96 THE POPULAR SCIENCE MONTHLY 



which lias heen the philosophy of organic chemistry for the past 

 thirty-five years, and the guiding thought in practically all of the best 

 work in organic chemistry from 1874 to the present time. 



The second great work of Van't Hoff had also to do primarily with 

 the chemistry of carbon. In 1867 the Norwegian physicist, Guldberg, 

 and his son-in-law Waage, the chemist, both of Christiania, announced 

 the law of the effect of mass or quantity on chemical reaction — the law 

 of mass action. This was published in the " Announcements " of the 

 University of Christiania, and very little attention was paid to it for 

 some time. Guldberg and Waage applied their law to comparatively 

 few reactions. 



Van't Hoff, shortly after the publication of his brief paper of eleven 

 pages in Dutch, on " The Arrangement of the Atoms in Space," took 

 up experimentally the study of the velocities of chemical reactions and 

 the conditions of chemical equilibria, from the standpoint of the law of 

 mass action. He, his assistants and students, carried out an elaborate 

 series of investigations in which the law of mass action was applied to 

 a large number of chemical reactions, and shown to hold. The results 

 of this work were published in French, under the French equivalent of 

 " Studies in Chemical Dynamics." In this work the whole subject of 

 chemical dynamics and chemical equilibrium was placed upon a scien- 

 tific basis, and for the first time. 



The third and greatest work of Van't Hoff had to with the relation 

 between solutions and gases. Through his colleague — the botanist, De 

 Vries, the attention of Van't Hoff was called to the osmotic pressure 

 measurements that had been made by the botanist Wilhelm Pfeffer. A 

 comparison of the results obtained by Pfeffer with the gas pressures 

 exerted by gases containing the same number of gaseous molecules in a 

 given volume that the solution contained dissolved molecules in the 

 same volume, showed that the gas-pressure was exactly equal to the 

 osmotic pressure- — in a word, the laws of gas-pressure apply to the 

 osmotic pressure of solutions. 



Van't Hoff showed that the laws of gas-pressure apply to the os- 

 motic pressure of solutions of non-electrolytes, i. e., those substances 

 whose aqueous solutions do not conduct the current. He also pointed 

 out that the laws of gas-pressure do not apply to the osmotic pressure 

 of a single electrolyte — a single acid, base or salt. Arrhenius explained 

 the apparent discrepancy in the case of electrolytes by means of the 

 theory of electrolytic dissociation, which says that acids, bases and 

 salts in aqueous solution are broken down into charged parts or ions. 



The question arises why is it so important to have shown that the 

 laws of gas-pressure apply to the osmotic pressure of solutions? We 

 know more about matter in the gaseous state than in any other state 

 of aggregation. We can deal with gases from the standpoint of the 



