THE COLLOIDAL STATE 



93 



tides will remain suspended. The method is a possible one but 

 not usually very successful. It works best with glass, which 

 if very finely powdered by grinding in a mortar will form a perma- 

 nent colloidal suspension. When an oil is mechanically shaken 

 in water under suitable conditions, a stable emulsion results. 



Electrical dispersion is the most satisfactory method for making 

 colloidal suspensions of metals. The apparatus was devised 

 by Bredig and modified by Burton. A heavy wire of the desired 

 kind (copper wire does well, as it is very pure and not costly) 

 is fastened to the clapper of an electric bell. 

 A second piece of the same wire is clamped 

 near the first in such a way that the free points 

 of the two are about an eighth of an inch apart 

 and immersed in water. The wires are con- 

 nected to a 110- volt house current with four 

 or six lamps (two or three amperes) in the 

 circuit (Fig. 69). The bell clapper is set 

 going; its vibration keeps the two wires con- 

 tinually making and breaking contact so as to 

 maintain an electric arc. After being in 

 operation some twenty or thirty minutes, a 

 suspension of the metal is formed due either 

 to the disintegrating effect of the electric arc 

 on the metal electrodes or to the formation of electrical dispersion of 



metalhc colloids. 



a gas of the metal, by the heat of the arc, 

 which condenses into particles of colloidal size. The heavier 

 particles settle, and the finer invisible ones remain in suspension, 

 giving the water a translucent appearance of characteristic color. 

 If the metal electrodes are of gold, a red, violet, or blue dispersion 

 is obtained. 



Chemical methods of preparing colloidal dispersions are 

 numerous and varied. They are termed pepft^a^ton methods. To 

 peptize is to disperse. If the term is used in its broadest mean- 

 ing it includes mechanical and electrical methods, but it is better 

 to limit the expression to methods involving the addition of a 

 peptizing agent. (The word is derived from "peptone," itself 

 a peptizing agent.) In a sense, water peptizes gelatin, glue, 

 gums, etc., in that it disperses them until a colloidal solution 

 results The dispersal of clay in water through the addition of 

 ammonium hydroxide or through leaching with acid, the emuL 



Fig. 69.— The Bre- 

 dig-Burton method of 









