296 COLLOIDS 



paper behaves in many respects like a colloid, this experiment 

 also illustrates the phenomenon of mutual adsorption by col- 

 loids which is the principle underlying most processes of dyeing 

 and staining, and also enzyme actions and other processes 

 taking place in the living organism. 



It is, of course, easy to understand that if adsorption takes 

 place so readily between colloids, such as filter paper and congo 

 red, both of which bear negative charges in water, the pheno- 

 menon must take place still more easily between oppositely 

 charged colloids in which the mutual electrical discharge facili- 

 tates the deposition. 



Numerous practical applications of adsorption from solu- 

 tions are known, as for example in the removal of colouring 

 matter in the purification of cane sugar, or in the removal of 

 fusel oil from crude spirit by filtration through charcoal. 



Other substances besides charcoal, such as fuller's earth and 

 china clay, have been similarly used on account of the large 

 surfaces which they present. 



From what has been said with regard to the structure of 

 gels and the assumption that they present a sort of network 

 with a considerable development of internal surface, it is easy to 

 find an explanation of the use of isinglass for clearing a turbid 

 solution or for the fact that colouring matter may be extracted 

 from a solution by precipitating gelatinous aluminium hydrox- 

 ide in it. 



Thermodynamical considerations, coupled with experimen- 

 tal measurements, show the fact that true adsorption takes 

 place according to well-defined mathematical laws which en- 

 able one to decide definitely whether a certain phenomenon is 

 due to physical adsorption or to chemical reaction ; thus, it 

 has been found that a relatively larger amount of the total 

 substance in solution is withdrawn from a dilute than from a 

 strong solution. 



It has been calculated that the total surface presented by 

 the particles of a red colloidal gold solution containing O'5 

 gram of gold per litre amounts to about 8 sq. metres. It is, 

 therefore, easy to understand that with such an enormous de- 

 velopment of surface there is the possibility for a marked 

 manifestation of adsorption by suspensoids, and this readily 

 explains the tenacity with which most such colloidal solutions 



