Gelatine

Gelatine. That a substance, the solution of which possesses the property of setting to a jelly-like mass, could be obtained from animal products, has been known from comparatively early times. Few early details of it, however, are existent. It is at present chiefly prepared from the hides, sinews, bones, hoofs, horns, etc., of various animals by the action of hot water or steam, or a mixture of both. The various processes employed vary somewhat in their details, but the following may be regarded as typical: - The hide, etc., is cut or broken up into small pieces, which are digested for some days with a weak soda solution at a temperature of about 60° to 70° Fahr. They are then placed in an air-tight chamber until quite soft, and are afterwards washed by cold water in a revolving cylinder. After bleaching, which is effected by means of sulphur dioxide, S02, they are well pressed and are thoroughly digested with hot water until as much as possible is dissolved out. The resulting liquor is strained and allowed to set upon slate or marble slabs. The crude gelatine is then cut up into strips and purified by redissolving, again straining, and allowing to cool. Pure gelatine is a hard, tough, transparent, tasteless, and inodorous substance. In dry air it-remains unchanged, but if exposed to moisture it decomposes and putrifies, becoming at first slightly acid, but then alkaline, owing to the formation of ammonia. If heated, it clears and evolves fumes with a very pungent odour. Its composition varies with the source and manufacture, but generally approximates to carbon 50, oxygen 25, nitrogen 18, hydrogen 7 per cent., and frequently also contains small quantities of sulphur. No satisfactory formula representing the composition has been forthcoming. It is heavier than. water, and if placed in cold water it swells considerably, absorbing from 5 to' 10 times its weight of the liquid, and at about 85° Fahr. (30° C.) it dissolves in the absorbed water. If hot water'be employed for its solution, a liquid is obtained, which sets on cooling if the quantity of gelatih'e present is not less than 1 part to 100 of water. It is insoluble in dilute acids and alkalies, but in strong; acids it generally dissolves or decomposes. It is precipitated from its solutions by tannic acid, 1 part in 5,000 of water being detectable by this means. When used as an article of food, the gelatine should be pure and carefully manufactured; its value for this purpose, however, appears to be much exaggerated in popular opinion. Isinglass (q.v.) and glue (q.v.) are respectively pure and impure forms of gelatine. It finds much employment in the arts and for manufacturing purposes. Thus, in photography it is very extensively used, as for the preparation of dry plates [Gelatinobromide], and in various printing processes. It is also employed largely in the dyeing industry, in electrotyping, waterproofing, in the manufacture of printing ink, etc., while it finds applications for many purely scientific purposes - e.g. bacteria culture [Bacteria] - and, in the household, is used largely for making jellies and various culinary mysteries.