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NA TURE 



[December 31, 1S91 



analogy, nevertheless theoretical speculations and practical 

 researches are indicated, which, in the long run, must 

 throw more light on the question. 



But, in spite of all this, the book is not satisfying. The 

 main objections which have been urged against the 

 physical theory still exist. 



To the fundamental question—" Is solution a physical 

 or a chemical process ? " — the answers are various. The 

 opening definition and much that follows seem quite 

 decisive on this point : " Solutions are homogeneous 

 7nixtures" 



Dissolved substances obey gaseous laws because 



" the molecules of the solvent in the interior of the solu- 

 tion act equally in all directions on each molecule of the 

 dissolved substance, these molecules are all free to move 

 as if there were, on the whole, no action upon them. 

 Hence it follows that the kinetic energy of the molecules 

 of. the dissolved substance is equal to that of the gas at 

 the same temperature." 



The deviations of concentrated solutions from the 

 simple gaseous laws are explained by the fact that in 

 such cases the osmotic pressure is high, and that " com- 

 pound gases of simple composition show marked devia- 

 tions from the gaseous laws at such pressures." 



The inference from such statements obviously is, that 

 solution is purely physical ;■ to the dissolved substance are 

 to be ascribed even the deviations from the gaseous laws ; 

 the solvent may be ignored. This is, indeed, the logical 

 outcome of the physical theory. 



On the other hand, evidence such as the following has 

 to be considered : — 



" Every liquid is capable of taking up every gas, and 

 combining therewith to form a homogeneous liquid or 

 solution. . . . Two classes of these gas-solutions are to 

 be distinguished. ... In cases belonging to the second 

 class, e.g. in a solution of hydrogen chloride in water, 

 we have sufficient grounds to assert that chemical change 

 occurs." 



The distinction drawn between crystalloids and colloids 

 js of the same order as the above : — 



" Those of the first group (crystalloids) dissolve in 

 water with more or less marked changes of temperature ; 

 they raise the boiling-points, lower the freezing-points, 

 and generally exert a marked influence on the properties, 

 of their solutions. The others (colloids) do not exhibit 

 all these properties : their solutions are mechanical 

 jnixtures rather than compotutds." 



Experiment has shown that the molecular weight of 

 the same dissolved substance, obtained by the Raoult 

 methods, varies in many cases with the solvent. In order 

 to make theory harmonize with practice, this explanation 

 is given : — 



" We know that iodine, sulphur, and many other sub- 

 stances exist in different molecular conditions. It is not, 

 then, to be wondered at that a definite substance should 

 exhibit different molecular conditions when dissolved in 

 different solvents. The different solvents act like different 

 temperatures or pressures." 



The notion of a passive solvent evidently does not here 

 apply. Even on making allowance for a loose use of the 

 terms mixture and compound, it is hard to see how these 

 latter statements accord with the ideas of the functions of 

 the solvent and dissolved substance derived from those 

 quoted previously. 



NO. 1 157, VOL. 45] 



That the book is a portion of a larger treatise is evident, 

 to its detriment, in several ways. One instance, which 

 can hardly escape observation, is the absence of any de- 

 tailed account of the support to the physical theory which 

 has been drawn from the electrolysis of solutions. At 

 first sight, it is difficult to conceive that, in a work on the 

 physical theory, of which the hypothesis of electrolytic 

 dissociation is an integral part, no mention should be 

 made of the quantitative estimate of the degree of dis- 

 sociation which has been derived from a study of electric 

 conductivity. The reason is, that electro-chemistry is 

 treated in Vol. 1 1, of the " Lehrbuch," and a second edition 

 of this volume is not yet published. It would have been 

 judicious to have delayed publication of this book till 

 portions of the subject of electro-chemistry could have 

 been included. 



It would have been desirable, it seems to us, to have 

 made some adequate reference to other theories which 

 have been put forward in explanation of the phenomena of 

 solution. The only statement which can be construed 

 into an allusion to the hydrate theory occurs when treat- 

 ing of the point as to whether or not a salt in aqueous 

 solution is united with its water of crystallization. And 

 here the question is somewhat contemptuously disposed 

 of:— 



" The endeavours of many investigators to find proofs in 

 favour of the existence in solutions of combined water of 

 crystallization have not led to results which can be 

 received without objection ; these endeavours may there- 

 fore be passed over." 



Fault might well be found on the score of incomplete- 

 ness with much of the evidence put forward in portions 

 of the book. The chapter on salt solutions is one 

 of the most striking ; it is, indeed, the only one which 

 has for its theme the dissociation hypothesis ; and bear- 

 ing in mind the contention which this hypothesis has 

 created, here if anywhere the matter put forward should 

 have been beyond criticism. Tables are given of com- 

 pressibilities, surface-tensions, viscosities, &c. ; while, to 

 begin with, these properties are not defined, and several 

 necessary details are omitted. Viscosity may be taken 

 as a special and perhaps the worst example. Two tables 

 are given with numerical values for viscosities. Whether 

 these are absolute coefficients in dynes or relative times 

 of transpiration is not stated. They are in reality relative 

 values, the transpiration time of water under the experi- 

 mental conditions being taken as unity. The numbers 

 in the first table are said to have been " determined with 

 half normal solutions, and referred to equivalent quantities 

 of salt." The meaning of this rather redundant sentence 

 is not quite clear. As a matter of fact, the observa- 

 tions were taken with half normal solutions, and referred 

 to normal solutions by means of Arrhenius's formula con- 

 necting viscosity with concentration. Nothing whatever 

 is said about the strength of the solutions used for the 

 observations in the second table. They also relate to 

 normal solutions, and were obtained by a similar but not 

 identical method. The most important omission, however, 

 and one occurring in the case of other properties, is that 

 of the temperature of observation. When it is remem- 

 bered that at 100° the viscosity coefficient of water is only 

 one-fifth what it is at o", the influence of temperature on 

 viscosity is apparent. The difficulty in attempting to 



