THE NEW THEORY OF SOLUTIONS. 21 



potassium ferrocyanide, and adjusting the concentrations 

 of the solutions till the copper sulphate enclosed within 

 the semi- permeable precipitate, which is at once formed 

 around the drop, ceases to lose or gain water. Whether 

 water does pass to or from the drop may be conveniently 

 ascertained by finding by the use of Topler's apparatus if 

 the refractive index of the liquid near the drop varies. 

 Another substance may be added to A, one of the 

 membrane-forming solutions. The isotonic concentration 

 of B, the other membrane former, is then found. On the 

 assumption that the osmotic pressure of the substance is 

 added to that of A, the concentration of B isotonic 

 with the substance may readily be ascertained. 



A remarkable method of obtaining isotonic solutions, 

 inasmuch as no semi-permeable membrane is present, con- 

 sists in the use of red blood corpuscles. If defibrinated 

 blood be introduced into water or a weak aqueous solution, 

 the colouring matter of the corpuscles becomes uniformly 

 diffused throughout the liquid which is tinted reddish- 

 brown. In a strong solution the corpuscles settle to the 

 bottom of the containing vessel, leaving the upper layers 

 of liquid colourless. Solutions to which the blood cor- 

 puscles are just on the point of communicating a red tint 

 are isotonic. The values of the concentrations of isotonic 

 solutions obtained in this way, using the blood of the ox, 

 agree satisfactorily with the results given by De Vries. 

 Different series of isotonic solutions may be obtained by 

 using the blood of different animals. Hamburger, to whom 

 this method is due, finds that in a dilute solution water 

 enters the corpuscle until the osmotic pressure of its con- 

 tents is equal to that of the solution. During this process 

 a quantity of dissolved substance also enters the corpuscle, 

 which apparently gives up to the solution an isotonic 

 amount of dissolved material from its own contents. If 

 the amount of water entering is sufficiently large, the 

 corpuscle will be ruptured and the solution will be 

 coloured, and the concentration of the contents of the 

 corpuscle at which rupture first occurs is the standard 

 concentration with which the surrounding solutions are 



