THE CELL; THE COLLOIDAL CONDITION 17 



Sols and Gels. — Liquid colloidal systems are commonly called 

 sols and, therefore, include the suspensoids and emulsoids men- 

 tioned above, as well as any colloidal system which seems to have 

 the qualities of a solution and can be poured from one container 

 to another. Colloidal systems, on the other hand, which are more 

 or less rigid and behave like solids are called gels. Because of its 

 peculiar structure a gel may appear solid and yet contain only a 

 small amount of solid matter. Thus 1 g. of agar-agar can cause 

 99 g. of water to set to a rigid formation. Although a plant may 

 be mostly water, it may nevertheless be firm owing to this property 

 of gels. True gels may be formed from lyophile colloids as seen in 

 the case of custards, gelatin, fruit jellies, heated egg white, muscle 

 tissue, etc. False gels or coagula, on the contrary, are precipitates 

 of lyophobe colloids and are of less importance in biological phe- 

 nomena. 



Colloids in Physiology. — Colloids are of the highest degree 

 of importance in physiology, and among their properties which 

 make them so important are (1) the ability of many of them to 

 take up large quantities of water as cited in the preceding para- 

 graph, (2) the large surface involved, and (3) the possession of 

 electrical charges. 



If a cube 1 cm. on an edge is measured it will be seen to have a 

 total surface of 6 sq. cm. If this cube is then divided into cubes of 

 0.1 cm. on an edge, there will be 1,000 such cubes and a total sur- 

 face of 60 sq. cm. (6,000 faces X 0.01 sq. cm. which is the area 

 of one face). The following table shows the increase in surface 

 as the colloidal state is approached and the number of cubes in- 

 creases : 



This means that a little cube of starch 1 cm. on an edge when 

 broken down into particles of colloidal size as found in starch 

 paste, would have as much surface exposed to the solution as is 

 found on a table 10 meters long and 6 meters broad ! 



Willard Gibbs of Yale University showed that in any system, 

 according to the second law of thermodynamics, the amount of 



