304 THE COLLOIDAL STATE 



by dipping a piece of filter paper into a dilute aqueous solu- 

 tion of congo red ; after a short time the dye will have accumu- 

 lated on the surface of the paper, leaving the solution much 

 lighter in colour. 



Moreover, since congo red itself is in colloidal solution and 

 filter paper behaves in many respects like a colloid, this ex- 

 periment also illustrates the phenomenon of mutual adsorption 

 by colloids which is the principle underlying most processes 

 of dyeing and staining, and also enzyme actions and other 

 processes taking place in the living organism. 



In this connexion there is an interesting experiment due 

 to Bayliss * which is designed to show that although in the 

 process of dyeing adsorption upon the surface to be dyed may 

 be the first step, yet chemical reaction between the dye and 

 the fibre may follow as a second stage. The experiment con- 

 sists in shaking up a blue solution of the acid of congo red 

 with well-washed aluminium hydroxide ; the latter at once 

 adsorbs the blue colour from solution, and settles down on 

 standing ; if it is now heated, the physically adsorbed congo 

 red acid combines with the aluminium hydroxide to form the 

 aluminium salt, a chemical reaction which is marked by the 

 change of colour from blue to red. 



In the same way Bayliss holds that in the case of enzyme 

 action adsorption of the substrate upon the surface of the 

 enzyme is the first stage, and that then, in consequence of the 

 intimate contact between the two, mass action accelerates the 

 reaction. 



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 (see p. 308). 



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 



* Bayliss : " Z. chem. Ind. Koll.," 1908, 3, 224. 



