WORK OF VAN"^T HOFF BRUNI. 777 



Duhem pitilessly remarked, one must in order to save this principle 

 admit also, to the ranks of external energy, the heat absorbed in 

 endothermic processes. The principle in question would amount, 

 then, to the following statement : " Every process which does not ab- 

 sorb heat develops it"; that is, in order to remain true, it must 

 "vanish into a ridiculous tautology." 



Now, the principle of van't Hoff gives us the key to these contra- 

 dictions ; since a lowering of temperature favors the processes whose 

 accomplishment is accompanied by a development of heat, it is the 

 exothermic reactions which must be produced by preference at the 

 low temperatures. And as the ordinary conditions of temperature 

 of our surroundings and of common chemical operations represent 

 zones sufficiently low in the complete scale of possible temperatures, 

 it is natural that the rule of Thomsen should be verified in them, 

 with a first approximation. Berthelot's principle would become rig- 

 orously true only at absolute zero. But to the high temperatures 

 correspond principally the endothermic processes, and chemists living 

 at a temperature of 3,000 degrees would sooner formulate a principle 

 of miniotium work * * *. 



From this time on the work of van't Hoff is divided into two 

 branches. While, following, as he will of course do, in addition, all 

 the rest of his life the experimental research of heterogeneous equilib- 

 riums, and particularly of condensed systems, he devotes the best of 

 his activity in the field of theory to the study of another fundamental 

 problem — the theory of dilute solutions and the investigation of the 

 molecular state of dissolved substances. 



The theory of solutions, in its general lines at least, is now so well 

 known that it would be superfluous to examine it here, but it may 

 be interesting to consider its genesis. It would not be difficult to fol- 

 low its course through the work of our scientist ; but he has himself 

 taken pains to make a clear exposition of it in a discourse delivered 

 before the Chemical Society of Berlin in 1890: "Wie die Theorie 

 der Losungen entstand." One must seek its origin in the " Etudes " 

 above referred to. He tries to find out the affinitj^ which keeps water 

 in solutions and in hydrated salts, and as Mitscherlich had previously 

 done, he thought of founding a measure of it in the diminution of the 

 vapor tension of these systems as compared with pure water ; but the 

 absolute value of these differences seems to him too slight in compari- 

 son with the strength which he felt even the smallest chemical forces 

 shall have. He then asks himself if this attraction of water can not 

 be measured in a more direct manner. With this question on his lips, 

 as he himself relates, he comes out of the laboratory one day and 

 meets his colleague de Vries. The latter, who was then busy with 

 osmotic experiments, puts him in touch with the classic researches of 

 Pfeffer on the direct measurement of osmotic pressure. And there 



