COLLOIDAL MIXTURES. 273 



Colloidal Suspensions. 



Methods of Preparation. — The mixtures of this class have been, for 

 the most part, prepared artificially. The principles of some of the 

 methods which have been employed for this purpose may, therefore, be 

 first described. 



Of these principles the most important one is, that when an in- 

 soluble substance is produced in the absence of electrolytes by a reac- 

 tion between two chemical compounds, it almost invariably separates 

 in the state of a colloidal suspension. By the term electrolyte is here 

 meant any dissolved substance which is a good conductor of electricity, 

 one, therefore, whose molecules are, according to the ionic theory, 

 largely dissociated into electrically charged atoms or atom groups 

 called ions. Most salts and strong acids or bases are such electrolytes ; 

 but water, neutral organic substances like alcohol or sugar, and very 

 weak acids or bases, are not. Electrolytes must not be present in con- 

 siderable quantity, for the reason that ions coagulate these suspensions. 

 Thus, when a saturated solution of hydrogen sulphide, a slightly ion- 

 ized substance, is added to one of arsenious oxide, also slightly ionized, 

 no coagulated precipitate of arsenious sulphide results, but only a 

 turbid yellow liquid, which, when poured through filter-paper, leaves 

 nothing behind. It will be noted that in this case the other product 

 of the reaction is water, an un-ionized compound. If this reaction be 

 carried out with a solution of arsenious chloride, instead of with one of 

 the oxide, the ordinary precipitate of arsenious sulphide is obtained; 

 for, in this case, the hydrochloric acid produced by the reaction, being 

 largely dissociated into hydrogen and chlorine ions, coagulates the col- 

 loidal suspension. So, also, upon adding hydrochloric acid to the col- 

 loidal mixture resulting from the former experiment, a large precipi- 

 tate is immediately produced. As a second illustration of this method, 

 hydrogen sulphide water may be added to a solution of mercuric cyanide. 

 In this case also a black opaque colloidal suspension of the sulphide 

 results; for the three substances involved in the reaction, hydrogen 

 sulphide, mercuric cyanide, and hydrocyanic acid, are non-electrolytes; 

 but, upon the addition of hydrochloric acid, or, still better, of magne- 

 sium chloride, to this solution, the precipitate immediately coagulates. 

 It is not necessary, of course, that electrolytes be entirely excluded, 

 but only that they be not present at any point at such a concentration 

 as will produce coagulation. The method is, therefore, of fairly gen- 

 eral applicability. Thus, a colloidal suspension of Prussian blue can 

 be prepared by mixing dilute solutions of nearly equivalent quantities 

 of ferric chloride and potassium ferrocyanide ; for the other product of 

 the reaction, potassium chloride, has a coagulating effect only at higher 

 concentrations. 



A second method which has until recently been even more com- 



VOL. LXVII. — 18. 



