_Decompositions._--When aqueous solutions of gelatin are heated under pressure, or in presence of glycerin and other bodies which raise the boiling-point, or more slowly at lower temperatures, they gradually lose the power of gelatinising on cooling, the gelatin being converted into modifications soluble in cold water, but still capable of being precipitated by tannin. Hofmeister[31] states that the gelatin takes up 3 molecules water and is split up into _hemicollin_, soluble in alcohol and not precipitated by platinic chloride solution; and _semiglutin_, insoluble in alcohol and precipitated by platinic chloride solution. Both are precipitated by mercuric chloride. Dry gelatin is soluble in glycerin at high temperatures, but probably suffers a similar change. Hence high temperatures and long-continued heating must be avoided in gelatin manufacture; and in making printers’ roller compositions, which are mixtures of gelatin and glycerin, the gelatin must be swollen with water and melted at a low temperature with the glycerin. [31] Bied. Centr., 1880, p. 772, and Ch. Soc. Abs., 1881, p. 294. Gelatin is also converted into soluble forms (peptones), perhaps identical with the above, by the action of heat in presence of dilute acids and alkalies. These, like gelatin, are precipitated by tannin and by metaphosphoric acid.[32] Heated for longer periods or to higher temperatures with aqueous solutions of the caustic alkalies, baryta, or lime, gelatin is gradually broken down into simpler and simpler products, ending in nitrogen or ammonia, water and carbonic acid. Among the intermediate products may be mentioned various acids of the amido-acetic series, as amido-acetic (glycocine, glycocoll), amido-propionic (alanine), and amido-caproic (leucine); and of the amido-succinic series (amido-succinic = aspartic acid).[33] [32] Lorenz, Pflüger’s Arch., xlvii. pp. 189-95; Journ. Chem. Soc., 1891, A. p. 477. [33] Compare Schützenberger, Comptes Rend., cii. pp. 1296-9; Journ. Chem. Soc., 1886, A. p. 818. Treatment with acids produces very similar effects. The first products are soluble peptones. Paal[34] on treating 100 parts of gelatin on the water-bath with 160 parts water and 40 parts concentrated HCl till the product was soluble in absolute alcohol, obtained, on purification, a white hygroscopic mass of peptone salts containing 10-12 per cent. of hydrochloric acid.[35] [34] Berichte, xxv. pp. 1202-36; Journ. Chem. Soc., 1892, A. p. 895. [35] See also Buchner and Curtius, Ber., xix. pp. 850-9; Journ. Chem. Soc., 1886, A. p. 635. The products of digestion of gelatin with gastric and pancreatic juice are peptones which do not differ materially from gelatin in ultimate composition, and the action is probably mainly hydrolytic.[36] [36] Chittenden and Solly, Journ. Chem. Soc., 1891, A. p. 849.