4o6 THE COLLOIDAL STATE 



therefore, that the surface layers will be most concentrated 

 in aqueous solutions of these substances. Direct evidence of 

 this may be obtained in the case of many solutions ; for 

 example, some dyes, such as methyl violet, on keeping, become 

 so concentrated at the surface as to cover themselves with a 

 film ; the same applies to solutions of albumen. By blowing 

 bubbles into such a solution and so increasing the surface, 

 Ramsden * was able to remove the major portion of the 

 dissolved substance from the solution by taking away the 

 froth. Indeed, the tendency to froth in liquids is usually a 

 manifestation of the greater concentration of dissolved sub- 

 stance at the surface with the resultant lowering of surface 

 tension. Ramsden was further able to show that when a 

 mixture of albumen and saponin is shaken up with water, the 

 froth is richer in saponin since this substance lowers the 

 surface tension of water more than does the albumen. This 

 same phenomenon, no doubt, also explains the inactivation of 

 some enzymes which results from mere shaking, and it has 

 been shown that the froth of such solutions has greater activity 

 than the rest of the liquid. 



The interface between the disperse phase and the con- 

 tinuous phase of any colloidal solution represents a surface 

 at which increased concentration can take place and hence 

 the tendency for adsorption which is so characteristic a pro- 

 perty of colloids. 



The concentration of a dissolved substance upon the sur- 

 face of a solid introduced into a solution may be illustrated 

 by dipping a piece of filter paper into a dilute aqueous solution 

 of Congo red ; after a short time the dye will have accumulated 

 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. 



* Ramsden : " Z. physik. Chem.," 1904. 47» 336- 



