EVOLUTION OF COLLOIDS 145 



dilute solutions, for all the colloid tends to 

 accumulate at the interface. The frothing of 

 soap solutions, and of all albuminous solutions, 

 with formation of a lather of bubbles is an 

 example of this, and what happens between 

 air and a colloidal solution can also happen 

 between such a solution and solid or fluid 

 particles suspended in it. Such an effect is 

 seen in the suspension of fat in milk or cream. 

 Colloids such as gum arabic are similarly 

 used by the pharmacist to hold solid drugs 

 in suspension. 



When the colloidal particles have once 

 been attracted into the surface layer they 

 form a highly concentrated solution or fluid 

 film there, and such concentration leads to 

 formation of solid films as well as to the 

 synthesis and building up of more complex 

 substances in living cells. 



High concentration in the surface layer 

 acts much as increasing pressure does in 

 causing gases to liquefy or condense, and 

 favours chemical union. 



An example of such action is seen in the 

 skin which forms, and again re-forms as often 

 as it is skimmed off, on the surface of a glass 

 of warmed milk while cooling, or on a cup of 

 cocoa and milk. Familiar as this observation 

 is, it furnishes an example of the manner in 



