January 28, 1892 



NATURE 



293 



explanation of phenomena whenever they are properly 

 understood. It will be urged by some that such informa- 

 tion is out of place in a practical manual, and that it 

 belongs to theoretical works and lectures on the subject ; 

 in our experience, however, the great difficulty in labora- 

 tory teaching is to make the student associate his prac- 

 tical work with what he hears in the lecture-room or 

 reads in his study, and that it is only by continually 

 drawing his attention to the bearing of his experiments 

 that the latter are made to have any great educational 

 value. In short, it is only in this way that the instruction 

 in a chemical laboratory materially differs from that ob- 

 tainable in a kitchen, and that a work on practical 

 physiological chemistry will be raised above the level of 

 a hand-book on cookery. By amplifying their work on 

 the lines indicated, we believe that its value to the 

 student would be much enhanced, for in its present form 

 it can only be used to much purpose under the guidance 

 of an accomplished and energetic demonstrator. 



In conclusion, vve must point out an error which should 

 hardly occur in a work professing to be the result of ex- 

 perience, still less in a second edition. On p. 29 the 

 student is told to prepare lactic acid as follows : — " Place 

 50 c.c. of milk in a warm chamber for several weeks 

 until it becomes strongly acid. Shake the ether, and 

 decant the ethereal extract. Evaporate the ether, add 

 extract residue with water. It is strongly acid, and 

 yields crystals \sic\ of lactic acid." The passage ob- 

 viously contains several printers' errors, but the crystalline 

 nature of lactic acid is new to us. 



OUR BOOK SHELF. 



Problems in Chemical Arithmetic. By E. J. Cox, F.C.S. 



Pp. 76. (London: Percival and Co., 1891.) 

 This book contains a series of arithmetical examples 

 chosen to meet the requirements of the examinations 

 held by the Science and Art Department in the ele- 

 mentary stage of chemistry. There does not seem to be 

 any outstanding feature to distinguish the book from 

 others of its kind ; indeed, setting aside the actual exer- 

 cises, which may be useful to the teacher, the explana- 

 tions of the principles involved in the calculations are, 

 as a rule, meagre, and frequently inaccurate. Informa- 

 tion such as the following is, to say the least of it, 

 faulty : " Whatever may be the weight of any given 

 volume of water, an equal volume of mercury under 

 similar conditions will be 136 times as heavy." This 

 conclusion was made to follow as a result of an apparently 

 practical method of obtaining specific gravity, although 

 no mention whatever was made of temperature or its 

 effects in the description of the process. 



The author keeps on repeating, without any qualifying 

 clause, that the formula of a compound represents the 

 molecule of the compound ; and the student is led to 

 infer (pp. 19 and 20) that the empirical formula as found 

 by analysis serves to fix molecular weight. As a con- 

 sequence of the above idea, several problems are set — 

 such as, Find the molecular weight of starch, of fluor- 

 spar, &c. — which are misleading, since they cannot be 

 solved in practice. 



The relationship between vapour-density and mole- 

 cular weight is given, but the use of the former as a 

 means of determining the latter is not even hinted at ; 

 and the author prefers to find the value of the ratio, 

 molecular weight to vapour-density, by the use of 

 numerical examples, rather than by a general process 



NO. I 161. VOL. 45] 



of reasoning, although all the necessary points have 

 previously been stated. 



The examples include a series selected from the De- 

 partment examination papers, and a table of contents 

 and list of answers are included. 



If the working of elementary problems in chemistry is 

 to be an intellectual process, founded upon an apprecia- 

 tion of fact and theory, which may be supplemented but 

 has not to be corrected by the student as he progresses, 

 books such as the above fail to fulfil this end. 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex' 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of ^ \'W&jl. 

 No notice is taken of anonymous communications.] 



The Theory of Solutions. 



In Nature of December 31 {p. 193) occurs a review by 

 "J. W. R." of my book on "Solutions," which gives me 

 occasion to enter, in a few words, upon the questions there 

 brought forward, and to set right some errors, which have 

 recently appeared ia other places as well. 



First, of all I wish to express to Mr. J. W. R. my cordial 

 thanks for the thorough and careful manner in which he has 

 made himself acquamted with the contents of my book. I 

 have no intention to discuss the objections made, some of which 

 I am quite willing to recognize as well founded, but to make 

 clear one important question in which I do not seem to be 

 properly understood by my critic. 



Mr. J. VV. R. begins his discussion with the words, " To 

 the fundamental question — ' Is solution a physical or a chemical 

 process?' — the answers are various" ; and out of this variety 

 he evidently finds against me a reproach. I have intentionally 

 neither set up this question nor sought to answer it, for I hold it 

 to be unclear and therefore very harmful. To the question, 

 " Is gas-formation a chemical or a physical process?" would be 

 answered, " In certain cases, as in the development of carbon 

 dioxide out of champagne, a physical one ; in others, as the 

 development of carbon dioxide from limestone, a chemical 

 one ; and in many cases, as in the development of hydrogen 

 from palladium hydride, one would be in doubt what to 

 answer." The question set up is faulty in implicitly assuming 

 that solution must be either a physical or a chemical process, 

 and by this prepossession he is hindered from recognizing that I 

 was entirely justitied in placing the physical or chemical side of 

 the question in the foreground according to the nature of the 

 case. However, in case Mr. J- W. R. is not satisfied with this 

 explanation, and insists upon setting up this question, I must 

 postpone further discussion upon it until he shall have given 

 me a sufficient definition of the ideas "physical" and 

 "chemical " processes, and of their distinction. I know of no 

 such definition, and have consequently not made use of the 

 expressions. 



From the definition of solution given by me, Mr. J. W. R. 

 concludes that I am a representative of the " physical theory" 

 of solutions, in contrast to which he places the "chemical 

 theory." I cannot repeat energetically enough that I have 

 never recognized such a contrast, and that I cannot at all admit 

 the existence of such a contrast. It has never been maintained, 

 either by me or by any other representative of the newer theory 

 of solutions, that no interaction takes place between the solvent 

 and the dissolved substance ; on the contrary, I have for years 

 directly encouraged research work directed towards making clear 

 the nature of such interactions. What distinguishes the new 

 theory of solutions, founded by van 't HofiF, from the others is 

 that it has succeeded in discovering and bringing into connection 

 a series of properties of solutions, which can be treated entirely 

 independently of the question of a possible interaction between the 

 parts of the dissolved substance and of the solvent. 



All these properties hang together with the fact that in the 

 making of a solution or in the altering its concentration there is 

 developed or absorbed a definite amount of free or available 

 energy, which is equal for equimolecular quantities of different 

 substances, and is independent of the nature of the solvent. The 

 amount of this free energy is the same as in the analogous 

 processes with gases. These are purely experimental facts. 



