THE COLLOIDAL STATE 



107 



In his book "Les Atomes," Perrin has given the path of colloidal 

 particles in Brownian movement (Fig. 77). 



This activity of colloidal particles is believed to be and indeed 

 must be largely responsible for keeping them in suspension. 

 It is, therefore, one of the factors upon which the stability of 

 colloidal dispersions depends. 



Practically all that has been said about Brownian movement 

 so far is apphcable to protoplasm, the particles of which are often 

 in an active state of motion. The ultramicroscopic picture of 



Fig. 77. — The path of colloidal particles in Brownian movement. 



Perrin.) 



{From 



the Plasmodium of a slime mold may also resemble the Milky 

 Way with every star trembling. But it is not necessary to view 

 protoplasm ultramicroscopically in order to observe the Brownian 

 movement of its particles. In a shme mold or an amoeba, many 

 particles may be in Brownian motion (apart from the movement 

 due to the streaming of protoplasm). They may also at times 

 be quiet, indicating a temporarily high consistency of the 

 protoplasm. 



Electrical Charge. — Colloidal particles possess an electrical 

 charge similar to that of an ion; in fact, the particles may be 

 regarded as colossal ions. Colloidal particles of platinum, gold, 

 and silver are negative; lead, iron, and copper are positive. 

 Among colloidal dyes, indigo and Prussian blue are negative; 

 Magdala red and Bismarck brown are positive. Living "col- 



