Proteids are an important class of substances, mostly of animal origin, but occurring also in the vegetable kingdom, of which egg albumen may be taken as a good example. The various members of the class are closely related to each other, and amongst them they make up the greater portion of the animal organism. The classification of the proteids is given in the article Animal Chemistry (q.v.). The most careful analyses of the various proteids show that they all closely approximate to the same ultimate composition, and contain about 53.5 per cent. of carbon, 7 of hydrogen, 15.5 of nitrogen, 22.5 of oxygen, and from .9 to 1.6 of sulphur. The majority of the proteids exist in two modifications, the one soluble and the other insoluble in water. The latter modification can be obtained from the former by the addition of alcohol or ether, or of many mineral acids or metallic salts to their aqueous solutions. Coagulation also takes place in most cases by the application of heat, as in the case of egg albumen in boiling water. The proteids are all dissolved by strong solution of acetic acid, and by phosphoric acid. They are also dissolved by alkalies with formation of alkali sulphide. When heated with solution of mercuric nitrate containing a little nitrous acid, they assume a violet-red colour; and when the solution of a proteid substance in acetic acid is mixed with strong sulphuric acid, a violet- coloured solution is obtained, which in the spectrum shows characteristic absorption bands.
By the action of the gastric juice, of pepsin and dilute hydrochloric acid, and of several other ferments, the proteids are eventually converted into peptones. The latter are soluble in water, and are not coagulated by heating. See ALBUMEN, CASEIN, FIBRIN, and GLOBULINS.