162 



PHYSIOLOGY 



the direction of their movement reveals the nature of their change. 

 Thus colloidal ferric hydrate is 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 

 we obtain a colloidal suspension of albumin. When thoroughly 



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

 recorded by cinematograph and ultra-microscope. (HENRI.) 



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 condition of these particles must play a considerable 

 part in keeping them asunder and therefore in preventing their aggrega- 

 tion and precipitation. This is shown by the fact that any agency 

 which will tend to discharge them will cause precipitation and coagu- 

 lation. Among such agencies is the passage of a constant current, 

 just mentioned. To the same action is due the coagulative or pre- 

 cipitating 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 precipita- 

 tion of the electro- positive ferric hydrate the acid ion of the salt 

 H the determining factor, the coagulative power increasing rapidly 



