[26] Compt. Rend., lxxxii. pp. 262-4.
The addition of a molecule of water would make a difference in the percentage composition indicated by these formulæ which would be less than their probable experimental error, and the change may therefore be one of hydration.
Gelatin certainly contains both carboxyl and amido-groups, and is capable of combining with both acids and alkalies (see [p. 84]).
Reimer[27] obtained what he supposed to be pure unaltered fibre-substance by digestion of purified hide with 1⁄2 per cent. acetic acid for many days and subsequent neutralisation. His analysis showed C = 48·45 per cent., H = 6·66 per cent., N = 18·45 per cent., O = 26·44 per cent., thus deviating considerably from direct analysis of unaltered skin. It is obvious that little weight can be placed on this result, Reimer’s precipitate being probably a mere decomposition product.
[27] Ding. Polyt., ccv. p. 164.
Hofmeister[28] notes that on heating gelatin it loses water and forms an anhydride which he considers identical with collagen or hide-fibre. When gelatin is dried at a temperature of 130° C. it becomes incapable of solution in water, even at boiling temperature, and can only be dissolved by heating under pressure. It is certain that collagen (hide-fibre, ossein) is less easily soluble in hot water than ordinary gelatin.
[28] Bied. Centr., 1880, p. 772.
So far as our present knowledge goes, we may regard hide-fibre as merely an organised and perhaps dehydrated gelatin.
Gelatin or glutin (not to be confounded with the gluten of cereals), when pure and dry is a colourless, transparent solid of horny toughness and of sp. gr. 1·3. It begins to melt about 140° C., at the same time undergoing decomposition. It is insoluble in hydrocarbons, in ether, or in strong alcohol. In cold water it swells to a transparent jelly, absorbing several times its weight of water, but does not dissolve. In hot water it is soluble, but a solution containing even 1 per cent. of good gelatin sets to a weak jelly on cooling. Gelatin jellies melt at temperatures which vary considerably with the quality or freedom from degradation products, but which within pretty wide limits (5-10 per cent.) are little affected by the concentration. A 10 per cent. solution of best hard gelatin melts about 38° C., while low glue may fail to set at 15° C. A useful technical test for the setting power of gelatin, based on this fact, consists in placing an angular fragment of the jelly in a small tube attached to a thermometer, and stirring in a beaker of water, which is slowly heated till the jelly melts, when the temperature is noted. The exact point is perhaps more easily seen if the tube is drawn to a conical point. The jelly may also be allowed to set in capillary tubes open at the bottom, and the moment noted when water rises into the tube. The temperature of fusion is raised by the addition of formaldehyde, salts of chromium, alumina and ferric salts, which produce a tanning effect, and in a less degree by sulphates, tartrates, acetates, some other salts, and diminished by iodides, bromides, chlorides and nitrates.[29] Solutions of gelatin too weak or too warm to gelatinise possess considerable viscosity. Gelatin may therefore be estimated, in the absence of other viscous matters, by the viscosimeter, an instrument which measures the time taken by a liquid in flowing through a capillary tube.[30] The firmness of a jelly, which is often important for commercial purposes, is frequently measured by Lipowitz’s method, in which a slightly convex disc, conveniently of exactly 1 cm. diameter, and cemented to the bottom of a thistle-head funnel tube, is loaded gradually with mercury till it sinks in the jelly. The jelly (5 or 10 per cent.) should be allowed to set some hours before the test is made.
[29] See Pascheles, ‘Versuche über Quellung,’ Archiv für ges. Path., Bd. 71.