GELATIN (Fr. gelatine; Germ. Lein. Gallerte). This term is applied to an im portant principle obtained by boiling certain animal substances in water, and filtering or straining the solution, which, if sufficiently concentrated, gelatinises, or concretes into a translucent tremulous mass on cooling, which may be again liquefied and gelatinised by heat and cold. Many varieties of gelatin occur in commerce, of which glue is perhaps the most important : it is obtained by boiling the refuse pieces of skin and hide, and the scrapings and clippings from the tan-yard, in a sufficient quantity of water, till a sample taken out of the boiler forms, on cooling, a stiff jelly ; the solution is then strained whilst hot, and run into coolers, where it concretes, and is afterwards cut by a wire into slices, which are dried upon nets. Membranes, tendons, cartilage, horn-shavings, and other similar sub stances also yield a jelly, which, however, is less stiff and binding than the former, espe cially when obtained from young animals : size is a jelly of this description. Isinglass, which consists of several parts of the entrails of fish, and especially the sound, &c. of the sturgeon, yields a very pure and tasteless jelly, which is chiefly used for the table ; the jelly of calves' feet and hartshorn-shavings is some what similar.
As jelly cannot be extracted by cold water, and as we have no direct evidence of its existence in the various substances from which it is obtained previous to the action of boiling water, and, moreover, as it does not occur in any of the animal fluids or secretions, it has been regarded by some chemists, and especially by Berzelius, as a product of the action of water and heat, and not as a mere ediict. Ile compares its formation to the conversion of starch into gum and sugar, and remarks that in both cases the change is ac celerated by the presence of dilute acids.
Pure gelatin is colourless, transparent, in odorous, insipid, and neither acid nor alkaline ; heat softens it and exhales a peculiar odour, and it burns with smoke and flame, leaving a bulky coal, difficult of incineration and containing phosphate of lime: it yields much ammonia, and the other ordinary products of analogous animal compounds, when subjected to destructive distillation.
In cold water dry gelatin swells and be comes opaque, and when gently heated it dissolves and forms a clear colourless solution, which gelatinises when cold. According to Dr. Rostock, one part of isinglass to 100 parts of water yields a perfect jelly, but with 180 of water it does not concrete.* Those modifications of gelatin which are the least soluble in hot water yield the strongest jelly. When the same portion of jelly is repeatedly liquefied and cooled, it gradually loses the property of gelatinising, and becomes so far modified as to leave a brownish gummy residue when evaporated, which readily dissolves in cold water. L. Gmelin kept a solution of isinglass in a sealed tube for several weeks at the temperature of 212° : it was thus changed to the consistency of turpentine, was deliquescent, soluble in cold water, and par tially so in alcohol.
An aqueous solution of gelatin exposed for some time to the air at the temperature of 60° to 70° becomes at first thinner and sour, and afterwards ammoniacal and fetid : the addition of acetic acid prevents the putrefac tion without impairing the adhesive power of the gelatin.
Gelatin is insoluble in alcohol and ether, and in the fixed and volatile oils. When a strung aqueous solution of gelatin is dropped into alcohol, it forms a white adhesive and elastic mass, which adheres strongly to the glass, and which, like dry gelatin, softenS, but does not dissolve in cold water.
When chlorine is passed through a warm and somewhat concentrated solution of gelatin, each bubble becomes covered with an elastic film, and deposits, on bursting, a white, tough viscid matter ; the whole of the gelatin is thus precipitated, and free muriatic acid is formed. This chloride of gelatin is insoluble in water and alcohol, and remains acid, and smells of chlorine, even after it has been kneaded in warm water Dissolved in caustic am monia in a tube over mercury, it evolves nitrogen and becomes mucilaginous. It is soluble in acetic acid; but the solution, though rendered turbid by dilution, gives no preci pitate by ferro-cyanuret of potassium, so that the gelatin is not thus converted into albumen, No analogous compound is produced either by iodine or bromine.