50 Diffusion and Osmotic Pressure 



appear to have a bearing in this connection: A somewhat 

 concentrated solution of mineral salts was thickened by the 

 addition of enough gelatin to make a firm mass at ordinary 

 temperatures. On the surface of this mass were placed single 

 drops of a dilute solution having the same chemical nature 

 as the one contained within the gelatin plate, and the whole 

 was kept in a moist chamber. After four or five hours it 

 was always noted that the drops of liquid had disappeared ; 

 they had been absorbed into the colloid mass. If, however, 

 the more dilute solution were contained within the gelatin 

 plate, and drops of a concentrated solution were placed upon its 

 surface, it took very much longer for total absorption to occur. 

 For the first few hours there was usually even an observable 

 increase in the size of the liquid drops. Eventually absorp- 

 tion occurred, but it was often at the end of a period of more 

 than twenty-four hours. Of course, if there had been a 

 semi-permeable membrane between the drops and the gel- 

 atin, absorption would not have taken place. The gelatin 

 mass is not semi-permeable, but seems merely to retard the 

 process of difPusion of crystalloid solutes. 



If a mass of such gelatin, containing a strong osmotic 

 solution and surrounded by a semi-permeable membrane, be 

 placed in water or a weaker solution, this membrane will be 

 stretched by the internal pressure practically as though no 

 colloid were present, and a state of equilibrium will be 

 reached only when the resilience of the membrane equals 

 the osmotic pressure within. This has been demonstrated 

 experimentally by Traube and PfefEer.' In such a case the 

 osmotic pressure of the colloid is of such an order as to be 

 negligible. 



Now, any mass of protoplasm is very much the same sort 

 of a colloid mass as the gelatin just described. Its outer 



IM. Traube, "Experimente zur Theorie der ZeUenbildung vind Endosmose," 

 ATch. f. Anat. u. Physiol., Physiol. Abth., Jahrg. 1867, pp. 87-165. Also Pfeffeb- 

 EwAET, Physiology of Plants, Cambridge, 1900, p. 106. 



