each modification being capable, tmder suitable conditions, of being converted into the other modifications, or, as we may call them, ■ modes ' of motion. How far these modes of motion can exist in the free aether itself requires careful consideration, because it by no means follows that a mode of motion which readily occurs within a solid mass of atoms, will also be possible within a free gas such as aether, .^ther gas may or may not consist of atoms all of the same size and shape ; but its atoms are evidently much smaller than those of hydrogen. Thej? are so small that they pass with freedom through openings amd interstices which are quite impassable even to hydrogen atoms, and they are present between atoms which are separated only by the atlmost infinitesimally small distances which distinguish the association of atoms, constituting a chemical molecule, from a single atom with a bulk equad to the bulks of such atoms in the form of one single atom. The fact that the aether intervening between atoms, united in chemicad combination, is no longer free, shows, however, that the aether atoms have a size which, minute ais it is, yet prevents their passing freely between the surfaces of larger atoms 58 THE NATURE OF THINGS when these are closely approximated. The modes of motion, then, from which we have to start are those which can occur in a free ocean of gas composed of very fine particles. The nature of the movements which are likely to exist under such circumstances can be inferred by analogy from the movements which we observe in the free atmosphere, and, to a large extent also, from those in an ocean of water. Movements which take place in the open air are obviously greatly modified when they pass into restricted channels and air spaces. Thus the move- ments of extended torrents of air, or water, when driven into narrow channels, or through the branches and leaves of trees, and so on, produce new modes of motion quite different in their effect on our senses from those due to more unrestrained movements in the free atmosphere or in the open ocean. The sounds produced by wind in the pipes of an organ, or in a wire stretched across the course of the wind, impress our senses in a manner quite different from the impressions produced by wind passing through the upper regions of the terrestrial atmosphere. The movements of the free aether may be classified as movements of transference, and movements arising from collisions of particles of aether, from one cause or another, moving in different directions or at different rates. The latter class includes wave motion, and movements characteristic of heat. With regard to movements of transference, there are strong grounds for believing that the aether within and beyond the furthest limits of our powers of observa- tion is not stationary, but is flowing rapidly not only in one definite direction, but at the same time MODES OF MOTION 59 in circular or elliptical whirls, resembling to some extent the movement of air in cyclones, etc. It is these cyclonic movements which collect particles coarser than those of the aether into masses such as those of the sun, stars, and planets, and which keep up the movements of these in their orbits, revolutions, etc. We will not here discuss the reasons for adopting this view, or show how, on this theory, a satisfactory explanation of the laws of gravitation is afforded without assuming the existence of an imaginary attraction of matter for matter. All that need be assumed at present is the familiar fact that matter in motion, by its impact on matter, imparts to it motion, and has its own motion modified at the same time, according to laws which are for the most part well understood. General motions of trans- ference of aether, in the regions of free aether, no doubt resemble similar movements of air in the free upper regions of the terrestrial atmosphere, carry- ing along with them various bodies floating in the aether gas. When, however, any difference occurs in the density of the aether, that is in the number of its atoms contained in a given space, any atoms larger than the particles of aether wiU receive more blows from the aether on the denser side than on the less dense side, aind the laurger atoms will consequently be driven into the less dense regions of aether ; and if the aether recovers equihbrium, they will not move back again, because the impact will then be equal on all sides, though the energy of the larger atoms will no longer be the same as that of the aether round about, whether such motion, or energy, be 60 THE NATURE OP THINGS Tendered latent or not, because motion cannot be destroyed. Two atoms, larger than the particles of aether, when thus driven into the regions of lower aetheric pressure, will stop each other when they collide, if equal in size, and will imprison between their colliding surfaces a little asther at lower pressure than that of the aether external to them, thus forming molecules or producing chemical com- bination. On such a nucleus a large mass may be built up in the aether ; atoms, and combinations of atoms, larger than those of the free aether, gradually congregating together until equiUbrium is attained. These masses, in turn, will be subject to the impacts, or pressure of the surrounding aether, reacting of course upon the aether according to the usual laws of motion. These masses will, according to their average density, which is determined simply by the propor- tion of the space occupied by them and completely filled with matter, tend to pass into regions of lower pressure in the aether ; but we have now to reckon with the resultant motion acquired, and the masses will evidently move along in the direction in which the resultant of the combined forces acting upon them guides them. On these lines, it must be admitted that all the movements of the stars and planets, with gravitation, both terrestrial and inter- sidereal, can be explained by the proved and accepted laws of motion, without assvuning any occult force, or anything but what we know by our bodily senses to exist. If our readers accept our contention that the weight of an atom is determined by the dimensions of the space which it occupies to the exclusion of all MODES OP MOTION 61 other matter, the accepted teachings of science remain much as before, but they will be seen to have a solid foundation resting on fundamental facts which our bodily senses make it impossible for us to doubt in our daily life, even if we delude ourselves into fancying that we can, theoretically at least, know anj^iing at aU which is not directly, or indirectly, derived from our bodily senses. The particles of matter which are formed by the aggregation of atoms, larger than the atoms or particles of aether, with intervening particles of aether, become congregated into large masses forming the sun, stars, and planets ; and we have next to consider what will be the effect of movements, in the free surroimding aether gas, acting on these masses of larger atoms, and on the aether particles which fill more or less completely the interstices in the masses, gaseous, liquid, or sohd, as the case may be. The aether gas is here confined within channels and inter- stices of very varied shape and capacity, and the motions of which the aether, so confined, is capable, will evidently be determined by the character of the spaces which it occupies. The stroke of a wave upon the open mouth of a tube blocked at the other end, and filled with liquid or gas, is, in its essence, the same as the stroke of a hammer on the head of a nail which has its point resting upon solid material which it does not penetrate. It causes a very rapid longitudinal vibration. An oblique stroke is partly, and a transverse one is altogether, analogous to a sharp pluck upon an elastic string fixed at the ends, and this form of vibration is the Scime in its general features as that of light. The wave in the external 62 THE NATURE OF THINGS ffither, which produces a light wave by its impact on a tube or thin layer of aether confined within inelastic boundaries, is not necessarily itself a wave of light. Again, an aether wave, striking a mass of particles thrown together like the sands of the sea- shore, causes a to-and-fro motion of these particles and of the intervening particles of aether, producing what we perceive as heat. Movements of condensation and rarefaction of aether in a longitudinal direction give rise to elec- trical waves, and magnetism results from the effects of movements of aether when the movements are circular hke those in currents carried round in the coils of a wire wound about the core of an electro- magnet. Light and heat probably do not exist as such in the ocean of free aether outside the terrestrial atmosphere. They are due to modifications of the movements of the free extra-terrestrial aether pro- duced by these movements acting upon the aether confined within the channels and interstices of the coarser matter which forms the gaseous, liquid, and solid material of which the earth, planets, etc., are composed. This is merely a more general state- ment of what has been already recognized by writers on physics. Thus, in Daniells' " Text Book of Physics " we read that " It would perhaps indeed be more correct to say that we designate under the one name — heat — two totally distinct forms of energy. The one of these is the energy of a wave motion in the ether, passing from a hot body to surrounding objects across the intervening space, as from the sun to our earth, or from a hot fire to the colder objects upon which it shines : this we call radiant MODES OF MOTION 63 heat. The other form is that of a confused oscilla- tory disturbance of the particles of a body ..." The transverse vibrations of light could hardly occur as such in the open aether. They are probably due to the separation of the elementary movements of aether waves when passing into the channels and interstices of cocirser matter. Light vibrations, in turn, when they strike masses of matter which are not transparent — that is to say, matter which has no regular layers of aether capable of light vibrations — are destroyed as such, and converted into heat vibrations. Electricity appears to be due to differ- ences in the density of the asther, with resulting currents and vibrations, the latter of which may be either " electrical,' or those of light or beat, accord- ing to circumstsinces. Electrical waves are waves in the aether similar in nature to the waves of extra- terrestrial aether, and they are similarly liable to be broken up into heat or light vibrations, etc. Neither heat, hght, nor electricity can penetrate the chemical atom. They can impart to it move- ment, or energy, and if the atom is near the higher limit of atomic size, they might even break it up into atoms of smaller size, ' transmuting ' it into atoms of other elements. The movements due to Ught, heat, and electricity can of course be modified into other forms of motion, producing sound, chemical changes, and so on. A mode of motion to which vast results have been attributed is that due to what is called ' attraction.' Matter is said to attract matter, and we are asked to beUeve that a mighty rock is perpetually strain- ing to unite itself to a neighbouring rock, or that a 64 THE NATURE OF THINGS point, or line, at the centre of the earth, is hugged by the rest of the terrestrial matter in its vain strivings to attain this highly coveted position. It is not enough, however, to ridicule the idea of ' attraction ' (ludicrous as it is unless regarded as a mathematical figment, introduced to serve a purpose similar to that served by a; or 3/ in algebra), because there is undoubtedly something which draws or pushes matter towards the centre of the earth ; and iron filings undoubtedly do fly to embrace the end of a magnet. Chemical atoms, previously dissociated, unquestionably do press up to one another, and become firmly attached together. We must therefore either content ourselves by saying that we believe that some unknown force or forces impel them to act in this way, and that we choose to call such force or forces ' attraction,' or we must try to explain what actual causes are at work. The latter expedient, if practicable, is of course the more satisfactory, and in order to obtain a clue to the actual causes, we cannot do better than look round for some similar effects, the causes of which can be more definitely recognized. We know how a ship at sea, in a storm, seems sometimes to be, as it were, drawn or attracted on to the rocks, or a boat in a river into the centre of a whirlpool, and we observe how a limpet attaches itself to a rock. These suggest to us that asther gas, moving like an aerial cyclone, or a whirlpool, may draw terrestrial matter towards the centre of the earth, or that the higher aetherial pressure outside a chemical molecule may press the atoms firmly together when the pressure of the aether between them is reduced MODES OF MOTION 65 below that outside them. Further investigation shows that this explanation conforms to the resiilts of critical observation so as to render it in a very high degree probable. Agciin. when matter ' repels ' matter a satisfac- tory explanation is obtained when we consider that the stroke and coimterstroke of material particles, in motion between the repelling and repelled matter, suffice to account for the apparent repulsion of two larger bodies between which such particles intervene. The introduction of imaginary forces, or lines of force, is no doubt in such cases useful for mathe- matical purposes, when no definite explanation is available ; but when these come to be regarded as. actually existing, scientific progress is seriously- retarded. Newton himself was obviously not satisfiedL wth the idea of attraction of matter for matter as. actually explaining gravitation, and he adopted it merely because it enabled him to study satisfactorily the laws of gravitation. There are, however, some, in these days, who seem to maintain that any real knowledge of what we call the nature of things is impossible, and to them of course the most convenient fiction is the best. Arguments founded on fictions, or imaginary facts, can however only be regarded as fictitious, tmtil they are shown to be in accordance with facts. Heat, light, sound, electricity, jmd magnetism, all depend for their phenomena on the motion or energy of matter, and our special senses of sight, hearing, etc.. enable us to distinguish special forms or modes of motion within certain limits ; but the fact of this special perception of special modes of motion should 5 66 THE NATURE OF THINGS not lead us to forget that the sEune modes of motion may — and they undoubtedly do — exist outside the hmits by which our powers of so perceiving them are restricted. Our senses enable us to perceive certain movements of matter, including the movements of matter which forms part of our own bodies, and also checks to movements, whether these imply transfer of motion, or conversion of movement into latent energy. Our senses perceive nothing else, whether it be our sight, or our hearing, or our touch, or even our minds, that are affected. Motion which we cannot directly perceive may, however, be indirectly revealed to us when motion which we cannot perceive is con- verted into motion which can be appreciated by one or other of our perceptive senses. We cannot, for instance, directly perceive the individual movements of chemical atoms, and it is only, as' Lucretius puts it, when the congregation of the elementary atoms comes up to the size of the motes seen in the sun- beam, that we begin to recognize their movement by our sight. Even when we increase the range of our vision by the help of the microscope, it is only comparatively large congregations of atoms that make the so-called ' Brownian movements ' faintly perceptible by our sight. The invisible world no doubt far exceeds with its movements the visible world, both in extent and in importance, but yet it is only by our senses that we can have any knowledge of the existence of such world, or of its movements. Any attempt to go beyond the knowledge derived directly, or indirectly, from our physical senses, is Uke mathematical calculations such as the multi- MODES OP MOTION 67 plication or division of infinity or zero, empty and unprofitable. A mode of motion or energy to which we may here refer is that which has been recognized under the terms 'capillary attraction,' 'surface tension,' etc. Here we have matter apparently overcoming the impiilses of gravitation, and proceeding without obvious reason to some extent in a direction opposite to that in which gravitation urges it. If a very narrow (capillary) tube of glass be plunged in water, or other Kquid that wets the glass, the level of the liquid is perceptibly higher inside the tube than outside it, to a degree which varies with the nature of the liquid and the diameter of the tube ; and if the tube containing the liquid is suspended in the air, with an open end downwards, a droplet wiU heing outside the tube without falling. In the case of larger tubes the level near the sides is not the same as that of the liquid further away from the sides. When the Hquid wets a tube the surface in the tube is concave ; when, as with mercury and glass, the liquid does not wet the tube, the surface is convex. In the case of Uquids containing salts in solution, the salts will sometimes creep up the sides of the vessel, and even over its edge. These phenomena are evidently due to the difference between the adhesion of the molecules to the Wcills of the vessel, and their adhesion to one another. This adhesion should not be attributed to any imaginary ' molecular attrac- tion,' but simply to the different degree of pressure of gas, between the molecules and the walls of the containing vessel, and between the molecules them- selves. A river, flowing swiftly between its banks. 68 THE NATURE OP THINGS becomes quite concave in the middle, because of the friction between the water and the banks and the adhesion of the water to the banks. In a tube, the surface of the liquid is concave if the adhesion of the liquid to the walls of the vessel is more powerful than that of the particles to one another, and convex if the reverse is the case. The pressure of gravity in the one case is least resisted in the centre of the liquid ; in the other case it is least resisted at the edge. Elasticity, in solids such as indiarubber, is due, not to stretching and resiliency of the atoms of which it is composed, but to the lowering and raising again of the pressure of the sether, or other gas, imprisbned between the atoms, molecules, or particles of which the rubber is composed. We will, however, discuss these phenomena more fully in a later chapter. We refer to them here to dispose of the idea that there is in them any occult ' attraction,' or source of motion or energy, other than that transference of motion or energy from matter to matter which takes place, without increase or diminution of the motion or energy concerned, according to well-known laws. It is not necessary for our purpose to refer here to other modifications which motion undergoes under special conditions. Enough has been said to show that aU motion of matter is derived solely from previous motion of matter, whether it reveals itself to us as light, heat, electricity, magnetism, sound, or any other of the modifications of motion which affect our senses directly or indirectly. Our senses cannot indeed perceive anjrthing but motion, or the tendency to motion spoken of as pressure or strain. MODES OF MOTION 69 The eye perceives the motion characteristic of light ; the ear perceives the motions of sound, and so on. The movements which we do not directly perceive must, however, be more than those which we do so perceive, and the ordinary laws of motion are no doubt applicable to unperceived motions, just as they are to those which are most obvious to our senses. 70 CHAPTER V. HEAT. Turn porro quaecumque Igni flammata cremantur, Si nil prseterea, tamen ex se ea corpora tradunt, Unde Ignem jacere, et Lumen summittere possint ; Scintillasque agere, ac late differre Favillam.^ — (Lucretius.) Moreover, whatever things are burnt up, set in flames by fire, If they yield nothing else, yet give out from themselves those particles of matter Whereby they are able to cast forth Fire and to emit Light, And to throw out sparks and cast ashes far and wide. TLTEAT, like other forms of motion, and like matter, -*■ -*■ cannot be created or destroyed, though it may be produced from motion, or energy, which has not the characteristics of heat : and it may be changed so as to disappear as heat in the ordinary sense of the word. Heat cannot exist as such in the chemical atom, though an atom can by its movements produce heat. If an atom were capable of becoming hotter or colder — ^er se — ^it would mean that movement was possible within the atom itself, and that would inevitably be accompanied by expansion and contrac- tion of the atom, and this, again, would produce variation of the atomic weight. When, however, atoms join together to form molecules or chemical compounds, with aether gas imprisoned between them, there can evidently be variation in the close- HEAT 71 ness of approximation of the atoms to one another, and any motion of the imprisoned aether must tend to cause such variation. The ' molecular weight ' would therefore be more variable than the ' atomic weight,' and there are reasons for thinking that this is so, but a discussion of the question would involve too great a digression. If a number of atoms were brought together at absolute zero of temperature, which would mean complete absence of motion, relatively to one another, they might remain free, whatever their shape might be ; but if they are not at rest, or moving uniformly together, the resulting collisions will force out some of the aether intercepted between their surfaces, and so produce adhesion, in the manner described as characteristic of the union of atoms to form molecules or chemical compounds, except when the shape of the atoms prevent it. Heat outside a molecule, must mean increased bombardment of the molecule by the surrounding particles, aetheric or other, and this must produce an alternating compression and rebound upon the component atoms and the inter- vening aether. If this process becomes sufficiently violent the atoms wiU be forced asunder, and the imprisoned aether will be set free. Cooling, or slackening of the heat movements, wiU cause the atoms to adhere again, and the molecules vdll be re-formed. The same of course applies to chemical compounds. This is quite analogous to what happens in regard to the adhesion of smooth surfaces in air. If they are sharply brought into collision, they adhere ; but if they are violentty plucked or forced asunder, they separate again. 72 THE NATURE OP THINGS The theory that molecular and chemical combina- tion depend on the imprisonment of aether, or at all events of some form of gas, between the component atoms, and groups of atoms, necessarily impUes that a sufficiently high degree of heat vibration must cause dissociation of the atoms, and there is abundant evidence that such dissociation is in fact produced by heat. On the other hand, if atoms attracted one another with nothing intervening, there would be nothing, in the case of atoms of the same size and weight, to lead to their dissociation, even if the bombardment from without became extremely violent owing to increased heat. They would move together as one atom. If, however, heat causes expansion of a molecule, it may be argued, as we have already indicated, that on our own showing the weight of the molecule must increase because it is now larger, and must displace a larger bulk of aether ; but it is evident that the increased bulk of aether gas displaced will now contain a smaller relative number of aetheric atoms, though the range and vigour of the movement of the aetheric atoms has been increased. When we pass beyond the molecule to the various congregations of molecules, the move- ments due to heat become more complex, consisting, outside the molecules, of the heat movements of the aether which is more or less free, its movements within the molecules, and the movements of the molecules relatively to one another, and so on, until we reach the readily perceptible effects produced by heat on large masses of gaseous, liquid, or soUd matter, the character of which effects we need not here discuss. HEAT 73 Heat is usually described in text-books under the names Radiant Heat, Diffused Heat, Latent Heat, etc., and in considering the nature of heat, it is necessary to explain how these varieties in the manifestations of heat arise. The heat transmitted from the sun to the earth is an example of radiant heat, and heat is also radiated from fires, heated ■ radiators,' and so on. It is perhaps best to consider the nature of heat radiated from the sun in the first place, because it is free from some of the complicating circimistances which must be taken into account when we deal with heat radiated from a fire or any hot terrestrial substance. The waves of the sea, breaking upon the shore, are not heat waves, but yet they produce heat. Similarly, the aethereal waves which beat upon the earth are not heat waves, though they are largely converted into heat when broken up by their impact upon the earth. Part of the energy of these waves pla3's its part in producing or modifying movement of the earth, part produces light, and part produces heat. The light itself is partly converted into heat when it strikes the ground, and during its passage through the air. Radiemt heat, Uke light, consists of alternating condensations and rarefactions of aether constituting what are described as waves or transverse undula- tions. The wave-length, that is the distance between the points of greatest and least condensation, is greater in the case of heat waves of radiant heat than in the case of light, the distinction between them being due to the fact that the eyes cannot perceive, as light, waves whose wave-length is above a certain point. The so-called " chemical 74 THE NATURE OF THINGS rays ' are those of shorter wave-length, whose wave- length renders them most effective in influencing the sether imprisoned between the atoms which constitute molecules and chemical compounds. Diffused heat to radiant-heat waves are what the motions of the surf and sand are to the waves of the sea. The waves no longer pass along in regular sequence, but they are broken up and give rise to confused to-and-fro movements in all directions, producing expansion, rise of temperature, and all the familiar results whereby heat is distinguished from cold. Cold, in its popular significance, is of course, like heat, merely a relative term, indicating a lower degree of heat-movements, as compared with heat, which indicates a relatively higher degree of such movements. Latent heat is heat which disappears as such, being expended in altering the state, or arrangement, of the substance in which it becomes latent. Like other forms of motion, heat in under- going this change is not in the least degree annihil- ated, and if the substance is restored to precisely the same state, or arrangement, as before, the exact amount of heat, which had become latent, again reappears as what is often called ' sensible ' heat, — that is to say, as active heat-movements, or as some other form of energy. Heat produces important changes in the state of matter, turning sohds into liquids, and solids and liquids into gases. The way it produces these changes is by imparting motion to the particles of which matter is composed. At " absolute zero ' there are no heat movements ; and at this temperature, if it could be attained, matter would, perhaps, exist in HEAT 76 the solid state only. As the temperature rises from a point near absolute zero, the particles of a solid begin to move to-and-fro in relation to one another, any aether, or other gas, imprisoned or intervening between them partaking in the movements. When the range of their movements becomes wide enough to allow one particle to pass clear of another, in the movement of approach to one another, the condition of liquefaction begins, and the readiness with which this commences, and the mobility of the resulting liqxoid, depends to a large extent on the size and shape of the particles. When a beam of light is focussed on ice, it melts away certain portions first, revealing beautiful cr\'stalline forms ; but although this probably indicates a difference in the size and shape of larger particles, or conglomerations of particles, it does not imply difference in the size and shape of the molecules of water ; for when the ice is completely melted, there are no longer any indications of any difference of size or shape in the separate particles of water. The nature of the adhesion of particles of ice, though similar in character to that of atoms in a molecule, is not identical, since the imprisoned gas, in the former case, is not only much more loosely enclosed, but no doubt includes water vapour, and air, or a mixture of them. This quite accounts for the adhesion of the ice particles being destroyed by an amount of heat which produces no dissociation of the atoms forming the water molecules. When a liquid is heated, the casting off of molecules, or minute particles, of water increases, and finally the liquid is converted into gaseous molecules free from 76 THE NATURE OF THINGS -the modified adhesion which there is between them in the liquid state, and, with extreme heat, even the molecular adhesion may at last be overcome, the atoms themselves being set free. Let us now see what effect heat produces on gases when the temperature is raised to a degree insufficient to produce dissociation of atoms, or when the gases are ones in which the atoms are already dissociated, as in the case of the gaseous forms of monatomic elements. We find that the quantity of heat required to heat a given volume — say, one litre — of a gas, up to a certain temperature, is half of what is required to produce the same temperature in two volumes — ^two litres — of the same gas when com- pressed into the same space as before — one litre. The reverse would be the case if the atoms in one of the two litres coalesced with those in the other litre so as to produce the same number of atoms— though now doubled in size and atomic weight — ^in the space of one litre. In general, the quantities of heat required to raise given weights of different simple gases through one degree of temperature (that is to say their ' specific heats ') are inversely, and not directly, as the sizes of their atoms or their atomic weights. We have already given our reasons for believing that the weight of an atom depends on the quantity of jether displaced by it, or in other words on its size, and we need not therefore repeat our reasons for adopting this view. We may consequently assert that the specific heats — ^for equal weights — of different simple gases are, roughly speaking, inversely as the size of their atoms. Accordingly, HEAT 7T the heat required to raise one htre of hydrogea through one degree of temperature will be sixteen times that required to raise a litre of oxj-gen, measured at the same temperature and pressure through the same range of temperature, if it be true that a given volume of all simple gases — measured at the same temperature and pressure — contains the same number of atoms. This is simply saying that the quantity of heat-energy required to impjirt the same amount of heat-movement to two atoms is inversely proportional to their atomic weights (or sizes). At first sight this seems Uke sa^dng that the energy required to give a certain movement to a. heavy body is less than that required to give the same movement to a lighter body in the proportion of their weights. We must, however, first of all consider what a degree of temperature really means, when we speak of a gas being raised one degree in temperature. Temperature is usually measiu^d by the expansion of mercury, alcohol, or air, and we must consider what is indicated by such expansion. The same rise of temperature causes mercury, within certain limits, to expand tsW of its bulk at zero centigrade, and causes air to expand ^ttt of its volume at zero, but it is most convenient to consider what heat expan- sion mecins in the case of air, or other suitable gas, because the question in this case is less compUcated. If the volume is kept constant, the pressure increases in the same regular fashion as the increase of volume when expansion is allowed, and this is independent of the weight (or size) of the atoms or molecules. Consequently, in order that their pressure may be 78 THE NATURE OP THINGS equal, the smaller atoms must be moving with a velocity greater than that of the larger atoms in the inverse ratio of their sizes or atomic weights. Therefore, if the quantity of heat requisite to give a velocity to the smaller atoms greater than that of the larger atoms in the inverse ratio of the sizes of the atoms is also in the inverse ratio of the atomic sizes or weights, it follows that the same quantity of heat produces the same increase of momentum in the case of small atoms as it does in the case of the larger atoms. If the atoms vary only in size, the difference of their weight being due only to the amount of aether gas displaced by them, this amounts to saying that equal quantities of heat produce equal momentum in all particles floating in sether, what- ever their size may be. A litre of hydrogen gas requires sixteen times as much heat to raise its temperature one degree as does a litre of oxygen, measured at the same temperature and pressure ; but for the same number of atoms of hydrogen, which have only one-sixteenth the weight of those of oxygen, to possess the same momentum, the velocity of the hydrogen atoms must be sixteen times as great as that of the oxygen atoms if we neglect the aether. As long as gases are so perfectly gaseous that equal volumes of them contain equal numbers of atoms, we need only consider the movements of the aether in wMch the atoms float, the movements of the atoms depending on the movements of the aether, as a small particle in the sea moves with the waves of the water in which it floats. If this be so, it would seem to follow necessarily that when two gases such HEAT 79 as hydrogen and oxygen are at the same temperature and pressure, the atoms or molecules of hydrogen must be moving with a velocity sixteen times as great as that of the oxygen atoms or molecules. The question, however, arises as to whether there may not be sixteen times as many molecujes of hydrogen in the same volume as there are molecules of oxygen ; and if not, how this comes about. For the strong grounds for the belief that the numbers of molecules of different simple gases contained in the same volumes, at the same tempera- ture and pressure, are identical, the reader can refer to chemical treatises ; but we may make some remarks as to the w^ay in which this occurs. Presiun- ably the motion or energy of the aether is the same in both cases, and we have to consider whether particles, whose sizes and weights are represented respectively by the numbers i and i6, when thrown into the same undulating gas are more hkely to move with velocities in the inverse ratio of their bulks (or weights), or to congregate in niunbers in- versely as their bulks (or weights). Taking each gas separately, its molecules, being equal in bulk and weight, would by their impact produce the same effect one on another in both gases, and there is no reason why the hydrogen atoms should congregate in greater numbers than the oxygen atoms whUe each has abundance of free space for the movements of its molecules. As soon as the two gases are brought together, in free communication with one another, it is obvious that they wiU mix with one another, owing to incessant imdulations of the aether, emd the consequent movements of the floating molecules, just 80 THE NATURE OF THINGS as particles, large and small, mix together in the sea. If, however, there were in a given space more mole- cules of the same size and kind than in an equal adjoining space, freely communicating with the other space, the more numerous molecules must collide more frequently, and therefore the pressure will not be the same at the boundary between the two spaces. No difference of pressure arises from differences of the atomic weights or bulks of gases freely exposed in the common ocean of aether gas, and the molecules of each will disperse independently one of another without altering the general pressure. Difference of pressure arises when the molecules are forcibly crowded together, the aether which moves freely through the compressing enclosures remaining practically unaffected in pressure. The pressure of a gas is altered also by heat, which increases the velocity of motion of the molecules, which therefore repel molecules at a lower temperature, and become themselves more dispersed. When, however, we are dealing with compound gases, which contain imprisoned aether, then heat may cause sudden alterations ot pressure in the aether itself, and alterations of pressure in the aether in such cases are sometimes of high explosive violence. The fact that it is only roughly true to say that at ordinary temperatures the quantity of heat energy required to raise a gas through one degree of temperature is inversely proportional to the atomic size or weight of the atoms or molecules of the gas, though the statement is probably nearly exact at very high temperatures, is a necessary consequence of the conditions which we have described as explaining HEAT 81 the mode of adhesion of atoms, and larger particles, to one another. In order to understand more clearly why the specific heats of simple gases are inversely as their atomic weights, let us begin by considering what would be the changes in the specific heat of a litre of free aether when we gradually displace the aether by the introduction of atoms or molecules of simple gases. The atonadc weight of aether being lower than that of any other known gas, its specific heat is at the highest point when it includes no other gaseous atom or molecule. If we introduce molecules of hydrogen, the specific heat of the hydrogen, being greatly lower, soon becomes measurable. The same result occurs if we introduce oxygen molecules, but the specific heat is still lower when the number of the molecules of oxygen is the same as that of the molecules of hydrogen. In this case the quantity of aether displaced by the oxygen is — ^if we are right in assuming that the sizes of atoms are proportional to their atomic weights — sixteen times that dis- placed by the hydrogen. Why then is the specific heat lower for the oxygen than for the hydrogen? If we concern ourselves merely with the molecules contained within the space of a Utre, at constant volume, we have simply to consider the to-and-fro heat movements of the molecules, and their resulting collisions. In the case of each gas individually, the colliding molecules are of the same size, and displace equal quantities of aether ; consequently we have the same conditions as if aether masses of dimensions equal to the bulk of the molecules (subject to correc- tion for imprisoned aether) were to become absolutely 6 82 THE NATURE OP THINGS solid. Taking the two gases separately, without regarding the aether, if the same amount of heat energy were divided between the same numbers of the molecules of both gases, the larger molecules would acquire a velocity only xV of that acquired by the smaller (hydrogen) molecules. To make the heat momentum of the hydrogen equal to that of the oxygen, as it must be if their temperatures are the same, the hydrogen would require sixteen times the amount of heat energy imparted to the oxygen ; but the movement of the molecules is simply that of solid particles floating in a gas (aether) and partak- ing of its movements. Therefore, if sixteen times the energy is communicated to a litre of aether, with hydrogen molecules floating in it, this only suffices to impart to the hydrogen molecules the same momen- tum as a sixteenth part of this energy would impart to the same number of molecules of oxygen similarly floating in the same quantity of aether, but displacing sixteen times as much of it. When the molecules of a gas, floating in aether as we have described them, are crowded together so as to clog one another's motion, or as soon as their movements become sufficiently slow, they will begin to coalesce and to form liquid or solid particles. We should therefore expect that the specific heat of an element of high atomic weight would require a higher temperature before its specific heat becomes uniform than does an element of lower atomic weight, and it is well known that a fairly perfect gas must be obtained in order to reach uniformity in the specific heat. The elements of higher atomic weight in general require a higher temperature to HEAT 83 convert them into true gases, and consequently a higher temperature is necessary before the specific heat becomes uniform. The same considerations apply to the case of molecules or chemical com- pounds as to that of atoms, only that we have to allow for what we may caU ' elasticity,' namely, the alteration of density in the aether imprisoned between the combined atoms. In comparing the specific heats of two litres of two elementary gases, one litre of each, the gases being measured at the same temperature and pressure, with the specific heat of a gas which results from the combination of the two litres, we shall, if the whole of the two gases enter completely into com- bination without alteration of volume, have, roughly speaking, the same state of affairs as if the size of the atoms of the combining gases joined together were that of a single elementary atom of the same size as a molecule of the gas resvdting from the combination. Then the molecular weight of the gas resulting from the combination will be to the molecular weight of either of the component gases in the same proportion as the specific heat of the component selected to the specific heat of the com- pound gas. Let us take some examples : — Hydrogen and chlorine form two volumes of hydrochloric acid gas (without condensation) when one volume of hydrogen is added to one volume of chlorine under suitable conditions. In order to calculate the specific heat of hydrochloric acid gas, we have the proportion : 365 (the molecular weight of hydrochloric acid) : 2 (the molecular weight of hydrogen) : : 3409 (the specific heat of hydrogen) 84 THE NATURE OF THINGS : S (the specific heat of hydrochloric acid). Hence, ^ 6-8i8 . oc 36-5 X S = 2 X 3"409. or S = -^ = 186. which is not far from "185, the figiire obtained experimentally. On the other hand, if we substitute the molecular weight of chlorine for that of hydrogen, we have — ^6 s : 71 : : 121 : S, and b =-^ — , which gives '242 as the specific heat of hydrochloric acid. In the case of chlorine the imperfect character of the gas probably accounts for the excess over the actual fact. The specific heat in this case is of course that for constant weight. Let us next take the case of nitrogen and oxygen combining to form nitrous oxide gas — ^NjO. In this case two litres of nitrogen added to one litre of oxygen unite to form two litres only of nitrous oxide gas. Here the molecular weight of nitrogen multi- plied by the specific heat of nitrogen (constant weights) = the molecular weight of nitrous oxide X the specific heat of nitrous oxide (S) ; or „ 28 X "244 . __ S = —-33 = -1552, 44 but the result must be multiplied by f , because of the contraction in volume, which gives us '233, the experimental number being '226. Using oxygen, a lower result is obtained. Stated in this way this method of calculation is the same as if founded on the well known rule that the ' atomic,' or ' molecular ' heats of gases are approximately equal ; atomic and HEAT 85 molecular heat being the products of the multi- plication of the atomic weight by the specific heat, and of the molecular weight by the specific heat respectively. We have, however, tried to explain how this rule is founded on recognized principles in regard to motion or energy, and to point out the way in which we may hope to explain satisfactorily the rather wide departures from the rule. The explanation of the reasons for exceptions to a rule often proves the rule more convincingly than almost anjiihing else. As regards the difference between the calculated and experimental specific heats, we must remember that in the case of simple elementary gases we are dealing with molecules consisting of two atoms only when dealing with the more ' perfect ' gases, but when dealing with chlorine and bromine we have to do with larger agglomerations of molecules. Taking hydrogen as a fairly perfect gas composed of mole- cules consisting of two atoms, we ought to be able to deduce with reasonable approximation to accuracy the specific heats for constant weights of the other elementciry gases by dividing the specific heat of hydrogen by the atomic weight of the element — the atomic weight of hydrogen being taken as i. In the case of ox\-gen this gives us "213 as compared with the experimental figure "218 ; for nitrogen we obtain '2435 instead of "244 ; for chorine "096 instead of "121 ; and for bromine "0424 instead of "055. When we consider that the larger molecules of oxygen are likely to enclose a larger amount of imprisoned aether than hydrogen molecules, it is hkely that the calculated specific heat will be too 86 THE NATURE OP THINGS low, because heat will be expended owing to the expansion of the inter-molecular aether ; but we cannot attempt to explain the irregular divergencies from the calculated specific heats in the case of the different gases until we have determined the shapes of the atoms as well as their sizes. In the case of chlorine, and still more of bromine, we have no doubt to take into account a certain amount of adhesion of some kind between the molecules themselves. In calculating the specific heat of compounds, the above method of calculation soon fails, and we have to adopt the rule that the specific heat (for constant weight) of a compound multiplied by the molecular weight of the compound is equal to the sum of the atomic weights of the component elements, multiplied in each case by the number of atoms entering into the combination, and by the specific heat of the individual element. When this method is employed we do not need to take into account the contraction which sometimes occurs when combination takes place. Thus, return- ing to the case of nitrous oxide, we now have the following equation, in which S is the specific heat of nitrous oxide, i.e., £, 28 X '244 + 16 X 218 b = 23_! = -224. 44 This is nearer to the experimental number "226 than the result '223 obtained by the other method. In the case of hydrochloric acid this method gives ■186, as before. The former method fails altogether in the case of aqueous vapour, and many other compound gases. For reasons which we have already HEAT 87 discussed, aqueous vapour is likely to show some abnormality, but by the fonner method we get '189 as the calculated result, which compares with "480 the experimental figure. On the other hand, by the last method the calculated specific heat is "572. This is nearer, but the discrepancy needs explanation. The explanation is probably to be found in the heat expended in expanding the aether confined within the conglomerations of molecules in a vapour which is far removed from the conditions of a ' perfect ' gas. In the case of sulphuretted hydrogen, taking the specific heat of hydrogen as 3'409, and that of sulphur vapour as 0T712, we have the equation : g _ 32 X 0-1712 + 2 X 3409 _ „.-„ 34 "^^ against 0^243 experimentally. In the case of aqueous vapour the excess of the calculated figure over the expenmental one is "092, and in the case of sulphur- etted hydrogen vapour it is 'loS. These are nearly the same, and, as sulphur has a higher atomic weight than oxygen, one might expect the excess to be a little greater in the compound of hydrogen with sulphur than its compound with oxygen. Let us next try to calculate the specific heat of carbon, as to which there is much doubt, from the more easily ascertained specific heats of oxygen and carbon dioxide. We now have the equation — o'2i692 (the specific heat of carbon dioxide) = 12 S(s ^specific heatV ^ .^^g/the specific heat\ \ of Ccirbon / \ of oxygen. / 44 88 THE NATURE OF THINGS This gives oig6 as the specific heat of carbon, £md this is what Wulner and Bettendorf make it to be. Taking o'igG as the specific heat of carbon, we cal- culate the specific heat of turpentine (CiqHio) by the equation — 120 X "196 + 16 X 3'409 V, J 136 which gives o'573 as compared with the experimental figure "463. Considering the necessary corrections, and the uncertainties of the figures on which we base our calculations, this is not a wide difference. We have said enough to show that the above method of calculation affords a useful means of checking results otherwise obtained, and to support our theory as to the basis of the connection between the specific heats of elements and their compounds. In the case of gases composed of perfectly free atoms the atomic heats — that is, the atomic weights multiplied by the specific heats for constant weights — should be identical, because the movement of an atom should require an amount of energy directly proportional to the weight of an atom, and the momentum — heat momentum — should be the same for atoms of different sizes, if the energy — heat energy — applied is the same, but if the same number of atoms have the same momentum collectively, but different atomic weights, then the velocity of movement of the different atoms must be inversely proportional to the weights of the atoms for the same number of two kinds of atoms of different atomic weights to have the same collective momentum or pressure. Consequently, HEAT 89 for the pressure of the same number of hydrogen atoms and of oxygen atoms to exert the same pressure, the hydrogen atoms being only one- sixteenth the weight of the oxygen atoms, the former must be moving with sixteen times the velocity of the latter ; or, if, as we maintain, the weight of the atom is in each case equal to the weight (or bulk) of the aether displaced, the tether in which the atoms of hydrogen are floating must be vibrating with velocity sixteen times as great as that of the aether in which the oxygen atoms float. If, on the other hand, it were the case that hydrogen atoms moving in a vacuum were the same size as the oxj^en atoms, but only one-sixteenth of their weight, the energy required to give them the same momentum as the oxj'gen atoms would be sixteen times as great, or there would have to be sixteen times as many hydrogen atoms in a given space for the hydrogen to have the same pressvire as oxygen. In other words, on this supposition we should as before have the speafic heats inversely as the size of the atoms, if the number of atoms were the same, or the same if the number of atoms of hydrogen were sixteen times as numerous as the oxygen. The number of atoms in a given space is pretty certainly the same at the same temperature and pressure, and therefore the weights of the atoms (or their sizes according to our theory) must be inversely as the specific heats (for constant weights) of the gases. In order to ascertain whether or not the sizes of atoms are equal but their weights different, or are of sizes directly proportional to their weights, we will consider what would be the difference 90 THE NATURE OP THINGS between the results if two litres of different gases, at equal temperature and pressure, are brought together in a strictly confined space of two litres capacity without any agitation or mixing of them together : (i) If the molecules differ equally in size and weight ; (2) If the molecules are the same in size but differ widely in weight. In the first case, when the molecules bombard one another the result is like that of a shower of bullets coming in collision with a shower of rigid footballs, and there will obviously be great commotion among the molecules with brisk diffusion. In the second case, the molecules being equal in size and momentum, but with different velocities, they would bombard one another much as they do in a gas all of one kind, and there is no reason why any great amount of diffusion should take place beyond the movements that would occur in a single gas. These statements, however, need further explana- tion. When we say that in case (2) the momentum of the molecules is equal, we are in fact saying that the pressures, or the combined momentum of all the molecules, is equal in the two gases ; while in the first case, since the combined momentum of the larger molecules is equal to that of the smaller molecules, the velocity of the smaller molecules must be greater in the inverse ratio of their size or weight to that of the larger molecules. If, as is probably the case, the aether in which the molecules float is moving with a velocity in the inverse ratio of the size of the molecules floating in it, and if the molecules depend for their weight on the amount of aether HEAT 91 displaced by them, the result is the same, and the sether of the two litres would obviously diffuse together until equilibrium was established. Experi- ment proves that the diffusion between gases of widely different atomic weights proceeds much more rapidly than with gases of nearly equal atomic weights. It has indeed been proved that the rate of diffusion of gases through unglazed earthenware is inversely as the square roots of their densities, and the diffusion is no doubt similar in character if earthenware is not interposed between the gases. Moreover, the fact that aether diffuses with the utmost freedom through the walls of an earthenware vessel, while other gases pass with increasing difficulty as their molecular weights increase, indicates that the atoms or molecules which pass through most readily are in all probability the smallest. We next come to the ' convection ' and ' conduc- tion ' of heat. Convection is merely an expression of the familiar fact that a body in passing from one position to Emother carries with it movements which have been previously communicated to it, or which are possessed by it. Thus the earth, in its motion round the sun, carries with it the motion of revolu- tion round its own axis, which belongs to it, and so on. Part of the heat, which is spoken of as carried by conduction, is no doubt really conveyed by the aether as it flows through a conducting material ; but this portion is in the form of radiant heat. The remaining part consists of movements which aie imparted to the particles of the conducting material on one side, and given out to matter in contact with it on the other side. 92 THE NATURE OF THINGS Radiant heat, as we have already explained, is heat-motion of the aether gas, and it gives rise to ordinary or diffused heat under certain circum- stances. Diffused, or ordinary heat, is the heat- motion of the coarser particles composed of chemical atoms other than aether atoms. The passage of radiant heat through a gas, a liquid, or a solid, is not in its rate proportional to that of ordinary or diffused heat, or in other words, the diathermancy of a substance is not proportional to its conductivity. Air transmits radiant heat readily ; but it conducts ordinary heat very badly. Rock salt is exceedingly transparent (so to speak) to radiant heat ; but its conducting power for ordinary heat is greatly inferior to that of metals, which are usually excellent conductors of heat, but not at all good in transmitting radiant heat. This is easily explained by consider- ing the probable disposition of the aether in the various substances. Obviously the density of a substance is not the only quality which determines its diathermancy or conductivity ; but the density, by diminishing the quantity of included aether gas, must have an influence. At first sight there are difficulties that look likely to be insurmountable if we adopt this view, but they vanish to a large extent on analysis. For instance, why should air be so bad a conductor ? Air, like other gases, floats, or is dissolved, so to speak, in a large quantity of aether gas, and we must therefore consider what the conductivity of aether is for heat. Space, filled with pure aether, is what is comn\pnIy called a vacuum, and the conducting power of a vacuum is almost non-existent, though it HEAT 93 transmits radiant heat readily enough. Each naolecule of the nitrogen and oxj'gen, which consti- tute air, is surrounded most of the time by non- conducting cether, and consequently its apparent conduction is really a kind of convection, the heat being for the most part only transmitted from one molecule to another when they collide. In the case of metals, on the other hand, the molecules are closely adherent, and heat passes through readily by direct conduction. That this is the true explanation of the nature of conduction appears from the fact that the rate of conduction is not the same in different directions in some substances which have different structure in different directions. Wood affords good examples of this, because the structure is readily discerned. Thus the conducting power is much greater along the direction of the fibres in all cases, and the influence of the density of the wood is small in comparison with the influence of the direction of the fibres. In the same way with metals: the conducting power is much greater in metals, or in the same metal, when the structure is fibrous, than when it is not so. Diathermancy, to some extent, tends to follow the same rule, because a fibrous or lamellar structure tends to produce long unbroken tubes or layers largely filled with aether, along which the waves of radiant heat are propagated. A sub- stance which conducts well in the sohd state, with regular structure, loses its conducting power if it is ground up into fine powder, because then there are few fibres or layers of closely adherent molecules, and the particles of the substance and the enclosed 94 THE NATURE OF THINGS aether gas are jumbled together in confusion. This is true throughout the whole range of gases, liquids, and solids, and there can be no doubt that it is analogous to the rules as to the transmission of sound, and due to similar causes. The apparent conductivity of hydrogen is probably due to the rapid convection of heat by the molecules, with their high specific heat, which is due to their rapid movement. This explanation of conductivity is, with obviously necessary corrections, a general guide to the conductivity of all substances, and it is founded on elementary laws applicable to matter and motion. It enables us to avoid the unscientific method (which is merely a resort of ignorance), whereby new peculiar ' properties,' etc., are imagined as a method of overcoming each difficulty in explain- ing natural phenomena, as it arises. We will next deal very briefly vidth the connection between heat and electricity and magnetism, though we shall have to revert to the subject when we have expounded our views on the nature of electricity and magnetism. Our object here will be merely to show that the electric and magnetic effects produced by heat are consistent with the views we have taken of the nature of heat as ' a mode of motion ' partly of atoms, molecules, and particles of matter, and partly of aether atoms, which are of course, if we are right in our theory, themselves simply very minute atomic particles of matter in a gaseous state. We shall, later on, contend for the view that electricity is a mode or modes of motion, as Tyndall beheved it to be, and we shall try to show that whereas hght is due to transverse vibrations or HEAT 95 movements of the aether, so electric waves axe due to longitudinal vibrations or movements of it. Heat is produced by resistance to electric move- ments, and heat again produces resistance to them. Cooling of a wire conducting electricity causes a decrease in the resistance offered by it to some extent ; probably because transverse and irregular movements tend to divert the movements passing down the minute channels, or fibres, of a wire from the hne in which they are flowing, so diverting and breaking up the movements along the wire, and thus increasing the resistance of the wire to the electric current. The thermopile depends for its usefulness on the fact that when two metals such as antimony and bismuth are closely joined at one end, and electrically connected at the other ends, heating of the united ends causes an electric current to flow from the bismuth to the antimony at the heated junction, and from the antimony to the bismuth through the cold junction. A possible explanation of this is that the greater conductivity of the bismuth in the longitudinal as compared with the transverse direction tends to convert the heat movements into electric ones, the difference of conducting power causing the longitudinal electric movements to pass in the direction of least resistance, namely, from the hot bismuth to the hot antimony. If the junction be cooled, instead of heated, the current is in the other direction, because the longitudinal move- ments in the bismuth being in excess of the transverse ones, comparatively, the checking of the movements in the bismuth affects the longitudinal component more than is the case in the antimony, and so 96 THE NATURE OP THINGS again a current flows from the higher to the lower pressure. Another way of explaining the naatter may be arrived at by considering the effect of the different specific heats of the two substances. We have contended that the velocity of the movements are greater in the substance of higher specific heat, and the same amount of heat produces greater elevation of temperature in the bismuth, which has a lower specific heat than antimony, and so may cause a current of aether from the hot bis- muth to the cooler antimony. The same degree of cold, on the other hand, is likely to have the opposite result as regards the current arising. This, however, is not a complete explanation, because the current is not proportional to the difference of specific heat between various pairs of metals. Moreover a current is produced by heating a junction of hotter and colder pieces of the same metal. Two things, however, may be inferred : (i) That heat is changed into electric current ; (2) That the current passes in the direction of least resistance. In this connection it may be noted that thermal and electric conductivities are nearly in direct proportion. The heat therefore passes in the direction of least resistance more freely than in the other direction, and part of the heat is converted into electricity perhaps by a simple separation of the longitudinal from the transverse constituent of the motion. When the current is produced in the same metal the heat has probably caused a change in the structure of the metal and affected the charac- ter of its conductivity. If there were no difference HEAT 97 in the resisting, or conducting, power of the metcils, there would probably be no definite current, because the currents would be equal and opposite. It must be remembered that heat affects the power of resist- ing an electric current to a different degree in different substances. In the case of metals heat diminishes the electric conductivity, but in the case of some ■ non-conductors ' it rather increases it. We must now brieflj' consider a few instances showing the relation between heat and magnetism. If a piece of silver is moved rapidlji' between the poles of two opposite magnets, it becomes heated by the magnetic resistance, although the silver is neither attracted nor repelled by the magnets. The reason for this will appear more clearly when we have discussed the nature of magnetism ; but it is e\-idently due to the state of the aether gas between the magnets, and it is manifestly not due to the air or to any occult ' attraction,* or ' repulsion.' A rapid movement of the aether between the poles pro'vides a rational explanation, and it mil be shown later on that a rotatory current of aether is the probable source of magnetic phenomena. If an ordinary magnet is heated, the magnetism is dim- inished, and it is even destroyed by very extreme heat. This, in all probability, is due to the effect of heat on the arrangement of the molecules, the peculiar structure of the magnet being thrown into greater or less confusion by the action of the heat. The chemical effects of heat, and the effects of chemical action in releasing heat, or rendering it atent, are due, as we have already explained, to the influence of heat movements on the sether gas 7 98 THE NATURE OP THINGS imprisoned between the atoms of which molecules and chemical compounds are built up, and to the results of setting free, or rarefying, or condensing, the imprisoned aether, and of imprisoning, or con- densing, or rarefying, free aether. Of course the effect upon the movements of the atoms, or mole- cules, of the chemical substances, and upon their arrangement, must be taken into account as well as the effects upon the movements and state of the aether. 99 CHAPTER VI. LIGHT. In quo jam genere'st solis lux, et vapor ejus Propterea quia sunt e primis facta minutis Quae quasi truduntur, perque Aeris intervallum Non dubitant transire, sequent! concita plaga : Suppeditatur enim confertim lumine lumen, Et quasi protelo stimulatur fulgure, fulgur. — {Lucretius.) Of this kind are the light of the sun and its vapour, Since they are formed from minute primary particles Which are, as it were, thrust out and which pass easily Through the interspaces of the air, urged on by a succession of strokes ; For light follows close upon light, and one flash Urges on another, like oxen driven in a team. T UCRETIUS'S description of light is quite -'— ' consistent with the generally accepted wave- theory, and it emphasizes the fact that it is a mode of motion of minute primary particles, or as we should put it, aether particles. We appreciate this form of motion by the special sense of sight, and this tends to make us regard it as different from what we recognize as motion by our other senses. Light is, by our sense of sight, limited to a very narrow range, comparatively, of even the kind of motion of which it forms a part. The movements which we recognize as light are of the kind described, by a somewhat rough analogy with the movements of water, as ' waves ' ; but they are transverse and 100 THE NATURE OP THINGS not longitudinal. They resemble in fact the move- ment which passes along a tight string when it is sharply plucked to one side and allowed to fly back. The movement, in the same way, passes rapidly at right angles to the exciting movement along the length of the string, without the particles moving sensibly in the direction in which the wave is so rapidly propagated. We will not delay to explain the grounds for this view of the nature of light, or for preferring it to other views, such as that light is due to the rapid emission of particles at an enormous speed from the luminous origin of the light. The text-books on light can be studied on the subject. The limits of our vision enable us to appreciate as light only undulations within certain degrees of wave-length ; but it must be remembered that the aether vibrates in similar fashion also in wave-lengths greater and less than those which our eyes can perceive. On the side on which the waves have a length too great for our eyes to perceive them as light, they are perceptible as radiant heat ; and on the other side, where the wave-length is too short, they indi- cate their existence by their chemical effects, and are hence designated as ' chemical rays.' It must be noted, however, that the eyes, like certain substances, have the power of, as it were, separating the light component from waves which are not simply light waves, and they appear to be able to convert other forms of energy into light. Thus, if the eyes are closed in the dark and pressure is applied to the eyeballs, bright light of all colours may be seen. This must mean that the energy LIGHT 101 applied to the exterior of the eyeballs is converted into light, or that it converts into light energy already latent in the eyes. There is nothing surpris- ing in this when we remember how ordinary energj' can be converted into heat, electricity, sound, and even into magnetism. Moreover, light is produced by striking flint with iron, by resistance to an electric current, by chemical action, and so on. When there- fore we consider how light travels from the sun and stars, we must remember that it may travel not in the form of light all the way, but that the compo- site general waves of aether maj', on striking the atmosphere of the earth, be split up more or less into light waves, as well as into heat movements, electricit}-, etc., though if the eye were outside the atmosphere of the earth, it is quite likely that it would be able, for itself, to separate out the light component from the aetheric waves impinging upon it, or possibly the light of the sun might then be blinding in its character. The fact of course is that light is limited by the capacity of our eyes to perceive aetheric movement, and we have by experiment ascertained what is the nature of the movements which our eye perceives. When the eye fails to perceive the movements, we cease to call them light, cdthough quite of the same form. In fact, aetheric movements generally are classified according to the way in which we perceive them, and not, in ordinary language, according to their broad physical characteristics. This method of classification has many advan- tages, but it might be possible to discuss physics much more comprehensively if we recognized the 102 THE NATURE OF THINGS fact that our senses are all of them powers of appreciating motion of matter (including aether as matter), and then considered what varieties of motion can exist, and whether our senses recognize them, and, if so, how. Latent energy we appreciate by the disappearance and subsequent reappearance of motion, and by other indirect methods ; while we have abundant evidence to show that motion cannot be destroyed, although it may become imperceptible or latent. We will not, however, pursue this subject any further in this connection. The differences of wave-length, or wave-frequency, within the limits of visible light, give rise to differ- ences of colour. All the colours which can be appre- ciated by the eye can apparently be produced by suitable degrees of pressure on the ball of the closed eye in the dark. By means of prisms, etc., light from sources external to the eye may be split up into portions of wave-length such as to give the various colours separately. The colours of various substances are due to their influence upon the light after it falls upon them, and before it leaves them, by reflection after traversing them to some degree of depth. This may be due to greater power of reflecting light of certain wave-lengths, or the sub- stance may alter the wave-length of the light while within its substance. In any case, what happens is that white light, falling on a coloured substance, is spht up, and light of a particular wave-length is turned back, or returned, while light of other wave- lengths is retained, or absorbed, or altered so as no longer to be light. Similarly, when light is trans- mitted through certain coloured media, only light LIGHT 103 of certain wave-lengths passes through. The usual way of speaking of light being spht up into varieties of light of different wave-lengths is not to be con- strued as meaning that varieties of light of these various wave-lengths coexist in white light, any more than it is to be supposed that motion in a particular direction consists of two or more linear motions in different directions which, if brought to bear simul- taneously on a body, would cause it to move in that particular direction. ^ther vibrating with the wave-length correspond- ing to white light may imder certain circumstances be changed so as to produce independent vibrations of wave-lengths associated with certain colours, which, added together, would produce white light ; or it may be changed so as to produce certain coloTired vibrations, with heat, and other vibrations which are not light. Thus white light falling on a substance may produce in it colour, heat, and chemical changes. Very frequently it does so. Red light, being of longer wave-length than violet light, more readily changes to heat (radiant), which has longer wave-length ; and violet light more readily changes to invisible rays which are capable of producing chemical changes, and so on. Waves, which are neither light nor heat, may be changed so as to produce both heat and light, etc. Again, chemical vibrations added to heat vibrations may produce light waves. Some substances, and some living creatures, are capable of giving forth light. This is spoken of as ' fluorescence and phosphorescence.' There are three wajrs in which this may be done. Invisible rays 104 THE NATURE OF THINGS may be altered in their wave-length so as to become visible, or chemical changes in the animal or sub- stance may produce light, or light falling on them may be stored up and rendered latent and subse- quently given out again. This is analogous to what happens with heat. Some substances are opaque to light, the light waves being changed so as to become invisible, or being rendered latent. Light passing through the glass of a greenhouse falls upon the ground, etc., and is converted into heat, which does not pass through glass so readily as light does, and hence the heat within the greenhouse accumulates. Fine powders, even of a substance which is transparent to light, are opaque, for reasons to which we have already referred, — namely, the breaking up of the channels along which the light is propagated. ' A substance is not always good for transmitting radiant heat when it is very good for transmitting light, and vice versa. Thus glass, which is very transparent to light, does not allow radiant heat to pass through it at all well, and it is important to ascertain, as far as we can, what is the reason of this. The fact that heat rays near the red end of the spectrum pass through glass much more readily than those further away, and whose wave-length is therefore greater, indicates that the increase of wave- length is the determining factor. Now, when we consider the structure of glass, as shown by its 'fracture,' it seems pretty clear that the reason of this is that the aether channels, which appear to be narrow in one direction and very wide in the direc- tion at right angles to this, do not allow the free LIGHT 105 vibration of the £ether when its wave-length is too great. Again, it is easy to understand that a substance like rock salt, which transmits the waves of radiant heat excellently, may break up light waves, with their shorter wave-length, by diverting the shorter waves into side channels or by admitting more than one wave of light in such a way as to cause them to be more or less destructive of their regular sizes. The colour black is usually described as a negation of light, and it is said that when light falls on a black substance it is completely absorbed by it, even though it be a black diamond. A substance which appears red in the daylight looks black at night, though it can still be distinguished, and evidently reflects raj's which affect the sight. Again, a sheet of paper which is white in daylight looks black at night if it is dark enough. Absence of light is certainly not the same as the light reaching the eyes from a black substance ; neither is that which reaches our eyes from a white material the same as colourless light. The intensity of the light is apparently the determining factor in the production of the colours white and black, and intensity of light is governed by greater or less amplitude of the light waves, and not by their length. Therefore white and black, and the intermediate shades of grey, cannot rightly be placed in the same category as the colours of the spectrum, since they are simply the extremes of the capacity of the eye for the appreciation of the amplitude of the waves of light. Thus, when a substance gives out light rays insufficiently intense to make it clearly luminous, it continues black, and 106 THE NATURE OP THINGS when the intensity is great enough it becomes white. Between the black condition of luminosity, and the white, the waves can under suitable conditions produce waves of length corresponding to the colours of the spectrum, and it is a reasonable supposition that the wave-lengths necessary to produce the colours of the spectrum may be consistent with an amphtude smeill enough to produce black, or great enough to produce white. If we regard the power of appreciating light as limited to rays with wave-lengths between that of the extreme red and that of the extreme violet, it may still quite possibly be the case that all these rays become black when their amplitude is sufficiently small, and that they all become white if their ampli- tude is sufficiently great. Coloured substances do in fact all become black in veiy weak light, and they all become white if the light is extremely intense. This therefore is the interpretation which we feel justified in adopting as accounting for the so-called ' colours ' white and black. The black lines due to diffraction are much more probably caused by alternating diminution of the amplitude of the waves of light than to the complete mutual destruction of the waves, or even to the lowering or raising of their wave-lengths so as to make them invisible. Opacity of a substance to light may be due either to its reflection of the light that falls upon it, or to its power of turning light into other forms of energy, of absorbing it, to use the ordinary expression. Substances which reflect most of the light falling upon them are usually white, and those which absorb nearly all the light are black, because with light LIGHT 107 falling on a spot in a smooth surface the reflected wave meeting the incident wave tends to increase its amplitude, whereas such light waves as escape back- wards from near the surface of an absorbing sub- stance are diminished in amplitude sufficiently to dimmish the amplitude of the incident waves near the surface. Since heat and light are different only in wave- length, it is reasonable to infer that what applies to heat will also apply to light, with any necessary allowances. We must therefore consider what specific heat suggests if we consider it in relation to light. 'Specific luminosity,' as we may term it, is the amount of energy required to produce a certain amount of light movement. Now we have already noted that the specific heat of a gas composed of elementary atoms, or molecules, or of a chemical compound (with necessary corrections), is inversely as the atomic or molecular weight, and we can there- fore infer, not that the element of highest atomic, or the compound of highest molecular, weight, will be capable of producing the brightest light, but that they will produce the brightest light for the least expenditure of energy. Hence we can infer that the element of highest atomic weight will produce light most economically, so to speak, when rendered luminous. Consequently, with the electric light the elements of the highest atomic weight, if otherwise suitable, will give the most economical electric light, or with gas the most economical incandescent light. We therefore use filaments of metals with high atomic weight, in preference, to produce electric light, and we use incandescent mantles made with 108 THE NATURE OF THINGS suitable compounds of metals of the highest atomic weights for illuminating purposes. Platinum, os- mium, etc., are consequently used for illuminating purposes, even at much greater cost. In considering the specific luminosity of solid substances, it must be remembered, however, that the structure of the solid greatly influences its specific luminosity, as it also affects the specific heat. Transparent substances are those which allow light to pass freely through them, without altering its wave-length so that it ceases to be light. This property depends largely on their structure, and as it is often different in different directions, so the transparency may differ according to the direction in which the light passes through the sub- stance. Again, the substance may affect the ampli- tude of the light waves transmitted, and so produce a change in the intensity of the light. Convection of light occurs when luminous particles or materials are conveyed from one place to another. Conduction of light, as apart from conduction of heat, appears not to have received any attention, so far as one can judge by the text-books. The chief reason for this no doubt is that there are no good conductors of luminosity. Still, it would be inter- esting to have the results of experiments such as might be obtained by heating wires of different metals at a single spot up to red or white heat, and then noting how far the luminosity of the wire extended. With thin wires it should be possible to obtain measurements which would be instructive, especially as regards the question whether or not conductivity is the same for transverse vibrations of LIGHT 109 different wave-lengths, e.g., that for dark heat and that for light waves. Light produces chemical effects as does heat, but the effects of light, and of the rays beyond the violet end of the spectrum, are different from those of heat. The chemical effects of light of different wave-lengths, as shown when light is subjected to analysis, are likewise not the same. The explanation of these differences is to be found by considering how waves of different wave-length will affect the aether im- prisoned between the atoms in different chemical molecules and compounds ; and it is easy to see that waves of short wave-length would readily produce chemical effects different from those produced by waves of longer wave-length. In some cases light energy will be absorbed, or rendered latent, and in some cases the changes produced by light acting on a substance will be capable of reproducing light again, when light from without is absent and the substance reacts to the state in which it was before the light affected it. This, no doubt, is what occurs in some forms of " phosphorescence,' where an unheated substance yields a faint luminosity in the dark, without any permanent change in the sub- stance. The theory we have put forward as to the nature of chemical combination, and molecular constitution, affords as satisfactory an explanation of this pheno- menon as could be hoped for, and it is diflScult even to suggest any reasonable explanation otherwise. No chemical change, in the ordinary sense, is neces- sary, because a mere shifting of the atoms in the molecules of a simple elementary substance may 110 THE NATURE OP THINGS create a strain in the imprisoned aether, such that a condition of very unstable equilibrium soon arises, when the light waves cease, culminating in a toppling over of the atoms into their original position, with resulting commotion in the aether, and consequent light waves like the echo of the original light. The influence of light in developing colours in plants is, no doubt, partly due to definite chemical changes produced by it, but the gradual adaptation of the interstices filled with aether to the wave-length of certain colours probably plays its part. The curious fact that animals have a great tendency to conform in their colours to the colour of their sur- roundings may, to some extent at all events, arise in the same way, the resistance to the prevalent rays becoming most developed. The tendency to blacken- ing of the skin of human beings, in the bright light of the tropics, is perhaps a similair protective change, the amplitude of the intense light waves being reduced in the skin and returned in that reduced condition; while in dark places the rays of low amplitude are rendered more ample and partly thrown back in that form, though feebly, the much greater transparency of the skin allowing a freer passage, in the deeper structures, of the deficient supply of light and also of heat rays. We will now consider what connection there is between temperature and luminosity. Temperature, as a measure of heat, is really a measure of heat momentum, or, for the same substance, inversely, of wave-length, and therefore when any substance reaches a certain temperature it becomes luminous. The intensity of the illumination is however different LIGHT 111 in different substances, and this depends on the amplitude of the vibrations. The vibration of a thin wire, transversely, is not the same as that of a thick wire, and neither is that of a gas, liquid, or solid, composed of large molecules, or particles, the same as that of those composed of small ones. The flame of burning hydrogen (burning in air) is not so lumin- ous as that of carbon ; and platinum, raised to the temperature of either of these, is much more luminous. The flame of burning coal gas is by no means so bright as the incandescent mantle containing metals of high atomic weight placed in these flames, and therefore at the same temperature. Consequently, since the limits of wave-length which are perceptible as light by our eyes are not very wide, our only method of increasing luminosity is to increase as far as possible the size of the atoms or molecules which we cause to vibrate within the limits of wave-length which affect our vision. If we cut off the wave-lengths above the red end of the spectrum, we remove the unnecessary heat effects, if they are not desirable ; and if we cut off the vibrations of wave-length shorter than the limits of vision, we remove the chemical effects, which may, in many cases, be undesirable in connection with illumination. The electric light of an incandescent filament is free from much of the heat of some other lights ; but it has a considerable amount of the chemical effect of ultra-violet Ta.ys, which may perhaps be detrimental or inconvenient to the eyesight. Therefore a globe which is transparent to light rays but opaque to chemical rays is desirable for illuminating purposes when a light rich in chemical rays is employed. 112 THE NATURE OF THINGS Light, as we have already mentioned, may become latent, like heat, but usually the wave movement is also made slower when light becomes latent, and it consequently does not appear again as light, even if it has not been definitely converted into chemical or other totally different forms of energy. As might be expected, the conversion of light into heat, and of heat into light, is of very frequent occurrence, since it is merely a matter of alteration of wave- length, or wave-frequency, and light being narrow in its limits, and of high frequency of waves, is of course very readily reduced to dark heat by any obstacles in its way. It is interesting to note the effect of light upon electricity, and of electricity on light, as indicating their several natures. Resistance to electricity con- verts its energy into heat and light, and heat and light offer resistance to electricity. Evidently, there- fore, the vibrations of heat and light are not the same as those of electricity. Substances transparent to light and heat are nearly always, if not always, bad conductors of electricity. The view that heat and light vibrations are transverse, while those of electricity are longitudinal, seems to afford a ready explanation of this. Light falling upon selenium increases its conductivity for electricity, evidently by altering the arrangement of its particles ; and it produces a similar effect upon rubber. The effect of magnets in rotating the plane of polarization of light is also apparently due to the effect of the magnetism on the arrangement of the glass, or other substance, subjected to the influence of the magnets. All this goes to show that the shape and size of the interstices LIGHT 113 of the material through which Hght passes affects its character in important respects ; and this fact affords strong confirmation of the view that light consists of vibrations or undulations of a gas consist- ing of very minute gaseous particles, such as we have maintained aether must be. Light has evidently a motive power, since it is able to alter the position of the particles of solid bodies, and to cause combination of atoms and dissolution of their combination, effects which can only be produced by expenditure of energy. Moreover, light can be converted into heat, and can in various ways, directly or indirectly, produce motion. Therefore, by the principle estabUshed by innumerable and incontrovertible proofs — that energy cannot be created or destroyed — ^it must possess energy in the form of motion. It is a mode or form of motion, and motion which can be transmitted with enormous rapidity through the medium of invisible particles of matter distributed between the earth, sun, and stars, and also entering into the interstices of dense solids. The limits of the kind of motion recognized as light are determined by the capacity of the eye to appre- hend the motion by the special sense of vision possessed by the eye. The fact, however, that the eye ceases to perceive this motion as light, and that it begins on the other hand to be perceived by other bodily senses, must not lead us to suppose that there is therefore necessarily any fundamental change in the general character of the motion. Heat reveals itself to us in a very different way from light ; but it differs from light merely in such secondary respects as wave-length, etc. ; while heat 8 114 THE NATURE OP THINGS itself differs in no fundamental respect from other forms of motion, — that is to say, motion of matter, for, as we contend, motion is unknown to us except as motion of matter. X-rays, again, show how light and electric vibrations are quite closely connected, so that the consideration of the physics of heat, light, electricity, magnetism, and chemical action, might be combined into a general consideration of the movements of sether particles, and the resulting motion and other effects produced by them acting on grosser matter which differs from aether in no fundamental respect, but only in the size of its ultimate particles, etc. We must refer briefly to polarization of light, though we are concerned with it here chiefly as indicating the nature of light, and are assuming that our readers are acquainted with the teaching of elementary text-books on the subject. Polarization of Ught shows very clearly the effect produced by the character of the aether channels in certain substances such as tourmaline, the vibrations, while retaining the general characters of light, being split up into component vibrations, — that is to say, vibra- tions which, added together, would yield the composite vibrations of ordinary light. These well-recognized phenomena support the view which we have already expressed, that heat, light, electrical vibrations, etc., are themselves merely component parts of larger combined undulations, which are propagated through the oceans of sether, and broken up in their passage through the atmosphere and solid and liquid terrestrial matter. It is thus, by the changes they produce in the character of motion to which they LIGHT 115 are exposed, that material things convey to our senses such varied impressions, producing from light colours, the outlines of form and shape, shadows, and brightness, etc., and convey thereby invaluable information as to the nature and condi- tion of things, so far as they can be appreciated by our eyes. 116 CHAPTER VII. ELECTRICITY AND MAGNETISM. Inter utrumque igitur cum Coeli tempora constant. Turn variae causae concurrunt Fulminis omnes. Nam fretus ipse anni permiscet Frigus et jEstum : Quorum utrumque opus est fabricanda ad Fulmina nobis, Ut discordia sit rerum, magnoque tumultu Iguibus et Ventis furibundus fluctuet Aer. * ' * * * * Hoc est igniferi naturam fulminis ipsam Perspicere, et qua vi faciat rem quamque videre. — {Lucretius.) When the weather conditions existing between the seasons of the yeair are present. Then the various causes of Lightning all simultaneously occur. For the interval between the seasons of the year mixes together Cold and Heat, Of both of which there is need for the formation of Lightning, So that there may be a discord of things, and so that the Air May fluctuate with Fires and Winds in a state full of fury. ***** This gives a clear perception of the very nature of Lightning And of the energy whereby it produces the various phenomena. T UCRETIUS recognized that electricity, or ■*— ' lightning, is derived from energy exerted under certain conditions, and his account of a way in which lightning is produced is in accordance with facts. A violent mixture of hot and cold air must cause a violent commotion in the gaseous aether in which the particles of air and of aqueous vapour are float- ing. This commotion gives rise to alterations of ELECTRICITY AND MAGNETISM 117 density in the aether, and to undulations, or waves, in the aether. The movements of the aether arising from differences of density in adjacent masses of it, with the waves produced, are the source of electiic phenomena in whatever way electricity is generated. In order to show that this is so, it is important to consider how the density of aether can be measured, or estimated, either relatively or absolutely, or both. The usual method of estimating density by weight is not available, because there is not at present amy method of weighing aether otherwise than in the surrounding aether. Weight is, however, only am expression of the momentum with which a body is urged towairds the centre of the earth, and we may therefore compaire the relative momentum of a given volume of aether with that of a similar volume of aether, both volumes being subject to the same force or forces, in order to ascertain their relative densities or masses. If a htre of aether gas, subject to the same moving force, contains twice as many particles of aether as another litre, its momentum will be twice as great ; or, in other words, it will weigh twice as much. On the other hand, if under the same circumstances its momentum is twice as great, it will contain twice as many particles of aether ; or, in other words, its density wUl be twice as great. Under suitable circumstances, therefore, electric 'potential' may be taken as a measure of aether density. When two masses of aether have different electric potentiad, and are in free communication with one another, motion will result, just as it does when two masses of water at different levels are in free communication ; the 118 THE NATURE OF THINGS water at the higher level rapidly running down to a lower level, till the levels of both become the same. In the same way aether at higher potential in free communication with aether of lower potential runs down in potential until the two masses are of equal potential. In the same manner a litre of denser gas in free communication with a litre less dense rapidly assumes a density midway between its own density and that of the litre with which it communicates, the litre of the less dense gas being raised in density, and that of the more dense being lowered. This way of regarding electric potential involves the acceptance of the ' one-fluid theory,' as it is called, of the nature of electricity, and this was the theory adopted by Franklin, as opposed to the ' two- fluid theory,' which represents electric equilibrium as resulting from a mixture of two fluids, the separa- tion of which gives rise to a force tending to bring them together again ; the one fluid, ' positive electricity,' attracting, and being attracted by the other fluid, ' negative electricity,' somewhat after the fashion of so-called ' chemical affinity.' We have already dwelt upon the absurdity of inventing imaginary ' attractions ' or ' repulsions ' to account for phenomena which are readily explained as due to ordinary motion, or motile energy, of matter. A third theory, which might be called the mathematical theory, gets rid of material limitations and difficul- ties by means of mathematical abstractions, which, useful as they are when properly employed, do not express the actual nature of things as revealed, in the only way they can be revealed, by our powers of perception through our bodily senses. ELECTRICITY AND MAGNETISM 119 By the one-fluid theory, aether gas being regarded as the fluid, positive and negative electricity are merely relative terms, and the ' attraction ' of the one for the other is simply the tendency of a fluid at high level, or pressure, to run down to a common and uniform level with that of fluid at a lower level or pressure. Again, two masses of the same fluid at a higher level, or pressure, placed one on each side of a mass of the same fluid at lower level, or pressure, will repel each other, since each exerts pressure on the intervening fluid in a direction opposite to that of the other. And similarly, two masses at lower pressure, one on either side of a mass at higher pressure, wiU appear to repel each other, since the higher intervening pressure will tend to drive them apart. Thus the repulsion of similar kinds of electricity for one another may be explained without inventing any imaginary power of repulsion, which does not in fact, in any other sense, exist at all. We will now apply this general principle to well- known and characteristic electrical phenomena. If a glass tube be rubbed with silk, both the tube and the silk being very dry, the tube will be found to have acquired the power of attracting light bodies such as bits of paper. Glass is a very bad conductor of electricity, and consequently we may infer that the sether included in its interstices is, as it were, bottled up in them, so that it does not readily pass in from the other side to replace any aether that is pumped out of them. It may be that particles of air act like miniature corks, so to speak, in closing up the interstices, while the silk, by its friction. 120 THE NATURE OF THINGS momentarily removes them, at the same time pumping out aether, the air particles quickly returning again, and again corking up the aether at a rather lower pressure. When the glass tube is brought near paper, the aether tends to force its way through the intervening air, thrusting the bits of paper towards the tube, and by degrees the aether which is at higher pressure makes its way into the aether at lower pressure in the interstices of the glass, and when a common level is reached the glass has lost its electrical properties. Sometimes the bits of paper, when attracted, remain in contact with the glass, and sometimes they are alternately attracted and repelled, the latter effect probably being due to the alternating dislodgement and return to their position, as ' corks ' to the glass interstices, of the air particles. Similar results are obtained by friction of resin, sulphur, amber, and other things, while even a good conductor, such as a metal bar, can be electrified by friction if held by a handle of glass or other non- conductor. In the latter case friction near the glass handle will produce electricity in the remote end of the metal bar, because the bar conducts well, and allows the aether to flow easily from one part of the bar to another. Again, if an electrified body is brought into contact with another not electrified, and if they are both conductors and are insulated, the electrified body will share its charge of electricity with the one that is not electrified. In other words, aether will flow from the body containing it at higher pressure to the one containing aether at lower pressure, after the manner of all liquids and gases at different levels or pressures when a free communi- cation between them is permitted. ELECTRICITY AND MAGNETISM 121 The electricity produced by rubbing resin with flannel is, however, not identical with that produced by rubbing glass with silk, for the one attracts what the other repels, and vice versa. The two kinds of electricity neutralize one another. This latter fact is a strong indication that the difference consists in a difference of level, or pressure, in the enclosed aether. It is, however, not easy to say at first sight which kind of electricity represents the higher pressure in the aether, and which the lower ; but this does not much affect the matter as far as the question as to the nature of electricity is concerned. If friction with silk pumps aether out of glass, then friction with flannel pumps aether into resin, and although particles of air may act as ' corks ' or ' plugs ' to prevent the outer aether from flowing freely into glass, it is more difiicult to understand how they can prevent the aether 'bottled up' at higher pressure from escaping into the aether outside. Evidently, however, the different character of the substance, and of the mouths of their channels and interstices, will easily account for the difference in their electrical properties. Indeed, the particles of the glass, resin, etc., may quite conceivably act like valves, in the one case only outwards, and in the other only inwards. There is, at all events, no greater difficulty in such a supposition than there is in any other explanation of the nature of electricity which has been put forward. The supposition of two fluids is very improbable, and that of two forms of motion brings us back to the point on which we have so strongly insisted — namely, that motion is not an adequate explanation, unless we specffy what it is that moves. 122 THE NATURE OP THINGS We have to accept the existence of aether, in some form or shape, to explain heat, light, etc. ; and when its existence as a gas, consisting of very minute particles, will explain all these physical phenomena satisfactorily, it is unscientific to invent new and admittedly imaginary substrata as a basis for ex- planatory theories. Of course, in speaking of piimping the aether from one substance by rubbing it with another, and of confining aether under positive or negative pressure by plugs or valves, we must not be understood too literally. We employ a near analogy to render our explanation of what happens more easily intelligible. In some substances aether is, under ordinary circum- stances, no doubt at higher pressure than in some other substances. The substances are consequently electro-positive, or electro-negative, in relation to one another. The results of friction may therefore depend on bringing the aetheric channels in two different substances into direct communication, so that aether flows from the one to the other, thus temporarily making the aether contained in the one substance acquire temporarily a pressure higher than normal, and that in the other a pressure lower than normal. The electrification perceived is the excess above the normal or the deficiency below it. When both substances are insulated, the one gains practic- ally the whole of the electricity (i.e., aether) lost by the other. In the case of a non-conductor, electricity produced by friction remains, for a time at all events, in the part where it is produced ; but in a conductor it spreads over the surface, and is apparently limited ELECTRICITY AND MAGNETISM 123 to the surface. This is due to the same cause as the repulsion of like kinds of electricity. The pressure of a river prevented from flowing on to the ocean is perceptible only on the surface of the restraining obstacle, and not in the water a little way behind it, and a spherical mass of gas, when the restraining globe retaining it at higher pressure than the gas outside is forced to expand, exerts its pressure perceptibly at the surface as before, and not at the place from which the restraining globe has moved. This leads us to consider next what is meant by ' induction ' of electricity. If an electrified sphere is placed near the end of an insulated cylindrical conductor, it induces on the end of the cylinder next the sphere electricity of the opposite kind, and in the end farthest away electricity of the same kind. Suppose the sphere to be positively electrified, or rather to be charged with aether at higher pressure than the free sether in the intervening air, then the intervening aether is driven back by the pressure of the aether in the sphere, and repels the aether from the near end of the cylinder into the further end, with the result that the near end has sether at a lower pressure in it, and the further end has aether at a higher pressure ; or in other words, the electricity at the near end of the cyUnder is of the opposite kind to that in the sphere, and the electricity in the far end of the cylinder is of the same kind as that in the sphere. We are here dealing with statical electricity, not with currents ; but as the pressure is due to the confined movements of the aether paLrticles, the energy is passed on through the intervening 124 THE NATURE OF THINGS aether to the aether confined in the cylinder, the total quantity of which continues practically unaltered. The transmission of the impacts of aether particles passes freely through non-conductors such as glass in which the aether is itself confined, and it is in- dependent of any definite transference of aether from the sphere to the cylinder. This is a little difficult to follow unless we remem- ber that aether being quite elastic from its nature, the sphere and cylinder are set in and permeated with a completely elastic fluid, which, having no boundaries within finite distance, resembles in many ways a jelly. The fact that an electrical 'charge' is equally distributed all over the surface of a spherical conductor, while there is no electrical force in the enclosed space, proves the law of inverse squares, which applies in the case of moving particles of equal size colliding with one another, or in the case of a gas at uniform pressure. Thus, if a series of concentric shells be imagined, the pressure at the centre, being evenly distributed over the surfaces of these shells, will decrease in the ratio of the squares of their radii, because the areas of the surfaces of the shells increase in the ratio of the squares of their radii, and the same amount of energy being distributed over each of the surfaces will be, as regards its intensity, in the inverse ratio of the surface over which it is spread. This is the same law that applies in the case of so-called ' attraction,' but there is no assumption beyond that of the presence of motile energy which obviously exists. Consequently, the theory that electricity is due merely to the motile energy, and consequent pressure, ELECTRICITY AND MAGNETISM 125 of aether gas, is consistent with the observed funda- mental phenomena of ' static ' electricity. The variation of electrical 'density' according to the shape of the body, and the extreme condensation of it in sharp points, is strong evidence in favour of the view titat an electric charge represents aether of density different from that of the surrounding aether. An exactly analogous condition exists if a vessel containing a Uquid or gas under a pressure is drawn out to a point. WTien an exit through this point is permitted, the Uquid or gas is dischai^ed violently, just as aether is discharged from a pointed conductor. The liquid is seen as a jet, and the gas reveals its discharge by the "wind' it produces. The jet of aether, on the other hand, appears as a spark, because aether in violent motion produces a flame in air or other gases. In fact, the only reason why these and other aetheric movements and pressures appear different in their nature from those of coarser gases, is really the fact that they do not affect our senses in the same way. Insulation and conduction of electricity, in like manner, are quite analogous to the bottling up of ordinary gases, and their discharge through apertures and channels which allow them to escape or flow away. The zigzag course of a spark of electricity, or a flash of lightning, is evidently due to the resistance of the air, etc., to the discharge, which would otherwise take place in a straight line, subject of course to the shape and direction of the aperture or channel through which it is discharged. The characteristic brush or fan-shaped discharge is no doubt due to this influence, being quite similar 126 THE NATURE OF THINGS to the discharge of a liquid or ordinary gas under similar conditions. If an electric discharge takes place through rarefied air, or gas, the resistance to the discharge decreases as the rarefaction increases, and the luminosity becomes very faint, and would apparently completely disappear if a true vacuum — that is, a space containing nothing but aether — could be attained. This bears out our view that move- ments of aether, however violent, outside the atmo- sphere, do not render it luminous in the absence of coarser matter. The fact that an electric discharge can take place through a vacuum shows conclusively that there is, in an electric discharge, a discharge of material particles, for to anyone but the abstract mathematician the idea of abstract motion project- ing itself through an absolute vacuum is contrary to common sense. This phenomenon is, moreover, much more in accordance with the supposition of the transference of a gas at higher pressure into a region where the gas is at lower pressure, than it is with that of a mutual ' attraction ' of two diverse fluids, which, in that case, might be expected to fly to meet one another, creating a commotion in the middle of the tube when they met. The electric spark, as might be expected, is capable of either causing combination of chemical elements, or, under other conditions, of decomposing them when combined. The dashing together of chemical atoms, with the imprisoning between them of aether particles, produces chemical combination, as we have already explained ; and the battering of chemical compounds held together, as we have explained that they are, by imprisoned aether at lower pressure ELECTRICITY AND MAGNETISM 127 than that external to them, is of course capable of shaking them asunder. The idea of attraction and repulsion as accounting for these chemical effects requires transcendent powers of imagination and versatility, combined with a neglect of well-established physical facts and generalizations ; and since the recognized properties of a gas composed of extremely minute particles, such as can in innumerable ways be proved to exist, affords a full explanation with but few difficulties, we surely need not hesitate to regard the electric spark as due to the discharge of aether gas passing from a region of higher pressure to one of lower pressure, like any other gas or liquid. The charging jof an electric condenser is quite analogous to the filling of a jar or sponge with a liquid or gas ; and the surrounding non-conducting material is analogous to the walls of a vessel contain- ing liquid or gas. In both cases the possibility of raising the level or pressure above that in the immedi- ate neighbourhood is due to the restraining influence of an enclosing material, which prevents an outflow. The main objection to such explanation of the nature of electricity seems to be its extreme simplicity, which ought of course, by the best rules of science, to be a recommendation of it. At the same time we must not neglect the fact that the structure of an electrified body is often modified by the presence in it of an electric charge ; but this again is quite analogous to what occurs when a substance is charged with a liquid or an ordinary gas. A piece of wood sodden with water has not the same structure as a dry one, and water charged with gas is not the same as water not aerated. 128 THE NATURE OP THINGS Atmospheric electricity is, in its main features, easily explained by the theory that electricity is due to variations of density in the aether. It has been observed that the remarkable absence of steadiness in the electric potential of the atmosphere is compar- able only with wind pressure among meteorological phenomena. Lucretius, as we have already men- tioned, pointed out that mixtures of hot and cold air, with violent movements of the air, produce light- ning ; and it is clear that air particles by their violent commotion must produce violent movements of the intermingled aether gas, and since different degrees of motion among the aether particles imply different degrees of density in the sether, the move- ments of the air will evidently produce temporarily different degrees of density in the sether, when indeed they are not themselves produced by the movements of the sether itself. The amount of moisture in the air, and its changes of state from gas to liquid or solid, and vice versa, will evidently produce changes of density in the surrounding sether, and the greater conductivity of moisture, as compared with dry air, will have import- ant effects which we need not enumerate. The violent discharges of lightning, which sometimes even scoop large trees out of the ground and pitch them away to some distance, water spouts, and other phenomena, show that wide variations of density in the sether can occur near together, under favourable atmospheric conditions. The interpretation of these phenomena by other theories of the nature of electri- city are admittedly unsatisfactory. Their explanation according to this theory is so simple and obvious that it need not be dwelt upon. ELECTRICITY AND MAGNETISM 129 Friction, as for instance that of two currents of air, or clouds, travelling quickly in different directions, in layers which are in contact with one another, do, no doubt, as Lucretius himself observed, produce lightning; but the production of electricity by friction is itself but one example of the many ways in which difference of aether density, or in other words, of electric potential, is brought about. The difference between static and current electricity is quite the same as that between the pressure of an ordiaary gas imder restraint, and the flow of it if an exit is available. We have, therefore, in considering current electricity, merely to take account of the driving force due to difference of aetheric pressure or momentum, and the resistance that is offered to the flow of the current which results. The established conclusions, as set forth in treatises on electricity, are of course not inconsistent with our theory, if, as we maintain, it is the true naturaj explanation of it. There are, however, a few points presenting some little dif&culty to which it is well to refer briefly. In the electrolysis of metaUic salts, or of water alone, we have decomposition of the compounds, with deposition of one component on the positive pole, and the liberation of the other at the negative pole. To explain this action of the electric current, we need only suppose that the current causes temporary decomposition throughout its course. At the positive pole, the component whose molecules or particles are electro-negative, that is to say, which contain aether gas at a lower pressure, attach themselves to the positive pole, while the electro-positive particles similarly are set free or 9 130 THE NATURE OP THINGS deposited at the electro-negative, their enclosed aether being at higher pressure. In the intermediate space an extremely rapid decomposition and re- combination takes place, so that no decomposition is apparent here. This process may be compared to the rapid to-and-fro movements of the armature of a magnet made and unmade by rapid reversal of the magnetizing current. The extreme rapidity of the process makes it not directly perceptible. We have spoken of particles and molecules as being electro-positive and electro-negative, and this description needs to be further elucidated. The terms are relative ones, particles which are electro- positive to one kind of particles being electro-negative to another. From what has already been said about chemical atoms, it is evident that they cannot them- selves be electrified with either negative or positive electricity, and the electric condition therefore depends entirely on the condition of the aether contained in the molecules or compounds. This is a much more probable h3^othesis than that which casts aside the well-established nature of atoms, and supposes them to be, in part at all events, composed of minute particles of so-called ' electricity ' or ' electrons.' The electron theory brings us no nearer to a clear understanding of the nature of electricity, unless we imagine further ' attractions ' in accordance with the methods so dear to abstract mathematicians. Our theory gives a simpler explanation of the per- ceived phenomena without assuming anything more than the existence of aether gas with its variations of motion and motile energy. We are quite willing ELECTRICITY AND MAGNETISM 131 to admit the existence of electrons. We have no doubt that they do exist ; but they are particles of ffither external to chemical atoms, and not forming part of the substance of the atoms with which they are associated, or to which they are adherent. The way in which sether particles can ' adhere ' to atoms will be more fully explained in the later chapter on Astronomy, and we will here only remark that when two particles collide in a vacuum, if they are perfectly incompressible, they adhere if they are moving in directly opposite directions. The theory of electrons, if they are regarded in this way as aether atoms, supplies much useful information in regard to the size, etc., of aether atoms. Whatever doubt might remam as to the nature of electricity is removed when we come to the consider- ation of electric waves. Here the connection of electric waves with those of light becomes unmistak- able owing to the identical rate of their propagation, etc. There can be no doubt that electric waves are waves in the same medium as those of light, namely, aether, and we have therefore only to consider in what electric waves and light waves differ. There are excellent grounds for regarding heat and light waves as transverse vibrations, roughly resembling the waves passing along a tightly stretched string plucked sharply sidewa^'s. Evidently aether must also be capable, like air, of longitudinal vibrations such as pass along a bar when struck sharply on one end, or such as the waves of sound which pass through air. Waves of this kind in the aether agree entirety in nature with electric waves and the associated phenomena, and are evidently identiccil. Hence, 132 THE NATURE OF THINGS without supposing the transmission of electrons, or electric fluid, at inconceivable velocities through the air, we have merely the now familiar propagation of waves through the sether which we already know must be capable of such undulations. It thus appears that, as often happens, two apparently contradictory theories, when rightly understood, have each of them something of truth in them, and each of them something that is not true. There is, we maintain, but one fluid concerned in electricity, but there are two forms of motion in it — actual transference of electric fluid, and to-and- fro undulations of it. A good example of the differ- ence between movements of transference of aether, and undulatory electric movements of it, is afforded by the forked flash of lightning as compared with so-called sheet lightning. In the lightning flash there is undoubtedly a violent current of aether, the power of which is often enormous, while sheet lightning is manifestly due to undulatory movements of aether spreading widely through the whole mass of it. Powerful electric vibrations in the atmosphere give rise to light vibrations, owing to the influence of the air which converts part of the longitudinal or electric waves into transverse or light waves. In pure aether alone light would not be produced, either by currents of asther or by its waves. Let us now proceed to consider the nature of Magnetism, which presents considerable difficulties whatever theory is adopted, and which undoubtedly strongly suggests the idea of 'attraction' in the ordinary sense of the word. Lucretius, following, we may suppose, the teaching of Epicurus, shows so ELECTRICITY AXD MAGNETISM 1S3 keen and accurate an insight into the nature of physical phenomena generally, that we ma\' well look to him for inspiration on this difficult question. He fully recognizes the difficulty, and quotes many instances showing that there is a constant flow and movement of minute particles through the inter- stices of soUds, etc. He has already explained that lightning is composed of exceedingly minute particles. The attraction of a magnet for iron he maintains to be due to a difference in density of gas, as between the magnet and the iron, the inability of some of the minute gaseous particles to escape from it causing the iron to be dri\'en up to the magnet. His explanation is, we believe, quite right in piinciple, and the much wider knowledge which we now possess of magnetism adds fresh weight to his arguments. The earth is itself a large magnet, and its magnetism can best be explained as being due to circular currents of aether gas around it. That such currents of aether run roimd the earth we shall explain in a subsequent chapter on Astronomy. The magnetism of an electro-magnet is plainly' due to the diculation of an electric or setheric current around the iron core of the magnet. It therefore seems probable that an ordinary magnet, or piece of magnetic ore, owes its magnetism to some similar movement of the aether within its substance. \Mience comes the motion or energy which enables a magnet to do work without any apparent loss of energy ? In the case of the earth, regarded as a magnet, there is the unending energy of its rotation, accompanied, and as we shall contend, caused, by the rotatory movement of the aether gas. The unceasing flow of 134 THE NATURE OP THINGS the aether supplies the energy of the magnet, and the modification of the constant aetheric current which is brought about by the arrangement of the aether-filled channels in the magnet, gives rise to the phenomena of magnetism. When a bar of iron is magnetized, there is undoubtedly a change produced in the structure of the bar ; and the change in its structure affects the direction and character of the flow of aether through it, with the result that it becomes a magnet. Tyndall, in his book on " Heat a Mode of Motion," describes an experiment in which a silver medal is hung between the poles of an electro-magnet. It is neither attracted nor repelled, but, if it is moved, resistance is encountered. This resistance is not due to the air, and, as he remarks, it must be due to the ' interstellar meditim,' — that is, to aether gas. There seems then to be no escape from the conclusion that a magnet acts by its influence on aether, and that influence cannot be motion or pressure derived directly from the magnet itself, because the magnet expends no energy, and energy cannot be created from nothing, as we have more than once explained. It therefore remains for us to determine what effect a magnet produces on the movement of the aether. In all probability the effect produced by a magnet is the imparting of a circular or rotatory motion to a powerful current already existing independently of the magnet. The solenoid, which practically amounts to a series of connected rings through which an electric current is passed, acts exactly like a magnet, and therefore, regarding an electric current as a current of aether, we have experimental proof that ELECTRICITY AND MAGNETISM 185 such a current produces the effects produced by a magnet. The magnetization of a bar of iron depends on the setting of its particles in such a way as to give the necessarj- direction to a current of aether passing through it ; and the permanence or tran- sience of the magnetism depends on the firmness with which this setting of the particles takes place. It follows from what has been said that all the phenomena of magnetism can be explained on the supposition that a permanent magnet owes its properties to a constant flow of electricity (that is, aether), in a manner similar to the flow of an electric current through a solenoid, or S5'stem of solenoids ; and we need not therefore for our purpose follow out the various manifestations and effects of magnet- ism. There is strong independent e^^.dence that the magnetism of the earth is due to constant currents of aether, and that such currents are unceasing and but slightly variable. We need to make no fresh assumption to explain the nature of artificial or natural magnets ; and magnetism, like other physical forces, is merely a mode of motion, or a modification of energy-, dependent on the nature and arrangement of the material particles affected by the motion or energy- which exists abundantly throughout the universe, changing its form and undergoing modifications of great variet5^ vnthout its amount being either increased or diminished. ' Perpetual Motion ' is often spoken of as a dream which can never be realized, and although in a sense this is true, j-et in fact it is assuredly true that motion, like matter, cannot in any way be created or destroyed. All phenomena of motion must for 136 THE NATURE OF THINGS their ultimate explanation be traced back to pre- existent motion of matter. A magnet cannot impart motion to iron except by means of matter, either itself actually moving or carrying with it energy in the form of latent motion. Attraction through space, devoid of matter, is purely imaginary, and is an imagination contrary to the whole vast extent of scientific and unscientific experience. 137 CHAPTER VIII. SOUND, AND OTHER PHYSICAL PHENOMENA. Principio. auditur Sonus, et Vox omnis, in Aureis Insinuata suo pepulere ubi corpore sensum : Corpoream quoque emm Vocem constare fatendum'st Et Sonitum, quoniam possunt impellere Sensus. — [Lucretius.) In the first place Sounds are heard, and all Voices, when they are insinuated Into the Ears and make an impulse on the Sense of Hearing By their material basis ; for it must be confessed That Voice and Sound are material, since they can By their impulse afiect the Senses. TN the case of sound it is not disputed that the -'■ impulse is conveyed to the sense of hearing through a material medium, and this fact supports the more general statement that a material medium is necessary for the transmission of any impulse to any of our senses. In the case of sound, air is the chief medium, and it is known to consist of particles of matter, namely, oxygen and nitrogen, etc. The sense ol touch also is affected only by coarser matter, and it is only when senses are affected through the rarer medium of sether gas that any difficulty has been experienced in accept- ing the statement so positively made by Lucretius. That this general statement is fully justified we have already mciintained. 138 THE NATURE OF THINGS A bell hammered in a space vacuous except for the presence of aether gas produces no audible sound. In hydrogen slightly audible sounds are produced, the molecules of hydrogen being much smaller than those present in air. Since the atoms, or molecules, of aether are certainly very much smaller than those of hydrogen, the fact of their not carrying sound is no argument against the opinion that aether is gaseous in nature, but rather the reverse. In general, the motion of sound is feebler in a lighter medium than in a heavier one. The motion producing sound is a wave motion propagated longitudinally, and not transversely like light. We have already contended that it is the same in character as that of electric waves, except that electric waves are waves of jether, while those of sound are waves of air ; that is to say, alternating condensations and rarefactions of aether, instead of air, as in the case of sound. By suitable means sound waves can be converted into electric waves, and vice versa, as in the telephone. The velocity of transmission of sound depends on two conditions — the elasticity and the density of the medium through which it passes. The elasticity of a gas is measured by its pressure, or the weight it can sustain. If the density remains the same, the increase of elasticity implies increased motion of the particles of the gas. Therefore, if air is contained in a closed vessel which cannot expand, and the motion of its particles is increased by heating it, sound travels through it more rapidly. In the atmosphere sound travels more rapidly in warm weather than in cold, if the density as indicated by the barometer is the same. This is no doubt due to the fact that SOUND 139 when, with the same number of particles in a given space, the motion of the particles is increased, the particles come more frequently into collision, and accordingly undulatory movements are transmitted more rapidly from particle to particle. If electric waves in aether gas are, as we assert, strictly analogous to sound waves in air, we have to explain why the waves of electricity in aether travel so enormously faster than sound waves in air, their velocity being the same as that of light. Is aether more elastic than air, or less dense, or both ? Before answering this question, we need to examine more closely what we mean by 'density' and by 'elas- ticity ' in this connection ; for nothing baffies us, in scientific investigations, so much as the use of words transferred from one set of conditions to another without sufficient consideration of the extent to which the new conditions involve a modification of the precise meaning of the words employed. Under ordinary circumstances the density of a gas is measured by weight ; but in the case of aether, which cannot be duectly weighed, we must use some other method for estimating its density. We have maintained that the density of a gas consisting of particles much coarser than those of aether is measured by the amount of aether displaced by the gaseous particles of the gas whose density is considered. The vaistly greater rapidity of the trans- mission of waves through aether, as compared with that through air, might then be due either to the lower density of the aether or to its greater elasticity as compared with air. The weight of a given volume of aether is equal to the weight of the aether displaced 140 THE NATURE OP THINGS by the particles of air within that volume, together with the weight of the aether not so displaced ; but the elasticity of the air and aether mast be equal, because if the elasticity or pressure of either the air or the aether were greater than that of the other in contact with it, movement would occur until equilibrium was established. Accordingly it would appear that the air must be denser than the aether, and by the rule that the velocity of propagation of undulations is inversely proportional to the density of the medium through which they are transmitted, aether should be many million times less dense than air. It must be remembered, however, that when we compare the density of two ordinary gases at equal pressure, the density depends on the molecular weight, and hence we ought only to infer that the weight of a molecule of air is much greater than that of a molecule of aether, although the weight of a given volume of sether might be greater than that of the same volume of air measured at the same pressure without aether. If the size of a molecule or atom is a measure of its weight, we must conclude that the aether molecule is several million times smaller than the molecule of air. Consequently, in a given volume of air we have several million (about twenty-five million) molecules of asther displaced by one molecule of air, and we infer that a molecule of asther occupies about one-twenty-five-miUionth of the space occupied by a molecule of air. When, however, we say that the density of a given volume of air is measured by the number of aether atoms or molecules displaced, in that volume, by air molecules (that is by mixed SOUND 141 oxygen and nitrogen molecules), it must be remem- bered that the density of air varies greatly, and it follows that the density of the aether in which the air is, as it were, dissolved, varies similarly. The number of aether atoms in intersidereal space is greater in a given volume than it is in the air on the surface of the earth, but the earth as a whole, including its atmosphere, has a weight or density equal to the average density of the aether displaced by it ; or, in other words, the number of aether atoms displaced by the earth have a weight equal to that of the earth, the density of the aether just outside the terrestrial atmosphere being equal to the average density of the earth as a whole, including the enclosed aether. The density of a block of gold or lead in intersidereal space need not be equal to the weight of aether displaced by it ; but the gold or lead would in that case be changed in density there, or would be rapidly driven into regions where the aether is at such pressure that the weight of the gold, together with the aether enclosed in it, would be equal to the weight of the same volume of the surrounding aether, or where its motion is stopped by some solid obstacle such as the earth. The absolute density of any solid, liquid, or gas, is the proportion of space abso- lutely occupied by it ; that is to say, the space apparently occupied less the space empty or un- occupied by matter. We shall show in the next chapter, on Astronomy, that as we pass from the outer confines of the terrestrial atmosphere, the proportion of space occupied by matter, other than aether, gradually increases towards the centre of the earth, and at 142 THE NATURE OF THINGS the same time the active motion of the matter decreases, and in part becomes latent. The weight of a body is determined by its absolute density, as defined above, and by its active motion. In short, a cubic centimetre of space occupied by matter, without any vacuous space included, would have the same weight whatever the nature of the matter might be if its motion was the same ; but the diversity in the nature of substances depends on the relative proportion of space absolutely filled by the substance and by aether respectively, and the shape of the portions thus absolutely filled — or, in other words, on the size and shape of the component atoms. This is not easy to appreciate, through the great difficulty of freeing ourselves from preconceived notions stereotyped in the language we use. The absolute density of gold or lead is greater than that of free aether around the earth, and hence gold is borne towards the centre of the earth, sinking downward in aether just as it sinks in water. This is probably, as we shall try to explain in the next chapter, owing to the centripetal movement or pressure of the aether acting as gravity towards the centre of the earth, the momentum of the outside aether driving bodies with greater mass per unit volume into regions where, owing to the decrease of movement, the pressure outwards is lowered. If a block of lead floated in aether just like a volume of the surrounding aether of equal dimensions, it would be equal in density and elasticity to the surrounding aether. If it were lower in density it would float in the direction of aether at lower density, that is aether containing fewer atoms in unit volume ; and if higher in density it would sink. SOUND 143 The fact that electric undulations, which are similar to those which, in air, give rise to sound, travel so vastly faster than the latter, is therefore due not to the lower density of a cubic centimetre of aether compared with a cubic centimetre of air, but to the vastly lower atomic weight of aether, which, as we have contended, means vastly smaller size of its atoms. Waves of sound and electric waves are both bent towards the earth under normal conditions. In the case of sound, this is explained by the greater density of the air near the surface of the earth ; but in the case of electric waves, it does not necessarily prove that the aether is more dense near the surface of the earth, because the larger admixture of air with the aether near the ground would tend to make the electric waves travel more slowly, and so cause downward bending of them even though the density of the aether itself were no higher. Waves of sound in the air no doubt impart undulatory motion to some extent to the aether in contact with the air, but undulatory movements of aether do not excite the sensation of sound. In Kent, during the summers of 1915 and 1916, but not during the winters of the same years, the sounds of the guns fired in the fighting in the north of France, about 130 miles away, were heard continu- ally with very great distinctness, although the sound at intervening places was at the same time either inaudible or less clearly audible. The most probable explanation of this rather remarkable phenomenon, which could not be adequately accounted for by mere bending of the waves of sound towards the solid earth, was that the sound falling from below 144 THE NATURE OF THINGS on the limiting surface of the atmosphere at the ' critical angle ' for the ' rays ' of sound passing from air to sether was, like light passing from water to air, totally reflected. The sine of the ' critical angle ' is the reciprocal of the index of refraction for rays of sound passing from air to aether (or to a so-called vacuum), or to hydrogen perhaps; and hence, given the refractive index — fi — and the distance at which the sound of the guns was most distinct, the height of the atmosphere might be calculated ; or, assuming the height of the atmo- sphere, the refractive index might be computed. The height of the atmosphere is greater when ex- panded by heat in the summer, and the sound is heard at a greater distance in hot weather. The vibrations which give rise to sound are reflected, refracted, and so on, as are those which give rise to light, and we can argue by analogy from the one to the other if we make adequate allowance for the difference in the media — air and aether — and for the difference in kind of the vibrations. In the case of electric waves, as compsu-ed with those of sound, there is only the difference of the media to be con- sidered, since the vibrations are the same in character, being longitudinal vibrations, in the one case of sether, and in the other of air. We will not discuss all the various phenomena of sound, because they simply require an analysis and explanation of the changes and modifications possible in the case of the vibrations on which sound depends, and we are here merely dealing with the nature of things, and such phenomena as appear most suitable for the purpose of determining that nature. In the EARTHQUAKES 145 case of sound, it is generally recognized that it is due to a vibratoty motion of material particles — usually air particles, — so we need not labour the point. We wDl now proceed to discuss the nature of certain physical phenomena which do not come directly under such general headings as chemical combination, heat, light, electricity, magnetism, sound, etc. One set of these, which have always attracted much attention, is that of earthquakes. The explanations hitherto given, although probably accounting for some forms of earthquake, do not seem to explain at all adequately the greater number of the more violent and widely extended ones. We have already maintained that the pressure of aether gas is not the same imder all circumstances and in all places. We shall later on try to show that in the main ocean of free aether there are, as in the air, variations of density causing currents, cyclones, and anticyclones analogous to those with which we are familiar in the atmosphere. The pressure of aether at the smface of the earth is probably lower than it is in the regions just outside the atmosphere, and the pressure of aether within soUds is, we may beUeve, sometime so different from that of the surrounding aether that, when from any cause free communication is permitted, violent explosions result, producing heat, light, electricity, etc. We may therefore reasonably infer that, from one cause or another, the pressuie of aether within the crust of the earth becomes, at certain times and in certain places, widely different from that of neighbouring aether, and that the rush of aether from ID 146 THE NATURE OF THINGS regions of higher pressure to regions of lower pressure through or \vithin the crust of the earth produces very violent disturbances. Such explosive move- ments afford very probable and adequate explanation of earthquakes, their greater prevalence in certain places and districts being capable of ready interpre- tation if the influences tending to produce excep- tionally high or low pressure of sether in certain regions are ascertained. Any conditions producing weaker or stronger resistance to the re-estabUshment of equilibrium in adjacent regions of unequal pressure of aether will of course exercise important influence as regards the occurrence of earthquakes, etc. Eruptions of volcanoes, with associated earthquakes, are no doubt often due to violent explosions result- ing from chemical action in large masses of explosive material in subterranean regions ; but earthquakes are often experienced over large tracts of the earth with no evidence of any such occurrence to account satisfactorily for them. Not only may explosive escapes of aether gas pent up in the substance of the earth give rise to earth- quakes, but another possible cause of them is the dashing of large irregular waves of aether against the earth. The approach of a comet is probably preceded by waves or currents of aether, and similarly followed, and these are obviously capable of causing shocks to the earth where they strike it. Currents and waves due to other causes may of course simi- larly strike the earth without any visible signs of their approach, the visibUity of such currents or waves depending on their carrying with them coarser gases or materials which become luminous. The CAPILLARITY 147 association of thunder and lightning with volcanic eruptions and earthquakes shows that the equili- brium of the aether is disturbed, with the result of violent discharges of aether passing from regions of higher pressure to those where the pressure is lower. Descriptions of violent earthquakes suggest that gra\'ity itself is temporarily and locally overcome, or even reversed, so that hea\-5- masses of matter are displaced and thrown about, causing damage far beyond what could be accounted for by the shaking of the ground from contractions, displacements, or commotions of the crust of the earth. Violent movements of the aether, from whatever cause, offer a more adequate explanation of these strange phenomena. We have alreadj', on pages 67 and 68, referred briefly to ' capillarity,' with which ' surface tension ' is closely connected ; but we maj^ discuss the matter a little further here. In the case of solids, we observe how the presence of water prevents surface particles being removed by the wind, and how in many cases dr\-ing of the surface loosens the adhesion of the surface particles, with the formation of dust. In the case of liquids, such as w^ter, compression, or cold, increases the adhesion of the particles, and the surface may become solid ; while lowering the com- pression, or heating, diminishes the adhesion, and particles of vapour are freely cast off from the surface. In the case of gases, the surface becomes non-existent, or ill-defined, but there is still a tendency to adhesion of the particles, over and above the powerful adhesion of atoms together to form mole- cules. The heavier gases in air tend to adhere so as 148 THE NATURE OF THINGS to form definite surfaces when they are kept at rest. Thus carbon dioxide accumulates, almost like a liquid, at the bottom of wells, and so on. We must, moreover, remember that the surface of water, under the influence of strong cdr currents, becomes very irregular, and abundant particles are blown off from the surface and diffused in the air. This process becomes greatly exaggerated in the case of geises, so that we are apt to overlook altogether the tendency that there actually is to the formation of an adherent surface. Air in a small vacuous space, that is a small space with little in it but aether, appears to diffuse pretty evenly throughout the whole space ; but the fact that the atmosphere clings together, and clings to the earth, is clear evidence of adhesion between the particles of it, and there is probably a limiting but very irregular surface to it, or it would be sure to drift away by degrees. No doubt the waves on the surface of the atmosphere are much higher than those on the sea, and the ' spray ' is no doubt much more abundant. Still, the conditions, mutatis mutandis, are similar. We have already, on pages 143 and 144, alluded to evidence of internal reflection from the outer surface of the terrestrial atmosphere. Even if we adopted (though we do not adopt it) the idea that the material particles of air are ' attracted ' to the earth, it must be admitted that the attraction diminishes as the square of the distance from the centre of the earth, or thereabouts, and if the atmosphere dwindles gradually into a true vacuum, we must either suppose a limiting surface where the attraction becomes equal CAPILLARITY 149 to the centrifugal force, or one where the adhesion of the particles equals the centrifugal tendenc}'. In both cases the " spray ' would never come back if the space outside were vacuous ; but if there is a material gas outside, the spray would come back as the spray thrown into the air from the surface of the sea returns to it, either directly or indirectly, in the form of rain, etc. It has, indeed, been recognized that it is neces- sary to the explanation of capUlarity and surface tension to recognize that there is adhesion of the particles or molecules of liquids to one another ; but here again the same fatuous device is resorted to of imagining mutual ' attraction' of the particles, and as the attraction of gravity cannot here be used, since capillarity to a certain extent overcomes gravita- tion, the attraction is called ' molecular attraction,' and is supposed to act only at the most minute distances and in a way not consistent with the laws governing other kinds of attraction. The adhesion is no doubt similar to other forms of adhesion and due to higher pressure outside the adhering particles than that between them. The intervening aether in this kind of adhesion is, however, probably not pure aether, as it seems to be in the case of atoms adhering in chemical combination or to form molecules. The molecules or particles are no doubt separated either by molecules or atoms of the same substance, or by those of some other gas coarser than aether, such as air. This affords an explaination of the essential difference between the adhesion of chemically com- bined atoms and that of particles of matter, or of molecules with molecules, when not chemically com- bined. Two atoms could not readily adhere by the 150 THE NATURE OP THINGS interposition of atoms of the same size as themselves, but the intervention of even one atom or molecule of at all nearly the same size may effectively prevent adhesion through the intervention of aether gas under the conditions which give rise to chemical combina- tion. Assumiag that the adhesion of particles of matter is due to the cause to which we have attributed it, we can explain the various degrees of adhesion or viscosity in liquids in a manner analogous to that by which we have explained the degrees of firmness of chemical combination or of so-called chemical affinity. The sizes and shapes of the particles must evidently affect the adhesion of the conglomerate particles which they form, just as the sizes and shapes of atoms and molecules influence chemical combina- tion ; and just as heat will break up or bring about chemical combination, so will it break up or bring about adhesion of the grosser particles of matter. Hence heat turns water into steam, and, in general, turns liquids into gases, or on the other hand assists in the welding of half -melted iron. The substances whose molecules are large will tend to be liquid or solid, other things being equal. This, as we have already pointed out, is very clearly indicated in the series of carbon compounds, and the influence of the shape of particles appears in the allotropic forms of carbon — charcoal, graphite, and diamond. The degree of adhesion, in the case of liquids, can readily be estimated by measuring it against gravity. Their adhesion, as we have men- tioned, does not by any means depend solely on the size of the adhering particles. Their shape has neces- CAPILLARITY 151 sarily a most important influence. Two round balls of lead will not adhere to the same degree as the same amount of lead beaten out into sheets. The adhesion of a particle of a substance is usually not the same when we compare its adhesion to another of the same kind with its adhesion to a particle of a different substance. MercuiA' does not adhere to glass to the same degree as it adheres to more mercury, and similarly with other Uquids which are convex or concave at the surface in a capillary glass tube. In the case of liquids which exhibit a con- ca\ity, the adhesion is greater to glass than it is to more of the same liquid, and less when there is a convex surface. Mercury does not ' wet ' glass, and if the idea of ' molecular attraction ' is adopted, there is nothing for it but to suppose that the glass, instead of attracting the mercury, actually repels it. We have then to invent the notion of ' molecular repulsion,' and must even recognize an attitude of neutrality Uable to be converted into attraction or repulsion by a change of temperature. On such lines we need have little difficulty in explaining an5rthing whatever. A liquid which does not wet glass is, no doubt, one which, by ordinary ph3^cal laws of motion or pressure, adheres more vigorously to other liquid of the same kind than it does to glass. If a substance is substituted for glass, to which the liquid, in the way we have described, adheres more firmly than it does to its fellow particles, it wets the glass. If however we took a liquid which preserved a flat surface in a capiUary glass tube, then on the molecular attraction theory we must suppose that its particles and those 152 THE NATURE OP THINGS of the glass neither attract nor repel one another, and it will be possible for a substance with powerful molecular attraction for one substance to be repellent to another and neutral to a third. If any difficulty still arises, we might easily imagine an alternating, or reversible, or variable attraction or repulsion. Language can always theoretically cut any Gordian knot. As in the fable of Proteus, we ought, how- ever, to wait till the substance has returned to its original form before we can hope to obtain any useful information from it, and in the meantime we should tie it tightly to material facts and recognized prin- ciples, by the methods of Euclid and strict logic. The adhesion of a liquid whose surface is concave in a capillary tube is evidently greater as between the liquid and the substance of the tube than between the particles of the liquid themselves, and the reverse when the surface is convex. This accounts also for the gradual creeping of a liquid up the sides of a tube or vessel, and sometimes even over the edges of it, and also for the separation of a solid in solution from the liquid itself which has it in solution. When the surface of the liquid is convex, the greater adhe- sion to one another of the particles of the liquid pulls the liquid away from the sides of the tube until the adhesion is overcome by gravity. Gravity in the same way limits the depth of the concavity of liquids with concave capillary surfaces. There is nothing essentially different between what happens in a capillary tube and what occurs in an open vessel ; but in the latter case the very small concavity or convexity is due to displacement of a much larger number of liquid particles, with the result that the PHYSICAL ADHESION 153 effect upon the surface becomes imperceptible, or very nearly so. When a drop of a liquid, such as oil, is placed on the surface of water, the adhesion of the oil particles to one another is greater than their adhesion to the water, and the same is the case with a drop of water on a surface of oil. Therefore they do not mix. The oil, acted on by gravity, spreads out on the surface of the water until the adhesion of its particles to one another is equal to the influence of gravity, tending to bring them to a level. When alcohol is dropped on water, the adhesion of the alcohol particles to those of water is much stronger than that of alcohol to alcohol, or water to water. Hence they mix very rapidly, and the more powerful adhesion resulting diminishes the bulk of the mixed liquids as compared with the sum of their bulks while separate. If we take two liquids which adhere to each other with the same vigour as that with which they adhere individ- ually to their fellow particles, they will mix when stirred, but not if brought into contact without stirring or shaking or any motion of their particles. If the adhesion of the two liquids is the same as the adhesion of the particles of the one liquid infer se, but not the same as that of those of the other liqiiid, they will mix, but in all cases the movement will be from the looser to the firmer adhesion. In short, if two liquids are brought together, they wiU mix if the adhesion of the particles of the two different liquids is firmer than the adhesion of the particles of either liquid individually. When we come to consider the solution of a solid in a liquid, the adhesion together of the particles of 154 THE NATURE OP THINGS the solid may be firmer than the adhesion of the particles of the solid to those of the liquid, and yet they may mix (that is, the solid may dissolve in the liquid) if the particles of the solid and liquid adhere together more firmly than the particles of the liquid or the solid individually. A solid will not dissolve in a liquid if their several particles do not adhere together more firmly than the particles of either the liquid or the solid. Saturation of a liquid occurs when the adhesion of the substance, being dissolved, to the liquid as modified by the solution that has already taken place, becomes equal to the adhesion of the particles of the substance to one another. If now another substance is added to the solvent so as to lower the strength of the solvent's adhesion to the substance previously dissolved, some of the substance wiU be thrown out of solution again, and so on. A liquid must evidently be soluble in all proportions in the same liquid as itself, and if two different liquids are soluble in all proportions, it follows that their mutual adhesion is never less than that of one of them to itself. When two chemical substances in solution are brought together they will, if possible, combine, if by their combination, after decomposition in the case of many compounds, a substance is produced less soluble than either of them. This implies that the adhesion of the particles of the two chemical substances to one another is stronger than the adhesion of either of them to the solvent, and that the adhesion of the particles of the chemically combined substances to the solvent is less than that of either of them when uncombined. PHYSICAL ADHESION 155 We may therefore state the rule by saying, that if two substances in solution are brought together, they will combine if the adhesion of the substance formed by their chemical combination to the solvent is less than the adhesion to the solvent of either of them uncombined. Regarded in this wav, the question su^ests itself whether the converse is true, namely, that "if the solubility' of a chemical com- pound is greater than that of either of its components, it will be decomposed by solution." When decompo- sition is almost instantaneous, it is practically impos- sible to test the point, but where the difference of solubilit\' is small, or the decomposition slow, some evidence may be obtained. In other words, if a chemical compound by breaking up will diminish its solubility in a particular solvent, does solution tend to break it up? This woiild simpl}' mean that if two particles are pulled awaj- from each other by a force or forces greater than the strength of their adhesicm, they will separate, and the pull, which was strong enough to separate the particles, will retain them apart until overcome bv another stronger force. Soluble salts of bismuth, lead, iron, etc., are thus gradually broken up when dissolved in water. The adhesion due to chemical combination is set off against the adhesion of atoms to form molecules if the compoond is broken up into elements, or against the adhesion due to the chemical combina- tion in the component compounds into which the dissolved compound is broken up. Thus, with silver nitrate, the silver itself is gradually deposited, while with an iron salt, a hydrate or oxide is deposited. The ' chemical ' adhesion has therefore to be taken 156 THE NATURE OP THINGS into account as well as the ' physical ' adhesion which determines solubility'. In the place of a multitude of imaginary forces of chemical affinity, molecular attraction or repulsion, and so on, we simply have to consider the adhesion of particles which come into collision, the adhesion being due to the interception of gas rendered lower in pressure than the surrounding gas, the higher pressure of which — be it aether or what not — keeps the particles in contact with a firmness varying according to the circumstances. In the case of gases, each gas forms particles as large as the circum- stances allow, and if no chemical action takes place, there will still be a certain modified adhesion between the particles, which will be equal so long as the pressure is uniform, and the adhesion being the same, they will exchange partners in response to the slightest impulse, and consequently diffuse freely together. In the case of a mixture of gases, chemical combination may take place if the adhesion of the atoms or molecules of the different gases to one another is greater than the adhesion of the different kinds of particles individually. The diffusion of the gases together will proceed until the adhesion of the various particles becomes uniform in adjacent volumes of the mixed gas. If the adhesion of the mixed particles is equal to the chemical adhesion of the constituent atoms or molecules, or greater than it, combination wUl not occur. If the adhesion of the particles of one kind of gas to one another be not the same as that of the particles of another kind to one another, they would exchange partners with one another xmless the adhesion of the different BERTHOLLET'S LAWS 157 kinds of particles together were less than that of the same kinds of particles to each other. When several different gases are mixed, the matter obviously becomes more complicated, and equilibrium only occurs when the relative proportions of the gases is suitable, if chemical combination does not take place. Thus the mixture of oxygen and nitrogen in air is in definite proportions, but not in equal propor- tions, because the adhesion of oxygen to oxygen is not the same as that of nitrogen to nitrogen, nor is the adhesion of oxygen to nitrogen stronger than the internal adhesion of either oxygen or nitrogen alone. In order that oxygen and nitrogen should mix in equal proportions without chemical change, there would have to be equal adhesion between oxygen and oxygen and nitrogen and nitrogen, unless the adhesion of oxygen to nitrogen were stronger than that of either of the gases individually. In actual fact air does not consist solely of oxygen and nitrogen in the atmosphere, so that the question is really more complicated than it would be if there were a pure mixture of the two gases only. Berthollet's law is that " two bodies in solution will always decompose each other, if it be possible, by double decomposition, to produce a new body less soluble than either of the two original substances." We have, in our statement above, widened this law so as to apply to elements in solution as well as compounds, and to compounds which unite without decomposition as well as those which first decompose. Berthollet's second law is that " two bodies when mixed or heated together will always decompose each other, if it be possible, by double decomposition. 158 THE NATURE OF THINGS to produce a new body more volatile than either of the two original substances." This again may be more widely stated by saying that two bodies, when mixed or heated together, if it be possible for them by combination, before or after decomposition, to produce a new body more volatile than either of the original substances, will combine, or form new combinations, as the case may be. There are exceptions to these rules, because, as we have already explained, the relative strength of chemical adhesion must be considered. Carbon and oxygen heated together form carbon monoxide, or dioxide, gas : hydrochloric acid and sodium hydrogen carbonate form carbon dioxide ; and so on. In the case of carbon and oxygen the physical adhesion of carbon to carbon is much stronger than that of oxygen to oxygen, and heating leads to an inter- mediate strength of adhesion, that of oxygen to carbon. In the case of hydrochloric acid acting on bread soda, carbon dioxide, which is more volatile than either of the original substances, is formed ; but on the other hand, the average volatility, or adhesion inter se, of sodium chloride and of water, is less than that of bread soda and hydrochloric acid. When ammonia gas and hydrochloric acid gas are mixed, the above rule appears to fail, the less volatile ammonium chloride and water being formed. In this case the. physical, as apart from the chemical, adhesion of the two compounds is very low, and the chemical adhesion dominates altogether. If we add hydrochloric acid to nitrate of potassium, we obtain the volatile oxides of nitrogen, and potassium chloride, and water. In this case the adhesion of PHYSICAL ADHESION 159 potassium chloride is greater physiccJly than that of potassium nitrate, and that of hydrochloric acid is greater than that of the nitrogen oxides, while the adhesion of water is intermediate. The adhesive force of nitrate of potassium, added to that of hydrochloric acid, is however equal to that of potas- sium chloride added to that of the nitrogen oxides and that of water proportionately, including both physical and chemical adhesion, with the addition or subtraction of any heat energy given out or absorbed. Consequently, we may sum up by saying that when two or more substances are intimately mixed, the sum of the adhesive forces of the original sub- stances are equal to the sum of the adhesive forces of the resulting substances, allowing for heat given out or absorbed. Similar considerations apply to what are called ' solid solutions,' and they account for the proportions of metals in definite alloys, etc. The nature of what we call ' physical adhesion ' is very well seen in the case of malleable metals such as iron. Two pieces of heated iron hammered together become firmly adherent, the hammering forcing out intervening gas and so causing adhesion, part of the motion or energy of the hammering being thus rendered latent. All adhesion may in fact be regarded as ' latent energy.' It cannot be produced without absorption of energy, which is of course given out when the adhesion is dissolved. This law therefore implies that there is a tendency to equili- brium of different degrees of latent energy under the influence of heat or mixing. Among the phenomena which have been brought within our powers of observation by the use of 160 THE NATURE OF THINGS powerful microscopes are the so-called ' Brownian movements.' These are incessant irregular move- ments of microscopic grains in liquids. They indicate that the elementary particles of a liquid are incessantly changing their position relatively to one another, and by their colUsion with the micro- scopic grains producing motion in the latter. This bears out the views we have already expressed as to the explanation of solidity, liquidity, and the gaseous condition, and at the same time indicates motion, which is shown experimentally not to be due to convection currents in the liquids themselves. The movements can, for instance, be observed in the microscopic bubbles of liquid enclosed in quartz, and they evidently continue for indefinite periods, and perhaps eternally. The adhesions to one another of the particles of a liquid are identical, if the liquid is pure, and conse- quently the slightest movement will transfer one particle from its adhesion to another particle into adhesion with a third one, and so on. In the case of viscous fluids such as glycerin, the Brownian movement, as might be expected, is found to be slower than in the case of less viscous fluids. The original source of the movements is, we may reason- ably believe, the movement of the aether, which takes place within the minute cavities of quartz as well as in free air or intersidereal space. In solids, however, as far as the actually solid portions are concerned, the adhesion is much firmer, and although it seems certain that particles even of solids are not absolutely fixed in position, yet they are practi cally stationary in relation to one another. PHYSICAL ADHESION 161 In the case of gcises, the particles appear to be free from mutual adhesion, or even appear to repel each other, yet they undoubtedly do to some extent adhere together, although as regards particles of the same gas they interchange positions quite freely. Gases of different kinds, like liquids, vary in the degree of the mutual adhesion of their particles, some being more viscous, as we may call it, than others. Solids as weU as liquids give off vapour, and, as the adhesion of the particles must in each case be overcome, it is interesting to note whether or not the vapour pressure of water and ice, both at zero, is the same. On the theory put forward here it is evident that the vapour pressure of water should be slightly higher than that of ice at the same temperature. Experiments on the point appear to prove that this is so. Many other examples might be given showing that the theory of adhesion, which we have adopted, explains better than any other the results of physical observation and experiment; but we wiU leave readers to examine for themselves whether or not the various accepted laws, and their recognized exceptions, are consistent with the explanation we have given of the nature of the mutual adhesion of material particles in solids, liquids, and gases. IX 162 CHAPTER IX. ASTRONOMY AND GRAVITATION, Illud in his rebus longe fuge credere, Memmi, In medium Summse (quod dicunt) omnia niti Atque ideo Mundi naturam stare sine ullis Ictibus externis, neque quoquam posse resolvi Summa atque Ima quod in Medium sint omnia nixa. — (Lucretius.) In these matters keep far away, Memmius, from believing That all things push towards the centre (as they call it) of the Universe, And that thus the nature of the world stands firm, without any External forces, and cannot in any way be broken up, Top or Bottom, because all things push to the Centre. T UCRETIUS, if we judge by the above quotation, -■— ' would have ridiculed with more caustic wit than ourselves the idea of explaining gravitation as an ' attraction ' towards the centre of the Earth or Sun. A pull towards a centre is as absurd as an imaginary push in that direction if there is nothing that pulls. There is, however, nothing absurd in supposing surrounding matter to be pulled or pushed towards the centre of the Earth or Sun or Universe if we can give a reasonable explanation of what it is that pushes or pulls. The existence of more or less empty space in the Universe seems to be proved, as Lucretius maintains, but the idea of the trans- mission of energy through a real vacuum, without ASTRONOMY AND GRAVITATION 163 material transport, is one from which we ought to keep far away. We have maintained that matter and motion exist, and that they can be neither created nor destroyed, and we contend that the facts of Astronomj' and Gra\-itation can be adequately explained witliout departing from this general' principle. Let us suppose an extended space filled with aether gas composed of atoms uniform in size and mass, and with nothing material intervening between the atoms, and let us suppose the atoms to be incompressible, and, under the existing conditions, indivisible. If from any cause a large absolutely vacuous space be formed within this gas, while the gas is in motion or under pressure, it will flow into the vacuum. Let us then consider what will happen if two of the gaseous atoms of exactly similar size, shape, and mass move through such a vacuum in precisely opposite directions along the same line with equal velocities and come into direct coUision, both being quite incom- pressible. Would each of them pass its motion on to the other, or would they both come to a standstill, the motion of the one neutralizing the equal and opposite motion of the other ? At first sight either result seems possible. Both atoms might rebound each with the same velocity as before, but with exchange of directions. This would involve no loss of energy, which we consider impossible. The atoms being incompressible and indi\'isible, no heat can be generated within the atoms themselves, since heat is merely a mode of motion due to movements of partides to-and-fro in a substance composed of separate particles. The two atoms may, however. 164 THE NATURE OP THINGS cease moving while continuing to push each other with equal energy in opposite directions. This would mean that they adhered to one another with an amount of energy equal to their individual energies added together, the combined energy becoming ' latent.' If each atom rebounded with the same energy as before collision, there would indeed be no loss or gain of energy involved, but there would be a complete change of direction of the movement of each atom. This, in algebraical language, would be expressed by saying that +a added to —a was equal, not to zero, but to — « added to +a, the signs of the two elements of energy being exchanged. If, how- ever, we accept the equation +a — a =o, this would imply that the two atoms would come to rest, and that the energy would disappear or become latent. Regarded experimentally, we find that in propor- tion as two impinging bodies become less and less compressible, so do they tend, other things being equal, less and less to rebound. It may therefore be reasonably inferred that the two atoms would adhere after their collision in a vacuum, their move- ment being stopped, and the whole of the energy of both of them becoming latent. When other atoms rushed into the vacuum from all sides and collided with the two stationary and adherent atoms, they would similarly tend to adhere to the nucleus already formed, until the vacuum was filled up. The force of impact would continually decrease as the distance traversed by inrushing atoms diminished. If the vacuum were spherical, the pressure would increase as we passed to the centre in the inverse ratio of the distance from the centre. This will appear ASTRONOMY AND GRAVITATION 165 readily from the following illustration. Suppose a hollow indiarubber ball of perfect elasticitj' to have the air inside it instant!}' replaced by a complete vacuum. The ball would contract uniformly into a small solid indiarubber sphere. As the ball con- tracted, the elastic pressure originally distributed over the whole of its surface would be concentrated on spherical surfaces, contracting concentrically, and, as these surfaces are proportional to the square of the radii of the spherical balls, the pressure would increase in intensity in the same proportion as that in which the surfaces diminish in area. When equilibrium has become established the energy due to influx into the vacuum becomes fixed in the form of a centripetal push towards the centre of the original vacuum, and there remains perma- nentiy in the ocean of aether a region of greater density, where the vacuum had been. There need be no tendency to dissipation of the energ}' thus rendered latent, in the case of a conglomeration of incompressible and inelastic atoms. If a vacuum such as we have imagined is not accurately spherical, the influx of atoms will evidentiy not be directiy centripetal, and rotation wUl ensue analogous to that which occurs in an aerial cyclone. The final result will then be a spheroid, part of the original energy producing adhesion and part producing rotation. ^\^len the outside aether has settied down again into a condition of equilibrium, the centripetal and centrifugal forces will be equal, and both will be equal to — , v being the velocity of a particle and r the radius of the circle described by the particle in 166 THE NATURE OP THINGS its movement. As the radius of a solid or adherent sphere increases, the velocity of a particle on its surface will increase in the ratio of the circumference of the circle it describes around the axis of rotation of the sphere, if it continues to rotate as a whole ; since a particle far from the axis will complete a revolution in the same time as one nearer to the axis. The circumference of a circle is represented by the expression a^r, the circumference increasing in length with increase of the radius, but 2 7r times more rapidly. Consequently, the centrifugal force of particles borne round on the surfaces of concentric spheres, — . increases, as we pass from the axis of rotation, according to the expression . or /[■jr'r. The centripetal force on a sphere, formed as we have described above, decreases in the inverse ratio of the square of the radius. Consequently, with a sphere of the kind described, the centripetal and centrifugal forces must after a time become equal. When this equality is established, particles will no longer adhere to the surface of the sphere, which accordingly reaches its limit of size as an adherent mass, a limit evidently determined by the dimensions of the vacuum which gives rise to the formation of the sphere. The maximum limit of the adherent sphere, or spheroid, formed by a cyclonic movement of sether gas, being thus attained, let us consider what conditions will prevail in the gas just outside the sphere, a gas composed of incompressible atoms, either in direct contact with one another, or ASTRONOMY AND GRAVITATION 167 with nothing intervening between them. Any original currents of the gas, independent of the cyclonic movement, will be still flowing, while close to the surface of the sphere equal centrifugal and centripetal forces act on the gaseous atoms. By the rule of the parallelogram (or triangle) of forces, the resultant of the centripetal and centrifugal forces, acting at right angles to one another, will act at an angle of 45° to each of them, and will tend to maintain the existing rotation. If the general mass of aether is flowing in any direction, as we have supposed that it is. the rotating sphere will be borne along in the current. If the current is in the direction of the axis of rotation, as it would be under the conditions assumed, the resultant current would pass, not circularly round the sphere, but in a spiral manner. This spiral circulation of jether arotmd a sphere imparts to it properties resembling those of a magnet. This movement will give rise to undulatory vibrations, extending to a great distance, and gradually decreas- ing in power in the ratio of the square of the distance, since the surfaces of concentric spherical shells increase as the square of their radii, and therefore, when the same amount of enei^ is spread over them in succession, as they expand continuously, the intensity of the energy will decrease in the same ratio. The greater crowding together of particles of aether, or other matter, towards the centre, may prevent an aether particle from travelling quite through to the centre of the Earth after equilibrium has been attained, but aether and other matter will move, or 168 THE NATURE OF THINGS tend to move, in that direction. If it is immersed in a so-called vacuum, that is in pure aether, matter moves towards the centre of the Earth at the same rate, whatever its nature may be, until it is stopped by solid matter. If it moves through water or air, its motion is greater or less according to its weight ; that is, according to the relative proportion of aether displaced by it in a given volume of it — and according to the surface which is exposed to the resistance of the matter through which it passes by displacing it. When it can overcome the resistance no longer, it ceases to proceed further centripetally, though partaking still in the general rotatory motion. Notwithstanding the difficulties involved, this interpretation of gravitation, as due to a cyclonic movement in a gas composed of incompressible atoms, and such as the intersidereal aether may reasonably be supposed to be, is much more satis- factory, as an interpretation of the phenomena actually observed, than the assumption of a most improbable property of matter whereby it exerts a power of attraction upon other matter across space, either empty or occupied, in a manner quite incon- sistent with experience or indeed with common sense. To recast the whole system of astronomy, or even a small fraction of it, on the lines of the theory herein outlined, cannot of course be here attempted. It would require a vast amount of labour and excep- tional mathematical attainments. It is quite clear, however, that we must regard energy as well as matter as being incapable of either creation or destruction, and we must regard the transference of ASTRONOMY AND GRAVITATION 169 energy from matter to matter as taldng place only through direct contact of matter with matter. The Smi must not be supposed to draw the Earth towards it through empty space. It may, however, impart to an intervening material gas movement which is transmitted through that gas to the Earth, and the planets, etc. The same general principle applies to light, heat, electric and magnetic energy, as we have already explained. They are all uncreatable and indestructible, although interchangeable and capable of becoming latent, and they can only be transferred from matter to matter by direct contact of matter \vith matter. The theory of " attraction ' has sufficed to enable accurate deductions to be made, and most of these deductions wiU no doubt be practically unaffected by the adoption of the theory outlined above ; but it is alAvays much safer and better to argue from actual fact than from an imaginary supposition, even though the latter leads to conclusions which are in the main in accordance with observed facts. Moreover, as has been shown, the recognition of the existence and properties of the actually existing intersidereal gas, commonly called ' aether,' affords at the same time real explanations of many other things in nature as well as gravitation. If then we regard the Earth, or one of the planets, as a cyclonic sjrstem in the unlimited ocean of aether, we may next proceed to consider how two cj-clonic sj'stems in aether gas can ' attract ' one another, as they certainly appear to do. If two rotatory move- ments meet, they obey the ordinary laws governing motion. When they are in the same direction, they 170 THE NATURE OF THINGS are superposed, and similarly, mutatis mutandis, when the}^ are in opposite directions. If we compare the rotatory movement connected with the Sun, and with the Earth, respectively, that which is connected with the Sun is manifestly greater than that connected with the Earth. The movements emanating from the Sun will then dominate those from the Earth, and will also, breaking up against the Earth, produce surf-like vibrations in the form of heat, or vibrations perpendicular to straight lines connecting the Sun and the Earth and constituting light, the latter being the component which acts tangentially to the surface of the sphere which is interposed as an obstacle in the way of the waves, and so on. Light, as we have said before, may quite probably not exist as such outside the terrestrial atmosphere, — that is to say, outside the limit where adhesion of the particles of matter ceases ; neither does it exist as such when the adhesion is so complete as to break up the tangential, or tiansverse, vibrations and to convert them into heat. Thus we have waves from the Sun partly expended in temporarily producing light in the terrestrial atmosphere, and partly expended in producing heat in the substance of the Earth, including its atmosphere. Waves of terres- trial origin, except so far as reflected waves from the Sun are concerned, do not give any appreciable light to the Sun, or to the other planets, because the movements originating from the Ecirth are wiped out by the more powerful ones of solar origin ; and the same is the case with movements arising from the planets. Now that equilibrium has been established ASTRONOMY AND GRAVITATION 171 in the solar s\^tein, we have spheres of adherent matter immersed in an indefinitely, if not infinitely, extended ocean of gas, composed of unattached, incompressible, and indivisible atoms ; and it has to be considered how their permanent rotatory and circumambient motions are maintained. If the aether is moving, as we have described it (and it is at least as likely to be moving as to be still), we can readily suppose it to have a steady general drift in one direction, and there are cogent reasons, into which we need not now enter, for believing that the whole solar s^-stem is in motion. Let us then suppose the aether to be flowing steadih' in one direction, and, at the moment when the Earth has completed the process of its formation, we have a sphere floating in a current, the motion of which is superposed on the cyclonic movement described above. This movement simply carries along with it the cyclonic s\'stem which we have inferred to exist, and does not destroy the cyclonic movement finally established. The apparent attrac- tion of the Sun for the planets can therefore be explained by supposing the planets to be carried round the Sun in the manner in which they do in fact travel round it, the gas in which they float having a centripetal pressure towards the centre of the Sun, while the centrifugal force due to their motion is equal to the centripetal force. If the centripetal pressure around the Sun is everywhere inversely proportional to the dimensions of the surfaces of concentric spheres drawn round the centre of the Sun, and passing through points under consideration, which would be the case with a gas, such as we have 172 THE NATURE OF THINGS assumed the iEther to be, moving in conformity with the movements of the planets floating in it, then the centripetal pressure of the gas, as we pass out from the centre of the Sun, will be in the inverse ratio of the square of the radius (that is, the distance from the centre of the Sun), since the surface of a sphere is equal to 4^^^ -pjjjg jg ju accordance with the law of gravitation. According to Kepler's laws, the squares of the periodic times of the planets are to each other as the cubes of their distances from the Sun, and, if we regard each planet as a point on a sphere with the centre of the Sun as its centre, we have the squares of the periodic times of the planets in the same ratio to each other as the volumes of the spheres on the surfaces of which they may be supposed to travel, since the volumes of concentric spheres are in the ratio of the cubes of their radii. If these spheres are supposed to rotate round a common axis, each as a whole, it does not follow that points on the surface of any one of them all travel with the same velocity, since the point situated on the equator of such a sphere would traverse a much wider circle than one situated nearer the poles ; but it does follow that the periodic times of all points on each sphere would be the same if they travel in circles described on the surface of such sphere. The law of Kepler would therefore be explained if we can suppose the planets to be carried around the Sun as points on concentric globes rotating on an axis passing through the centre of the Sun at right angles to the plane of the circles on which the planets travel. Instead, however, of a solid globe we might ASTRONOMY AND GRAVITATION 173 have a sphere like a soap bubble, in which a particle may be travelling around with the rotation of the sphere, and at the same time moving after the fashion of a screw in the substance of the bubble, the orbit being thus elliptical and in a plane not at right angles to the axis of the sphere. In the case of each planet, as we have pointed out before, the centrifugal and centripetal forces must be equal, or the planet would approach or recede from the Sun or the centre round which it revolved. The centrifugal force affecting a planet may be expressed as — , and the centripetal force as —, V being the velocity of the planet's motion, and r its distance from the Sun, — the accelerating force at the Sun's centre being taken as unity. Therefore, from what has been said it v^ I I follows that — = — , or D^ = -. In other words. r r^ r the square of the velocity of a planet varies inversely as its distance from the Sun when we compare planets at different distances from the Sun : that is to say, v~ : v'^ : : r' : r or v:v':: V/ : Vr, a-"^ since the spaces traversed are respectively 3 ^ r and 2t/, T2 : T'2 : : ^3 : /a, T and T' being the periodic times, and r and / being the distances of the planets respectively from the axis of the Sun. Taking the distances from the axis of the Sun as being practi- cally the same as the distances from the centre of the Sun, Kepler's law therefore follows from the supposition that the planets travel round the Sun on the surfaces of concentric spheres rotating round the Sun's axis, or prolonged axis, with velocities the 174 THE NATURE OF THINGS square of which are inversely as their distances from the Sun, the surfaces of the imaginary spheres being such that all points on each one of them travel with a velocity defined by the equation — v^ = ^, in which r is the radius of the imaginary sphere, or the distance of every point on its surface from the centre of the Sun. As we have already stated, - is the centrifugal force acting on a particle travelling on a circle with radius r, and -^ is the centripetal force required to counteract it. The centripetal force diminishes, as we pass away from the Sun, in the inverse ratio of the square of the distance, but the velocity of the planets revolving round the Sun may also diminish, and, according to our theory, would diminish. It is not necessary of course that the imaginary concentric spheres should revolve as though they were solid globes. The centrifugal force must indeed, as regards the several planets, diminish as we pass away from the Sun, because, where the planets revolve, the centripetal and centrifugal forces must be equal unless the planets oscillate to and from the Sun. Since the centripetal force steadily diminishes as we go away from the Sun, there will evidently come a time at last when the centrifugal force will no longer be equal to the centripetal, and revolution will there- fore cease. We shall then have reached the limits of the solar system, and the general currents of the sether will overcome the enfeebled centripetal force acting towards the centre of the Sun. In the case ASTRONOMY AND GRAVITATION 175 of the terrestrial system the same argument applies to the Moon in relation to the Earth. We have as before — = ^, u being the velocity of the Moon in its path around the Earth, r the distance of the Moon from the centre of the Earth, and g the accelerating force of gravity near the centre of the Earth. If we take 981 as the acceleration due to gravity at the surface of the Earth, it will be about ■27 at the distance of the Moon from the Earth. If this — the centripetal force — is equal, as it must be, V- to the centiifugcd force, it must be equal to — , v being the velocity of revolution of the Moon around the Earth, and r its distance from the Earth. Using the ascertained values for v and r, we obtain '27 V » approximately as the value of — . There is no other sateUite of the Earth, the centripetal force being probably too weak beyond the Moon to restrain the cmxents in the ather acting centrifugally. Bearing in mind the analogy of a cyclonic system, it is easy to form an idea of the solar and planetary systems, and gravitation generally, without pre- supposing anything more than the famihar move- ments of gases, with solid masses floating in them, and subject to the so-called ' laws ' governing such motion. From this point of view, regarding the Sun and planets as masses of matter floating in ather gas, by the recognized law of pneumatics and hydrostatics, the weight of the Sun, and of each planet, must be equal to that of the aether which it displaces; or in other words, the density of each 176 THE NATURE OF THINGS planet (taking the average) must be equal to the density of the sether in which it floats. If the calculated densities of the Sun and planets are to be depended on, they indicate the density of the aether in the region in which respectively they float. According to this, the density of the aether in the region of Neptune, the most distant of the well-known planets, is nearly the same as that in the solar region. The highest density of the intervening aether, on the same hypothesis is in the region of Mercury, the planet nearest the Sun. The lowest is in the region of Saturn. There is thus, in the solar system, a central region of relatively low pressure surrounded by a zone of high pressure, outside of which comes a zone of the lowest pressure of all, with finally a pressure about equal to that at the centre. This is distinctly suggestive of a cyclonic system, although, like aerial cyclones, not very easy to explain exactly, and of course allowance must be made for the fact that it is in the interior of an unlimited ocean of gas, and not within the narrow limits of the terrestrial atmosphere. Moreover, the condition is not that which exists while the cyclonic disturbance is in its active state, but it is the condition into which the disturbance settles down when the forces brought into play have settled down into a condition of equilibrium. It will no doubt be objected that we have explained gravitation as due to a centripetal pressure arising from gas at a higher pressure tending to pass towards regions of lower pressure, or rushing into a vacuum, and that unless there is a temporary abrogation of the law of gravitation, we are supposing ASTRONOMY AND GRAVITATION 177 a tendency for the gas at low pressure in the region of Saturn to pass into the region of higher pressure in the region of Mercury, or even through that into the relatively lower pressure in the region of the Sun itself. It must, however, be remembered that the solar S5'stem has already attained a condition of equilibrium in which velocity of revolution must be taken into consideration in conjunction with centripetal pres- sure ; and, as we have explained, the centrifugal pressure, — , in the case of each planet equals the g centripetal pressure — . Since g is a constant, s^x y must be equal for aU the planets — ^i; being the velocity of the planet in its revolution roimd the Sun, and r its distance from the Sun. According to the accepted values for v and r this is approximately the case. The centripetal force is the same, of course, as that commonly ascribed to gravitation, and the centri- petal force has to be multiplied by the mass of a planet in reckoning the total pressure, or so-called attraction, impelling the planet towards the Sun. In hke manner the centrifugal force must be multi- plied by the mass. The condition of equilibrium therefore depends altogether on the distance from the Sun and the velocity of revolution of the planet, if it be true that the centripetal force is one which is a constant near the Sun's centre, and which diminishes as the square of the distance from the Sun's centre as we pass outwards. The velocity of revolution of a planet round the Sun is consequently the variable 12 178 THE NATURE OF THINGS factor which is most in need of explanation. The ordinary theory of attraction gives no clue to the determining agent. It explains the centripetal force as due to attraction exerted by masses of matter upon one another, while the centrifugal forces neces- sary to bring about the movements of the planets round the Sun, as they actually exist, are attributed to original impulses acting tangentially to the orbits of the planets. The explanation of the origin of these impulses which undoubtedly exist, or have existed, is most unsatisfactory so far as any explanation has been attempted hitherto. It has been said that the Divine Arm impressed them according to the orbits of the heavenly bodies at the Creation. This use of the deus ex machina simply indicates an inability to deal with the question satisfactorily otherwise. The cyclonic theory which we have put forward is, .on the other hand, a simple physical explanation, and one strongly supported by analogy, not to speak of its being in principle perhaps the oldest explanation which has come down to us in ancient scientific writings. There are serious objections to any theory of original impulses of various strengths remaining unexhausted and undiminished age after age, without reinforcement from without, unless we suppose the planets to travel in an absolute vacuum. If this were so, the transmission of light and heat through a perfect vacuum involves very improbable assump- tions. The existence of a medium, commonly known as aether, for such transmission, is indeed now almost universally admitted ; though highly ingeni- ous but very unconvincing attempts have been made ASTRONOMY AND GRAVITATION 179 to show that jether need not necessaxily be composed of actual matter. Our contention is that gravitation and the movements of the planets can be best explained in accordance with the common-sense view (which Newton probably held, though he hesitated to maintain it), that intersidereal space is occupied by a chemically inert gas composed of very minute particles. 180 CHAPTER X. LIFE AND VITAL PROCESSES. Nunc Animam quoque ut ia membris cognoscere possis Esse, neque Harmoniam corpus retinere solere : Principio fit, uti detracto corpore multo Saepe tamen nobis in membris vita moretur. Atque eadem rursus cum corpora pauca caloris Diffugere, forasque per os est editus aer, Deserit extemplo venas atque ossa relinquit : Noscere ut hinc possis, non sequas omnia parteis Corpora habere, neque ex aequo fulcire salutem ; Sed magis base, Venti quae sunt, calidique vaporis Semina, curare in membris ut vita moretur. Est igitur Calor, ac ventus vitalis in ipso Corpore, qui nobis moribundus deserit artus. — (Lucretius.) Now in order that you may be able to recognize that there is Life also In the members, and that the body is not accustomed to maintain Harmony : It happens, in the first place, that when much of the Body is removed Still our Life often lingers on in the members. And that it also, when a few particles of heat have been lost And air has been given out by the mouth. At once deserts the veins and leaves the bones. So that you may recognize from this that all substances have not equal functions. And do not to an equal extent support health and life ; But those particles which are gaseous or which form warm vapour [members. Have more effect in maintaining the retention of life in the There are therefore Warmth and vital Gas in the Body itself Which desert our limbs when we are at the point of death. /^ DOLING of the body, and alteration in the ^-^ gaseous condition of the blood by cessation of the oxygenating process of respiration, are immedi- LIFE AND VITAL PROCESSES 181 ate precursors of death in the body, and to a large extent the causes of it, so that Lucretius is very accurate in what he says, although his knowledge of physiological processes must have been very imper- fect. We have, in previous chapters, complained repeatedly of the way in which imaginary forces are invented for the purpose of explaining physical phenomena, and here again we find the same method employed. Instead of going back to the recognized properties of matter and the laws of motion for the explanation of life and vital processes, we find an imaginary vital force introduced and credited with all sorts of qualities to suit any difficulty which arises. Moreover, to suit various creeds and philo- sophical systems, life, under names such as " soul,' ' spirit,' etc., has been developed into a sort of independent being which passes into a body and again passes out of it. The body has hence been regarded as a prison-house of the living inhabitant, and has, with perverse consistency, been maltreated accordingly. It is difiicult, however, to tell where the line has been drawn between the ' vital principle ' and the equally imaginary " thinking principle.' The immortality of the soul is a dogma asserted with the most absolute confidence by generation after generation, without anything approaching an accepted or intelligible definition of what is meant by the soul. Sometimes we might suppose that it was an incorporeal individual being, the presence of which in a body imparted life to the body, which ceased to live at the instant when the soul flew away and left it. At other times it would appear as though life was considered as something apart from the soul. 182 THE NATURE OP THINGS life being common to man and other animals (living things), while the soul was an immaterial being to which the higher intelligence of man was attributed, and which was by most people denied to exist in lower animals. The popular conception of the human soul seems to be usually that of a human being completely divested of the conditions due to matter. It is, like abstract ideas in general, arrived at by a process of abstracting, or neglecting, everything except the particular quality to which attention is directed. Thus a man is supposed to remain alive, capable of inteUigent thought, and even action, apart from the matter which constitutes his body, which is regarded as distinct from his being, just as sweetness may be regarded as existing apart from a sweet substance such as sugar or honey. The body is then considered to have a relation to the soul such as a man's coat, or the tools he uses, have to his body ; or as a vehicle for the soul, as sugar or honey may be considered to be mere vehicles for sweetness. In principle this is much the same as the notion, which we deprecate, that motion or energy exist apart from matter, but at the same time localized and individualized as we know to be the case with material bodies. Life, in the proper sense of the word, can be attributed to plants as well as animals. It therefore does not imply consciousness, for we cannot suppose that plants have anything like the consciousness possessed by animals. Animals, moreover, can lose consciousness without ceasing to live. We ourselves, under the influence of an anaesthetic such as chloro- form, or that of deep sleep, can absolutely lose LIFE AND VITAL PROCESSES 183 consciousness without loss of life or stoppage of vital processes. Locomotion, or voluntary power of move- ment, is also not an essential attribute of life, being absent in living plants, and, under the influence of anaesthetics, or paralj^c conditions, also in living animals. Neither does life include sensation, since it exists without sensibility. We can, in dealing with the nature of life, therefore exclude all those qualities and powers which are usually included under the terms ' mind ' and ' senses ' ; and they will be considered in the next chapter. Life, as far as we know, can only arise from pre- existent life, under the conditions which now exist. In former times it was believed to originate from inanimate matter under suitable conditions. It was, for instance, thought that living animals were pro- duced in processes of putrefaction, and it was not considered absurd to suppose that swarms of bees might be formed in the putrefying carcase of a calf. Even up to quite recent times it has been contended that some of the lower forms of life may originate irom inanimate matter. Close investigation has, however, always resulted in disproving apparent cases of life originating anew. It is therefore now almost universally admitted that, by one process or another, every living plant or animal has been derived from a pre-existent living thing. It might therefore be argued that life, like matter or motion, is neither created nor destroyed ; but it cannot be contended that vital energy remains always the same in quantity when traced through its various changes. A minute amount of life may evidently become multiplied and increased almost indefinitely. 184 THE NATURE OF THINGS but the energy developed in the form of vital energy does not indicate creation of any new energy, nor does the destruction of life, as such, imply any actual destruction of energy. Vital energy is merely a modification of other forms of energy, from which it may be formed, and into which it may be restored, without the slightest gain or loss. All that can reasonably be maintained is that Uving matter is alone capable of converting inanimate matter into living matter. Whatever be the mode of growth or reproduction, it is quite clear that plants and animals alike do convert inanimate matter into living matter, and cannot grow or multiply without using for the purpose an equivalent amount of suitable pre-existent chemical elements or compounds, which can be recovered in precisely the same quantities from the living structures into the composition of which they enter. The same is true of the energy or motion appearing as vital energy or motion. A comparatively small amount of energy may be directly transferred from the original living matter to that which is newly formed, but most of it is derived from chemical energy or other forms of ordinary energy. All that can be maintained, there- fore, is that living matter can be formed from its inanimate constituents only, under certain condi- tions which, as far as we know at present, require the presence and influence of living matter. This is not unlike what occurs in the formation of inanimate chemical compounds, which often require for their formation very exact conditions, both as regards the constitution of the necessary ingredients and the amount and character of the heat, light, or LIFE AND VITAL PROCESSES 186 other form of energy required for the formation of the compounds. In the case of simple forms of living matter, such as protoplasm, or whatever we call it, there must first of all be very precise and compli- cated conditions and materials present in order to bring it into existence ; and, secondly, there must be a continuous supply of suitable material — food — with energy in appropriate forms, in order to main- tain its existence and to enable it to produce more living matter like itself. It is thus in fact a chemical compound which continually undergoes certain chemi- cal changes, and which, under suitable circumstances, renews itself with but little permanent alteration. At the same time it affords conditions under which more of the same compound is formed afresh, so that, when the original living matter finally becomes permanently changed so as to lose the properties essential to the continuation of the conditions and processes which constitute life, fresh material has been provided which replaces the original living mat- ter, often in increased quantity. This, again, goes through a similar process, and accordingly, while the necessary conditions Eire present, living matter, not- withstanding its perpetual change, decay, and partial death, remains immortal, though the matter which was alive has died, but not before it has converted inanimate matter into living matter of its own kind. This, in turn, reproduces itself and ceases to live. If we compare living matter with what remains of it when suddenly deprived of life, the chief differ- ence is that death has stopped the chemical and physical changes and processes characteristic of life. It is much the same as if a very elaborate and com- 186 THE NATURE OF THINGS plex machine stops working, the machine remaining for the time nearly the same as before. When death is caused by a sudden violent shock, the stoppage of the working of the machine is practically the only change which has taken place at the moment of death, though important chemical and physical changes result with great rapidity when the machinery of life stops working. In some instances, indeed, the machinery of life can by artificial means be set going again, and, if the changes which quickly supervene upon death have not proceeded too far, life may be fully restored. If life were anything more than a complex set of motions, physical and chemical, it would be very unlikely that the application of stimulus to some part of the vital machinery would recall life when it had definitely left the body. The heart, which is to the body, in many respects, what a mainspring is to a watch, can often be made to beat again after having for a substantial time stopped completely, and if the blood still remains quite liquid, and uncoagulated, the renewed action of the heart may restore life. It must be remem- bered also that, as Lucretius has well explained, each part of the living body is itself alive, and that its life may be independently destroyed without affecting the life of the rest of the body, unless indeed the life of the part which has died is essential to the general life of the body ; and even then death does not occur at the same moment throughout the whole system. The heart of an animal, if carefully removed and kept under suitable conditions, can be revived and made to beat again naturally, quite a long time after removal, and after the rest of the body has for some time been unquestionably devoid of life. LIFE AXD VITAL PROCESSES 137 The popular idea of life as a sort of individual entit\% whicli gives life to an animal on entering it. and which leaves it lifeless when it departs, cannot therefore be reasonably maintained, unless we sup- pose that it may leave the body piecemeal, in which case it can hardlj' be imagined that the fragments of life can be patched together again. Consequently, the supposition that the life ot a himian being con- tinues to exist as an indi\ndual, after the body associ- ated with it is bereft of it. seems quite untenable. On the other hand, the \iew that the life of man, or other animal, is simply highly specialized energj-. which energizes, in \-arious degrees though in close co-ordination, the limbs and organs of animals, is one which affords an explanation of life on a scientific, physical, and chemical basis, to which no soxmd objection can be raised, although a detailed explana- tion of all the phenomena of life is much too difficult and comjdicated to be as comjdete and satisfactory as we might wish. life, as we have alreadj- remarked, is not known to arise othenvise than from pre-edstent Bfe, and the question su^^ts itself whether we suppose that life has alwa\-s existed as such, or that it has wigin- ated, as far as the earth is concerned, at some time and in some way, from other fcmns of eneigv, such as heat, light, electricity, nragnetism. chemical energy, and so on. Evidraitly vital energy is not a definite form of energv distinct from th