or JELLY, an animal product which is extracted by long-continued boiling in water from all the hard and solid parts, such as the skin, cartilages, bones, ligaments, tendons, membranes, and muscles. By the slow evaporation of the water which thus holds it in solution, the gelatine may be obtained in a state of purity, when it appears as a hard, brittle, semi-transparent substance, which breaks with a glassy fracture. One of the most striking characteristics of gelatine is the property of forming a tremulous jelly when its solution in boiling water cools, with that of liquefying again on the application of heat. These alternate solutions and desiccations may be repeated for any number of times without changing its chemical constitution. Isinglass, glue, and size are various forms of gelatine, the first being this substance in a state of comparative purity. Solid gelatine undergoes no change if it be kept perfectly dry; but in the form of solution or of jelly it putrefies rapidly. Tannin added to a solution of gelatine occasions a copious white precipitate. This is a compound of tannin and gelatine, which collects into an elastic adhesive mass, that soon dries in the air, and forms a brittle resinous-like substance, which is perfectly insoluble in water, and not susceptible of putrefaction. It is this action of tannin upon gelatine that is the foundation of the art of tanning leather. The ultimate components of gelatine are 47·8 carbon, 7·9 hydrogen, 16·9 nitrogen, 27·4 oxygen.

Gelatine is insoluble in alcohol, but when already in solution in water it is not precipitated by that fluid. Acids dissolve it with great facility, even when much diluted, especially when aided by heat. The nitric acid effects its decomposition, during which nitrogen, and then nitrous gas, are disengaged in considerable quantities; and oxalic and malic acids are evolved, and may be obtained from the residuum. Sulphuric acid, with the assistance of heat, partly converts it into a substance resembling sugar. Chlorine combines with gelatine, forming a white substance, which assumes the form of filaments. The pure liquid alkalies dissolve gelatine very readily. The solution is a brown viscid substance, which possesses none of the properties of soap, and is not precipitated by acids. This property of remaining dissolved after acids are added to the alkaline solution, distinguishes gelatine from albumen, fibrin, and other animal products, and is therefore a valuable mode of discriminating its presence, and of separating it from them in analysis. It is precipitated by several of the metallic salts and oxides, but not so unequivocally as to afford satisfactory tests of its presence.

Gelatine is a nutritious article of food, and is very largely used as such; but as it cannot yield albumen, fibrin, or