[30] See Prollius, Ding. Polyt. Journ., ccxlix. p. 425, who employs a 1 per cent. solution; also Stützer, Zeit. Ann. Ch., xxxi. pp. 501-15.

Solutions of gelatin from skin and bone are powerfully lævorotatory to polarised light. At 30° C. (A)D = -130°, but temperature and the reaction of the solution have much influence on the value found.

Gelatin is precipitated from aqueous solution by the addition of strong alcohol and concentrated solutions of ammonium sulphate and some other salts. Many other colloid bodies such as dextrin and gums behave similarly. In the absence of these substances, precipitation by alcohol may be utilised for the technical analysis of gelatins and glues, printers’ roller compositions and gelatin confectionery. 25 c.c. of the gelatinous solution, which is preferably of about 10 per cent., is placed in a small beaker tared together with a glass stirring rod, and thrice its volume of absolute alcohol added. On stirring, the gelatin sets firmly on the rod and sides of the beaker, and may be washed with dilute alcohol or even with cold water, dried and weighed. A very pure French gelatin gave 98·6 per cent., while a common bone-glue only yielded about 60 per cent. precipitate. Absolute alcohol withdraws water from gelatin-jelly, leaving a horny mass. Gelatin may also be precipitated completely by saturating its solution with sodium chloride, and then acidifying slightly with sulphuric or hydrochloric acid; and masses of jelly become hardened in acidified salt solution as in alcohol, though a neutral solution has little effect. The cause of this is difficult of explanation, but its bearing on the pickling of sheep-skins ([p. 89]) and the production of white leather ([p. 186]) is obvious.

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.