April 4, 1895] 



NA TURE 



531 



(7) electrolytic dissociation, (8) the physical properties 

 of salt solutions, (9) colloidal solutions, and (10) the 

 absolute size of molecules. Sections three and four are 

 concerned with the subject of affinity. The author treats 

 that part of this subject which is connected with the 

 transformations of matter under the headings : (i) the law 

 of chemical mass-action, (2) chemical statics in (a) homo- 

 geneous, and {h) heterogeneous systems, (3) chemical 

 equilibrium in salt solutions, and (4) chemical kinetics. 

 In dealing with the energetics of chemical affinity. Prof. 

 Nernst treats of (i) the applications of the first law of 

 heat, (2) temperature and (a) complete, {b) incomplete, 

 chemical equilibrium, (3) temperature and the velocity 

 of reactions, (4) heat and chemical energy, (5) electro- 

 chemistry, (6) photochemistry. An appendix contains 

 accounts of the more important developments of 

 theoretical chemistry in the year 1893. 



The ground covered by the book is evidently very 

 large, and very different from that traversed by the 

 books on theoretical chemistry published ten years ago. 

 Theoretical chemistry, as understood by Nernst, is 

 based on Avogadro's law, and van't Hoffs extension of 

 that law to dilute solutions, the law of mass-action, and 

 the laws of thermodynamics. It would not be very far 

 from the truth to say that the book is concerned with 

 the meaning of the sign of equality in chemical equations, 

 and that the connotation of that sign is elucidated by 

 applications of the laws of Avogadro and van't Hoff, and 

 the law of mass-action, bound together and lighted up by 

 the laws of thermodynamics. 



The essential characteristics of the book, so far as I 

 can judge, are the exceeding clearness in the statement 

 of each problem of theoretical chemistry, the cutting out 

 of irrelevant issues, and then the binding together of the 

 apparently detached discussions into an harmonious 

 whole by the application of the general principles of the 

 molecular-atomic theory, and of chemical energetics. 



The treatment of osmotic pressure ,may be chosen as 

 an illustration of these characteristics of Prof. Nernst's 

 book. The meaning of the term osmotic pressure is 

 made clear on pp. 118-119, by the description of a theo- 

 retical experiment on the diffusion of sugar in aqueous 

 solution through a semi-permeable partition fitted with 

 a movable piston. Unfortunately, the translator has 

 rendered Losnngsmittel by " solvent material " ; had he 

 used the term " solvent " (as he does in some other 

 passages), the meaning would have been better conveyed. 

 Then follow lucid paragraphs on the methods of measur- 

 ing osmotic pressure (I wish the translator had not 

 allowed the phrase to pass, " ether containing consider- 

 able benzene"), admirably illustrating the bearings on 

 this problem of determinations of vapour-pressures, boil- 

 ing points, and freezing points. Having thus, by adher- 

 ing strictly to the problem under discussion, arrived at 

 a clear conception of osmotic pressure, the author pro- 

 ceeds, in a few brief and clear paragraphs, to consider 

 the circumstances which condition the osmotic pressures 

 of solutions, viz. the concentration of the solution, the 

 temperature, the nature of the dissolved substance, and 

 the nature of the solvent. The outcome of the matter is 

 then summed up in the statement — the lowering of the 

 freezing point of a dilute solution is proportional to the 

 number of molecules of the dissolved substance. This 

 NO. 1327, VOL. 51] 



summary at once iiiggesls an inquiry into the range of 

 applicability of the law of osmotic pressure, and the laws 

 of solution in general ; and the inquiry leads to a state- 

 ment of van't Hoffs extension of the law of Avogadro. 

 The experimental det eimination of molecular weights by 

 the application of the \an't Hoff generalisation is de- 

 scribed (pp. 224-231) in that part of the book which deals 

 with the determination of molecular weights. When the 

 author comes to treat of dissociation he returns to the 

 subject of osmotic pr essure, and shows how an appli- 

 cation of the hypothesis of dissociation to salts in dilute 

 aqueous solutions leads to a far-reaching theory of 

 chemical change, and he propounds this theory in a clear 

 and practical manner. In one of the chapters dealing 

 with thermochemistry (pp. 565-567), the authcr, follow- 

 ing van't Hoff, b rings the subjects of osmotic pressure 

 and electrolytic dissociation within the range of thermo- 

 dynamical methods, and, by a fundamental equation, 

 connects the e quilibrium of a chemical system with such 

 conditions as temperature, pressure, and dissociation. 

 Had Prof. Nernst been tempted to discuss such a side 

 issue as the part played by the solvent in bringing about 

 electrolytic dissociation, he could not have arrived at the 

 very general results to which his strictly accurate and 

 limited method of inquiry have led him. He does, indeed, 

 devote a paragraph or two to this matter of the action of 

 the solvent {e.g. pp. 232, 444). He is careful to note the 

 great interest and importance of the question ; at the 

 same time he draws attention to the fact, often over- 

 looked, that no definite answer can be given to questions 

 regarding the existence of compounds of molecules of 

 the solvent with molecules of the dissolved substance by 

 the study of the osmotic pressures of dilute sclutions, 

 because the existence of such compounds would not 

 affect the os motic pressures of fairly dilute solutions. 



Prof. Nernst dismisses the so-called hydrate theory 

 of solution in a short and somewhat contemptuous para- 

 graph (p. 444) ; he speaks of this conception as having 

 no theoretical basis, as having led to no general laws, 

 and as based on an uncritical examination of experi- 

 mental data. 



A very admirable feature of the book is the care 

 taken to warn the student against drawing unsound and 

 inapplicable deductions from sound generalisations. For 

 instance, at the beginning of the chapter on the physica 

 prop erties of salt solutions (p. 331), the statement is 

 enun ciated that the properties of an aqueous solution 

 of a salt arc made up, additivcly, of the properties of 

 the free, ions. And then the illegitimate and misleading 

 inferences which may be, and some of which have been, 

 deduced f rom this generalisation are noted in a few 

 sentences, before the true meaning and applicabiHty of 

 the statement are developed. 



I s hould like to deal at length with the author's treat- 

 ment of the law of mass-action, and the many appli- 

 cations and developments of this law ; but space forbids. 

 Among the applications of the law of mass-action I 

 would ask the student to note that which serves to explain 

 the discrepancies among the values for the strengths 

 of acids obtained by Ostwald by employing different 

 methods of measurement (p. 438). 



The volume literally abounds in suggestions ; new 

 light is thrown on almost every question of theoretical 



