Gelatin is insoluble in alcohol, ether, and cold water, but swells in the last, absorbing about 40 per cent. It is soluble in hot water, but is reprecipitated on the addition of a sufficient quantity of alcohol, resembling in this respect gum, dextrin, and many other substances. It is soluble in glycerin, with the aid of heat, and in concentrated sulphuric acid in the cold. Moist gelatin exposed to the air rapidly putrefies. It first becomes very acid, from formation of butyric (and perhaps other) acids, but afterwards alkaline, from evolution of ammonia. Boiled with concentrated potash, it yields leucin (amidocaproic acid, C_{6}H_{15}NO_{2}), glycocin (sugar of gelatin), and other substances. The same products are obtained by boiling with sulphuric acid, and probably also more gradually, and in greater or less proportions, by the prolonged action of lime or barium hydrate, by putrefaction, and by any other influence which tends to resolve the gelatin molecule into its simpler parts. Gelatin is precipitated by all tannins, even from very dilute solution. A solution containing 2/10000 parts is rendered turbid by infusion of gall-nuts or gallotannic acid. The precipitate is soluble in excess of gelatin. Solution of gelatin dissolves considerable quantities of lime phosphate, hence this is always largely present in common glue. Gelatin is precipitated by mercuric chloride, in this respect resembling peptones; but not by potassium ferrocyanide, by which it is distinguished from albuminoids; and it differs from albumen in not being coagulated by heat. On the contrary, by prolonged boiling glue loses the property of gelatinising, and becomes soluble in cold water, being split up into two peptones; semi-glutin, which is insoluble in alcohol, and precipitated by platinic chloride; and hemicollin, which is soluble in alcohol, and not precipitated by platinic chloride. Both are precipitated by mercuric chloride (see Hofmeister, abst. Chem. Soc. Jour. 1881, p. 294). Gelatin or glue with about 3 per cent. of potassium dichromate becomes insoluble when exposed to the light, from the formation of a chromium compound. This reaction is the base of several modern photographic processes, and has been used for waterproofing and for cementing glass, &c.