COLLOIDS 



57 



To ascertain the nature of the charge various methods may be em- 

 ployed, of which the following are important: 



1. The method of electrophoresis. The colloid solution is placed in a 

 U-tube, each side of which carries a platinum electrode dipping into the 

 solution. After a strong continuous electric current has been allowed 

 to pass for some time through the solution, it will be found that the 

 colloid collects at the anode (where the current enters) when it is a 

 negative colloid (since unlike electric charges attract each other), and 

 at the cathode when it is positive. In the case of colored solutions, the 

 migration can be readily seen, but otherwise it may be necessary to ana- 

 lyze the solution at the two poles. 



Fig. 15. Capillary analysis of colloids. Strips of filter paper, after being suspended with 

 the lower ends dipping into colloidal solutions. Those on the right hand were positive colloids, 

 which did not rise in the strips, but formed a sharp line of demarcation at the lower end on 

 account of precipitation. Those on the left hand were negative colloids. (From W. Ostwald.) 



2. The method of capillary analysis. For this purpose a long strip of 

 filter paper is arranged vertically over the solution, with its lower end 

 dipping into it. In the case of negative colloids the colloid, as well as 

 the dispersion medium, rises uniformly on the strip of paper (it may be 

 to a height of 20 cm.) ; whereas with positive colloids the dispersion 

 medium alone rises, the colloid itself doing so only to a very slight ex- 

 tent, but becoming so highly concentrated at the interface between the 

 solution and the paper that it coagulates on the end of the strip of paper, 

 where it forms a sharp line of demarcation (Fig. 15). 



3. The method of mutual precipitation of colloids. When a positive 



