146 



PHYSIOLOGY 



electro-positive and travels from anode to cathode. Silicic acid, in the 

 presence of a trace of alkali, is electro-negative, and the same is true of a 

 hydrosol of gold. When a current is passed through these hydrosols, the 

 colloidal particles travel to the anode, where they are precipitated. In 

 certain colloids the charge varies according to the conditions under which 

 they are brought into solution. If, for instance, egg-white be diluted ten 

 times with distilled water, filtered and boiled, no precipitate occurs, but 



A. 



FIG. 28. Movements of two particles of india-rubber latex in colloidal solution, recorded by 

 cinematograph and ultra -microscope. (HENKI.) 



we obtain a colloidal suspension of albumin. When thoroughly dialysed, 

 this protein is insoluble in pure water, but is soluble in traces of either acid 

 or alkali. In acid solution the protein particles carry a positive charge, 

 whereas in alkaline solution their charge is negative. The charged condi- 

 tion of these particles must play a considerable part in keeping them asunder 

 and therefore in preventing their aggregation and precipitation. This is 

 shown by the fact that any agency which will tend to discharge them will 

 cause precipitation and coagulation. Among such agencies is the passage 

 of a constant current, just mentioned. To the same action is due the 

 coagulative or precipitating effects of neutral salts. Thus any of the 

 colloids we have mentioned, ferric hydrate, silica, gold, or boiled albumen, 

 are thrown down by the addition of traces of neutral salts, and it is found 

 that in this process they carry with them some of the ion with the opposite 

 charge to that of the colloidal particle. Thus, in the precipitation of the 

 electro-positive ferric hydrate the acid ion of the salt is the determining 

 factor, the coagulative power increasing rapidly with the valency of the acid. 

 On the other hand, in the precipitation of a gold sol the electro-positive ion 

 is the effective agent, and here again the coagulative effect is enormously 

 increased by a rise in valency. This is shown in the following Tables, 

 where it will be seen that, in the coagulation of gold, barium chloride, with 

 the divalent Ba", is seven times as powerful as K 2 S0 4 , containing the 

 univalent K'. On the other hand, in the precipitation of the electro-positive 



