COLLOIDS 59 



the state of dispersion of the colloid particles, and until we know all of 

 the factors which may influence this, measurements of osmotic pressures 

 of colloids can scarcely be of very much value. Nevertheless, that this 

 property has some physiologic bearing is clear from the effect which col- 

 loids have in restoring the blood pressure after hemorrhage (page 141). 



Further evidence that the osmotic pressure of colloids has not the 

 significance that it has in the case of molecular solutions is furnished by 

 the fact that the osmotic pressure is only approximately proportional 

 to the concentration of the solution; it may either increase or decrease 

 relatively to 'the strength of the solution. Temperature also has quite 

 a different influence on the osmotic pressure of colloids from that which 

 it has on the osmotic pressure of molecular solutions, and it frequently 

 has an influence which persists after the solution is brought back to its 

 original level. 



The influence of added substances on the osmotic pressure of colloidal 

 solutions is of considerable interest to the biologist, for, whereas in the 

 case of molecular solutions this is purely additive, in the case of col- 

 loids the added substance may at one time cause the osmotic pressure to 

 increase, at another, to decrease. It has been found that the osmotic 

 pressure of gelatin solutions at first decreases, then rapidly increases as 

 the H-ion concentration is raised. The addition of alkali increases the 

 osmotic pressure until a maximum is reached, beyond which it begins to 

 fall. Both acids and alkalies lessen the osmotic pressure of egg albu- 

 min. Electrolytes always decrease the osmotic pressure of gelatin and 

 albumin solutions, and the degree to which they exert this influence 

 depends on the nature of the cation and anion composing the electrolyte. 

 In the order of their depressing influence the cations arrange them- 

 selves: 



Heavy metals > alkaline earths > alkalies; 

 and the anions: 



S0 4 > Cl > N0 2 > Br > I > CNS. 



The influence of a given electrolyte varies extraordinarily with the reac- 

 tion of the colloid, a fact which must be carefully regarded in all work 

 in this field. 



