360 Dr. James Walker on the 



jihysical theory the reverse should be the case." Nowhere has 

 Arrhenius made any such statement : the deviation may be 

 in one direction or the other, depending on the nature of the 

 dissolved body and of the solvent. 



The objections urged by Mr. Pickering against the theory 

 of osmotic pressure in the Philosophical Magazine, xxix. 

 pp. 490-501, fall to the ground when we take into account 

 his neglect of electrolytic dissociation, his (then) ignorance of 

 the difference between Raoult's empirical value for the mole- 

 cular depression and the value deduced from van 't Hoff's 

 theory, and the odd selection of solvents to which Mr. Shaw 

 drew attention at the B. A. meeting (Report, 1890, p. 336). 

 We are given, for example, the molecular depression for water 

 dissolved in H 2 S0 4 ,H 2 0. One is inclined to look upon the 

 addition of water to this substance rather as the dilution of a 

 strong sulphuric-acid solution than as the dissolution of water 

 in a definite solvent. That such a hydrate as H 2 S0 4 ,H 2 can 

 be crystallized is no proof that it has previously existed as 

 such in solution. All experiments, for instance, have gone to 

 show that racemic acid and racemates do not exist in aqueous 

 solution although they are always obtained on evaporation. 

 Again, although ammonium chloride may be sublimed un- 

 changed, yet it does not exist as such in the vaporous state. 

 The fact, therefore, that certain hydrates can be obtained as 

 crystalline solids in nowise proves their existence in solutions 

 from which they separate ; so that Mr. Pickering's choice of 

 them for solvents renders his experimental results in their 

 application to the new theories worthless*. 



Mr. Pickering takes exception to the theory of dissociation 

 into ions on the ground that it " seems to be quite irrecon- 

 cilable with our ideas of the relative stability of various bodies, 



* In a footnote to the paper cited, p. 494, the following passage 

 occurs :— " Van 't Hoff's statement as to Raoult's position respecting the 

 high and low values of substances in water is certainly remarkable ; he 

 says (Phil. Mag. 1888, xxvi. p. 99), 'Raoult did not discover the exist- 

 ence of so-called normal [meaning small] molecular depression of freezing- 

 point and lowering of vapour-pressure until he investigated organic com- 

 pounds ; their behaviour is almost without exception regular.' As a matter 



of fact, Raoult investigated organic substances first " In this Mr. 



Pickering is perfectly accurate. The misunderstanding has arisen from a 

 somewhat ambiguous German sentence of which the above is a transla- 

 tion: it runs-- "so wurde auch die Existenz der sogenannten normalen 

 molekularen Gefrierpunktserniedrigung und Dampfdruckverminderung erst 

 aufgefunden, als Raoult sich den organischen Verbindungen zuwandte : 

 da eben tritt das normale Verhaiten fast ausnahmsios ein " (Zeit. physikal. 

 Chem. i. p. 501). The meaning is of course — "the existence of so-called 

 normal depression .... was not discovered until organic compounds were 

 investigated " (by Raoult). 



