COLLOIDS 37 



coagulation are still but little understood. In some cases it 

 o 



is a reversible phenomenon, thus gelatine coagulated by cold is 

 redissolved by heat ; whereas with other colloids the process 

 is irreversible, albumin coagulated by heat is not redissolved 

 on cooling. 



Colloids in a state of coagulation have a vacuolar or sponge- 

 like structure. The solvent is imprisoned in the vacuoles of 

 the clot, and is expelled little by little by its retraction. 

 Colloids diffused in water are usually called colloidal solutions, 

 but they are not true solutions. Such a pseudo-solution of a 

 colloid is called a "sol," while a colloid in a state of coagula- 

 tion is called a "gel." Colloidal solutions spread but little, 

 diffuse very slowly in the liquids of the body, and cannot 

 penetrate organic membranes. 



Colloidal solutions diffuse light, unlike crystalloid solutions, 

 which are transparent. We all know how the trajectory of a 

 beam of sunlight through a darkened room is rendered visible 

 by the particles of dust. In the same way if a colloidal solution 

 is illuminated by a transverse ray of light, the light is diffused 

 by the molecules of the colloid in semi-solution, and the liquid 

 appears faintly illuminated on a dark background. The light 

 diffused by a colloidal solution is polarized, which shows that 

 it is reflected light. 



Siedentopf and Sigmondy have applied this principle of 

 lateral illumination on a dark background to the construction 

 of the ultra-microscope. AYith the aid of this instrument 

 we may not only see, but count the particles in a colloidal 

 solution, which is in reality merely a pseudo-solution or 

 suspension, in contradistinction to the true solution of a 

 crystalloid. 



Colloidal solutions possess only a very feeble osmotic 

 pressure. The lowering of the freezing point and the other 

 corresponding constants are also quite insignificant. This 

 arises from the fact that the molecules of a colloid are 

 extremely large when compared with those of a crystalloid. 

 For example let us take colloidal substance whose molecular 

 weight is 2000. A solution containing 40 grammes per litre 

 would have an osmotic pressure only one-fiftieth of that of a 



