sence of an electric charge on the particles of a
sol and a determination of its sign can be made by 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 electric current through the contents of the
tube, keeping the electrodes 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, containing less than 1 per
cent of the dispersed solid; while emulsoids may be prepared with widely
varying proportions of the two component 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.

Surface adsorption is