22 



SCIENCE PROGRESS. 



compared. From this peculiar property whereby the 

 corpuscle, although losing part of its contents, takes up 

 an isotonic amount from the solution, it is possible to 

 obtain isotonic solutions even in the absence of a semi- 

 permeable membrane. 



By studying the electrical excitability of frog's muscle 

 (Nasse) or even the motions of bacteria (Wladimiroff) in 

 different solutions, attempts have been made to arrive at 

 the concentrations of isotonic solutions. 



Most of the relative measurements of osmotic pressure 

 relate to electrolytic solutions. 



Boyle s law for dilute solutions. — The only absolute 

 measurements on a well-defined indifferent substance 

 which can be used to test this law are those of Pfefter on 

 sugar. If the law holds, at constant temperature the 

 ratio of the osmotic pressure P to the concentration C 

 should be constant. For a sugar solution at tempera- 

 tures between i3°'5 and i4°7 Pfeffer gives the following 

 values : — 



Although the values of P/C vary by some 10 per cent., 

 considering" the difficulty of the observations their 

 constancy is satisfactory. The above measurements 

 were made with a membrane of copper ferrocyanide. 

 Much lower values for the osmotic pressure were 

 obtained for the same sugar solution when a membrane 

 of Prussian blue or calcium phosphate was employed, and 

 these facts have been cited as indicating that even with 

 artificial membranes, just as with animal bladder, the 

 nature of the membrane affects the results. It is easy 

 to show theoretically, however, as Ostwald has pointed 



