292 COLLOIDS 



never viscous. This difference is accounted for by assuming 

 that the non-viscous .solutions are, in reality, of the nature of 

 suspensions of extremely minute solid particles in a liquid 

 medium, whereas the viscous solutions consist of liquid particles 

 suspended in a liquid medium. 



Or, expressing it somewhat differently, it may be said that 

 in the one case the disperse phase is a solid and the continuous 

 phase a liquid, while in the other case both phases are liquid. 



On this assumption rests the classification of colloidal 

 solutions into Suspensoids and Emulsoids. 



PROPERTIES OF SUSPENSOIDS. 



Although only the inorganic colloidal solutions belong to 

 this class, so that they are relatively unimportant biologically, 

 a brief description of their properties is essential to a survey 

 of the whole subject. 



The methods of preparation of inorganic suspensoids such 

 as 



(a) The disintegration of metals by means of electric sparks 

 under water ; 



() The reduction of dilute solutions of salts of the metals 

 by various reducing agents ; 



(c) The passing of sulphuretted hydrogen through arseni- 

 ous acid, whereby the arsenic trisulphide, instead of 

 being precipitated, remains in colloidal solution, 

 all point to the presence of solid particles in such solutions. 



Examination under the ultramicroscope reveals the fact 

 that all these solutions contain extremely minute solid particles, 

 the surface of which, from mathematical considerations, must 

 be enormous in comparison to their mass. It is natural that 

 such particles should be electrically charged owing to contact 

 electrification, and they will therefore repel each other and so 

 prevent aggregation to larger particles with consequent pre- 

 cipitation. This, moreover, accounts for their susceptibility 

 to electrolytes, since the negatively charged particles become 

 discharged by the positive ion of the electrolyte, whereupon 

 the discharged particles are able to unite and form larger ones 

 and so be precipitated. 



This effect may be seen by the addition of a few drops 

 of hydrochloric acid to a colloidal arsenic sulphide solution 



