36 DYNAMICS OF LIVING MATTER 



the colloidal particles a more positive charge, while a trace of alkali 

 will give them a more negative charge. 



Hardy was the first to call attention to the fact that the electrical 

 charge of the colloidal particles - - or, in his opinion, the difference of 

 potential between the particles and the surrounding solution --is a 

 prerequisite for the stability of many colloidal solutions.* If in these 

 solutions the charges are removed from the particles, a precipitation 

 occurs through the clumping together of the small colloidal particles 

 to larger aggregates and the falling of these aggregates. Hardy 

 proved this in two ways: first, by carefully neutralizing acid white 

 of egg with NaHO, until the particles no longer migrated with the 

 positive or negative electric current. As soon as this occurred, a 

 slight mechanical agitation of the particles was sufficient to produce a 

 precipitation of the white of egg. The second proof consisted in show- 

 ing that when a constant current is sent through the solution, the par- 

 ticles that are carried to the electrode are precipitated. At the pole 

 the particles lose their charge and become isoelectric with the surround- 

 ing water. It is, however, not impossible that acid or alkaline white 

 of egg is soluble, while the neutral white of egg is insoluble, or less 

 soluble, in water. 



It had been known for a long time that water which was rendered 

 opaque through a suspension of small particles could be made clear 

 if salts were added to the suspension. A similar experience had been 

 made in connection with the precipitation of colloidal particles. It 

 was further known that the precipitating power of various electro- 

 lytes is a function of only one of the two ions, - - mostly the cation, 

 and that it increases with the valency of the active kind of ions. The 

 fact that Freundlich found in experiments with a sol of arsenic sulphide, 

 whose particles have a negative charge, that the precipitating force of 

 salts with a bivalent cation was about seventy times as large as that 

 of salts with a univalent cation, while salts with a trivalent metal pos- 

 sessed a precipitating force five hundred times as large as that of a uni- 

 valent cation, may serve as an example. Hardy added the important 

 fact that in the case of sols with negatively charged particles, the pre- 

 cipitation is due to the cations; while in the case of positive colloids 

 the precipitation is caused by the anions of the precipitating salt. 

 Hardy states that the precipitating power of an ion is an exponential 

 function of its valency.f Freundlich, however, has shown clearly 

 that where the cation of a salt causes the precipitation, the anion is 

 not without some effect It seems quite possible that the facts found 

 by Hardy indicate a purely chemical action of the precipitating salt. 



* Hardy, lot. cit. f Hardy, Jour, of Physiology, Vol. 24, p. 288, 1899. 



