As a result of the long series of investigations which were stimulated by Graham's work, the modern conception is that diffusibility is a _condition_ of matter when in minute subdivision, or in solution, in some liquid, as contrasted with its _state_, or condition, when existing alone. That is, the _state_ of a substance may be either gaseous, liquid, or solid; and its _condition_ when in solution may be either crystalloidal or colloidal. Substances which are in crystalloidal 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.