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Gelatine is colourless, transparent, devoid of taste and smell. It is usually brittle. Its S.G. is about 1.42, and it melts at 140° C. and decomposes. It is insoluble in organic solvents. When swelling in cold water it may absorb up to 12 times its own weight of water. The swollen product is called a
gpt
Jellies easily melt on heating and a colloidal solution of gelatine is obtained. This
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again to a jelly on cooling, even if only 1 per cent. gelatin (or less) be present. The solution is optically active and lævorotatory, but with very variable specific rotation. Some observers have thought that the different gelatins have different specific rotations and may so be distinguished. Gelatins are precipitated from solutions by many reagents, such as alcohol, formalin, quinone, metaphosphoric acid, tannins, and many salt solutions, _e.g._ those of aluminium, chromium and iron, and of mercuric chloride, zinc sulphate, ammonium sulphate, potassium carbonate, acidified brine. Many of these precipitations have analogies in leather manufacture (see Parts I. to IV.). The gelatin peptones or gelatoses are formed by hydrolysis with acids, alkalies, ferment or even by digestion with hot water only. A more detailed description of the properties of gelatine is given in Part V., Section I. Gelatine is sometimes called
gpt
and