Formic acid reduces the oxides of gold, silver, and mercury, to the metallic state, and is itself oxidized into carbonic acid. The alkaline formiates also possess the same properties.
Gelatine.
Symbol, C{13}H{10}O{5}N{2}. Atomic weight, 156.
This is an organic substance somewhat analogous to albumen, but differing from it in properties. It is obtained by subjecting bones, hoofs, horns, calves' feet, etc., to the action of boiling water. The jelly formed on cooling is termed size, or when dried or cut into slices, glue. Gelatine, as it is sold in the shops, is a pure form of glue. Isinglass is gelatine prepared, chiefly in Russia, from the air-bladders of certain species of sturgeon.
Properties of Gelatine.—Gelatine softens and swells up in cold water, but does not dissolve until heated: the hot solution, on cooling, forms a tremulous jelly. One ounce f cold water will retain about three grains of isinglass without gelatinizing; but much depends upon the temperature, a few degrees greatly affecting the result.
Gelatine forms no compound with oxide of silver analogous to the albuminate of silver; which fact explains the difference in the photographic properties of albumen and gelatine.
Glycerine.
Fatty bodies are resolved by treatment with an alkali into an acid—which combines with the alkali, forming a soap,—and glycerine, remaining in solution.
Pure glycerine, as obtained by Price's patent process of distillation, is a viscid liquid of sp. gr. about 1·23; miscible in all proportions with water and alcohol. It is peculiarly a neutral substance, exhibiting no tendency to combine with acids or bases. It has little or no action upon nitrate of silver in the dark, and reduces it very slowly even when exposed to light.