THE BEARING OF OSMOTIC PRESSURE 



77 



than the warmer. The results of experiment agree so closely 

 with the result of calculation that we can say that osmotic pres- 

 sure and gas pressure have the same temperature coefficients, 

 or, in other words, the law of Gay-Lussac applies to the osmotic 

 pressure of solutions. 



The third law of gas pressure that we wish to consider is the 

 law of Avogadro. This says that those gases which contain 

 the same number of parts or molecules in a given volume have 

 the same gas pressure. It is very easy to test this law for osmotic 

 pressure, and Van't Hoff did so in the following way. He took 

 hydrogen gas of a certain definite concentration, that is, contain- 

 ing a certain number of molecules in a given volume, and calcu- 

 lated its gas pressure. He then took a solution of cane sugar 

 containing the same number of molecules dissolved in a given 

 volume of the solvent that there were molecules of hydrogen in 

 the same volume of space. He compared the osmotic pressure 

 of the solution of cane sugar with the gas pressure of the hydrogen 

 gas, and found that the two pressures were exactly equal. This 

 was a most remarkable discovery. That the gas pressure exerted 

 by a gas particle should be exactly equal to the osmotic pressure 

 of a dissolved particle under the same conditions of concentration, 

 could never have been suspected until it was demonstrated experi- 

 mentally. 



Let me make a slight digression here to avoid confusion. Gas 

 pressure and osmotic pressure are identical in result, that is in 

 their magnitude. But this does not say that they have identical, 

 or even closely related causes. As we shall see, we do not today, 

 in my opinion, know the cause of osmotic pressure, while we have a 

 very satisfactory theory to account for gas pressure. The kinetic 

 theory of gases tells us that the cause of gas pressure is the bom- 

 bardment of the walls of the confining vessel by the rapidly mov- 

 ing gas particles. It is difficult, not to say impossible, to account 

 for osmotic pressure on the basis of any such dynamical theory. 

 We know that dissolved particles move through the soh^ent with 

 very small velocities, with velocities altogether too small to give 

 us osmotic pressure in any manner analogous to that with which 

 gases produce pressure on the walls of the vessels that contain 



