PROTECTIVE POWER 291 



precipitation of negatively charged colloidal particles of sewage 

 by the ferric hydroxide hydrosol. 



Similarly it has been suggested that the process of dyeing 

 is really a mutual gel formation between the colloidal dye and 

 the colloidal fibre ; similarly the interaction between toxin 

 and antitoxin, and the phenomenon of bacterial agglutina- 

 tion, etc., may be regarded as examples of the mutual pre- 

 cipitation of two colloids. 



D. Protective Power. 



Many organic substances such as gelatine, agar, etc., when 

 added in small quantity to inorganic colloidal solutions, can 

 prevent the precipitation of the latter by electrolytes ; under 

 these conditions the organic colloids are said to exert a pro- 

 tective action upon the inorganic colloid. 



It is not known in what way this protective action is 

 exerted, but it has been suggested that the particles of the 

 suspensoid become covered with a layer of gelatine and so 

 acquire the properties of gelatine particles. 



Suspensoids, so protected, can be evaporated to dryness, 

 and the residue when taken up with water will redissolve. 



The greatly increased stability thus acquired by the in- 

 organic colloid makes the process of value for the preparation 

 of colloidal solutions of the metals, particularly silver and 

 mercury, which are used for various medicinal purposes. 



A measure of protective power was first worked out by 

 Zsigmondy,* who defined as the gold number, the number of 

 milligrams of colloid which, when added to 10 c.c. of a bright 

 red colloidal gold solution containing from '0053 to '0058 per 

 cent of gold, is just insufficient to prevent the precipitation (as 

 shown by the colour change to violet) of the gold by i c.c. of 

 a solution of sodium chloride, containing lOO grams of salt in 

 900 CO. of water. 



CLASSIFICATION OF COLLOIDS. 



One of the most striking differences between organic and 



inorganic colloidal solutions is the much greater viscosity of 



the former. Thus, while even dilute solutions of gelatine are 



more or less sticky or viscous, inorganic colloidal solutions are 



* Zsigmondy: " Zeit. f. anal. Chem.," igoi, 40, 697. 



19* 



