OSMOTIC PRESSURE 11 



is an interesting instance of the mutual precipitation of 

 colloids. 



The interaction of ' toxins ' and ' antitoxins * in serum 

 therapy further illustrates the same property. 



It is probable that we have here also to do with the attrac- 

 tive action of extended surfaces, such as are presented by 

 gelatinous precipitates, whereby not only colloids but also to 

 a certain extent crystalloids are withdrawn from solution. 

 This attractive effect is known generally as absorption. 



Colloids exercise a very low osmotic pressure, and conse- 

 quently are assumed to have a very high molecular weight. 



The latter characteristic is of considerable importance in 

 considering the changes taking place in a cell. Modern research 

 has shown that substances, such as ordinary salt, which allow 

 the passage of electricity when they are dissolved in water 

 exist, at any rate in dilute solution, in a state of dissociation, 

 and the dissociated ions, as they are termed, obey in dilute 

 solution the laws of gaseous particles. They will tend rapidly 

 to diffuse throughout the solution. 



If, therefore, a dilute solution of salt is enclosed in a vessel 

 with semi-permeable walls, 1 i.e., walls which are permeable to 

 the molecules of the solvent but not to those of the dissolved 

 substance, there is a tendency for the ions to extend, they 

 cannot pass through the sides of the vessel ; but if the latter be 

 placed in clean water there will be a tendency for the water to 

 enter and thus a pressure will be created in the ulterior of the 

 vessel ; this is known as the osmotic pressure. This will 

 obviously depend on the concentration of the salt solution. 

 In dilute solutions it is proportional to the number of the 

 molecules of the dissolved salt present in a given volume of the 

 solution. 



In order that chemical activity may go on in the cell, it is 

 evident that it must be possible for crystalloidal bodies to 



1 A porous pot, in the pores of which copper ferrocyanide has been 

 precipitated, forms such a semi-permeable septum. 



