214 CHEMISTRY OF PLANT LIFE 



placing the solution in a U tube, with a layer of distilled water 

 above the sol in each arm of the tube, and then passing an elec- 

 tric current through the contents of the tube, keeping the elec- 

 trodes in the distilled water, so that the migration of the particles 

 toward one pole or the other can be observed by their appearance 

 in the clear water at that end of the tube; or by passing an electric 

 current through the observation chamber of an ultramicroscope, 

 in which the solution under examination has been placed, and 

 observing the migration of the particles across the field toward 

 either one or the other (positive or negative) electrode. 



Emulsoids and suspensoids differ in their properties in the 

 following respects. * Suspensoids are always very dilute, con- 

 taining less than 1 per cent of the dispersed solid; while emulsoids 

 may be prepared with widely varying proportions of the two com- 

 ponent liquids. ^Suspensoids have a viscosity which is only slightly 

 greater than that of the liquid phase when it exists alone, and their 

 viscosity varies with the proportion of dispersed solid which is 

 present in the sol; while emulsoids have a very high viscosity in 

 all cases. ^ Emulsoids usually form stiff gels when treated with 

 electrolytes; while suspensoids more commonly yield gelatinous 

 precipitates under the same conditions. 



Suspensoids and emulsoids which carry electric charges of 

 opposite sign mutually precipitate each other. But emulsoids 

 often protect suspensoids from precipitation by electrolytes, by 

 forming a protective film around the particles of the suspensoids, 

 which prevents the aggregation of the particles into the precipitate 

 form. 



ADSORPTION 



If a sol be precipitated or coagulated by the action of an 

 electrolyte, substances which may be present in solution in the 

 liquid of the sol are carried out of solution and appear in the gel or 

 precipitate. This phenomenon is known as " adsorption," which 

 means the accumulation of one substance or body upon the surface 

 of another body, as contrasted with " absorption," which means the 

 accumulation of one substance within the interior of another. 

 Since substances which are in the colloidal form have very large 

 relative surface areas, it follows that the opportunity for surface 

 adsorption on colloidal materials is very great. 



