Gelatine, an azotized substance obtained from various parts of the animal body, such as the white fibrous tissue, the skin and serous membranes, and cartilage, by boiling in water. The substance as it exists in the body is probably not precisely the same as that obtained by boiling, although it cannot be said with certainty that the proportions of its chemical constituents have been changed. No precise formula of equivalents has been established, and it is therefore usual to write the composition of gelatine in percentage parts by weight. According to Mulder it consists, in 100 parts, of carbon 50.40, hydrogen 6.64, nitrogen 18.34, and oxygen and sulphur 24.62, of which about 0.7, according to Verdeil, is sulphur; but the presence of sulphur is disputed, and gelatine, although an azotized, is not a proteine substance. Fremy and Scherer make the percentage of nitrogen rather less than that here given. The gelatine of commerce is prepared as follows: The skins of calves' heads and other thick pieces which are unfit for the manufacture of leather are first freed from hair and thoroughly cleaned of flesh and fat, and well washed.
They are then reduced by cutting machinery to small pieces or to a pulp, cold water being allowed to run through the pieces during this operation in order to remove all impurities. The pieces of skin or pulp are differently treated by different manufacturers in order to obtain the solution most readily, some employing the mechanical force of rollers in conjunction with the application of a temperature varying from 230° to 250° F. When the solution is obtained it is clarified with some albuminous matter, as the white of eggs or ox blood, and after settling is drawn off upon shallow coolers, as plates of glass or slates. When partially dry, so that it can be cut into convenient shapes for handling, it is removed upon nets or placed in a vacuum drying apparatus to complete the process of desiccation. In the course of the preparation the substance is flavored with essences. Bones and ivory also are made to yield gelatine by subjecting them, when crushed, to water boiling at high temperatures in a digester, or to the action of steam gradually raised to the pressure of 32 pounds to the square inch, and thus kept for 3 1/2 hours.
By this means their soluble portions are taken up, and the earthy matters, about 60 per cent. of the whole weight, are left behind, together with a soapy substance produced from the fat and lime of the bones. This residue is used for the manufacture of bone black, or the preparation of phosphorus, and is besides an excellent material for composts. The manufacture of gelatine has been largely carried on in France by first removing the earthy salts from bones by digesting them for many days in dilute hydrochloric acid, and afterward in boiling water.-For a long time gelatine was largely employed in the hospitals and pauper establishments of Paris, as a cheap and, as it was believed, very nutritive material for soups. Its value for this purpose was at last questioned, and the commission appointed to investigate its qualities reported unfavorably. It is, however, generally regarded as possessing nutritive properties, though in a less degree than fibrine and albumen; and even if insufficient itself to support life, its almost universal use in some form of food attests its importance as an article of diet.
It also finds numerous other uses, as for the clarifying of liquors, in the manufacture of cements, as a chemical test for tannin, and in pharmacy for coating pills and forming pouches or capsules in which disagreeable medicines may be concealed and swallowed without disgust. It is also applied in the dressing of silk and other stuffs. It is made by the French into thin transparent sheets called papier glace, which are used for copying drawings; and they also prepare from it artificial flowers richly colored to imitate the natural specimens, or presenting the appearance, in their glittering and semi-transparent substance, of flowers wet with dew or drenched with rain. Another ap-. plication of gelatine is for taking casts or forming moulds of objects presenting complicated forms, for retaining which plaster is nut sufficiently adhesive. A series of casts in imitation of ivory were produced in this substance in 1844 by M. Franchi, for which the prize of the London society of arts was awarded in 1846. He afterward obtained gelatine casts from moulds of the same substance, the lines being perfectly retained in their original sharpness, He also took casts in gelatine from flat models, which he applied to cylindrical bodies, thus saving much expense in the carving or construction of intricate models.-Pure gelatine is colorless, transparent, inodorous, and insipid.
It should be tested for smell by putting it in boiling water, as when dry the odor of glue may not be perceived. It softens and swells in cold water, but does not dissolve till heat is applied, a property which distinguishes it from fibrine and albumen. According to Bos-tock, one part of gelatine dissolved in 100 of water gelatinizes on cooling, but in 150 parts it remains liquid. When the solution is repeatedly warmed and cooled, especially if boiled, it loses its tendency to gelatinize, and becomes more and more soluble in cold water. Gelatine is soluble in all the dilute acids except tannic, in which respect it differs much from albumen. It is precipitated from aqueous solutions by excess of alcohol. Tannic acid is a very delicate test; when added to a solution of one part of gelatine in 5,000 parts of water, it will render it cloudy; when added to a strong solution, a dense curdy precipitate falls, which is the same substance as the basis of leather. Gelatine is rendered insoluble when mixed with chromic acid and exposed to the action of light.
This property is applied in the manufacture of imitations of ivory, and in the reproduction of photographic prints, according to the invention of Woodbury and Albert.-Gelatine which is obtained from the sounds of fishes is called isinglass, and an impure variety is known as glue. (See Glue, and Isinglass.)