THE COLLOIDAL STATE 111 



agglutination, etc., are of such biological importance that they 

 will be considered in more detail later (page 479). We may, 

 however, here ask an apparently simple question; if one ion of 

 the precipitating salt is carried down with the precipitate, what 

 happens to the other? It can hardly be left in the solution 

 uncompensated for. Either it must also be carried down (in a 

 purely passive way with the coagulum), or it is balanced by an 

 equivalent amount of oppositely charged ions possibly set free 

 from the surface of the particle. 



The theory of colloidal precipitation is based on the electrical 

 neutrahzation of charges. But if a particle can be decharged 

 in some other way, the result will be the same. Two positive or 

 two negative colloids may precipitate each other. They prob- 

 ably do so through chemical interaction of their stabilizing ions. 

 The ionic environment of one kind of particle (negative colloidal 

 sulphur) may react with that of another colloid (AS2S3) of the 

 same sign, so as to eliminate the ionic environment; thus is the 

 particle decharged, not physically (electrically) but by a chemical 

 reaction. 



Some interesting attempts have been made to apply colloidal 

 theories of precipitation to the elimination of fog, smoke, and 

 obnoxious fumes. English and French scientists considered the 

 possibility of dispelling fogs by sending powerful hertzian waves 

 (magnetic oscillations resembling light waves but much longer) 

 out into the air. Another application of this method is found in 

 a process now in use in the California oil fields for separating 

 emulsified (colloidal) water from crude oil. Efforts have been 

 made to handle the smoke nuisance of large cities in this way; 

 but the most successful application of the method is to gases in 

 smelter flues. The method is based on the fact that if a needle 

 point and a flat plate are connected to a high-potential current, 

 the air between them becomes highly charged with electricity 

 of the same sign as the needle point. Any body brought into 

 this space instantly receives a charge of the same sign. If this 

 body is free to move, as in the case of a floating particle, it will be 

 attracted to the plate of opposite charge. Plates attached to the 

 sides of smelter flues serve as the plate electrode. The point 

 electrodes are the fine tips of asbestos fibers; these are more 

 effective than wire and proved to be the key to the first com- 

 mercially successful installations. Another application of precipi- 



