WORK OF VAn't HOFF BRUNI. 781 



ebullition, and freezing of solutions — laws which are the necessary 

 consequence of this theory — have been and still are to-day verified 

 with the necessary accuracy. The thousands and thousands of 

 ebulliscopic and, above all, cryoscopic determinations which have 

 been and are daily being made in laboratories represent so many 

 confirmations and place the theory on so solid a foundation, that no 

 critic, however keen he may be, will be able to shake it. 



It can not be denied that our ideas on the nature of solutions — 

 of concentrated solutions in particular — and on the state of the dis- 

 solved substances have taken a new direction in the last few years 

 and are tending to-day to return to a theory of hydrates or, in 

 general, of solvates; that is, to admit the existence of combination 

 of the molecules of the solvent with the molecules of the dissolved 

 ions. But there is nothing in that which would conflict with the 

 theory of van't Hoff ; indeed, it is strong partisans of the latter who 

 have inaugurated the new movement. 



In this respect, again, the theory of van't Hoff is a limit theory; 

 it corresponds to the purely physical conception of the solution in 

 which the solvent has no other function than that of diluting, of 

 separating from one another, the molecules of the substance dissolved ; 

 reality differs more or less from this scheme. The deviations have 

 an insignificant effect only in the case of very dilute solutions and of 

 especially indifferent solvents; in default of these circumstances the 

 deviations cease to be negligible. 



It often happens in the history of science that at a certain time 

 two theories seem absolutely opposed to each other; one of them 

 emerges from the struggle victorious. In the course of tune, how- 

 ever, it is perceived that the contradiction was not by any means as 

 necessary as had been believed at first, but that each of the two theo- 

 ries represented an extreme and too simple solution of the problem, 

 and that the vanquished theory itself contained germs of truth, sus- 

 ceptible of development and adaptation. 



As Walden remarlced, one might have believed at Leeds 20 years 

 ago that a chemical theory of hydrates was irreconcilably opposed 

 to a physical theory of solutions ; to-day Pickering can have the sat- 

 isfaction of seeing rejuvenated his idea of combinations with the 

 solvent; but far from appearing as a negation, this idea appears 

 rather as a useful extension of the views of van't Hoff. The credit 

 for having propagated this new movement belongs to Ciamician, 

 who, as early as 1891, proposed admitting the fonnation of such 

 solvates to explain electrolytic dissociation. 



Permit me finally to draw your attention to the influence that the 

 most recent study on the Brownian movement and on the nature of 

 colloids has exercised on the questions that we are discussing. 

 These brilliant researches have established the continuous passage 



