On the subdivision, classification and nomenclature of the proteins much ink has been spilled, and it is impossible in this volume to go into the various systems which have been suggested. It should be noted, however, that some writers habitually use the terms "proteid" or "albuminoid" as synonyms for protein. The classification of proteins adopted in this work is used because it is the most suitable for a volume on industrial chemistry and has the additional merits that it is simple and is already used in several standard works on industrial chemistry. It is based upon the behaviour of the proteins towards water, a matter of obvious moment in manufacturing processes. On this basis proteins may be divided into albumins, keratins and gelatins. Cold water dissolves the albumins, does not affect the keratins, and only swells the gelatins. The behaviour in hot water confirms and elaborates the classification. When heated in water, the albumins coagulate at temperatures of 70°-75° C., the gelatins (if swollen) dissolve readily, whilst the keratins only dissolve at temperatures above 100° C. Albumins and keratins may be distinguished also from gelatins by adding acetic acid and potassium ferrocyanide to their aqueous solutions. Albumins and keratins give a precipitate, gelatins do not. Another distinguishing reaction is to boil with alcohol, wash with ether, and heat with hydrochloric acid (S.G. 1.2). Albumins give a violet colour, keratins and gelatins do not. =Albumins= may be first discussed. They are typified by the casein of milk and by white of egg. Their solutions in water are faintly alkaline, optically active, and lævorotatory. They are coagulated by heat and also by mineral acids, alcohol, and by many poisons. The temperature of coagulation (usually about 72° C.) is affected by mineral salts, the effect being in lyotrope order (see Part V., Section I.). The coagulated albumin behaves in most respects like a keratin. Some of the albumins (globulins) are, strictly speaking, not soluble in cold water, but readily dissolve in weak solutions of salt. The albumins are coagulated from these solutions, as usual, when heated. Into this special class fall myosin (of the muscles), fibrinogen (of the blood) and vitellin (of egg yolk). By a gentle or limited hydrolysis of the albumins with dilute acids in the cold, a group of compounds called albuminates are obtained. They dissolve in either acids or alkalies, and are precipitated by exact neutralization. They may also be "salted" out by adding sodium chloride or magnesium sulphate. They are not coagulated by heat. After further hydrolysis with either acids, alkalies or ferments, very soluble compounds are obtained called albumin peptones or albumoses. These are soluble in alkalies, acids and water, and are readily hydrolyzed further into amido acids and acid amides. They are very similar to the peptones obtained from keratins and gelatins. They are not coagulated by heat.