COLLOIDS 39 



solutions. Non-electrolytic solutions do not appear to provoke 

 this precipitation. This is not a chemical action, for an 

 exceedingly small quantity of an electrolyte is able to 

 precipitate an indefinite quantity of the colloid. The pre- 

 cipitation is probably due to the electric charges carried by 

 the dissociated ions of the electrolytes. 



When an electric current is passed through a colloid 

 solution, the course of the molecules of the colloid is some- 

 times towards the cathode and sometimes towards the anode, 

 according to the nature of the colloid and of the solvent. 

 This displacement would appear to indicate a difference of 

 electric potential between the molecules of the colloid and 

 those of the solvent. Hardy has shown that in an alkaline 

 solution the molecules of albumin travel towards the anode, 

 while in an acid solution they travel towards the cathode. 



Metallic Colloids. — Carey Lea and afterwards Crede suc- 

 ceeded in obtaining silver in colloidal solution by ordinary 

 chemical means. Professor Bredig has introduced a more 

 general method of obtaining a number of metals in colloidal 

 solutions in a state of great purity. He causes an electric- 

 arc to pass between two rods of the metal immersed in 

 distilled water. The cathode is thus pulverized into a very 

 tine powder which rests in suspension in the liquid, constitut- 

 ing a colloidal solution. Bredig has in this way prepared 

 sols of platinum, palladium, iridium, silver, and cadmium. 



Catalytic Properties of Colloids. — Catalysis is the property 

 possessed by certain bodies of initiating chemical reaction. 

 The mass of the catalyzing body has no definite proportion to 

 that of the substances entering into the reaction, and the 

 appearance of the catalyzer is in no way altered by the 

 reaction. 



Ostwald has shown that catalysis consists essentially in the 

 acceleration or retardation of chemical reactions which would 

 take place without the action of the catalyzer, but more 

 slowly. 



Catalytic reactions are very numerous in chemistry. The 

 inversion of sugar by acids, the etherization of alcohol by 

 sulphuric acid, the decomposition of hydrogen peroxide by 



