54 PHYSICAL CHARACTERS OF ORGANIC SUBSTANCES 



gradually swells till it passes back into the condition 

 of a sol. 



A gel which can thus pass back into a sol is called 

 a reversible gel. A piece of solid gelatine, for instance, 

 will absorb water till it passes into the gel, and finally 

 into the sol condition, in which the original^ gelatine has 

 disappeared from sight. Allowed to evaporate the sol 

 will lose water till the gel reappears and the gelatine 

 finally becomes solid again ; and these opposite pro- 

 cesses can be carried out an indefinite number of 

 times. 



Many organic colloids of the living body are in the 

 gel condition, e.g. cell walls, starch grains, gums, 

 mucilages, etc., and some of these, by the absorption 

 and loss of water, can pass into sols and back again. 

 Protoplasm itself may exist either as a sol or as a gel. 

 The internal surface of a gel, though less than that of 

 the disperse phase of a sol, is nevertheless very great, 

 and gels accordingly show those characters, such as 

 the power of adsorption, which depends upon internal 

 surface. 



Membrane Formation. — Gels may be formed from 

 sols in other ways than by loss of water. For instance, 

 when a sol is in contact with some other substance, 

 a film or membrane of gel structure is often formed 

 on the plane of contact. The protoplasmic sol always 

 forms a membrane or film on its free surface, and these 

 protoplasmic gel membranes play a viteil part in the 

 economy of the living cell. 



Difiusion. — A very important difference between 

 crystalloids and colloids, a difference used by Thomas 

 Graham, who first investigated the subject in the 

 middle of last century, to distinguish between them 

 is their relative diffusibility. 



