Book excerpt: The colloidal state of matter from the physical-chemical viewpoint. The elementary structure of matter. Adsorption. Electrical concepts and their importance in colloidal dispersion. The meaning of hydrogen-ion concentration and its importance in colloidal dispersion. Orientation. Gel structure and the donnan theory of membrane equilibrium. The protective action of colloids in dispersion. The dispersion of solids and liquids in gas. The theory of emulsions and emulsification. Dispersion of solids and liquids in liquids. The colloid mill and some of its applications. Laboratory methods and physical testing of properties.