MEASUREMENT OF OSMOTIC PRESSURE 545 



liquid or "osmotic pressure" was still dependent on the 

 individual properties of the particular membrane used. No 

 real progress could be made until this difficulty was overcome 

 by the discovery of " semi-permeable " membranes which would 

 stop completely the outward flow of the solute whilst still 

 permitting the solvent to pass inwards to the solution and 

 there develop the maximum osmotic pressure that was possible. 

 Such membranes were, in fact, discovered by Traube in 1865 

 in the form of floating films precipitated by the interaction of 

 two contiguous solutions. Traube then showed that if solutions 

 of copper sulphate and potassium ferrocyanide are brought 

 together, a floating membrane of copper ferrocyanide is pro- 

 duced which is permeable by water but impermeable by both 

 salts. According to the relative strengths of the two solutions, 

 water is drawn in one direction or the other through the 

 membrane which is so displaced that it always forms the 

 boundary between the two solutions. If. the boundary expand 

 or if the membrane be broken, a fresh precipitate is at once 

 produced by the interaction of the two membrane-forming 

 solutions. 



But whilst Traube's membranes possessed the property of 

 being semi-permeable, they were not suitable for quantitative 

 experiments, as they were incapable of supporting even the 

 smallest osmotic pressure. Great importance attaches there- 

 fore to the introduction by Pfeffer in 1876 of methods by 

 which Traube's membranes could be strengthened by pre- 

 cipitating them on linen or silk or parchment or best of all 

 in the pores of an unglazed porcelain battery-jar. With this 

 equipment, it was possible, for the first time, to make real 

 measurements of the maximum osmotic pressure set up in a 

 solution by the inflow of water through a semi-permeable 

 membrane. Even then, however, very few regularities were 

 discovered : the maximum pressure was found to be propor- 

 tional to the concentration of the solution but no indication 

 was obtained of any law by which the magnitude of the pressure 

 could be predicted. 



B. Van't Hoff's Equation 



In view of the obscurity in which the phenomena of osmosis 

 were involved, it would be difficult to exaggerate the dramatic 

 effect produced by the discovery, made by Van't Hoff in 1887, 



