74 



HARRY C. JONES 



he wanted to know something about the dynamics of a chemical 

 reaction. 



While in this state of mind, and while pondering over the nature 

 of chemical reactions, especially in solution, he met his colleague 

 in the University of Amsterdam, Hugo DeVries, then botanist 

 there, a man who has since become very famous, as is well known 

 to this company. DeVries thought that Van't Hoff might ob- 

 tain some aid in solving the problems that he had in mind from the 

 work of Pfeffer on the osmotic pressure of solutions of cane sugar 

 and a few other substances, which, as is well known, was pub- 

 lished as a monograph in 1877 under the title "Osmotische Unter- 

 suchungen." 



This was the introduction of Van't Hoff to the work of Pfeffer, 

 and this introduction marks an epoch in the development of the 

 science of chemistry. Let us now see why. 



Pfeffer had measured the osmotic pressure of certain solutions 

 of cane sugar and a few other compounds, to learn the order of 

 magnitude of this force. He had gone at the problem as a plant 

 physiologist would do, and had solved it from this standpoint. 

 He measured more accurately than any one else had done the mag- 

 nitude of osmotic pressure, and this was his aim. "WTiere Pfeffer 

 left this problem was where Van't Hoff took it up. 



Van't Hoff first observed from Pfeffer's data that there is a 

 very simple relation between osmotic pressure and the concen- 

 tration of the solution exerting it. This can be seen from the 

 following data: 



PEHCENTAGE CONCENTRATION 

 OF CANE SUGAR 

 C 



It will be observed that ^ is a constant, which means that 

 osmotic pressure is proportional to the concentration of the solu- 



