Large numbers of industrial processes are based upon colloidal phenomena. Many of these processes were known and practiced long before the nature of the phenomenon itself was understood. But with the coming of the knowledge of the nature, causes, and possibilities of the control, of the colloidal condition of the materials involved, immense improvements in the economy of the process, or the quality of the end-products, have been worked out, in many cases. Many volumes of treatises concerning the industrial applications of colloidal phenomena have been written. Any discussion of these would be out of place here; but the following list of examples will serve to illustrate the immense importance of these matters both in industry and to the needs of everyday life: the tanning of leather; the dyeing of fabrics; vulcanizing rubber; mercerizing cotton; sizing textile fabrics; manufacture of mucilages and glues; manufacture of hardened casein goods; manufacture of celluloid; production of colloidal graphite for lubrication; the prevention of the smoke nuisance by electric deposition; the purification of sewage; the manufacture of soaps; the manufacture of butter, cheese, and ice cream; fruit jellies, salad dressings, etc. This list could be extended to a great length, but is already long enough to emphasize the very great importance and practical value of colloidal phenomena in daily life. NATURAL COLLOIDAL PHENOMENA Many of the phenomena of nature are colloidal in character. These may be observed in the mineral, the animal, and the vegetable kingdoms. Here, again, a lengthy discussion of the nature of these phenomena would be out of place in this connection, and a few typical examples will serve to illustrate the general importance in nature of this property of matter. In the soil, the following properties are easily recognizable as definite colloidal phenomena: water-holding capacity of clays, silts, loams, etc.; adsorption (or "fixation") of soluble plant foods so that they are not readily leached out of the soil by drainage; flocculation and deflocculation of clay, etc. In the animal body; the contraction of muscles, the conveyance of nerve stimuli, etc., are undoubtedly accomplished by colloidal changes; and the existence of insoluble casein and fat in colloidal form in milk insures the proper nourishment of the young of nearly all species of animals. In both plants and animals, as will be pointed out in the following chapter, practically all the vital activities of the cell protoplasm are definite manifestations of colloidal phenomena. Enzymes perform their catalytic functions by reason of their colloidal form. Proteins exist in colloidal form and are the seat of all vital functions. The regulation of the passage of materials into and out of the cell is governed by minute changes in the electrolyte concentration, etc., which produce enormous changes in the colloidal character of the protoplasm.