206 KEPOET — 1886. 



periments were conducted with solutions containijig one equivalent in 

 grammes of the base or acid in one litre of the solution . Consequently 

 the. salt solutions obtained on neutralisation contain one equivalent in 

 grammes of the salt in the two litres. These solutions, though not 

 strictly comparable, are still nearly so, and are of the approximate 

 strength, MR. IIOH^O. 



The results' obtained by Ostwald are as follows : — 



(a) When a solution of a base (potash or soda) is neutralised by the 

 requisite amount of the solution of an acid (fourteen organic and inorganic 

 acids), the difference between the sum of the refractive indices of the 

 two solutions before mixing and twice the refractive index of the result- 

 ing salt solution is a value almost identical for both bases, no matter 

 what may be the acid. There is thus complete parallelism between the 

 change of refractive index and of molecular volume on neutralisation. 



(/3) The conclusion is tliat the alteration in the physical constants, 

 brought about by combination, has a constant value for each constituent 

 which enters into the combination, and is therefore independent of the 

 other constituents with which the first may combine. 



Thus there is little room for doubt that in other cases also alteration 

 in molecular volume will be accompanied by parallel changes in the refrac- 

 tion equivalent. Unfortunately, Ostwald's results are not of a form to 

 permit their conversion into refraction equivalents, or it would be pos- 

 sible to show, even more clearly, the close connection between these 

 physical constants. 



III. Saturation of Salt Solutions.^ 



It has long been known that a salt is able to drive another out of 

 solution in very many cases, partially or completely, while in other 

 cases the solubility of one or both salts is largely increased. 



When the two salts are capable of forming well-defined double salts, 

 then either salt added to a saturated solution of the other completely 

 expels it from solution. On the other hand, when the two salts are iso- 

 morpJwus, and are thus able to form mixed crystals, then the expulsion 

 from solution is onlj partial (Rudorff. Wiedem. ' Annalen,' xxv. 626). 



This may be explained as follows : — 



Double salts do not exist as such in solution ; the saturated solution 

 of a double salt is therefore not necessarily saturated for either of its 

 constituents, but may be able to dissolve more of one or other of the 

 single salts. As the amount, however, of this salt increases there 

 arrives a point at which the solution has become so rich in this salt (B) 

 that any one molecule of the other salt (A) may be regarded as being in 

 contact with a molecule of B ; aggregation or combination to form the 

 double salt (AB) is then possible, and crystallisation proceeds jjari passu 

 with the solution of B, and results finally in the complete expulsion of A 

 from the solution in cases where the attraction between A and B exceeds 

 the cohesion of either A or B. While, on the other hand, if this is not 

 the case the expulsion is only partial. This explanation is strongly 

 supported by the stability and definite character of the well-defined 

 double salts, which are totally expelled from solution, and by the insta- 

 bility of those salts which are only partially expelled, and also by the 



« Published Phil. Mag. January 1886. 



