Colloidal dispersions may be divided into two classes: those in which the colloidal state of the dispersed substance is an extrinsic one, and those in which the dispersed substance is intrinsically colloidal. The extrinsic colloids are obtained by indirect means, in which one substance is finely pulverized or precipitated as fine particles from a homogeneous phase and distributed in a medium in which the subdivided material is in the ordinary sense insoluble. The dispersed condition is thermodynamically unstable, but may be pseudo‐stable owing to the protective effect of an electric double‐layer or to the presence of an intrinsic colloid that sticks to the particles of the extrinsic colloid. Intrinsic colloids, on the other hand, spontaneously disperse into suitable liquids in virtue of an affinity between colloid and medium which closely resembles that between solvent and solute in ordinary solutions. As a matter of fact there is evidence for believing that some intrinsic colloids, such as egg albumin, hemoglobin, and several other proteins, form molecular solutions just as truly as sugar, alcohol, and similar materials of small molecular weight. The colloidal behavior of these protein solutions is a consequence of the large size of the dispersed particle, even though each particle may be a single molecule, exceeding in size, for instance, the crystalline chunks in a colloidal gold solution. Rubber, agar, gums, cellulose and polysaccharides and their derivatives (e. g., cellulose esters and ethers), some dyes and soaps, and various naturally occurring or synthetic polymeric materials of high molecular weight are also intrinsic colloids. Their dispersions in spontaneously dispersing liquids resemble in essential respects the systems ordinarily called solutions, and they also may with propriety be called solutions. The absence of a well‐defined solubility, which is sometimes offered as a feature distinguishing between solutions in the universally accepted sense and solutions of intrinsic colloids, is in fact, due to the failure of most intrinsic colloids to form a well‐defined solid (or liquid) phase of unique composition, upon the existence of which the quantitative concept of solubility depends.