552 INFECTION AND RESISTANCE 



repulsion. The extension of this idea has been of great value in 

 colloid investigation. The electrical repulsion will not, of course, 

 necessarily prevent the smaller particles from dissolving and deposit- 

 ing upon the larger ones, unless the solubility is affected by the 

 charge. Concerning this we know nothing. The fact that colloid 

 substances possess little or no solubility in the ordinary sense of the 

 word means such solution and deposition must, of necessity, be a 

 very slow process, and the colloid solution would thus appear to be 

 perfectly stable over very long periods of time. There are many who 

 believe that all such systems are only apparently stable, and that on 

 account of the absence of any sufficiently rapid means of transforma- 

 tion which would allow the stabilizing influences to operate rapidly 

 enough to be perceptible. 



Chemical Properties of Colloids 1. It is reasonable to suppose 

 that the chemical properties of colloid solutions are very much what 

 is to be expected from the chemical nature of the dispersed substance 

 as it is known under other conditions. The colloidal solutions of ar- 

 senic sulphid should therefore react very much as would be expected 

 of arsenic sulphid in general, except in so far as the substance is in a 

 finely divided state in the presence of a dispersing medium (water) 

 in which it is little soluble. Thus colloidal arsenic sulphid is soluble 

 in alkalies and alkaline sulphid just as is the massive form. If a rod 

 of zinc is suspended in a colloidal solution of arsenic sulphid there 

 takes place a slow reaction, lasting over weeks and even months, 

 whereby the sulphur of the sulphid unites with the zinc to form 

 colloidal zinc sulphid, while a black deposit, probably arsenic, is 

 found on the zinc. Chemical reactions with colloids are thus, as 

 a rule, very slow, as is to be expected, but otherwise not essentially 

 unusual. 



2. The exact chemical composition of the disperse phase in a 

 colloidal solution is probably not definitely known in any case. In 

 the case of colloidal metal solutions, such as gold and silver, the sus- 

 pended particles seem to be practically pure metals, but in most cases 

 the composition is very problematical. The 'great variation in the 

 properties of such solutions with variations in the methods of prep- 

 aration are undoubtedly to a great extent due to small differences in 

 composition. Thus the properties of arsenic sulphid vary greatly 

 with* the extent to which free hydrogen sulphid is removed from the 

 solution, which is probably due to the differences in the amount of 

 hydrogen sulphid absorbed or otherwise held by the arsenic sulphid. 

 Linder and Picton believed that amorphous copper sulphid was a 

 definite compound of copper sulphid with hydrogen sulphid. It has 

 also been found that amorphous copper sulphid suspended in water 

 continually deposits free sulphur, the cupric sulphid being at the 

 same time largely converted to cuprous. It seems to be rarely or 

 never the case that the disperse phase may be looked upon as a sub- 



