form, in true solution,
exist there in molecular or ionized condition; but, as will be pointed out
below, when in the colloidal condition they exist in aggregates which are
somewhat larger than molecules, but not large enough to be visible as
individual particles under the ordinary microscope, even under the highest
magnification which has yet been obtained. Colloidal particles are,
however, generally visible under the Zigmondy "ultramicroscope." (See
below.)

The use of the word "colloid" as a noun, or as the name for a substance
which is in the colloidal condition, is of the same nature as the use of
the words "gas," "liquid," and "solid," in such statements as "ice is a
solid," "water is a liquid," or "steam is a gas," etc.; i.e., the noun
represents a state or condition rather than an actual object or thing.
Hence, the expression "enzymes are colloids," means only that enzymes exist
in the colloidal condition, and not that enzymes represent a definite type
of substances having the group name "colloids."

THE COLLOIDAL CONDITION A DISPERSION PHENOMENON

When one substance is distributed through the mass of another substance,
the mixture is said to be a "two-phase system," composed of the _dispersed
phase_, or substance, and the _dispersion medium_, or _continuous_ phase,
through which the other substance is distributed. The following examples
illustrate the possibilities of such two-phase systems:

(1) Dispersion medium a gas.
(_a_) Disperse phase a liquid--mist in the air.
(_b_) Disperse phase a solid--smoke or dust in air.

(2) Dispersion medium a liquid.
(_a_) Disperse phase a gas--foams.
(_b_) Disperse phase a liquid--emulsions.
(_c_) Disperse phase a solid--suspensions.

(3) Dispersion medium a solid.
(_a_) Disperse phase a gas--solid foams, pumice stone, etc.
(_b_) Disperse phase a liquid--liquid inclusions in minerals.
(_c_) Disperse phase a solid--alloys, colored glass, etc.

Although the same general principles of physical chemistry apply to all
two-phase systems, the term "colloidal condition" is commonly used only in
connection with a particular type of dispersions, in which the dispersion
medium is a liquid and the dispersed material is either a solid or a
liquid.

Thorough and careful studies have shown that when a solid or a liquid is
introduced into another liquid, and becomes disperse