PHYSICAL CHEMISTRY 33 



A suspensoid colloid, as has been stated, is easily precipitated 

 by the addition of an electrolyte. If a small amount of an emul- 

 soid colloid is added to a suspensoid colloid solution, the latter is 

 much less easily precipitated. The suspensoid colloid is "pro- 

 tected" by the emulsoid colloid (albumin, for example). This 

 phenomenon is called the protective action of colloids. 



Much interest has centered around gel formation and gel 

 structure. Often a very small amount of material will form a 

 gel holding fairly rigid a relatively enormous volume of water. 

 The jelly like consistency of protoplasm is due to the colloidal 

 character of its constituents, so that gels are of the greatest 

 biological importance. Gels are now considered to be a net- 

 work of tiny crystal-like formations which hold the water in 

 large measure enmeshed. If a gel stands for some time, the 

 mass of crystals shrinks, squeezing out some of the liquid. This 

 process is called synresis or "bleeding." 



Emulsoid colloids enormously increase the viscosity of solu- 

 tions, suspensoid colloids scarcely at all. 



Adsorption Surface Tension. Colloidal particles have the 

 property of taking up other substances on their surfaces. This 

 process is called adsorption. It is of great biochemical interest, 

 since it undoubtedly plays a part in many important processes, 

 such as the formation of the temporary union between an en- 

 zyme and its substrate, the combination of toxins with anti- 

 toxins, the sensitization of leucocytes by opsonins, the taking 

 up of bacteria by leucocytes, the formation of compounds be- 

 tween proteins, lipins and other constituents of the cell. 



The process is closely related to surface tension. 



At the surface between two media, for example water and air, 

 the molecules of water in the surface layer are attracted down- 

 wards and to all sides by their fellows, but not up. The mole- 

 cules in the surface are thus drawn down, and form what is 

 called the surface film. There is a constant tendency for the 

 amount of free energy in a system to decrease. This is the 

 principle of Willard Gibbs. Now in the surface between two 

 liquids, or a liquid and a solid, the presence of dissolved sub- 



