780 ANNUAL BEPOKT SMITHSONIAN INSTITUTION, 1913. 



tial ally in the person of Sir William Ramsay, their contemporary, 

 who was already known through important studies, though he had 

 not yet given all he was to give in publications, which since have 

 made his name so universally famous. 



From this moment the success of the new school was rapid and tri- 

 umphant, the opposition which from all sides had been made against 

 it was soon almost completely appeased; to be sure it did not en- 

 tirely disappear, but it was only whispered, rather than expressed 

 openly and supported by definite arguments. 



The new theories were soon introduced even into elementary trea- 

 tises, and now they have penetrated and transformed all branches of 

 chemistry. 



Discussions on the theories of van't Hoff have, however, been again 

 renewed during recent years, both from theoretic and experimental 

 points of view. An American physical chemist, Kahlenberg, after 

 having carried out some measurements of osmotic pressure, claims to 

 have arrived at the conclusion that the theory of van't Hoff is insup- 

 portable. But so radical a conclusion is little in accord with the 

 modest experimental data on which it is based, data which, besides, 

 have been contradicted by other writers, among them Cohen, one of 

 the first pupils of van't Hoff. Another American, Morse, has shown 

 in a series of much more exact and thorough experimental works 

 that osmotic pressure follows the laws of van't Hoff up to a quite 

 high degree of concentration. Scarpa has confirmed the fact that its 

 variations with temperature follow the law of Gay-Lussac. 



Other chemists and physicists who have not thoroughly studied 

 the work of the master or who are acquainted with it only at second 

 hand have attempted to criticize the ideas of van't Hoff concerning 

 the mechanism of osmotic pressure. But that is like fighting wind- 

 mills. It is true that van't Hoff has sometimes made allusion to a 

 kinetic conception of solutions the parallelism of which with gases 

 he had shown; but although he has had recourse to this conception 

 in didactic exposition, he never used it to deduce his laws. Indeed, 

 he has always brought out the fact that the mechanism of osmotic 

 pressure and the manner of action of the semipermeable wall have 

 no influence on the deduction and the development of the theory. 



The truth is that the theory of dilute solutions (many forget this 

 adjective on which van't Hoff had always laid stress) represents 

 a limit law, like many other great natural laws, the value of which 

 no one has ever, on that account, denied. The truth is, also, that 

 the perfect semipermeable membrane is an ideal object impossible 

 ■ to attain and even now extremely difficult to approach. 



But even if this membrane were a pure abstraction the theor_y of 

 it would be none the less true, since all the laws of evaporation. 



