SUSPENSOIDS 389 



If osmotic pressure is ultimately caused by the impact of 

 particles upon the walls of the containing vessel, then the 

 more sluggish larger particles would produce fewer impacts 

 and therefore a lower osmotic pressure than the more rapidly 

 moving particles of molecular dimensions. Direct determina- 

 tions by Starling, Lillie, Moore and Roaf, Bayliss, and others 

 have indeed shown that colloids have a small but measur- 

 able osmotic pressure which is not due to any accidentally 

 adhering crystalloidal impurities. 



Assuming, then, that colloidal solutions are two phase 

 systems, a phase being any particle of matter bounded by its 

 own surface, two classes of such solutions are distinguished : — 



1. Suspensoids in which, as in a true suspension, the 

 discontinuous or disperse phase is a solid while the continuous 

 phase is a liquid ; and 



2. Emulsoids in which, as in an emulsion, both the con- 

 tinuous and the disperse phases are liquid. 



SUSPENSOIDS. 



Although the suspensoids are biologically of but slight 

 importance, since only the inorganic colloidal solutions belong 

 to this group, a brief description of their properties is essential 

 in a survey of the whole subject. 



Colloidal solutions of otherwise insoluble substances may 

 be obtained in a variety of ways — such as electrical disintegra- 

 tion of the metals, reduction of metallic salts, the formation 

 of the substance under special conditions or in particular 

 solvents, etc., for the details of which one of the many text- 

 books on Colloidal Chemistry may be consulted. 



GENERAL PROPERTIES OF SUSPENSOIDS. 

 A. Optical Properties. 



Suspensoid sols, as a general rule, appear more or less clear 

 to the unaided eye, but are frequently highly coloured. This 

 is notably so in the case of the metallic sols such as gold and 

 silver, which may be obtained in a variety of different shades, 

 depending on the method of preparation and the consequent 

 size of the particles. Thus gold sols may be either blue, 



