Fig. 2. 



82 J. van't Hoff on the Function of Osmotic Pressure 



It is evident that this state of 

 equilibrium might have been at- 

 tained in A without entry of 

 water if the vessel had been con- 

 structed with a piston, compress- 

 ing the solution with a pressure 

 equal to the osmotic pressure 

 (fig. 2). It follows moreover 

 that, by increasing or diminishing the pressure on the piston, 

 the state of concentration of the liquid can also be altered, 

 owing to the passage of water through the walls of the vessel 

 in an outward or inward direction. 



Such osmotic pressure has been experimentally investigated 

 by Pfeffer (Osmotische Untersuchungen, Leipsig, 1887). The 

 walls of the cell consisted of unglazed porcelain, rendered 

 impermeable to sugar though not to water, by filling it with 

 a solution of potassium ferrocyanide and placing it in a solu- 

 tion of copper sulphate. Owing to diffusion, the ferrocyanide 

 and the copper-salt come in contact after some time, and pro- 

 duce a membrane of copper ferrocyanide having the required 

 properties. Such a vessel is then filled with a one-per-cent. 

 solution of sugar ; it is then closed with a cork provided with 

 a manometer, and sunk in water ; the osmotic pressure gra- 

 dually rises, owing to entry of water, and the pressure due to 

 the entry of water is read off when it becomes constant. As 

 an example of the results obtained, it may be mentioned that 

 a one-per-cent. solution of sugar (which, owing to its con- 

 siderable mass, was not appreciably diluted on entry of water) 

 exerted at 6 0, 8 a pressure of 50*5 millim. of mercury — about 

 one fifteenth of an atmosphere. 



The porous membrane, such as that described, will be 

 termed in the following pages a "semipermeable membrane;" 

 and the conception will be made use of even where experi- 

 mental verification is lacking. The behaviour of solutions 

 may thus be studied in a manner strikingly analogous to that 

 employed in the study of gases, inasmuch as what is knowm as 

 " osmotic pressure " corresponds to pressure, or, as it is com- 

 monly but incorrectly termed, " tension " of a gas. It is right 

 to mention that this is no fanciful analogy, but a fundamental 

 one ; the mechanism which, according to our present views, 

 controls the pressure of gases and the osmotic pressure of liquids 

 is substantially the same. In the former case pressure is due 

 to the impacts of gaseous molecules on the walls of the con- 

 taining vessel, and in the latter to the impacts of the mole- 

 cules of the dissolved substance on the semipermeable mem- 

 brane, since the impacts of the molecules of the solvent, being 

 equal and opposed on each side of the vessel, may be neglected. 



