THE ENERGY OF MOLECULES IN SOLUTION 141 



cent, of the same salt, the concentration of the cell sap must be about 

 equivalent to a 0'595 per cent. NaCl solution. Solutions of different 

 salts, in which plasmolysis just occurs, must also be isotonic with one 

 another. Thus a TOl per cent, solution of KN0 3 is found to be 

 isotonic with a 0'58 per cent. NaCl solution. 



DETERMINATION BY HAMBURGER'S BLOOD-CORPUSCLE 

 METHOD. The limiting external layer of red blood corpuscles resembles 

 the primordial utricle of plant cells in being impermeable to a number 

 of dissolved substances. If, therefore, it be placed in a solution of 

 smaller concentration than the corpuscle contents, it will swell up and, 

 since it has no supporting cell wall, the increase in size will go on until 

 the corpuscle bursts, and its contained red colouring-matter, haemo- 

 globin, passes into solution in the surrounding fluid. If the corpuscles 

 be then allowed to settle or be centrifuged, the fact that haemolysis 

 has occurred is shown by the red colour of the clear supernatant fluid. 

 With a given sample of blood, the concentration of a potassium nitrate 

 solution is found at which the first traces of haemolysis occur. 

 In order to determine the osmotic pressure of a solution, say, of 

 sugar or of sodium chloride, these are also added in various dilutions 

 to blood corpuscles until we get solutions in which haemolysis just 

 occurs. These solutions will then be isotonic with the first determined 

 potassium nitrate solutions. As an example of this method may be 

 adduced the following results : 



OSMOTIC PRESSURE OF ELECTROLYTES. It will be noticed in 

 the last Table that the isotonic solutions of different salts contain these 

 salts in the proportion of their molecular weights, i.e. each solution 

 contains the same number of molecules of dissolved salt. For the term 

 isotonic we might therefore employ equimolecular. When, however, 

 these salts are compared with solutions of sugar, it is found that the 

 osmotic pressures of the salt solutions are double or nearly double 

 those of equimolecular solutions of sugar. The osmotic pressure 



