culation, the gold particles do not settle, even at
the slow rate as calculated above. They remain uniformly distributed
throughout the liquid for an indefinite period or time. The reason for
this phenomenon undoubtedly lies in the fact that these minute particles
carry an electric charge, which, is of the same sign for all of the
particles and results in a repellent action which keeps the particles in
constant motion. This constant motion may easily be conceived to keep the
particles uniformly distributed throughout the liquid, just as constant
shaking would keep those of a mechanical suspension uniformly distributed
through the mixture.

The sign of the electric charge on the particles of a sol may be either
negative or positive, depending upon the chemical nature and dielectric
constants of the two phases of the system. The proportion of the total
electric charge of the system which is of the opposite sign to that borne
by the dispersed particles is, of course, borne by the liquid which
constitutes the other phase. The origin of this electric charge on the
colloidal particles is, as yet, not known with certainty; but it seems
probable that it is due to a partial ionization of these small particles,
similar to, but not so complete as, that which takes place when compounds
which are soluble go into true solution in water, or other solvents which
bring about the dissociation of dissolved substances.

The conditions necessary to bring a solid substance into a colloidal
mixture with some liquid, or, in other words, to produce a suspensoid sol,
require that the proportion of liquid to solid shall be large and some
means of disintegrating the material which is to be dispersed into very
fine particles. Many common chemical reactions, if carried out in very
dilute solutions, result in the production of sols, especially if a small
amount of some emulsoid is present in the reacting mixture; sols produced
in this way are very stable, and the emulsoid which is used in stabilizing
the sol is known as a "protective colloid." Direct methods of
disintegration; such as reduction by chemical agents, discharge of a strong
electrical current through the substance which is to be dispersed while it
is submerged in the liquid, alternate treatment of finely ground material
with alkali and acid so as to frequently change the electric charge, etc.,
are utilized for bringing inorganic compounds into the colloidal state.

Suspensoids usually co