PROTECTIVE ACTION 393 



means of two solutions of night blue and alkali blue respec- 

 tively, as recommended by Wo. Ostwald. The same principle 

 has been worked into a complicated system of capillary 

 analysis by Goppelsroeder,* Freundlich,-j- and others. 



PROTECTIVE ACTION OF COLLOIDS. 



The sensitiveness of suspensoid sols to electric influences 

 can be considerably reduced by what are known as protective 

 colloids. 



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,J 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 100 grams of salt in 

 900 c.c. of water. 



Appended is a list of some of the commoner colloids with 

 their corresponding gold number taken from Zsigmondy's 

 paper :— 



* Goppelsroeder : " Kapillaranalyse," Basel, 1906. 

 t Freundlich : " Kapillarchemie," Leipzig, 1909. 

 J Zsigmondy : " Zeit. anal. Chem.," 1901, 40, 697. 



