ADSORPTION. GEL FORMATION 53 



of the particles or droplets. This is of very great 

 importance in determining the behaviour of the colloid 

 towards liquids, solutes and other sols, which may be 

 attracted to the surfaces of the disperse particles or 

 droplets, and held there by the surface energy, kt the 

 same time decreasing the surface tension of the dis- 

 perse phase. This process is called adsorption. The 

 living protoplasm of the cell is believed to be a colloid 

 sol of which the continuous phase is a solution of various 

 crystalloids, and the disperse phase consists of protein 

 and fat droplets or particles. These take up and hold 

 by adsorption the molecules and ions of solutes which 

 can diffuse into the cell and come within the range 

 of their surface energy. In this way large quantities 

 of various substances may be taken in and held in the 

 living cell. Dyes are taken up in the same way by 

 protoplasm and by the other organic colloids of the 

 plant or animal, and on this process largely depends 

 the staining of tissues employed in making microscopic 

 preparations. 



Gel Formatiou. — When hydrosols (sols with water 

 as the continuous phase) lose water, for instance by 

 evaporation, their disperse particles get closer and 

 closer together. In the case of certain sols, including 

 those whose disperse phases consist of large organic 

 molecules, such as those of gelatine for example, the 

 molecules or aggregates of molecules tend, on loss of 

 water, to run together in chains. As more water is 

 lost, we picture these chains becoming more and more 

 entangled with one another, the sol becomes more and 

 more viscous (treacle-like), till ultimately a jelly — 

 technically called a gel — ^is formed. On further loss 

 of water the gel shrinks, and eventually becomes solid. 

 If water is again added it is absorbed, and the substance 



