276 THEORY OF COLLOIDAL BEHAVIOR 



If we apply the conclusion drawn in this statement to the 

 proteins, it follows that we have no longer any right to insist 

 that proteins can form only colloidal solutions. If solutions of 

 gelatin and of crystalline egg albumin behave like crystalloidal 

 solutions in water and if gelatin solutions in alcohol behave like 

 colloidal solutions, we have no right to say that nevertheless 

 albumin and gelatin are in the colloidal state when dissolved in 

 water. Such an assumption is as arbitrary as to say that NaCl 

 is in a colloidal state when dissolved in water simply because it 

 is in a colloidal state when dissolved in benzol. The idea that 

 the two proteins mentioned must be* in the colloidal state when 

 dissolved in water is a survival from the time when it was custom- 

 ary to discriminate between colloidal and crystalloidal substances 

 instead of between colloidal and crystalloidal states, and the 

 terms emulsoids or hydrophilic colloids when applied to proteins 

 which require high concentrations of salt for their precipitation 

 are also a survival from that time. The fact that the molecules 

 of protein are large does not matter, since the chemical constitu- 

 tion of solute and solvent and not the mere size of the molecules of 

 solute determines the stability of their solution. The large size 

 introduces only interesting complications inasmuch as it makes it 

 possible that one and the same molecule may have groups with a 

 different degree of attraction for the molecules of water or solvent. 



In Burton's statement just quoted only one colloidal property 

 is taken into consideration; namely, the stability of colloidal 

 solutions. We have seen, however, that as far as the proteins 

 are concerned there are a number of other properties of colloidal 

 solutions which are all as characteristic for colloids as are the 

 conditions for the stability of the solutions; and that as a matter 

 of fact the stability of colloidal solutions depends on these other 

 properties, such as the P.D. and the osmotic pressure. It is, 

 therefore, no longer possible to base our definition of colloids 

 exclusively on data derived from a study of the stability of 

 colloidal solutions. 



The general characteristics of colloidal behavior may be stated 

 as follows : 



1. Low concentrations of neutral salts depress the osmotic 

 pressure, P.D., viscosity, and stability of colloidal solutions and 

 the swelling of gels. 



