The Nature of Osmotic Pressure. 



119 



A comparative, quantitative study of the hemolytic action of 

 these three substances shows that 



(1) The same degree of osmotic hemolysis is not produced by 

 identical "osmotic concentrations" of boric acid, formaldehyde 

 and urea nor by a corresponding lowering in the concentration of 

 the substances in the medium of suspension of the treated cor- 

 puscles. 



(2) As the treating concentration of the three hemolytic sub- 

 stances is correspondingly diminished, the ratio between that con- 

 centration and the final concentration, in the hemolytic experi- 

 ment, increases disproportionately with the different substances. 



(3) The osmotic hemolysis of corpuscles which have been 

 treated with the three hemolytic substances in the same "osmotic 

 concentrations" is not inhibited by identical concentrations of 

 sodium chloride nor of the hemolytic substances themselves. 



All of these facts contradict the usual assumption that os- 

 motic pressure is exerted directly by the solute. 



These facts are easily compatible with the alternative view 

 that osmotic pressure is merely the pressure of the water which 

 diffuses through a semipermeable membrane to the side of 

 higher "osmotic concentration," if, as is necessary, the factor of 

 time is taken into consideration. 



Under this conception, the degree of osmotic pressure devel- 

 oped depends not alone upon the original concentration of the 

 solute but upon the length of time during which an effective differ- 

 ence in concentration is maintained on the two sides of the mem- 

 brane. If the solutes are diffusible, as are all three of the hemo- 

 lytic agents under study, the degree of osmotic pressure developed 

 by them, under the conditions of the experiment, must depend, in 

 part, on the rate of diffusion of the respective substance. 



It is found that the diffusion rates of boric acid, formaldehyde 

 and urea are respectively 90 seconds, 30 seconds and less than 5 

 seconds. These differences correspond with the differences in 

 the hemolytic action of the three reagents mentioned above and 

 they confirm the view that osmotic pressure is not a direct prop- 

 erty of a solute but merely water pressure developed by the process 

 of diffusion. 



