112 PROTOPLASM 



tation by electrical decharging, but one which never got beyond 

 the speculative stage, is that suggested by Bancroft, who had 

 the idea that fog over airplane landing fields could be dispersed 

 by the scattering of charged sand from the airplane. The plane 

 would carry sand which could be electrically charged and spread 

 over the fog-covered field just before landing. The suggestion 

 is a fertile though impractical one, but it may lead to a similar, 

 yet commercially feasible, solution of the problem. 



The Suspensions. — If the conditions of colloidality are present 

 when one kind of matter (gas, liquid, or solid) is finely divided 

 and permanently suspended in another kind, then there should 

 be nine colloidal types. All but one of these exist, the exception 

 being gas dispersed in gas, which is always a molecular and never 

 a colloidal dispersion. The eight types of colloidal suspensions 

 are: 



Gas in liquid Liquid in solid 



Gas in solid Solid in gas 



Liquid in gas Solid in liquid 



Liquid in liquid Solid in solid 



Gas-in-liquid colloidal systems are foams. They are of fre- 

 quent occurrence in nature and present some important com- 

 mercial problems, such as the lather-forming qualities of soaps. 

 It should be noted that while foams of gas dispersed in a pure 

 liquid exist, they are rare and unstable. All naturally formed 

 gas-in-liquid systems, and most artificially made ones, contain 

 a third phase which surrounds and stabilizes the dispersed gas 

 bubbles. (We shall later see how this third phase functions.) 



Gas-in-solid systems belong chiefly to that class of colloids in 

 which both phases are continuous. These systems are more 

 typical of the second group of gel-forming colloids than of the 

 suspensions where we now put them. Pumice stone, charcoal, 

 and air-filled silica gel, the latter two of commercial value as 

 adsorbents, are examples. 



Liquid-in-gas systems, in the form of fogs and clouds, give the 

 meteorologist colloidal problems to think about. When air 

 rises to higher altitudes and lower temperatures, the invisible 

 molecules of water vapor which it contains aggregate to form 

 colloidal particles of water, and a cloud results. The cooling of 

 the air at night has the same effect and forms early-morning mist. 



