THE BEARING OF OSMOTIC PRESSURE 



75 



tion. This is in itself not surprising. To say that the osmotic 

 pressure of a solution is proportional to the concentration of the 

 solution, is simply to say that two molecules in solution exert 

 just twice the osmotic pressure of one molecule, and there are 

 many similar properties of substances — the one being a linear 

 function of the other. 



To an average mind this would mean nothing more than that 

 the one propert}' of the dissolved substance was proportional 

 to the other. But Van't Hoff's mind was not of the average 

 type. Indeed, very far from it. In looking over the history of 

 chemistry I can find no name which stands for more in the develop- 

 ment of scientific chemistry than J. H. Van't Hoff. Indeed, it is 

 a question in my mind whether anyone else has contributed quite 

 so much in so manj^ different directions. Van't Hoff is not only 

 one of the very greatest chemists of all time, but one of the great- 

 est men of science of his century. It is well known that he died 

 at the early age of fifty-nine in Berhn, somewhat more than a 

 year ago. 



p 



In the ratio — equals a constant, Van't Hoff saw an analogy to 



the law of Boyle for gases. This law is usually stated that the 

 pressure times the volume of a gas is a constant. But since the 

 concentration of a gas, or the number of parts in a given space, 

 varies inversely as the volume, we can say that the pressure of a 

 gas divided by the concentration is a constant. In a word, the 

 pressure of a gas varies as the concentration. 



These two properties, the pressure of a gas and the osmotic 

 pressure of a solution, might both obey Boyle's law as they do, 

 and yet have nothing very fundamental in common. There are 

 many properties of substances in nature which vary the one with 

 the other. 



Van't Hoff went farther than this. If there is any real relation 

 between solutions and gases, then other laws of gas pressure must 

 apply to the osmotic pressui'e of solutions. Let us see whether 

 this is true. 



Another well known law of gas pressure is that of Gay-Lussac. 

 This states that the pressure of a gas increases tts for every 



