Methods for Determining Osmotic Pressures. 165 
salt solution was “ balanced ” by the addition of a little calcium 
chloride. Osterhout therefore suggests that the salt, sodium chloride, 
has some effect on the protoplasm which is not purely osmotic. 
True’s 1 work on the physiological action of sodium chloride and 
potassium nitrate on Spirogyra cells also showed that these salts 
were not without some toxic action on protoplasm. 
2. Direct determinations. The osmotic pressure of various 
solutions has been measured directly by means of a semi-permeable 
or differential membrane of copper ferro-cyanide, first by Pfeffer, 
more recently by Morse and Fraser 2 and others, and by Berkeley 
and Hartley. 3 
3. Indirect determinations. The value of the osmotic pressure 
of any solution is, however, generally determined indirectly by 
utilising some other function of the solution which also depends on 
the amount of substance in solution. The osmotic pressure, the 
vapour pressure, the freezing point, and the boiling point are all 
functions of any solution which vary with its concentration, and the 
osmotic pressure has been calculated (a) from the difference between 
the vapour pressure of the solution and that of the solvent ( b ), from 
the depression of the freezing point of the solution below that of 
the pure solvent, and (c) from the elevation of its boiling point 
above that of the pure solvent. 
4. Physiological methods. Certain physiological methods have 
been used to estimate the value of the osmotic pressures of various 
solutions. 
The action of salt solutions on the red corpuscles of blood has 
been utilised to find what strengths of the solutions are isotonic. 
This has been done in two ways; the concentrations of different 
solutions that cause “ laking ” of blood have been compared, and 
the amount of decrease in volume of the corpuscles in different salt 
solutions has been measured by means of the haematocrit, and 
compared. 
The behaviour of certain bacteria in various salt solutions has 
also been used to determine what concentrations of these solutions 
are isotonic. 
As far as I know the majority of these methods have not been 
used for any extensive determinations of the osmotic pressure of 
1 “ The Physiological Action of certain Plasmolysing Agents." Bot. Gaz., 
Vol. 26, 1898, p. 407. 
2 Amer. Chem. Journ., 1905-1909. 
5 Phil. Trans. Roy. Soc., Series A, 1906. 
