<SS I'RIXCIPLKS OF rilKMJSTRY 



solved, 14 showing the existence of a chemical action between the solvent 

 and the substance dissolved which is of the same nature as in all other 

 forms of chemical relation. 1 ' 



Although an alteration of the external pressure does not usually 

 decompose solutions of solids, nevertheless the feeble development of 



*' Inasmuch us the decree of change exhibited in many properties on the formation of 

 solutions, is not large, so. owing to the insuflii-ient ac-curacy of observations, a proportion- 

 ality between this change and a change of composition may, in a first rough approximation 

 and especially \vithin narrow limits of change of composition, easily be imagined in cases 

 where it does not even exist. The conclusion of Michel and Kraft is particularly instruc- 

 tive in this respect: in lsf>4. on the basis of their incomplete researches, they supposed 

 the increment of the specific gravity of solutions to be proportional to the increment of 

 a salt in a given volume of a solution, which is only true for determinations of specific 

 gravity which are exact to the second decimal place an accuracy insufficient even for 

 technical determinations. Accurate measurements do not confirm a proportionality 

 either in this case or in many others where a ratio has been generally accepted ; as, for 

 example, for the rotatory power (with respect to the plane of polarisation i of solutions, and 

 for their capillarity, Arc. Nevertheless, such a method is not only still made use of, but 

 even has its advantages when applied to solutions within a limited scope as, for instance, 

 very weak solutions, and for a first acquaintance with the phenomena accompanying 

 solution, and also as a means for facilitating the application of mathematical analysis to 

 the investigation of the phenomenon of solution. Judging by the results obtained in my 

 researches on the specific' gravity of solutions, I think that in many cases it would be 

 nearer the truth to take the change of properties as proportional, not to the amount of a 

 substance dissolved, but, to the product of this quantity and the amount of water in 

 which it is dissolved; all the more so as many chemical re'ations vary in proportion to 

 the reacting masses, and a similar ratio has been established for many phenomena of 

 attraction studied by mechanics. This product is easily arrived at when the quantity of 

 water in the solutions to be compared is constant, as is shown in investigating the fall of 

 temperature in the formation of ice (nee footnote 41), p. IK)'. 



'' All the different forms of chemical reaction may be said to take place in the process 

 of solution, il \ CinnbiiKiiiona between the solvent and the substance dissolved, which 

 are more or less stable (more or less dissociated). This form of reaction is the most 

 probable, and is that most often observed. ('2 1 Reactions of substitution or of double, 

 ih-coiHjioxitiun between the molecules. Thus it may be supposed that in the solution of 

 sal-ammoniac, XII, Cl. the action of water produces ammonia, NH.,HO, and hydrochloric 

 acid. HC1. which are dissolved in the water and simultaneously attract each other. As 

 these solutions and many others do indeed exhibit signs which are sometimes indispu- 

 table of similar double decompositions (thus solutions of sal-ammoniac yield a certain 

 amount of ammoniai. it is probable that this form of reaction is more often met with 

 than is generally thought. (Mi Reactions of ixuHK'nmit or rcylnceiui'iit are also probably 

 met with in solution, all the more as here molecules of dim-rent kinds come into intimate 

 contact, and it is very likely that the configuration of the atoms in the molecules under 

 these influences is somewhat different from what it was in its original and isolated 

 state. One is led to this supposition especially from observations made on solutions of 

 substances which rotate the plane of polarisation land observations of this kind are very 

 sensitive with respect to the atomic structure of molecules), because they show, for 

 example (according to Schneider, iH.slj, that strong solutions of malic acid rotate the 

 plane of polarisation to the right, whilst its ammonium salts in all degrees of concentra- 

 tion rotate the plane of polarisation to the left. (4 1 Reactions of <li-<-<>nij>u.ii(H>n under 

 the influences of solution are not only rational of themselves, but. have in recent years 

 been recognised by Arrhenins, Ostwald. and others, particularly on the basis of electro- 

 lytical determinations. If a portion of the molecules of a solution occur in a condition of 

 decomposition, the other portion mav occur in a yet more complex state of combination, 



