THE NEW THEORY OF SOLUTIONS. 15 



and when this maximum is attained there is equilibrium 

 between solution and solvent — the concentration of the 

 solution, which was, of course, lessened by the entrance of 

 solvent, and the pressure remain the same. When this 

 constant state of things is established, the excess of 

 pressure on the solution-side of the semi-permeable mem- 

 brane over that on the solvent-side, is termed the osmotic 

 pressure of the solution. 



ORIGIN OF THE NEW THEORY. 



It is to this experimentally ascertained magnitude — 

 the osmotic pressure — which is in the main the outcome 

 of the labours of physiologists, that the new theory of 

 solutions, founded by van't Hoff, owes its origin. As 

 van't Hoff pointed out, the great practical advantage 

 which arises from the experimental demonstration of the 

 existence of osmotic pressure, no matter what its real 

 nature may be, lies in the fact that it enables the laws 

 of thermodynamics to be applied to solutions. If we 

 enclose a quantity of solution in a cylinder fitted with a 

 piston, and if the walls of the cylinder be semi-permeable, 

 on immersing the cylinder in water, the latter will 

 begin to enter the cylinder, and pressure will begin to 

 rise within it. If, however, we compress the solution by 

 means of the piston, entrance of water will be retarded, and 

 when the pressure exerted by the piston is equal to the 

 osmotic pressure, there will be equilibrium between solu- 

 tion and solvent. Moreover, by altering the pressure 

 on the piston, the concentration of the solution may be 

 altered just in the same way as a gas may be rarefied 

 or compressed, with the difference in the case of the 

 solution that change of volume is produced by the passage 

 of water through the semi-permeable membrane. By 

 taking care that the pressure on the piston is just equal 

 to the opposed pressure the above processes may be 

 made reversible, and it is possible to perform with a solu- 

 tion a reversible cycle of operations just as Carnot did 

 with a gas. In this way solutions as well as gases may 

 be treated thermodynamically. From such considerations 



