•■•.VfMfU' 
 
 vitmiagmmmmmmmmmmmmamm 
 
 HINTS ON THE TEACHING 
 
 OF 
 
 ELEMENTARY CHEMISTRY 
 
fil0mett UtttoJJSiitg Jilrarg 
 
 THE GIFT OF 
 
 . T^.dS^,...&,.Q. Z.oX.L.^^'i^ 
 
 ■A-.-l.^.U-^S v^Vv\o.v-. 
 
 CllCMI5T ftY i,IDnARY . 
 
Cornell University Library 
 
 arV11062 
 
 Hints on the teaching of eiementary chem 
 
 3 1924 031 271 681 
 .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/cu31924031271681 
 
HINTS 
 
 ON 
 
 THE TEACHING 
 
 OF 
 
 ELEMENTARY CHEMISTRY 
 
 m 
 SCHOOLS AND SCIENCE CLASSES 
 
 BY 
 
 WILLIAM A. TILDEN, D.Sc, F.R.S. 
 
 PROFESSOR OF CHEMISTRY IN THE ROYAL COLLEGE OF SCIENCE, LONDON 
 EXAMINER IN CHEMISTRY TO THE DEPARTMENT OF SCIENCE AND ART 
 
 LONDON 
 LONGMANS, GREEN, AND CO. 
 
 AND NEW YORK 
 1895 
 
 a 
 
 All right s reserved 
 
t\ . i M-U -^^ ^^ 
 
 BY THE SAME AUTHOR. 
 
 INTRODUCTION TO THE STUDY OF 
 CHEillCAL PHILOSOPHY. The Princi- 
 ples of Theoretical and Systematic Chemistry. 
 With 5 Woodcuts. With or without the 
 Answers of Problems. Fcp. Svo. 4J-. dd. 
 
 PRACTICAL CHEMISTRY. The Principles 
 of Qualitative Analysis. Fcp. Svo. \s. 6d. 
 
 LONGJL-\.NS, GREEN, &: CO. 
 London and New York. 
 
 C^' 
 
 -^U-.v,(Vi--...:~f:,,_, 
 
PREFACE 
 
 The issue of the new Syllabus of Inorganic and 
 Organic Chemistry by the Department of Science 
 and Art marks a very important advance in 
 the history of the teaching of these subjects in 
 this country. In this Syllabus the treatment 
 of Chemistry from the theoretical side remains 
 necessarily at the discretion of the teacher, though 
 from the order in which the subdivisions of the 
 matter for treatment are placed, it is obvious that, 
 in the early stages especially, it is considered desir- 
 able to keep theory in a subordinate position, and to 
 make use of it only when a sufficient foundation 
 of fact has been duly acquired by the pupil. 
 According to the experience of the Author, it is 
 scarcely possible to use a purely inductive method 
 in dealing with young students ; but, though this 
 may be admitted, the necessity of clearly distin- 
 guishing fact from hypothesis requires to be far 
 more definitely established in the minds of a large 
 proportion of the teachers, whose pupils present 
 
iv Teaching of Elementary Chemistry 
 
 themselves for the examinations of the Depart- 
 ment. 
 
 In the earliest stages, the learner's attention 
 and energy are usually wholly used up in the 
 process of exact observation. To see and accu- 
 rately record the whole of a given phenomenon 
 is enough for young boys and girls, and, until 
 sufficient practice and experience in this direction 
 have been gained, it is a better educational exercise 
 than the attempt to solve problems which, to 
 render them simple enough, require the neglect of 
 part of the phenomena. For example, the investi- 
 gations of the changes which occur when a piece 
 of sheet copper is heated in the air requires for a 
 complete answer observations (a) on the iridescent 
 colours first visible, {h) the black coating which 
 succeeds, (c) the red lining to the scales which may 
 be separated on cooling, {(T) the temperature at 
 which these changes occur, {e) the alteration in 
 weight of the mass, (/") changes in the surrounding 
 air, &c. Observation of all these facts is possible 
 even for the beginner ; but when he is in possession 
 of the whole, he would be a wonderful boy in- 
 deed who could, without assistance, infer that the 
 products contain oxygen as well as copper, and 
 that there are two oxides of copper of definite 
 composition, to say nothing about the colours of 
 thin films. 
 
 Chemistry is a science which, in its modern 
 form, has sprung up almost entirely within the 
 
Preface v 
 
 memory of living man, and its boundaries, already 
 far reaching, are being rapidly extended year by 
 year, and even month by month, by the labours 
 of an army of workers. This condition of growth 
 constitutes a feature of the science which should 
 render it as a study attractive in no ordinary degree ; 
 and the teacher who desires to make use of it as 
 an effective educational agent must devote some 
 part of his time to extending and consolidating 
 his own knowledge. Not that the very latest 
 discovery is necessarily more important than many 
 of the facts which have been long familiar, and 
 the teacher ought to be careful to exercise due 
 discretion in communicating to a class of young 
 pupils the most recent results of observation, 
 lest their sense of proportion should be unequal 
 to the strain of distinguishing the important 
 from the comparatively unimportant. The dis- 
 covery of argon in the atmosphere twelve months 
 ago possesses the greatest interest for the advanced 
 student ; but oxygen still ranks higher in im- 
 portance, from all practical as well as theoretical 
 points of view, than the newly discovered gas. 
 
 In the new Syllabus issued by the Department 
 the most important changes will be noticed in the 
 part which relates to the practical examinations. 
 Scientific chemistry is based almost wholly upon 
 observation and experiment, and the practical study 
 of this science is one of the most useful means of 
 developing the faculties of observation. But even 
 
vi Teaching of Elementary Chemistry 
 
 in the teaching of analytical chemistry, which is 
 especially dependent upon the practice of careful 
 and exact observation, the ' crammer ' has been 
 at work to such an extent as largely to destroy 
 the educational value of this important and in- 
 teresting subject, and, instead of being made to 
 record accurately the results of his own obser- 
 vations, the pupil is too often allowed to learn by 
 rote tables or schemes of analysis, and is led to 
 state, not what he sees, but what the book tells him 
 he ought to sec. A form of examination question 
 used by the Author which requires the candidate 
 to describe some operation which he has himself 
 conducted has often received answers so remarkable 
 as to show that the faculty of observation is in 
 some cases almost extinguished by the habit of 
 blind reliance upon a printed text. 
 
 The following chapters contain the substance 
 of a short course of Lectures given in July last to 
 the class of Teachers assembled for instruction at 
 the Royal College of Science ; and in the belief that 
 the hints contained in them would be useful to 
 others engaged in preparing for the ]May exami- 
 nations, as well as to Teachers of Elementary 
 Classes generally, they are now offered in a slightly 
 amplified form, and with a little more detail than 
 is possible in presenting the same subjects in the 
 lecture-room. 
 
 RoYAi. College of Science, London : 
 Set/ember 1895. 
 
CONTEXTS 
 
 CHAPTER PAGE 
 
 I. OBSERVATION— QUALITATIVE .... 1 
 
 II. OBSERVATION— QUANTITATIVE . . . . 12 
 
 III. OBSERVATION— QUANTITATIVE . . . .21 
 
 IV. CHEMICAL EQUIVALENTS — THE LAW OF 
 
 VOLUMES 31 
 
 NOTE TO CHAPTER IV 44 
 
 CHRONOLOGICAL TABLE, FOUNDERS OF 
 
 CHEMISTRY 42 
 
 V. MOLECULAR WEIGHTS AND FORMULAE . . . 46 
 
 VI. ATOMIC WEIGHTS— CLASSIFICATION . . .55 
 
 APPENDIX 70 
 
HIXTS ON THE TEACHING 
 
 ELEMENTARY CHEMISTRY 
 
 CHAPTER I 
 OBSER VA TION- Q U A LIT A TIVE 
 
 The history of chemistry affords numerous in- 
 stances of fallacies originating as a consequence 
 of imperfect observation. In some cases such 
 fallacies have been received as articles of belief 
 almost undisputed for centuries. The doctrine of 
 the transmutability of metals, and especially of the 
 base metals, into silver and gold is an example of 
 this kind. It is a fact that almost any sample of 
 lead, submitted to the process of cupellation, yields 
 a little bead of silver which often contains gold. 
 It was not an unnatural hypothesis that this silver 
 somehow originated in a change brought about in 
 the lead by the action of heat ; and since in different 
 operations the yield of precious metal was known 
 
 B 
 
2 Teaching of Elementary Chemistry 
 
 to vary, it seemed likely that by suitably changing 
 the conditions complete transmutation might be 
 brought about. 
 
 The doctrine of Phlogiston, again, seemed to 
 supply all that was wanted to explain the process 
 of combustion and to bring under a common theory 
 the phenomena of burning, rusting of metals, 
 decay of wood, and fermentation of saccharine 
 fluids. When a piece of wood or charcoal is 
 burned, the theory taught that the escaping 
 phlogiston was the cause of the appearance of fire, 
 the calx remaining in the ashes upon the hearth. 
 Metallic iron exposed to the air loses its phlogiston, 
 and becomes transformed into a calx more copious 
 than that of charcoal. The latter, then, is rich in 
 phlogiston, and ought to be able to impart some of 
 it to the calx of iron, and so restore it to the metallic 
 or reguline condition, and this it does when iron 
 ore or rust is heated with charcoal. All this is 
 true as far as it goes, but the defect in the theory 
 was that it did not take cognisance of the fact that 
 when metals are calcined the resulting calx is 
 heavier than the metal. 
 
 Another most instructive example of bad ob- 
 servation, and consequent erroneous theory, is 
 exhibited in the history of the element we now 
 call chlorine. This gas was discovered by Scheele 
 in 1 774, and called by him dephlogisticated muriatic 
 acid. In this he was quite right, for such a de- 
 signation means that the new gas consisted of 
 
Observation — Qualitative 3 
 
 muriatic acid deprived of its inflammable principle. 
 Berthollet, however, concluded from his experi- 
 ments that this body was composed of muriatic 
 acid and oxygen, and for a long time it was 
 thought that the hydrogen, which was undoubtedly 
 procurable from muriatic acid by the action of 
 metals upon the gas, was due to the accidental 
 presence of water. 
 
 Sir Humphry Davy established the elemental 
 character of the substance, and gave it the name 
 chlorine, which refers solely to the yellowish-green 
 colour of the gas, and not to any hypothesis as to 
 its nature. But Davy succeeded only very gra- 
 dually in getting rid of errors arising from the 
 presence of moisture in the imperfectly dried gas. 
 The principle adopted in his researches was the 
 very simple one which consists in attacking the 
 point in dispute directly. The question was 
 whether ' oxymuriatic acid ' containcfd oxygen, 
 and this he answered by a series of experiments in 
 which he proved that metals, charcoal, phosphorus, 
 and other bodies heated in the gas give compounds 
 recognisable as muriates, but never as oxides ■; and he 
 pointed out that the presence of oxygen or of hydro- 
 gen, or phlogiston, ' or of other principles, should 
 not be assumed where they cannot be detected.' ' 
 
 Among the chemists of the past there is none 
 whose writings are more instructive, or whose fame 
 
 ' For 3 complete account of Davy's work on Chlorine, see 
 'Alembic Club Reprints,' No. 9; W. F. Clay, Edinburgh. 
 
 B 3 
 
4 Teaching of Elementary Chemistry 
 
 is likely to be more enduring, than the industrious 
 Scheele. And the reason of this is that his work 
 was almost wholly experimental. As he himself 
 says, ' Conjectures can determine nothing with 
 certainty ; at least they can only bring small satis- 
 faction to a chemical philosopher who must have 
 his proofs in his hands.' ' Observation and experi- 
 ment constitute the first and the most important 
 business of all students of chemistry ; but, that they 
 may be fruitful, observation and experiment alike 
 must be carried on not listlessly, carelessly, with 
 ' lack-lustre eye,' but with concentrated attention 
 and senses awake and active. 
 
 With all the accumulated experiences of the 
 past, the art of observation is, however, still imper- 
 fectly learned, and the chemical journals teem with 
 descriptions of phenomena or of substances which 
 are either incomplete or inaccurate, or even wholly 
 erroneous. Writers of text-books are also far from 
 blameless in this matter, and the inability on the 
 part of students to describe correctly even very 
 simple things is often as much due to the per- 
 petuation of false statements in the books as to 
 want of attention or of skill on the part of the 
 student. One of the most flagrant cases is the 
 description of the usual process for the production 
 of sulphur dioxide. A well-known text-book 
 
 ' See Scheele's ' Chemical Treatise on Air and Fire,' of which 
 the most interesting part, relating to the discovery of oxygen, is 
 translated into No. 8 of the ' Alembic Club Reprints,' already 
 referred to. 
 
Observation — Qualitative 5 
 
 contains the following statement : ' This gas is 
 prepared by the action of hot concentrated sul- 
 phuric acid upon copper turnings. 
 
 Cu + 3H,SO,=CuSO, + 2lI,0 + SOj.' 
 
 That is the whole of the explanation given 
 in this case. An observer of the process, how- 
 ever, cannot fail to see that, so soon as the action 
 commences and the gas begins to escape, the 
 copper becomes black ; but the equation gives no 
 explanation of this fact, and students are actually- 
 led to believe that the residue ought to be blue. 
 According to the author's experience, candidates 
 at examinations in which a question occurs as to 
 the action of sulphuric acid upon copper do fre- 
 quently make the statement that the residue is 
 blue. Nor are students alone liable to fall into 
 this error. A teacher once seriously proposed in 
 the author's presence to assist the memory by 
 writing chemical equations in chalks coloured so 
 as to recall the appearance of the several materials 
 concerned. Selecting the equation referred to by 
 ^vay of illustration, he wrote the Cu in red, the 
 HjSO^ in white, and the CuSO, in blue chalk, 
 ignorant or forgetful of the fact that CuSO, is a 
 white substance. 
 
 The difficulty of making exact observations ot 
 comparatively simple facts is daily illustrated by 
 the reports of the law courts, where different wit- 
 nesses, actuated, we must believe, in many cases, 
 
6 Teaching of Elementary Chemistry 
 
 by no dishonest motive, often give wholly contra- 
 dictory testimony as to occurrences of the most 
 ordinary kind. Most people are also usually in- 
 capable of discriminating objects unless they differ 
 enormously in size or colour. This is illustrated 
 by the common names which have been applied 
 for centuries to many familiar plants. There is, 
 for example, the greater celandine and the lesser 
 celandine, which have little resemblance to each 
 other beyond the colour of the flowers. The plants 
 called water-lilies, again, have no botanical con- 
 nection with true lilies, neither do the flowers 
 resemble those of the lily. For a long time there 
 was, and there probably still exists, confusion in 
 the minds of the majority of people as to the two 
 forms of electric light and the ' incandescent ' gas- 
 burners now so common. Any kind of bright light 
 seems to be sufficient to deceive the untrained eye. 
 In order to cultivate the powers of observa- 
 tion, various branches of natural science have been 
 brought into use in schools, but none seem to 
 present so many advantages as are offered by 
 chemistry when rightly taught. As a science 
 based entirely upon the results of observation and 
 experiment, it is only by making experiment a 
 principal feature of the system of instruction that 
 these advantages can be secured. The observations 
 and experiments must also, as far as possible, be 
 the work of the pupil and not of the teacher, and 
 therefore exercises undertaken should be in the 
 
Observation — Qualitative 7 
 
 first instance of the simplest possible character, 
 and graduated so as to lead on to more difficult 
 operations, which should only be undertaken after 
 some time and after demonstration by the teacher. 
 It is a mistake to suppose that the great theories 
 of chemistry can be established by experiments 
 conducted wholly by beginners, but with due pre- 
 liminary instruction the more advanced student 
 may get a long way in this direction. 
 
 The object of the following series of exercises 
 is merely to suggest to teachers the kind of practi- 
 cal work which may be advantageously done by 
 the pupil, and to show in a general way how he 
 may proceed from very simple operations to work 
 of a more complex character. 
 
 The earliest examples provide material for 
 observation without requiring more than care and 
 attention. The teacher will have no difficulty in 
 devising an extended and almost infinite variety 
 of such simple exercises. It will be necessary to 
 insist upon a written account of each experiment 
 made by the student, with a statement in his own 
 words of what he sees, without at first requiring any 
 theoretical explanation or discussion. It will often 
 be advantageous to supply materials with which 
 the student is not already familiar. When his 
 observations have been successfully recorded, the 
 teacher may, if he thinks proper, tell him the 
 composition of the substance and explain the nature 
 of the changes which he has noticed. 
 
8 Teaching of Elementary Chemistry 
 
 Experiments to be done by each Pupil 
 
 I. Observation of the action of Iieat supplied by 
 the gradual application of a Bunsen flame to a dry 
 test-tube containing a little of the substance. 
 
 Any of the following or other substances may 
 be tried : Mercuric oxide, red lead, lead nitrate, 
 potassium chlorate, potassium nitrate, ammonium 
 nitrate, ammonium chloride, mercuric iodide. 
 
 Gases evolved may be tested for as follows : 
 
 (i) Notice colour, appearance of fumes, odour. 
 
 (2) Apply lighted wax taper first at the mouth 
 of the tube, then pushed inside. 
 
 (3) Hold in the gas strips of red and blue 
 litmus paper, moistened with water. 
 
 (4) Hold within the tube a drop of clear lime 
 water at end of a narrow glass tube, then gently 
 suck for a moment at the open end of the tube so 
 as to draw up the drop. 
 
 This series of tests is not intended to enable 
 the pupil to identify the gas, for that is a matter 
 of comparatively small importance at this stage. 
 After observations have been correctly recorded, 
 the teacher may suggest to the pupil other experi- 
 ments which appear to him desirable, such as the 
 collection of the gas in bulk or further examination 
 of the residue. 
 
 Some of the substances mentioned in the above 
 list will afford opportunities of testing the genuine- 
 ness and completeness of the student's work. For 
 
Observation — Qualiiative g 
 
 example, he may have read or learned that mercuric 
 oxide gives off oxygen and mercury when heated, 
 but unless he has tried the experiment or seen it 
 tried, he could not guess that the powder would 
 become black while hot, and that at a high tempe- 
 rature it would give a yellowish-coloured gas and 
 a small yellow sublimate upon the sides of the 
 tube, owing to the trace of nitrate which the com- 
 mercial red oxide invariably contains. 
 
 II. Crystallisation of salts from water, and 
 recognition of crystalline form, or at least differen- 
 tiation of one sort of crystal from another. 
 
 For example, make strong solutions of potas- 
 sium nitrate and ammonium chloride in hot water, 
 and pour into separate watch-glasses. On cooling 
 the long prisms of the nitre are easily distinguished 
 from the fern-leaf forms of the sal ammoniac. 
 Similarly alum, lead nitrate, and barium nitrate 
 will yield regular octahedrons and dodecahedrons, 
 and intermediate forms distinct enough to be 
 readily visible and easily sketched. Other salts 
 which crystalli.se from hot water are potassium 
 chlorate, sodium nitrate, magnesium and zinc 
 sulphates, copper sulphate, chrome alum, potas- 
 sium dichromate, &c. 
 
 III. Precipitates to be distinguished by differ- 
 ences of density or consistence. The following are 
 white precipitates differing in character : solution 
 of alum mixed with ammonia gives a gelatinous 
 precipitate ; calcium chloride with carbonate of 
 
I o Teaching of Elementary Chemistry 
 
 ammonia a flocculent precipitate which becomes 
 sandy on heating or after standing; calcium 
 chloride with dilute sulphuric acid a mass of minute 
 crystalline needles ; barium chloride with dilute 
 sulphuric acid a fine powder some of which may 
 remain suspended a long time ; silver nitrate with 
 a chloride a white precipitate which curdles and 
 on exposure to daylight assumes a purple tint, &c. 
 
 In like manner arsenious sulphide, lead chro- 
 mate, zinc chromate, stannic sulphide, silver iodide 
 are examples of precipitates which have a yellow 
 colour, but differ in tint and density. 
 
 IV. The action of st7-ong sulphuric acid upon sub- 
 stances resulting in the evolution of gas. 
 
 The following examples will serve : common 
 salt, potassium nitrate, red lead, potassium iodide, 
 sodium sulphite, sodium formate. 
 
 A few grams of the substance may be placed 
 in a test-tube and strong sulphuric acid added in 
 quantity sufficient to make a fluid paste. If heat 
 is applied it will be well to caution young students 
 as to the corrosive nature of the liquid, and direct 
 them to turn the open mouth of the tube away 
 from their own faces and from the persons of their 
 neighbours. The evolved gas may be tested as 
 already described under Section I. 
 
 In addition to such exercises as the foregoing, 
 specimens of well-crystallised compounds such as 
 potassium iodide, alum, sugar, zinc sulphate, or of 
 minerals such as galena, fluorspar, pyrites, calcite, 
 
observation — Qualitative 1 1 
 
 or specular iron may be given to be drawn and 
 described. And when qualitative analysis is begun, 
 the pupil should be taught in every case to write 
 down, before applying tests, an account of the appear- 
 ance and more obvious characters of the substance 
 analysed. 
 
1 2 Teaching of Elementary Chemistry 
 
 CHAPTER II 
 
 OBSER VA TION—QUANTITA Tl VE 
 
 Quantitative experiments may be gravimetric 
 or volumetric ; that is, they may take the form of 
 measurements of weight or of volume. Contrary 
 to general belief, quantitative work may be done 
 with very simple and inexpensive appliances, and 
 in the hands of even young students results may be 
 obtained which closely accord with theory. It is, 
 however, advisable in the first instance to set 
 exercises to be done independently of theory until 
 a series of experiments has given concordant 
 results, showing that the pupil has acquired suffi- 
 cient skill to be allowed to test for himself some 
 general law which he has already learnt. 
 
 The balance shown in the figure may be 
 obtained of several instrument makers for about 
 27X 6d., and a box of weights ranging from 100 
 grams to i milligram costs &s. 
 
 The simplest quantitative experiments are 
 those in which a gain or a loss of weight of a 
 single object, such as a crucible, has to be recorded. 
 The following are examples of the kind of experi- 
 
Ob servation — Qitantitative \ 3 
 
 merit referred to. They can all be done by 
 beginners with very slight supervision and with 
 satisfactory results. It is not necessary to weigh 
 to a smaller subdivision than 'Oi gram. 
 
 I. Formation of metallic oxides and sulphides. — 
 Magnesium may easily be converted into the oxide 
 by ignition in air, but to avoid loss it must be' 
 heated in a covered crucible. A clean ' half-ounce 
 
 Fig. I 
 
 covered porcelain crucible should be placed on the 
 left-hand pan of the balance, and counterpoised by 
 putting small shot into the lid of a pill-box placed 
 in the opposite pan. About 15 centimetres of 
 magnesium ribbon rubbed clean with sand-paper 
 is then coiled up in the crucible, and its weight 
 noted. It should weigh '3 to '4 gram. The cruci- 
 ble is then placed on a wire and tobacco pipe 
 
14 Teaching of Elementary Chemistry 
 
 triangle supported on a tripod or retort stand 
 ring over a Bunsen burner. The flame is then 
 gradually applied till the bottom of the crucible 
 is red-hot. The lid may be lifted slightly from 
 time to time, and in about a quarter of an hour the 
 glowing of the metal due to combustion will have 
 ceased and a grey mass will remain. The lid 
 should then be removed and a strong heat applied 
 a little longer till the mass becomes white, i gram 
 of magnesium gives i"66 gram of the oxide. In a 
 
 Fig. 2 
 
 test experiment conducted as above, -39 gram gave 
 •65 gram of oxide. This is in the proportion of 
 I to I '66. 
 
 Copper may be readily converted into the oxide ; 
 but as a simple roasting process would occupy too 
 much time, it is best to conyert the metal first into 
 nitrate, which is decomposed by subsequent heating. 
 Counterpoise a crucible as before, weigh out about 
 half a gram of thin sheet or wire, add to it five 
 or six drops of ordinary strong nitric acidj coyer 
 
Observation — Qtiantitative 1 5 
 
 the crucible and set it in a warm place till a dry- 
 green mass remains. Then heat over the Bunsen 
 flame gradually to redness. The resulting oxide 
 should of course be black and show no signs of 
 unchanged metal. Should any unchanged metal 
 be suspected, a few more drops of nitric acid 
 should be added and the process repeated. Iron 
 and tin may be dealt with in the same way. 
 
 I gram of copper gives r'25 gram oxide 
 I gram of iron „ 1-43 „ „ 
 
 I gram of tin „ 1-27 „ 
 
 Copper may also be converted into sulphide, 
 by heating it with sulphur. A counterpoised co- 
 vered crucible containing about i gram of flowers 
 of sulphur is placed over the Bunsen flame and 
 heated strongly till the vapour of sulphur is seen 
 escaping round the lid. A weighed piece of 
 copper (about '5 gram) is then dropped into the 
 crucible, the lid being lifted only slightly and im- 
 mediately replaced. The heating is continued till 
 the excess of sulphur is completely volatilised. 
 The crucible is then allowed to cool, and when cold 
 is weighed. A small quantity of sulphur should 
 be added to the contents of the crucible, which 
 should be rapidly heated again to redness, cooled 
 and reweighed. The second weight should be the 
 same as the first. 
 
 For the success of this experiment the air must 
 be excluded as much as possible from the crucible. 
 
1 6 Teaching of Elementary Chemistry 
 
 The lid must therefore not be removed while the 
 sulphide contained in the crucible is still hot. A 
 good way of preventing access of air is to finish 
 the heating in an atmosphere of coal gas supplied 
 through a tobacco pipe, attached to a rubber tube, 
 and having the bowl inverted over the crucible. 
 The escaping gas burns with a luminous flame 
 round the top of the crucible, i gram of copper 
 should theoretically give r25 gram of sulphide. 
 
 Fig. 3 
 
 An experiment conducted as described gave i'26 
 gram. 
 
 11. Decovtposition of compounds by heat.- — Pro- 
 ceeding in a similar manner, quantitative estima- 
 tions may be made of the products of various 
 chemical decompositions produced by heat in which 
 a fixed residue remains. For example, potassium 
 chlorate, heated, leaves potassium chloride ; magne- 
 sium carbonate and zinc carbonate leave a residue 
 
Observation — QtMulitative 1 7 
 
 of oxide ; crystallised sulphates of copper and 
 other metals lose their water and leave anhydrous 
 sulphates. Few remarks are necessary in con- 
 nection with such experiments as these. Salts 
 which are liable to crackle or decrepitate mu-;t 
 of course be heated in a closely covered crucible. 
 Copper sulphate, magnesium sulphate, zinc sul- 
 phate, and gypsum require to be heated only very 
 gently by means of a rose burner placed some 
 three inches below the bottom of the crucible. 
 Commercial carbonates of zinc and magnesium 
 have a nearly constant composition corresponding 
 respectively to the formulae ZnCO^. ZnTHO,^. H^O 
 and sMgCO.,. Mg(HO,,. 4H.P. 
 
 One gram of crystallised copper .sulphate should 
 yield '64 gram of anhydrous sulphate. An expe- 
 riment gave 1-39 gram, from 2* 16 grams, which 
 corresponds with theory. The residue was slightly 
 grey. 
 
 By such exercises as the foregoing, varied and 
 repeated according to the discretion of the teacher, 
 the student may be shown that he can for himself 
 verify the fundamental law of chemistry — namely 
 the Law of Definite Proportions, which a.sserts that 
 every chemical compound has a fixed and definite 
 composition. The experimental method may after 
 these exercises be modified so as to show that the 
 same results may be obtained though the materials 
 pass through a different series of processes. 
 
 III. Treatment of precipitates. — As an example 
 
 C 
 
1 8 Teaching of Elementary Chemistry 
 
 a piece of clean iron wire not more than '5 gram 
 in weight may be dissolved in a half-pint beaker 
 by a few cubic centimetres of a mixture of hydro- 
 chloric with a little nitric acid. The solution 
 is then diluted with water and excess of solu- 
 tion of ammonia added, the precipitate filtered off, 
 the filter grained, dried, and burnt to ashes in 
 a counterpoised crucible. The filter ash may be 
 neglected. By this process also it may be shown 
 that I gram of iron gives I "43 gram of the red oxide. 
 
 In order to prove to the pupil that it is usually 
 necessary to wash precipitates, the same experiment 
 may be performed with the substitution of caustic 
 soda for ammonia. After weighing the oxide of 
 iron, add some boiling water to the contents of the 
 ciucible, let the oxide settle, and pour off the water. 
 After repeating this once or twice, dry the whole 
 and weigh again. The weight will be slightly 
 reduced, and nearer to the theoretical amount. 
 The washings will turn red litmus paper blue. 
 
 IV. Replacement of metals by one another. — A 
 clean piece of zinc foil immersed in a somewhat 
 dilute (2 to 5 per cent.) solution of copper sulphate 
 in water becomes coated immediately with a black 
 or brown furry coating of metallic copper, which 
 continues to be thrown down till all the zinc has 
 dissolved. A few small bubbles of hydrogen 
 sometimes escape, but these may be neglected, as 
 they represent only an extremely minute quantity 
 of zinc. In order to show the ratio between the 
 
Observation — Quantitative i g 
 
 weight of zinc dissolved and copper deposited, the 
 clean zinc must be weighed and placed in a small 
 beaker containing the solution of copper, which 
 must be used in excess — that is, it must remain 
 blue after the zinc is dissolved. The action is over 
 when on feeling about with a glass rod no particles 
 of the zinc are encountered. It is advisable to 
 warm the solution toward the end. The reddish 
 precipitate of copper is allowed to settle, the 
 blue solution containing zinc and copper sulphates 
 poured off, and the precipitate brushed by means of 
 a camel-hair brush into a counterpoised crucible. 
 The washing is completed in the crucible, by 
 adding some boiling water, which, after settlement 
 of the copper, is poured off as completely as 
 possible. As the pulverulent metal is liable to 
 oxidise if heated in contact with the air, it should 
 be dried rapidly, and this may be accomplished 
 by adding to it a few cubic centimetres of strong 
 alcohol, pouring this away, and then placing the 
 crucible in a steam oven for a few minutes. It is 
 weighed when cold. Theoretically i gram of zinc 
 precipitates "97 gram of copper. By experiment 
 conducted in the manner described, '57 gram of 
 zinc gave '55 gram copper, which is in the propor- 
 tion of I to -965. 
 
 By a similar method of procedure : 
 
 I gram magnesium precipitates Z'6^ grams 
 
 of copper. 
 I gram copper precipitates 3 40 grams of silver. 
 
20 Teaching, of Elementary Chemistry 
 
 I gram magnesium precipitates 9 grams of silver 
 I gram zinc „ 332 „ „ „ 
 
 In the case of replacing silver by copper mag- 
 nesium or zinc, an excess of solution of silver nitrate 
 may be used. 
 
 From results obtained in this way the Laiv 
 of Reciprocal Proportions or Equivalents may be 
 tested by the pupil for himself He finds, for 
 example, by successive experiments, and calcu- 
 lating from the results, that i part of magnesium, 
 271 parts of zinc, and 2'62 parts of copper are 
 respectively required to precipitate 9 parts of silver. 
 From this he may infer that 271 parts of zinc and 
 262 parts of copper are equivalent to each other, 
 or are capable of doing an equal amount of chemical 
 work. This he can verify by recalculating the 
 result of his experiment in which copper is preci- 
 pitated by zinc, for 
 
 1 : '97 :: 271 : 2-62. 
 
 Similarly he may infer that I part of magnesium 
 is equivalent to 271 parts of zinc, though this 
 cannot be similarly verified by direct precipitation. 
 In a later chapter an experiment will be described 
 by which the weight of hydrogen corresponding 
 to these quantities of the metals can be experi- 
 mentally determined ; but as this requires more 
 careful manipulation, and the observation of 
 weight to the third place of decimals, it is not 
 within the capacity of pupils at this stage. 
 
21 
 
 CHAPTER III 
 
 OBSER FA TION- QUANTITA TIVE 
 
 Measurement of gases. — In the preceding 
 chapter the experiments described involve deter- 
 minations of gain or loss of weight. The pupil 
 may now proceed to make estimations of the volume 
 of gas obtained in various processes, and it is sur- 
 prising what close approximations to theoretical 
 results are possible by the use of the simplest 
 apparatus and by methods which appear to be 
 among the least exact. Estimations of the volume 
 of hydrogen evolved from acids by various metals 
 afford the best exercises to begin upon, and at first 
 all calculations may be neglected, inasmuch as the 
 solubility of hydrogen in water and the difference 
 between ordinary atmospheric conditions and nor- 
 mal temperatui'e and pressure have comparatively 
 little effect on the volume of gas collected. The 
 apparatus shown in the figure may be recommended 
 for its simplicity. It consists of a piece of tubing 
 about 20 cm. long, 6-8 mm. internal diameter, 
 drawn out at the top and bent into a curve, upon 
 
2 2 Teaching of Elementary Chemistry 
 
 which is fitted, by means of a short piece of rubber 
 tubing, a second piece of quill glass tubing, turned 
 up at the end so as to convey the gas into a jar. 
 The metal, zinc foil, or magnesium ribbon is 
 thrust into the wider tube, and, to prevent portions 
 of it from falling out of the open end, a little plug 
 of copper gauze is stuffed into the mouth. 
 
 To collect the gas a common stoneware pneu- 
 matic trough, with beehive shelf and cylindrical gas 
 
 Fig. 4 
 
 jar filled with water, may be used. To make the 
 experiment, the upturned end of the delivery tube 
 is put under the mouth of the jar, and a deep 
 beaker or cylinder containing dilute sulphuric acid 
 is brought under the tube containing the metal, 
 and the latter is slowly lowered into it. Evolution 
 of gas commences immediately, and the gas passes 
 into the jar. When the whole of the metal is dis- 
 solved, the tube may be removed and the level of 
 
Observation — Quantitative 23 
 
 the water in the jar is marked off by means of a 
 strip of paper label, which is affixed to the outside 
 of the jar, and the edge of which marl<s the surface 
 of the water or of the gas. The jar may then 
 be lifted out of the trough, water poured into it till 
 the surface is level with the edge of the paper 
 strip, and this water, which now occupies the space 
 taken up previously by the gas, is measured by 
 pouring it out into a graduated cylinder. 
 
 No attempt should be made to expel the whole 
 of the gas from the tube into the jar, for this 
 residual gas occupies the space previously filled by 
 air, which at the beginning of the experiment was 
 driven into the jar. When magnesium is used it 
 should be lowered into the acid very cautiously or 
 some bubbles of gas may be lost. 
 
 . A series of experiments should in the first 
 instance be made as described, and the resulting 
 volume of gas recorded in the notebook ; at the 
 same time the temperature of the water and the 
 height of the barometer may be noted, the correc- 
 tions being reserved by beginners, and applied 
 later, when they have learned the effect of changes 
 of temperature and pressure upon gases, and can 
 make the necessary calculations. 
 
 One gram of zinc gives theoretically 343 c.c. 
 of hydrogen at normal temperature and pressure. 
 An experiment in which '86 of zinc foil was used 
 gave 3 1 5 c.c. at 20° ; i gram would, therefore, have 
 given 366 c.c, or 341 c.c. at normal temperature. 
 
24 Teaching of Elementary Chemistry 
 
 One gram of magnesium gives, theoretically, 
 930 c.c. of hydrogen at normal temperature and 
 pressure. 
 
 By experiment, -38 gram of magnesium gave 
 385 c.c. of hydrogen at 20°: so that i gram would 
 have given 1013 c.c, or 943 c.c. at normal tem- 
 perature. 
 
 Here it may be remarked that the correction 
 required for pressure is usually much less than 
 the correction for temperature, and may very 
 commonly be neglected altogether for purposes 
 such as we have in view in these experi- 
 ments. 
 
 It may be observed that the measurement of 
 the gas evolved from acid by the metals may be 
 made use of for the purpose of estimating the 
 quantities of aluminium, zinc, and magnesium, 
 which are equivalent to one another, by calculat- 
 ing from the result obtained the amount of metal 
 required to yield the same amount, say lOOO 
 c.c, of hydrogen. 
 
 Another good form of experiment involving 
 the measurement of gas is the estimation of oxygen 
 in atmospheric air by means of phosphorus. A 
 graduated measuring cylinder may be u.sed for this 
 purpose, or simply a plain gas cylinder holding 
 about 250 c.c. A scratch is made about 5 cm. 
 from the mouth, and the capacity of the jar to this 
 mark is ascertained by filling with water and 
 measuring the water. The jar is then inverted in 
 
Observation — Quantitative 2 5 
 
 a water-trough, and the air admitted till the sur- 
 face of the water inside stands at the mark. The 
 air in the jar then occupies the bulk of the measured 
 quantity of water. 
 
 A piece of clean phosphorus, the size of a pea, 
 is now stuck at the end of a piece of copper wire, 
 and thrust up into the jar till it is near the top. 
 The whole is allowed to stand 6-8 hours, the phos- 
 phorus withdrawn, and the new level of the water 
 marked off with a strip of gummed paper. The 
 volume of the residual nitrogen can then be 
 measured by means of water as before. 
 
 Measurement of liquids. Neutralisation of acids 
 and alkalis. — The metric system of weights and 
 measures is a subject included in the syllabus of 
 the Science and Art Department, and every teacher 
 who professes to meet the requirements of the 
 Department will find it necessary to give occa- 
 sional demonstrations of the relations between the 
 several denominations of weight and measure. 
 This is one of those subjects which cannot be 
 acquired offhand, but demands repeated consi- 
 deration and use to make it familiar. Whether 
 it has already been learned or not by the pupils, it 
 will be advisable at this stage to begin operations 
 by allowing them to handle some of the common 
 measuring vessels of different forms, and to note 
 the relation of burettes to graduated cylinders, 
 flasks, &c. Volumetric exercises in great variety 
 may then be practised. 
 
26 Teaching of Elementary Chemistry 
 
 For the purposes now contemplated we may 
 assume, without appreciable error, that fresh oil of 
 vitriol, which has not been long exposed to the air, 
 may be regarded as consisting of real sulphuric 
 acid, HjSO,,. This assumption will be convenient 
 to the teacher, though it is unnecessary and un- 
 desirable to communicate at first any notions of 
 this kind to the pupil. A standard solution of 
 dilute sulphuric acid may be prepared by weighing 
 out 49 grams of oil of vitriol in a small beaker, 
 and pouring this into water, sufficient being used, 
 together with the rinsings of the beaker, to make 
 up looo c.c. 
 
 The next step is to make choice of indicators, 
 and it will be found that litmus or cochineal 
 is the most useful. The burette to be used may 
 be divided into "2 or "i c.c. With the aid of the 
 solution of sulphuric acid, a number of experi- 
 ments ma)' be quickly made on the neutralising 
 power of various common alkaline solutions, such 
 as solution of potash, soda, or ammonia, or of solid 
 alkaline substances such as sodium carbonate. 
 The only special precaution to be taken in dealing 
 with carbonates is that when litmus is used the 
 liquid requires to b; boiled in order to effect com- 
 plete decomposition and expulsion of the carbonic 
 acid. Details of the mode of operating are given 
 in so many manuals that it is unneces.sary to 
 describe the common process of alkalimetry or 
 acidimetry in this place. 
 
Observation — Quantitative 1 7 
 
 Suppose now it is found that 50 c.c. of the sul- 
 phuric acid are required to neutralise 
 
 a c.c. of solution of potash 
 b c.c. of solution of soda 
 or c grams of sodium carbonate. 
 
 We can apply either of these facts to ascertain 
 the neutralising power of other acids. Thus a c.c. 
 of solution of potash will be found to neutralise 
 
 m c.c. of hydrochloric acid 
 n c.c. of nitric acid 
 ox p grams of oxalic acid. 
 
 Then a, b, or c will neutralise not only 50 c.c. of 
 sulphuric acid, but m, n, or p of the other acids, 
 and so these quantities, a, b, c, in, n, p, are re- 
 spectively equivalent to one another. So important 
 is it to secure due and exact attention to fact 
 before permitting the introduction of theory, that 
 in this part of the work again the author would 
 impress upon teachers once more the advisability 
 of requiring young pupils to practise the obser- 
 vations of neutralising power before they are 
 allowed to use formulae. When the mere manipu- 
 lation has been mastered, the explanation may be 
 given of the use of the specific quantity of sul- 
 phuric acid taken for the preparation of the 
 standard solution, and the reactions may be ex- 
 pressed by equations. It will be well, however, 
 to confirm the result expressed in the equation in 
 
2 8 Teaching of Elementary Chemistry 
 
 a few cases by direct experiment. Thus the 
 amount of HCI in a sample of hydrochloric acid 
 having been determined volumetrically, as ex- 
 plained above, 50 c.c. of the liquid may be exactly 
 neutralised by soda, and the solution evaporated 
 to dryness in a small counterpoised porcelain dish 
 heated by a water bath. The salt when dry may 
 be weighed. Another more difficult process would 
 be to place a weighed quantity of salt in a small 
 flask connected with a bulb absorption apparatus 
 containing distilled water. On adding excess of 
 strong sulphuric acid to the salt and heating, the 
 whole of the hydrochloric acid gas may be driven 
 into the water, and the resulting solution tested 
 volumetrically by alkaline solution of known 
 strength. 
 
 For exercise in manipulation and observation 
 without the interference of theory there is no 
 better example than the application of the soap 
 test to the estimation of ' hardness ' in water. It is 
 not necessary to describe this process here, as an 
 account of it is to be found in nearly all books on 
 analysis. 
 
 The following statement occurred in a paper 
 sent up at the recent May examination (1895) 
 ' Common salt is called sometimes sodium chloride, 
 and this shows that it must contain chlorine by the 
 name.' The childish character of such an answer 
 may be thought to be exceptional ; but though it 
 is true that such a crude form of statement is not 
 
Observation — Quantitative 2 9 
 
 common, the fact remains that names and formula 
 are often used as expressions of fact when they re- 
 present the thing that is to be proved. An illus- 
 tration of this is almost certain to occur whenever 
 the examinees are requested to illustrate the law 
 of multiple proportions by referring to, say, the 
 oxides of nitrogen. The formulae, having been 
 committed to memory, are at once produced, the 
 candidates being apparently unconscious that any 
 reference to them is equivalent to begging the 
 whole question. If, however, the problem is stated 
 in another form without supplying the names of 
 the elements concerned, students seem to be in- 
 capable of attacking the question, and it remains 
 usually unanswered. Suppose the question to 
 stand as follows : ' Two elements A and B combine 
 in the following proportions : 
 
 A B 
 
 I . . . 96-28 
 
 II . . . 92-83 
 
 III . . . 89-62 
 
 IV . . . 86-62 
 
 372 
 
 7-17' 
 
 IQ-3S 
 
 13-38 
 
 Show that these combinations comply with the 
 law of multiple proportions.' 
 
 All that has to be done is to make one of these, 
 say A, a fixed quantity, and to calculate the several 
 proportions of the second element which are com- 
 bined with this quantity, and then see whether. 
 they stand in the simple relation to one another 
 required by the law. If in the above example we 
 calculate the amount of B which is in each com- 
 
30 Teaching of Elanentary Chemistry 
 
 pound 
 
 united 
 
 with 
 
 lOO parts 
 
 of A, the figures 
 
 stand 
 
 as follows : 
 
 
 
 
 
 
 
 
 
 A 
 
 r. 
 
 
 
 
 I 
 
 
 100 : 
 
 3-86 
 
 
 
 
 II 
 
 
 loo : 
 
 772 or 3-86 > 
 
 .■ '' 
 
 
 
 III 
 
 
 loo : 
 
 11-58 or 3-86 X 
 
 ■?, 
 
 
 
 IV 
 
 
 loo : 
 
 15-45 or 3-86 / 
 
 ■4 
 
 Q. 
 
 E.u. 
 
31 
 
 CHAPTER IV 
 
 CHEMICAL EQUIVALENTS—THE LAW OF 
 VOLUMES 
 
 In the last chapter experimental methods were 
 described by which the volumes of hydrogen 
 expelled from acids by the action of several 
 different metals could be estimated, and hence the 
 weight of the metals equivalent to one another 
 could be determined. 
 
 It is important for the foundation of chemical 
 theory to be able to substitute the mass for thelv 
 volume of the hydrogen in these and similar cases, 
 so as to be in a position to express the chemical 
 equivalents of the metals in terms of hydrogen 
 taken as the unit. In order to accomplish this, 
 we must know the weight of a unit volume of 
 hydrogen. This cannot be determined directly 
 by any experimental process, which is accurate 
 enough and at the same time sufficiently easy to 
 be attempted by young students. But an equi- 
 valent result may be attained by making use of a 
 form of experiment familiar in analytical chemistry, 
 in which the loss of weight consequent upon the 
 escape of gas from a light glass apparatus is deter- 
 
32 Teaching of Elementary Chemistry 
 
 mined. A plan has already been described by 
 which the hydrogen expelled from an acid by a 
 weighed quantity of metallic magnesium can be 
 collected and measured ; by the following device 
 the iveigJd of this hydrogen can be determined.' 
 
 The apparatus consists of a wide-mouthed 
 weighing bottle of thin blown glass. Through a 
 
 Fig. s 
 
 rubber stopper pass two tubes ; one as shown bent 
 at right angles and closed by a light rubber cap ; 
 the other shaped as shown in the figure, the bulb 
 being filled with loose fibrous asbestos. About 
 
 ' The suggestion that this can be accomplished was derived from 
 Professor J. Emerson Reynolds's ' Experimental Chemistry for 
 Junior Students' (Longmans), a book which deserves the attention 
 of all teachers. 
 
Chemical Eqtdvalents 33 
 
 5 c.c. of water are placed in the bottle, together 
 with the weighed quantity of metal. The bulb 
 tube is dipped into a beaker of strong sulphuric 
 acid till the lower part is filled, as shown in the 
 sketch. The outside of this tube is then washed 
 free from sulphuric acid, and the stopper bearing 
 the tube fixed in its place. The whole apparatus 
 is then placed on the pan of the balance, and 
 weighed carefully, the weight being noted to milli- 
 grams. The whole apparatus should not weigh 
 moi'e than 30 to 40 grams. The weight having 
 been recorded, a small piece of rubber tubing is 
 attached to the open end of the bulb tube, and by 
 blowing gently a few drops of sulphuric acid may 
 be expelled into the water, in which the metal 
 is immersed. Hydrogen is immediately evolved, 
 and, escaping through the tube containing the oil 
 of vitriol, is dried in its exit. When the whole of 
 the metal has been dissolved, the cap is removed 
 from the end of the right-angled tube, and air is 
 gently sucked through the rubber attached to the 
 bulb tube. The residual hydrogen is thus replaced 
 by air, and, after a few minutes, the apparatus is 
 ready to be weighed. 
 
 The following are the results of an experi- 
 ment : 
 
 Magnesium taken ■ 1 2 gr. 
 
 Weight of apparatus before solution of metal, 34'469 gr. 
 after „ „ 34-459 gr- 
 
 Loss of weight •010 gr. 
 D 
 
34 Teaching of Elementary Chemistry 
 
 Thus magnesium expels from dilute acid -^-^ of its 
 weight of hydrogen, or 12 parts of magnesium 
 replace i part by weight of hydrogen. Now, 
 having shown by previous experiments that i gram 
 of magnesium produces from dilute sulphuric acid 
 about 930 c.c. of hydrogen and yV of a gram 
 is "0833, this is the weight of 930 c.c. of the gas : 
 1000 c.c. of hydrogen would, therefore, weigh '0895, 
 which is near enough to the figure usually given, 
 viz. -0896 gram. 
 
 The determination of the composition of water 
 by weight is another experiment of the same order 
 which may be done by pupils at about the same 
 stage. 
 
 For this experiment we require a Kipp or 
 other apparatus for generating a stream of hydrogen 
 free from air, a couple of tubes containing pumice 
 stone wetted with strong sulphuric acid in order 
 to dry the gas, a tube containing pure dry oxide 
 of copper which can be weighed, and an apparatus 
 also light enough to be weighed, in which the 
 water formed can be collected. The shape of a 
 convenient apparatus for gathering the whole of 
 the water without loss by vaporisation is shown 
 in fig. 6, where a represents one of the tubes by 
 which the stream of hydrogen is rendered dry ; b 
 is a bulb tube of hard glass containing pure dry 
 black oxide of copper ; the end of this tube is 
 turned down at a right angle, and the extremity 
 cut off obliquely. This tube is suspended by means 
 
Chemical Equivalents 
 
 35 
 
 of a thin piece of iron or, better, platinum wire 
 from any suitable support, and by the same wire 
 it can be attached to the beam of the balance 
 when it has to be weighed. The thistle funnel 
 has a bulb on the stem, the lower end of which 
 dips through a cork into some oil of vitriol con- 
 tained in a short test-tube. There is a short exit 
 
 Fig. 6 
 
 \^ 
 
 tube from the cork for the escape of the excess of 
 hydrogen. The whole of c can be hung on the 
 balance by means of a wire loop. For the success of 
 the experiment, the copper oxide should be freshly 
 ignited, so as to ensure its dryness and freedom 
 from nitrate or other impurity, and the stream of 
 hydrogen should be carried through the whole series 
 of tubes at a moderate speed for 5 to 10 minutes 
 
36 Teaching of Elementary Chemistry 
 
 before the copper oxide is heated, l) and c are to 
 be weighed as accurately as possible at the 
 temperature of the air before and after the reduc- 
 tion of the oxide and formation of the water. The 
 gain of weight in c (water) should be to the loss 
 of weight in 1} (oxygen) in the ratio of 9 to 8. 
 
 The fact that oxj'gen combines with twice 
 its volume of hydrogen was observed by Cavendish 
 in the middle of the last century, but it was Gay- 
 Lussac ^ who established the generality of the prin- 
 ciple that gaseous combinations are formed by the 
 union of their constituents in very simple volumetric 
 proportions. Thus he observed that 
 
 100 vols, of oxygen combine with 200 vols, of 
 hydrogen. 
 
 1 00 vols, of muriatic acid combine with 1 00 vols, 
 of ammonia. 
 
 100 vols, of carbon dioxide combine with 200 
 vols, of ammonia. 
 
 He also drew attention to the established com- 
 position of ammonia, which consists of 100 vols, of 
 nitrogen to 300 vols, of hydrogen ; the composition 
 of sulphuric anhydride, which is formed of 100 vols, 
 of sulphur dioxide united to 50 vols, of ox}'gen ; 
 of carbonic anhydride, which is composed of 100 vols, 
 of carbonic oxide and 50 vols, of oxygen, and to 
 the composition of the oxides of nitrogen. From 
 all these facts Gay-Lussac drew conclusions which 
 supplied very important arguments in favour of 
 ' See ' Alembic Club Reprints,' No. 4, p. 15. 
 
Chemical Eqidvalents 2)1 
 
 Dalton's Atomic Theory, at that time still under 
 discussion, and far from being the generally accepted 
 basis of chemical theory it is at the present day. 
 
 The experimental demonstration of the volu- 
 metric composition of hydrogen chloride, water, and 
 ammonia is a matter of fundamental importance, 
 but is best exhibited by the more experienced hand 
 of the teacher. The methods generally in use for 
 this purpose were for the most part devised by 
 Hofmann, whose 'Introduction to Modern Chemis- 
 try,' though published a quarter of a century ago, 
 is still delightful and instructive reading. As all 
 these processes are described in the principal text- 
 books, and particularly in detail in Mr. Newth's 
 ' Chemical Lecture Experiments ' (Longmans), it is 
 unnecessary to repeat the account of them in this 
 place. One or two remarks, however, are neces- 
 sary to indicate what each experiment serves to 
 establish. 
 
 Hydrogen chloride. — It is usual to analyse 
 hydrogen chloride gas by means of sodium 
 amalgam. It must, however, be noted that this 
 experiment merely proves that 2 vols, of the gas 
 contain i vol. of hydrogen ; it gives no information 
 as to the volume of the chlorine with which this 
 I vol. of hydrogen is united. 
 
 The volume of the chlorine can only be shown 
 by collecting the gas which results from electrolysis 
 of the strong aqueous solution, and showing that it 
 contains half its volume of chlorine and half its 
 
38 Teaching of Elementary Chemistry 
 
 volume of hydrogen. The absorption of the 
 chlorine from the mixed gas is best effected by 
 means of concentrated solution of potassium iodide. 
 Not only is half the gas absorbed, but the liberated 
 iodine serves to demonstrate the nature of the 
 gas. 
 
 Water. — As to the composition of water, it must 
 be observed that the process of electrolysis so com- 
 monly resorted to proves only the ratio of the 
 hydrogen to the oxygen, and is therefore incomplete 
 as a demonstration of the composition of water. 
 To show the volumetric relation of water gas to its 
 constituents, the process must be synthetical — that is, 
 the method must be used which consists in uniting 
 2 vols, of hydrogen with i vol. of oxygen at a tem- 
 perature above 100° C. 
 
 Ammonia. — The ratio of the volume of hydrogen 
 to that of nitrogen in ammonia is shown by the 
 chlorine process ; but the proof that 3 vols, of 
 hydrogen and i vol. of nitrogen are contained in 
 2 vols, of ammonia is obtained only by decompos- 
 ing a measured quantity of ammonia by heat, and 
 subsequently analysing the resulting mixture of 
 hydrogen and nitrogen. This is usually accom- 
 plished by sparking the gas till its volume is doubled, 
 and then heating in the mixed gas a particle of 
 cupric oxide, which converts the hydrogen into 
 water. 
 
 The composition of ammonia cannot be con- 
 veniently demonstrated by exploding the gas with 
 
Chemical Eqtdvalents 39 
 
 oxygen ; for combustion with excess of oxygen 
 leads to the production of so considerable a 
 quantity of oxides of nitrogen as to render the 
 process worthless for analytical purposes. 
 
 From the experiments just referred to we learn 
 that equal volumes of chlorine, oxygen, and nitro- 
 gen combine respectively with one, two, and three 
 volumes of hydrogen. Hence, without any assump- 
 tions as to atoms or molecules, we may express the 
 composition of these gases by the formulae HCl, 
 HjO, H3N, in which the symbols H, CI, O, N re- 
 present unit volumes of the constituents. 
 
 We know also that sodium and other metals 
 acting upon these gases expel from them part of 
 the hydrogen they contain, and take the place of 
 the hydrogen so removed, the amount of hydrogen 
 replaced being one-half in the case of water, and 
 one-third in the case of ammonia, while from 
 hydrogen chloride the whole of the hydrogen is 
 removed at once. These facts are recorded in the 
 formulse. 
 
 In the earlier chapters great stress was laid upon 
 the importance of cultivating the powers of observa- 
 tion ; but, to make any progress with the theory 
 of chemistry, it is equally necessary to practise the 
 careful consideration and selection of evidence, and 
 to form habits of reasoning thoughtfully from facts 
 previously established. The confusion so often 
 existing in the minds of students is, in general, dufe 
 partly to the use of unfamiliar language, partly to 
 
40 Teaching of Elementary Chemistry 
 
 want of experience in observation and in reasoning. 
 This want of experience of phenomena and of the 
 nature of scientific evidence renders the condition 
 of the student-mind quite comparable with that of 
 the early investigators. Some knowledge of the 
 history of the development of modern chemistry 
 from the time of Boyle onwards is therefore very 
 valuable to the teacher. Frequent reference to a 
 good English dictionary even by the teacher is 
 also a practice which cannot be too strongly com- 
 mended. The statement by a pupil that ' metals 
 are hard and brittle, and are also malleable and 
 ductile,' is a proof that the wholesome custom of 
 explaining the meaning of words must have been 
 neglected by that boy's teacher. Endless examples 
 of the same defects are noticeable not only in the 
 papers sent in by candidates at the examinations 
 of the Science and Art Department, but at the 
 iMatriculation of the London University and other 
 examinations of junior students within the experi- 
 ence of the author. 
 
 In the matter of selecting evidence great mis- 
 takes are commonly made, in many cases arising 
 from mere indolent habit of mind or positive 
 inability to think. A question recently set requir- 
 ing evidence of the presence of oxygen in nitrous 
 and nitric oxides was generally answered by the 
 statement that ' they support combustion, therefore 
 they contain oxygen.' Here 'support combustion' 
 may be assumed to refer to the combustion of a 
 
Chemical Equivalents 41 
 
 candle. In the first place, therefore, it is not 
 true that they both support combustion ; but this 
 answer also ignores the fact that there are many 
 known examples of burning where there is no 
 oxygen. The proof required is as follows : Pass 
 the gas over heated copper, then subject the hot 
 copper oxide to a stream of hydrogen, and show 
 the production of water. To make the proof com- 
 plete, a knowledge of the composition of water 
 must be assumed, and the water must be fully 
 identified by observation of its physical as well as 
 chemical characters. 
 
 A similar case occurs in the comparison of the 
 gas obtained from chalk by heat or acids with that 
 which is produced by combustion of charcoal, by 
 fermentation of sugar or in animal respiration. It 
 is not sufficient to point to the white precipitate 
 with lime water as evidence of identity. ^\\& pro- 
 bability that they are the same is increased by 
 each additional character — taste and odour, solu- 
 bility, density, chemical character — which is found 
 to be alike in both, but certainty is not attained till 
 all available characters have been examined. 
 
 It may not be within the power of young stu- 
 dents to make an exhaustive investigation of this 
 kind, but the teacher should be careful to show that 
 there are different stages or degrees of probability 
 leading up to proof. 
 
f4i o; hi 
 
 i-i; 
 
 M ^ 
 
 
 ^ 
 
 
 oo 
 
 
 M 
 
 o-> 
 
 
 O-v 
 
 ro 
 
 
 !>. 
 
 
 
 (N 
 
 
 !>, 
 
 ^ 
 
 
 
 
 
 , , 
 
 ■X3 
 
 
 C 
 
 
 
 
 
 PQ 
 
 - CJ 
 
 
 o 
 
 
 
 CO -m -3 — 
 
 1 I 
 
 V o 
 
 1° 
 
 ^ I a-^-^-K 
 
 m 
 
-J 
 
 M 
 
 ^i q! S 
 
 O 
 
 o 
 
 PQ 
 
 o 
 
44 Teaching of Elementary Chemistry 
 
 Note to Chapter IV. 
 
 The preceding chronological table was drawn 
 up by the author some years ago, and it has been 
 found very useful in his own teaching. It supplies 
 the names of those chemists of the past who may 
 be regarded as founders of modern chemistry, 
 when they lived, and the length of each life, so as 
 to show at a glance who were contemporary at any 
 period. The table only includes the names of 
 deceased chemists who have substantially ad- 
 vanced the theory of chemistry by their discoveries 
 or by their writings. A crowd of well-known 
 names has necessarily been omitted, as represent- 
 ing contributors to the detail rather than to the 
 fundamental principles of the science, and in a few 
 cases it may be a question whether the names in 
 the list are all entitled to a place there. Klaproth, 
 for example, did a great deal of work relating to 
 the details of mineral analysis, and his name is 
 admitted, chiefly on the ground that improve- 
 ments in the art of analysis lent important aid 
 to the progress of the science. 
 
 A general survey of the work of the majority 
 of the chemists whose names appear in the table 
 is provided in a compact form in Mr. Pattison 
 Muir's volume entitled ' Heroes of Science — 
 Chemists' (Society for Promoting Christian Know- 
 ledge, 1883). 
 
 There is also Watts's translation of the ' History 
 
Chemical Equivalents 45 
 
 of Chemical Theory,' by the late Professor \'\'urtz 
 (Macmillan), which brings the story down to 1869, 
 now more than a quarter of a century ago. 
 
 Dr. Thorpe's charming volume of " Essaj's in 
 Historical Chemistr}' ' (Macmillan) gives a critical 
 review of the work and an account of the life of 
 many, but not all, of these founders of Chemical 
 Science. 
 
46 
 
 CHAPTER V 
 
 MOLECULAR WEIGHTS AND FOLiMULAL 
 
 The establishment of Gay-Lussac's ' Law of 
 Volumes ' was destined to lead to consequences 
 of tlie utmost importance for theoretical chemistry. 
 In 1811 Avogadro published his discussion' of 
 the constitution of gases upon the basis of Gay- 
 Lussac's law, and although it attracted compara- 
 tively little notice at the time, what is now known 
 as the law of Avogadro became half a century 
 later the foundation of the modern system of 
 molecular and atomic weights. The law of 
 Avogadro states that equal volumes of different 
 gases under the same conditions of temperature 
 and pressure contain the same number of mole- 
 cules. This statement is sometimes altered by 
 students, and even by teachers, into ' gaseous 
 molecules have the same or equal volumes.' This 
 is wrong, because we know next to nothing about 
 the dimensions of single molecules : all that we can 
 
 ' ' Essay on a Manner of Determining the relative Masses of the 
 Elementary Molecules of Bodies and the Proportions in which they 
 enter into these Compounds.' See ' Alembic Club Reprints,' No. 4, 
 p. 28. 
 
Molecular Weights and Forrmilce 47 
 
 say is that molecules of all gases require on the 
 average equal spaces to move about in. 
 
 The density of a gas or vapour is the mass 
 of unit volume, taking some gas, preferably the 
 lightest of all, as t'he standard. So that when we 
 say the density of oxygen is 16, of nitrogen 14, of 
 carbonic anhydride 22, and so on, we mean that 
 equal volumes of these gases weigh 16, 14, 22, &c. 
 units when the weight of the same volume of 
 hydrogen is i unit. Now it can be shown that 
 the molecule of hydrogen is divisible into two 
 equal parts. The hypothesis of the divisibility 
 of many molecules is discussed very clearly in 
 Avogadro's essay, to which reference has been 
 made. The argument may run in this way : 
 Equal volumes of hydrogen and chlorine interact 
 to form hydrogen chloride, the volume of which 
 is equal to the volume of the hydrogen added to 
 that of the chlorine. If Avogadro's hypothesis is 
 true, as we assume it to be, this process may be 
 expressed by saying that a molecule of hydrogen 
 reacting with a molecule of chlorine gives two 
 molecules of hydrogen chloride. Hence each 
 molecule of hydrogen chloride must contain half 
 the hydrogen contained in a molecule of hydrogen, 
 and half the chlorine contained in a molecule of 
 that gas. Hence the molecules of hydrogen and 
 of chlorine are respectively divisible into two equal 
 parts, and the molecular weight of either gas may 
 be conveniently taken to be the weight of two unit 
 
48 Teaching of Elementary Chemistry 
 
 volumes. The determination of the density of a gas 
 or vapour, therefore, leads directly to the molecular 
 weight of the gas, whether elementary or compound. 
 The experimental methods of determining the 
 densities of gases used by such great experimenters 
 as Regnault and Lord Rayleigh are exceedingly 
 simple in principle ; but attainment to great ac- 
 curacy requires much skill and experience, and 
 attention to many details. But, though the method 
 cannot be applied by beginners to the case of 
 hydrogen and the lighter gases, it may be used even 
 by young students with considerable success when 
 
 Fig. 7 
 
 the density of the gas is so great that the experi- 
 mental errors of weighing and those due to impurity 
 in the gas stand in small p'-^portion relatively to 
 the whole mass to be weighed. 
 
 Two globes are taken of the form shown in the 
 figure. They may be purchased in the shape of 
 large light pipettes, the stems of which may be 
 cut off four or five cm. from the globe and fitted 
 with light rubber caps. The globes should be so 
 chosen as to be as nearly as possible of the same 
 capacity — viz. 250 to 300 c.c. They should be hung 
 by thin wires at the opposite ends of the beam of 
 
Molecular Weights and Formulce 49 
 
 the balance without removing the pans, and the 
 lighter one supplemented by a small piece of 
 glass tubing filed down so as to supply an exact 
 counterpoise. One of the globes which is to hold 
 the gas must have its capacity determined once for 
 all by filling with water and running it out into a 
 measuring vessel. The globe is then dried and 
 filled with the gas by displacement of air, the globe 
 being held with the ends vertical, while the gas, 
 if heavier than air, is admitted below. 
 
 The ends of the tubes are then closed again 
 with the caps. On replacing the globe upon the 
 balance, weights will have to be added to the 
 opposite pan to restore equilibrium. These weights 
 cannot be taken as a measure of the mass of the 
 gas, as in the case of solids or liquids, inasmuch 
 as the volume of air displaced, by the gas in this 
 case, has an effect too great to be ignored. The 
 weight of this displaced air must be calculated and 
 added to the weights used in order to arrive at the 
 weight of the gas in the globe. The details of an 
 experiment will make the matter clear : 
 
 Capacity of globe 235 c.c. 
 
 Weight of I c.c. of air at normal temperature 
 and pressure = -001 2937 gram. At 17°, the tem- 
 perature of the laboratory, the weight of i c.c. of 
 air is '00122 very nearly. 235 c.c. therefore weigh 
 ■286 gram. Globe filled with carbon dioxide re- 
 quired for counterpoise ■13 gram. Then '286 + '13 
 = ■416 is the weight of 235 c.c. of the gas. 
 
 E 
 
50 Teaching of Elementary ' Chemistry 
 
 The density is therefore ------i'4S- 
 
 •280 
 
 Calculated density i'52 (air=i). 
 
 The determination of the vapour density of 
 various volatile solids or liquids can be so easily 
 accomplished by Victor Meyer's air-expulsion me- 
 thod, and the calculations involved are so simple, 
 that even young students may successfully practise 
 the operation. It is not necessary to give an ac- 
 count of the process in this place, as it is described 
 with full detail in many text-books. Ether (b.p. 35'^) 
 and chloroform (b.p. 61°) may be used as examples 
 of substances which volatilise readily and com- 
 pletely in boiling water ; alcohol (b.p. 78°) and 
 benzene (b.p. 81°) do not give results so good, 
 unless the water is saturated with common salt, 
 which brings up the boiling point to 1 10°. The 
 employment of higher temperatures requires rather 
 more skill than beginners usually possess, but a 
 bath of glycerine or oil, or any of the organic 
 liquids of constant boiling point mentioned in the 
 books, may be used when the pupil has had 
 sufficient practice. 
 
 A knowledge of molecular weights provides one 
 very important method by which atomic weights 
 may be chosen. The atomic theory explains 
 chemical phenomena by the assumption that com- 
 bination results from the close approximation and 
 connection of minute masses of elementary matter 
 called ' atoms,' because they are believed to be 
 
Molectdar Weights and ForimilcE 51 
 
 indivisible by the forces which operate in chemical 
 changes. Chemical decomposition in like manner 
 is the result of the separation of the atoms forming 
 a compound and their rearrangement into new 
 combinations. The hypothesis of atoms, however 
 it may have originated and developed in Dalton's 
 mind,^ is firmly established by the facts which are 
 embodied in the so-called laws of chemical com- 
 bination. It is a fact that (i) elements unite in 
 definite proportions ; that (2), if two elements 
 combine in several proportions, these proportions 
 bear a simple relation to one another ; that (3), if 
 two elements combine with a third, the proportions 
 in which they unite with this third element are the 
 same proportions or simple multiples of the same 
 proportions in which they combine together, should 
 combination between them ever occur ; and, lastly, 
 that (4) when gases unite together, the volumes so 
 uniting are represented by very simple proportions. 
 If now we accept the doctrine of Avogadro, the 
 molecular weight is proportional to the specific 
 gravity of the substance in vapour. For example, 
 the molecule of carbon dioxide is 22 times heavier 
 than the molecule of hydrogen, since their specific 
 
 gravities are 
 
 22 : I. 
 
 But for reasons already given, the molecule of 
 hydrogen is believed to be divisible into two equal 
 
 ' There seems to be some conflict of evidence on this point. 
 See ' Life of Dalton,' by Sir H. E. Roscoe, ' Century ' Series. 
 
52 Teaching of Elementary Chemistry 
 
 parts, or, in other words, consists of two atoms. 
 Hence, to avoid fractions, the relative molecular 
 weights are represented as 
 
 44 : 2. 
 Taking two volumes of hydrogen as the standard 
 volume for comparison of molecular quantities of 
 all gases, we have a means of fixing atomic weights. 
 For since by hypothesis the atom of an element 
 is an indivisible quantity, it is evident that, if we 
 measure off molecular proportions of all known 
 volatile compounds containing that element, the 
 smallest quantity of the element in question ever 
 observed in such molecular proportion must be 
 assumed to be one atom, until and unless some 
 other compound is afterwards found containing a 
 smaller quantity. This may be illustrated by the 
 case of oxygen, as shown in the following table : 
 
 
 Vapour 
 density, or 
 
 Weight of 
 
 Weight of 
 
 — 
 
 weight of 
 
 2 volumes 
 
 oxygen in 
 2 volumes 
 
 1 
 
 ! I volume 
 of vapour 
 
 9 
 
 of vapour 
 
 of vapour 
 
 Water 
 
 18 
 
 16 
 
 Carbonic oxide .... 
 
 14 
 
 28 
 
 16 1 
 
 Carbonic anhydride 
 
 22 
 
 44 
 
 1 
 
 32 
 
 Nitrous oxide 
 
 22 
 
 44 
 
 16 
 
 Nitric oxide 
 
 IS 
 
 30 
 
 16 
 
 Sulphurous anhydride . 
 
 32 
 
 64 
 
 32 
 
 Sulphuric anhydride 
 
 40 
 
 80 
 
 48 ' 
 
 Alcohol . . . . 
 
 23 
 
 46 
 
 16 i 
 
 Aldehyd ... 
 
 22 
 
 44 
 
 16 1 
 
 Acetic acid 
 
 30 
 
 60 
 
 32 
 
 As the smallest proportion of oxygen ever 
 
Mohadar Weig-hts and ForrmtlcE 
 
 oj 
 
 found in two volumes of the vapour of any of its 
 compounds is i6 units of weight, i6 is the value 
 assigned to the atomic weight. 
 
 One word of caution is necessary in conse- 
 quence of the confusion of phraseology still pre- 
 valent, even in the writings of chemical experts, 
 in reference to atomic weights. In some of the 
 principal chemical periodicals we find papers the 
 titles of which announce that they refer to the 
 ' atomic weights ' of various elements, as, for 
 example, in recent years the atomic weight of 
 oxygen, of chromium, of titanium, of silicon, of 
 tellurium, &c. In nearly all these cases the 
 inquiry relates not to the determination of the 
 atomic weight, but of the combining proportion or 
 equivalent of the element in question. In fact, it 
 may be stated as a general rule that two distinct 
 operations are required for the determination of the 
 exact value of the atomic weight of any element, 
 and that the second of these is commonly assumed 
 or ignored. First, the combining ratio, or equi- 
 valent, is determined with the utmost possible 
 accuracy by the analysis or synthesis of some one 
 or more of its compounds ; and, secondly, the 
 equivalent is converted into the atomic weight by 
 multiplying this experimental number by some 
 factor — I, 2, 3, 4, or more — which is chosen by 
 appeal to some other wholly different criterion, such 
 as that just referred to in the case of oxygen. The 
 atomic weight of oxygen, in fact, is not 1$ exactly 
 
54 Teaching of Elementary Chemistry 
 
 but some number closely approaching i6 which 
 is determined by analysis of water and other 
 oxides. 
 
 Other examples and other methods are given 
 in the next chapter. 
 
55 
 
 CHAPTER VI 
 
 A TO MIC WEIGHTS— CLASSIFICA TION 
 
 The calculation of the atomic weight of an element 
 must in all cases be preceded by the determination 
 of the quantity of that element which combines 
 with or replaces one unit weight of hydrogen. 
 This is the equivalent. The operations which 
 follow are of different kinds, according to the 
 character of the element concerned, the most 
 important being the following : 
 
 I. The vapour density method, described in the 
 last chapter. It is a mistake to assume, as is some- 
 times done, that the vapour densities of the elements 
 themselves serve as a guide to their atomic weights ; 
 for though it is true that the vapour densities and 
 atomic weights are identical in a few cases, this 
 affords no evidence towards the establishment of 
 the atomic weight without additional information, 
 and beside these are nearly as many other ele- 
 ments (Hg, Zn, Cd, As, P) whose vapour densities 
 do not coincide with their atomic weights. 
 
 II. Appeal to the Law of Dulong and Petit 
 
56 Teaching of Elementary Chemistry 
 
 relating to Specific Heat. Care is necessary in 
 expressing this law ; it relates only to the elements 
 in the solid state, and does not hold under ordinary 
 conditions for the solids, boron, carbon, silicon, and 
 beryllium. 
 
 The law may be stated in various ways : The 
 specific heat of a solid element (with the named 
 exceptions) is inversely as the atomic weight ; or, 
 the specific heat multiplied by the atomic weight 
 is a constant (about 6"4) ; or the atomic weight is 
 the quotient obtained by dividing 6'4 by the specific 
 heat. 
 
 The application of the law consists in finding 
 what multiple of the equivalent must be taken to 
 comply with the formula 
 
 11 Equiv. X Sp. H. = 6'4 (approx.), 
 
 the problem being to find the value of n. Now n 
 must always be a whole number, because the 
 atomic weight must always be identical with the 
 equivalent or some multiple of it, according to 
 the valency of the element. The atomic weight 
 obviously cannot be deduced directly from the 
 value of the specific heat, because no process is 
 known by which the specific heat can be deter- 
 mined with the same degree of accuracy as the 
 equivalent. This should be explained carefully, 
 for students, even at the Honours stage, are too 
 often impressed with the idea that the atomic 
 weight can be directly settled by this process alone 
 
Atomic Weights — Classification 57 
 
 and seem to be ignorant that the specific heat can 
 only be used by way of control. 
 
 III. Atomic weights can be deduced from 
 equivalents by appeal to the Periodic Lmv. 
 
 The statement of the general principle may run 
 thus : when the elements are ranged in a series in 
 the order of the numerical value of their atomic 
 weights, there is observed a revival of the same or 
 closely similar characters periodically, and usually 
 at every eighth term. This principle is usually 
 displayed in the following or some similar table. 
 
 The utility, of this table in the settlement of 
 atomic weights depends upon the credence which 
 is given to it. So many cases of doubtful atomic 
 weights have, however, been submitted to this 
 criterion, with results always tending to confirm 
 the law, that it is now generally accepted. An 
 example or two of its application will suffice to 
 make it clear. 
 
 The element tellurium has an atomic weight 
 which, deduced from different experimental esti- 
 mations of its equivalent, probably lies somewhere 
 between 127 and 128. 
 
 Tellurium is undoubtedly related to selenion 
 in the same way that selenion is related to sulphur. 
 Hence tellurium ought to stand in the table vertically 
 under selenion ; it cannot do so unless a number 
 is adopted less than 127, and therefore differing by 
 several units from the experimental result. In this 
 case the experimental value has been set aside by the 
 
58 Teaching of Elementary Chemistry 
 
 qffi 
 
 
 o o 
 
 
 
 
 
 
 
 ::3 
 
 
 
 
 
 
 
 
 
 — '-^ 
 
 
 — -^ 
 
 
 
 
 v-^-' 
 
 
 ■3'i 
 
 
 in 
 
 n-. 
 
 
 
 
 
 
 
 n-. 
 
 n iH 
 
 
 i-O 
 
 
 
 
 
 r^ 
 
 
 
 
 
 
 
 ro 
 
 
 00 
 
 
 ri 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 '^S 
 
 fe 
 
 U 
 
 
 W 
 
 
 •-H 
 
 
 
 
 
 ■Ss 
 
 
 
 > 
 
 
 
 ^ 
 
 
 
 
 H^ 
 
 
 ro 
 
 
 W 
 
 
 ^ 
 
 P-, 
 
 
 -< 
 
 
 
 
 CO 
 
 
 
 
 
 ^ 
 
 
 -o 
 
 
 
 H 
 
 
 
 
 
 
 
 
 N 
 
 
 
 t-^ 
 
 
 
 .- 
 
 
 OJ 
 
 o 
 
 o 
 
 u 
 
 t/1 
 
 U>-I 
 
 pq 
 
 u 
 
 
 J3 
 
Atomic Weights — Classification 59 
 
 majority of chemists in favour of the theoretical, 
 because it is thought to be more Hkely that an 
 undetected source of error has attended the expe- 
 riments than that the statement of the law should 
 be at fault. 
 
 The element beryllium is a metal which was 
 formerly supposed, chiefly on the evidence of the 
 specific heat ('445 at 10° to 100°), to have the 
 atomic weight 13-5, but it was observed that there 
 is no place in the scheme for an element having 
 the combination of properties exhibited by beryl- 
 lium associated with such a value for the atomic 
 weight. Then it was discovered that the specific 
 heat of beryllium is anomalous in the same sense 
 as the specific heats of boron, carbon, and silicon 
 especially, and, in a minor degree, of several other 
 elements of comparatively low atomic weight. 
 Redeterminations of the specific heat of beryllium 
 at higher temperatures (-62 at 400° to 500°), and 
 applications of these values in the formula express- 
 ing the law of Dulong and Petit, led to the value 
 9' I for the atomic weight, and this is now uni- 
 versally adopted. This value is confirmed by the 
 vapour density of the chloride. H^nce beryllium 
 is now recognised as the first term of the series to 
 which magnesium belongs. 
 
 When, therefore, the equivalent of an element 
 is known, and a question is proposed as to its atomic 
 weight, the answer may be obtained not only by 
 referring to the specific heat, but by recalling the 
 
6o Teaching of Elementary Chemistry 
 
 chief properties of the element, and then considering 
 whether, having such properties, it can possibly 
 occupy the position which would be assigned to it 
 if the equivalent is accepted as equal to the atomic 
 weight. Take the case of magnesium, the equivalent 
 of which is I2i. Magnesium is a light metal, 
 forming one salifiable oxide which is insoluble in 
 water ; it forms a sulphate which crystallises in 
 prisms containing water of crystallisation, and 
 having the same form as sulphate of zinc. On 
 referring to the table we see that there is no place 
 between carbon and nitrogen available for any 
 element, and that there is no element in any of the 
 columns headed by boron, carbon, or nitrogen which 
 exhibits properties similar to those of magnesium. 
 But the isomorphism of the sulphate with sulphate 
 of zinc at once gives the clue, and we see that the 
 equivalent must be doubled in order to supply the 
 value of the atomic weight. Magnesium with the 
 value 24-2 or thereabouts then naturally falls into 
 place as the first term of the series of which zinc 
 and cadmium are the succeeding members. 
 
 The confidence placed in this table of the 
 elem.ents is based upon the belief that the properties 
 of the elements are intimately dependent upon their 
 atomic weights, that the number of elements is 
 limited, and that their atomic weights can have 
 only certain values. All experience in the past 
 certainly tends to support this view. Thus in 1871 
 the places in the table now occupied by gallium. 
 
Atomic Weights — Classification 6i 
 
 germanium, and scandium were vacant. These 
 three elements have since been recognised v/ith the 
 properties attributed to them from a consideration 
 of their position in the table.' 
 
 There is a strong analogy between this manner 
 of displaying the connection between the elements 
 and their atomic weights and the process of group- 
 ing the compounds of carbon into homologous 
 series. Fifty years ago a considerable ntimber of 
 hydrocarbons, alcohols, aldehyds, bases &c. were 
 known, but for the most part they were known only 
 as individual substances without recognisable rela- 
 tions to one another. Then it was remarked, first 
 by Schiel, and afterwards by Dumas, that the 
 radicles of the alcohol and of the fatty acids 
 exhibited a regularity of composition, and that the 
 properties of the substances themselves which 
 formed the series were only gradually modified in 
 passing from term to term. Hence wood spirit, 
 common alcohol, and fusel oil were found to be 
 related to another, and even such apparently dis- 
 similar substances as formic acid and stearic acid 
 were recognised as terms of the same ' homo- 
 logous ' series. 
 
 The arrangenient of the elements themselves 
 into groups of closely related members of the same 
 type, with properties gradually modified through 
 successive terms, was a discovery of the same order, 
 
 ' For a fuller exposition, see MendeleefiPs ' Principles of 
 Chemistry,' vol. ii. 
 
62 Teaching of Elementary Chemistry 
 
 and since, as pointed out by Dumas, the values of 
 the atomic weights exhibit the same kind of rela- 
 tions to one another as the atomic weights of the 
 radicles in successive terms of a homologous series, 
 the analogy is very remarkable. A single example 
 will suffice : 
 
 (a = 7 
 
 Lithium 
 Sodium 
 Potassium 
 
 d=i6) 
 
 Atomic weight, 
 . . a 
 . . a + d 
 . a + 2d 
 
 (a=i5 d=i4) 
 
 Atomic weight. 
 
 Methyl a 
 
 Ethyl . . . . a + d 
 
 Propyl . . . a + 2d 
 
 Now the several short series of elements when 
 placed in parallel columns, as in the table given 
 above, stand towards another in much the same 
 position as heterologous series of carbon compounds ; 
 except, of course, that while the latter can be trans- 
 formed into one another, the former cannot. 
 
 The periodic law, then, is based upon and 
 includes the results of the earlier attempts at 
 classification of the elements. All classification is 
 founded upon the recognition of resemblances and 
 differences, upon finding similarity in the midst of 
 diversity ; but to be useful a .system must be based 
 upon a record of as many points of resemblance as 
 possible. Classification, therefore, implies, first, the 
 practical process of observation, and, secondly, 
 the survey of many observations, with a view to 
 finding resemblances, so constituting the logical 
 process of induction. In looking for indications 
 of relationships, some considerable experience is 
 
Atomic Weights — Classification 63 
 
 necessary in many cases in order to select rightly 
 among the phenomena observed and to assign to 
 each its share of importance, so as to escape from 
 the delusions to which the observer is exposed who 
 gives too much attention to differences of colour, 
 of density, or of mechanical condition. It is pro- 
 bably difficult for a young student to recognise 
 at first the close relation subsisting between such 
 substances as chlorine, bromine, and iodine. That 
 a green gas should have much connection with a 
 black shining solid, or even with a red liquid, 
 doubtless appears at first a difficult proposition ; 
 and the conception that these three substances 
 form a family having common features with minor 
 differences comes only after the characters of a 
 homologous series have been realised. It is on 
 this account unfortunate that so-called Organic 
 Chemistry should be separated from Inorganic 
 Chemistry so rigidly as it commonly is by teachers, 
 text-books, and boards of examiners. The idea 
 may, however, be gained by the study of a single 
 series of hydrocarbons — namely, the paraffins — in 
 which the relation of physical properties such as 
 volatility, density, melting point, solubility, to mole- 
 cular weight can be readily illustrated. As the busi- 
 ness of scientific chemistry is so largely made up of 
 the art of classification, one or two further exam- 
 ples may be given. 
 
 The word ' metal ' has received many applications, 
 and even lexicographers seem to be in doubt as to 
 
64 Teaching of Elementary Chemistry 
 
 its origin, but in chemistry it possesses a technical 
 signification which is generally recognised. Never- 
 theless it is remarkable how few chemical teachers 
 seem to have a clear idea of assigning to it a 
 definite connotation, to judge at least by the 
 answers given by candidates at examinations. 
 Few, perhaps, are to be found in the confusion of 
 mind exhibited by a student who, at a recent 
 examination, stated that ' metals differ from non- 
 metals, both by ending in uiii and having a 
 metallic lustre,' but the answers of the best show 
 that they have not learnt the art of observing 
 correctly and selecting judiciously. 
 
 Dr. Percy, in the Introduction to his great 
 work on Metallurgy, says : ' The term metal, like 
 the term acid, is rather conventional than strictly 
 scientific' On the other hand, Professor Roberts- 
 Austen, in his ' Introduction to the Study of Metal- 
 lurgy,' remarks that ' the physical aspects of metals 
 are so pronounced as to render it difficult to 
 abandon the old view that metals are sharply 
 defined from other elements, and form a class by 
 themselves.' The difficulty is to select characters 
 which may be regarded as diagnostic, marking off 
 the members of the class from other bodies which 
 do not belong to it. 
 
 The properties of weight and lustre seem 
 formerly to have made up the whole idea of a 
 metal, and the difficulty of getting rid of established 
 prejudice, and of seeing things as they are, is illus- 
 
Atomic Weights — Classification 65 
 
 trated in an amusing way by the well-known story 
 of Dr. Pearson, a friend of Sir Humphry Davy's, 
 who, on being shown a specimen of the newly dis- 
 covered potassium, and noticing its lustre, ex- 
 claimed, ' Why, of course it is a metal — how heavy 
 it is ! ' 
 
 Obviously, then, high density is not a property 
 exhibited by all metals, nor even by all the common 
 metals now that magnesium and aluminium are 
 sold in the shops. Metallic lustre again is a quality 
 exhibited by many substances, such as graphite, 
 galena, pyrites, iodine, which are evidently not 
 metals. 
 
 The following may be regarded as the characters 
 of the metals as a class when in a pure state. 
 All are more or less malleable and ductile, they 
 are relatively good conductors of heat and electricity, 
 and they form oxides which, when not too rich in 
 oxygen, are basylous and interact with acids. The 
 metals which present this assemblage of characters 
 also rarely form compounds with hydrogen, and 
 such of these compounds as are known are non- 
 volatile solids. 
 
 The elements known, for want of a better name, 
 as non-metals, though very diverse in physical 
 characters, agree in forming volatile compounds 
 with hydrogen and oxides which are, for the most 
 part, capable of giving rise to acids by uniting with 
 the elements of water. Natural objects, however, 
 do not admit of classification by the application of 
 
66 Teaching of Elementary Chemistry 
 
 any rigid system, and no definition can be accepted 
 absolutely and without reserve. The teacher will, 
 therefore, at the proper stage, point out there arc 
 many elements which stand in an intermediate 
 position between metal and non-metal, exhibiting, 
 like tellurium and antimony, some of the properties 
 of both classes of elements. 
 
 The difficulty of arriving at an agreement con- 
 cerning the application of terms is still more 
 forcibly displayed in the case of the words acid, 
 base, salt. Even at the present day each of these 
 words is encumbered with a residue of ancient 
 usac^e from which it is almo.st impossible to set it 
 free. The word ' salt ' is perhaps less difificult now 
 than it was a few years ago ; but what is a base ib 
 still a subject concerning which no two chemists 
 seem to be agreed. 
 
 Common salt is a food stuff known to man- 
 kind from very early times. The characters origin- 
 ally recognised were probably first its taste, then 
 perhaps its solubility in water. But sodium chloride 
 is not the only saline substance obtainable from the 
 earth ; there is nitre found commonly in tropical 
 countries and the soda, borax, magnesium sulphate, 
 sodium sulphate, and other crystalline deposits on 
 the shores of many lakes. These were doubtless 
 regarded in early times as closely akin to common 
 salt from their saline taste, solubility, and possibly 
 also from their crystalline appearance. Such was 
 probably the vague kind of notion prevailing almost 
 
Atomic Weights — -Classification 67 
 
 down to the time of Lavoisier. Then came the 
 dualistic theory of salts. Having found that metals 
 combine with oxygen, and that the products are 
 usually capable of dissolving in acids, with produc- 
 tion of salts, Lavoisier saw in these facts the basis 
 of a series of definitions, A salt is composed of an 
 acid united to a base : an acid is an oxide, usually 
 of a non-metallic element ; a base is the oxide of 
 a metal. If these views had stood the test of 
 experience, we should now be in possession of a 
 system consistent with itself As it is, this view 
 of the constitution of salts has been overthrown, 
 while its nomenclature has been retained. The 
 difficulty about the use of the term base arises from 
 the fact, that though we may still apply it to cer- 
 tain of the oxides of metals, these compounds do 
 not unite as a whole with- acids, for water is now 
 known to be formed in every case and eliminated 
 as a bye product. The essential components of 
 the idea of a base were in Lavoisier's time (i) 
 that it was an oxide, and (2) that it entered into 
 combination with an acid, tending to neutralise it. 
 The only substances which enter wholly into union 
 with acids are ammonia and its derivatives : these 
 are the only true bases, but they do not contain 
 oxygen. It is not very creditable to the leaders 
 among chemists that this state of confusion in a 
 comparatively important part of chemical nomen- 
 clature, otherwise reasonable and consistent, should 
 be allowed to continue. 
 
68 Teaching of Elementary Chemistry 
 
 As to ' acid,' everyone now admits that this 
 term is included under ' salt ; ' and to the question 
 ' what is an acid ? ' a complete and legitimate 
 answer would be ' a salt of hydrogen.' For- 
 tunately we now possess a criterion by which a 
 salt can be defined, no matter whether it be repre- 
 sented by the now obsolete dualistic symbol, or 
 by the unitary or by the more modern structural 
 formula. 
 
 Sodium sulphate, for example, is Na^O.SOj or 
 Xa^SO, or rXaO^SO^. 
 
 The question whether it is a salt is now re- 
 solved by another — Does it readily enter into double 
 decomposition, and is it an electrolyte ? By this 
 test we can at once distinguish salts from other 
 compounds which in formula resemble them ; 
 thus sodium chloride, XaCl, is a salt; carbon 
 chloride, CCI4, is not a salt. 
 
 On the other hand, if this criterion is accepted, 
 metallic oxides and hydroxides, the compounds to 
 which commonly the term base is applied, come 
 under the same category, caustic potash being 
 nearly as good an electrolyte as hydrochloric 
 acid. 
 
 These subjects are still undergoing investiga- 
 tion, and chemists are far from being united in the 
 interpretation of experimental results. The teacher 
 will do well to study current literature, and con- 
 sider carefully the facts for himself. 
 
 While, according to the experience of the 
 
Atomic Weights — Classification 69 
 
 author, it is unwise to communicate unsettled ques- 
 tions to mere beginners, as tending only to con- 
 fusion and adding to their difficulties, it is impera- 
 tive that all the facts leading up to any generalisa- 
 tion should be laid without reserve before advanced 
 students. They should be encouraged to judge 
 of evidence and draw conclusions for themselves, 
 being instructed, however, that, no matter how 
 firmly established or complete a theory ( = a view) 
 or hypothesis ( = supposition) may seem to be, it 
 is never final, and is certain to be sooner or later 
 absorbed into a more comprehensive system. One 
 of the greatest evils of working wholly by text- 
 books without personal contact with a teacher who 
 is master of his subject is that the student thus 
 taught invariably imbibes the idea that the subject 
 is complete, rounded off, and finished, and he sees 
 no room for further inquiry. This is not only 
 disastrous from the educational point of view, but 
 is a serious disadvantage to the student who learns 
 chemistry for the sake of its practical technical 
 applications. 
 
 One word in conclusion. Chemistry cannot be 
 learnt by reading alone. There comes a stage in 
 every student's career when much reading is very 
 necessary, but this is only reached after long 
 training of the eye and the hand. First of all 
 learn to see things clearly, and from first to last let 
 fact stand before all hypothesis. 
 
APPENDIX 
 
 EXTRACTS FROM THE SYLLABUS ISSUED BY THE 
 DEPARTMENT OF SCIENCE AND ART, 1895. 
 
 SUBJECT X.— IXORGAXIC CHEMISTRY, 
 THEORETICAL. 
 
 First Stage or Elemextary Course. 
 
 Chemical distinguished from physical changes. In- 
 destructibility of matter. Decomposible and undecom- 
 posible substances. Action of heated copper and 
 mercury on air. Action of heated iron on steam. Gas 
 formed by action of iron on steam passed over product 
 of heating copper in air gives water. Action of metals on 
 hydrochloric acid gas and upon solutions of hydrochloric 
 acid and sulphuric acid. Production of hydrogen. Pro- 
 duction of oxygen from red mercuric oxide. Formation 
 of water by combustion of hydrogen in air or by explo- 
 sion with oxygen in eudiometer. Identification of water 
 by its physical and chemical characters. Mixtures dis- 
 tinguished from compounds. Laws of definite and 
 multiple combination. Equivalents. Estimation of 
 equivalents illustrated experimentally by deposition of 
 such metals as silver or copper by zinc or magnesium and 
 weighing. Physical properties of gases as distinguished 
 from liquids and solids. Boyle's law. Law of expansion 
 
Appendix 7 1 
 
 of gases by heat (without problems). Phenomena of 
 gaseous diffusion. Molecules and atoms. Formation 
 and properties of products of burning carbon in excess of 
 air or oxygen. Comparison of gas so formed with gas 
 obtained by treating chalk &c. with acid. Composition 
 of the atmosphere and action of animal and vegetable life. 
 Systematic study of following elements and compounds, 
 their condition in nature, usual methods of isolation and 
 chief properties : hydrogen, oxygen, nitrogen, chlorine, 
 hydrogen chloride (bromine and iodine to be exhibited 
 and compared with chlorine), sulphur, sulphur dioxide, 
 sulphur trioxide, and sulphurous and sulphuric acids, and 
 a few of their most familiar salts, hydrogen sulphide, 
 nitric acid and chief nitrates, nitrous oxide (from ammo- 
 nium nitrate), nitric oxide (from nitric acid by copper), 
 ammonia, carbon and its two oxides, properties common 
 to acids in general. 
 
 Symbolic notation. Nomenclature. Formulse and 
 equations. Calculation of quantities by weight from 
 chemical equations (French system of weights). Division 
 of elements broadly into metals and non-metals. General 
 characters of metals. Basylous oxides and hydroxides, 
 such as lime, caustic soda, zinc oxide, black oxide of 
 copper, and litharge, and the interaction of these with 
 acids to foim salts and water. 
 
 Alternative First Stage or Elementary Course. 
 
 No substantial alteration has been made in this part 
 of the Syllabus, but a few corrections have been intro- 
 duced into the phraseology. 
 
 Second Stage or Advanced Course. 
 
 In addition to the subjects of the first syllabus of 
 the elementary course, students presenting themselves 
 
72 Teaching of Elementary Chemistry 
 
 for the advanced examination will be assumed to have 
 received instruction in the following : 
 
 The experimental methods by which the composition 
 of the following bodies has been accurately determined : 
 water, atmospheric air, hydrochloric acid, ammonia, the 
 gaseous oxides of nitrogen, the oxides of carbon, sul- 
 phuretted hydrogen. 
 
 Laws of gaseous combination of elements and com- 
 pounds. Reduction of gaseous volumes to standard 
 pressure and temperature. Calculation of quantities by 
 volume and by weight. 
 
 Atoms and molecules, atomic and molecular weights. 
 Law of Avogadro. Atomic and molecular formulse and 
 equations. Specific heat and atomic heat of the elements. 
 Graphic or constitutional formulae. Atomic value or 
 valency of the elements. Phenomena of dissociation. 
 Classification of the elements. 
 
 Water. — Causes of permanent and temporary hard- 
 ness in. Modes of softening. Suitability for domestic 
 purposes. Determination of composition of water by 
 weight and by volume. 
 
 Hydrogen Dioxide. — Its preparation and pro- 
 perties. 
 
 Ozone. — Production, properties and constitution. 
 Occurrence in nature. 
 
 Atmospheric Air. — Mode of ascertaining exact com- 
 position of. Carbon dioxide, amount contained in, and 
 determination of Aqueous vapour, determination of. 
 
 Chlorine. — Theory of bleaching. Composition and 
 preparation of bleaching powder. Oxides and the follow- 
 ing oxy-acids of chlorine, viz. : Hypochlorous, chloric 
 and perchloric acids, preparation, properties, and com- 
 position of. 
 
 Bromine, and hydrobromic and bromic acids. 
 
Appendix 73 
 
 Iodine, and hydriodic, iodic, and periodic acids. 
 
 Fluorine. — Hydrofluoric acid. 
 
 Phosphorus, its sources and its allotropic modifica- 
 tions ; its hydrides, chlorides, and oxides. Phosphoric 
 acids and the phosphates. Hypophosphorousand phos- 
 phorous acids. 
 
 Arsenic and its hydrides, chlorides, and oxides. 
 Arsenious and arsenic acids. Arsenites and arsenates. 
 Detection of arsenic. 
 
 Antimony and Bismuth. — Hydride, chloride and 
 oxides of antimony, chloride, oxides and salts of bismuth 
 to be studied chiefly with the object of showing the rela- 
 tions of these two elements to the members of the phos- 
 phorus group. 
 
 Boron, its occurrence and allotropic modifications. 
 Boron trioxide. Boric acid. 
 
 Silicon and silica. Silicic acid. Silicon hydride, 
 sihcon fluoride. Names and formulae of some of the 
 more important silicates (mineral). 
 
 The chief properties of the following metals, and the 
 composition and properties of their more important com- 
 pounds : potassium, sodium, ammonium, silver, mercury, 
 copper, zinc, cadmium, magnesium, barium, strontium, 
 calcium, tin, gold, aluminium, platinum, lead, chromium, 
 manganese, iron, cobalt, and nickel. 
 
 The processes of formation and crystallisation of the 
 chief salts of these metals should be practically de- 
 monstrated, and specimens should be prepared by the 
 students. 
 
 Manufacturing processes for the production of — • 
 
 Oxygen. 
 
 Chlorine, bromine, iodine. 
 Bleaching powder. 
 Sulphuric acid. 
 
 Hydrochloric acid. 
 Nitric acid. 
 
 Sodium carbonate and 
 caustic soda. 
 
 G 
 
74 Teaching of Elementary Chemistry 
 
 Lime. 
 
 Silver and gold. 
 
 Iron and steel. 
 
 Zinc. 
 
 Copper. 
 
 Aluminium. 
 
 Lead. 
 
 Sodium. 
 
 Mercury. 
 
 
 SUBJECT X./. -INORGANIC 
 
 CHEMISTRY, PRACTICAL 
 
 First Stage or Elementar'^' Course. 
 
 The practical knowledge of the candidate will in this 
 stage be tested both by a written and a practical exa- 
 mination. 
 
 L The subjects of the written examination will 
 include — 
 
 (fl) The preparation of the elements and compounds 
 enumerated in the elementary course of Sub- 
 ject X. (Theoretical), and the methods of ex- 
 perimentally demonstrating their properties. 
 (b) The principal reactions, both wet and dry, of the 
 following : lead, copper, iron, zinc, calcium, 
 potassium, ammonium, carbonates, nitrates, 
 sulphates, chlorides. 
 
 These questions will, as much as possible, be so 
 framed as to prevent answers being given by students who 
 have obtained their information merely from books and 
 oral instruction. Any student on whom it is intended to 
 claim payments in this stage may be called on by the 
 inspector of the Department, when visiting the Laboratory, 
 to repeat some of the experiments which he has had the 
 opportunity of witnessing. 
 
 The value of the answers will be greatly enhanced by 
 the neatness and clearness of the sketches ; provided 
 always, that an accurate knowledge of the construction 
 of the apparatus is exhibited. 
 
Appendix 7 5 
 
 II. The practical examination may consist of exercises 
 selected from any of the following divisions of experi- 
 mental work : 
 
 A. — Testing of two powders neithc^r of which will con- 
 tain more than two metaUic and oni' acid radicle from 
 the above list. The powders will be soluble in water or 
 in dilute acid. 
 
 The ejiperiments made must be carefully described, 
 and the analytical results clearly stated and confirmed by 
 more than one reaction. 
 
 No notes, books, or analytical tables may be consulted 
 during the examination. 
 
 B.— Carrying out experiments for which printed 
 instructions will be given in the paper, e.g. the observa- 
 tion of the effects of heat or of water, or acids &c. upon 
 the materials supplied. If a gas is evolved, the candidate 
 may be required to determine what it is. [Such a gas 
 will always be one of those referred to under Subject X., 
 First Stage, and of which the properties have been demon- 
 strated in the lectures.] 
 
 C. — Recognition of materials which have been 
 exhibited upon the lecture table, and the properties of 
 which have been demonstrated, e.g. sulphur, iodine, 
 charcoal, manganese dioxide, potassium chlorate, nitre, 
 sal-ammoniac, &c. 
 
 D. — Quantitative experiments of a simple kind, e.g. a 
 metal or a salt may be supplied to be weighed, dissolved 
 in acid, and the amount of evolved gas measured at the 
 temperature of the Laboratory. Or the carbon dioxide in 
 a carbonate may be estimated by solution in an acid, and 
 observation of the loss of weight. Or determination of 
 the weight of one metal, such as silver, which can be dis- 
 placed by another, such as magnesium. Or experiments 
 on the neutralisation of acids or alkalies. 
 
76 Teaching ojj Elenieniajy Chemistry 
 
 Second St^ce or Advanced Course. 
 
 The examinatior/ will consist of two parts : 
 
 I. A short writtf/i examination of about four questions, 
 with the object of 'testing the candidate's knowledge of 
 the theory of ordir.ary methods of qualitative analysis and 
 the preparation of such bodies as are enumerated in 
 Second Stage, Subject X. 
 
 II. A practical examination, in qualitative and very 
 simple quantitative analysis. One or two substances for 
 qualitative analysis, each containing not more than four 
 salt radicles, positive or negative, selected from the follow- 
 ing list : silver, lead, mercury, copper, bismuth, cadmium, 
 tin, arsenic, antimony, iron, manganese, aluminium, 
 chromium, zinc, cobalt, nickel, calcium, strontium, 
 barium, magnesium, potassium, sodium, ammonium, 
 oxides, hydroxides, chlorides, bromides, iodides, fluorides, 
 sulphides, sulphites, sulphates, chromates, carbonates, 
 phosphates, arsenates, borates, nitrates, nitrites, chlorates, 
 permanganates. 
 
 There might, therefore, be four metals in the form of 
 oxide, or three metals in the form of the same salt, or two 
 simple salts or one metal in the form of three salts. 
 
 One substance may be given to be tested quantita- 
 tively by means of a previously prepared volumetric solution. 
 
 The remarks with respect to inspection made in the 
 Elementary Stage apply also to this grade. 
 
 Three and a quarter hours will be allowed for the 
 examination in practical analysis, and one hour for the 
 written examination. 
 
 Note. — Candidates will not be allowed to communi- 
 cate with one another during the examination, but the 
 use of notes or text-books of analysis will be permitted. 
 
 Spottiswoode &" Co. Printers^ Neiv-street Square, London. 
 
(l(b