4362. To Purity Glycerine

4362.    To Purity Glycerine. Commercial glycerine is rendered pure by diluting it with water; it is then decolored with a little animal charcoal (see No. 1729 (Properties of Charcoal)), filtered, and evaporated to the consistence of a thin syrup, after which it is further evaporated in a vacuum, or over sulphuric acid, until it acquires a specific gravity of 1.265.

4363. To Purify Glycerine

4363.    To Purify Glycerine. Bottles sent out from wholesale and manufacturing houses, labeled, "Pure Glycerine," do not always contain what their labels declare. Some samples called pure are rich in lead, others contain chlorine, most are diluted with water, and the best is generally acid. It is necessary, therefore, to purify even the best retains the form of the bone, is washed in a stream of water, plunged in hot water, and again in cold, to remove all remains of acid, and sometimes put into a solution of carbonate of soda. "When well washed, it is dried on open baskets or nets. By steeping the raw gelatine in cold water, dissolving it in boiling water, evaporating the jelly, and cutting it into tablets, it may be dried and preserved in that form.

4364. Tests for the Purity of Glycerine

4364.    Tests for the Purity of Glycerine. Pure glycerine has a neutral reaction, and on evaporation in a porcelain dish leaves only a very slight carbonaceous crust, while the impure has a much greater percentage of coaly matter. The pure article does not become brown when treated, drop by drop, with concentrated sulphuric acid, even after several hours; the impure becomes brown even when but slightly adulterated. Pure glycerine, treated with pure nitric acid and a solution of nitrate of silver, does not become cloudy, while the impure exhibits a decidedly milky appearance. Sometimes the impure article becomes blackened with the sulphide of ammonium. Oxalate of ammonia produces a black clouding; lime-water sometimes causes a milky discoloration. Pure glycerine, however, constantly remains perfectly uncolored, and clear as water, the impure becoming colored to a greater or less extent. If a few drops are rubbed between the fingers, pure glycerine causes no fatty smell; the contrary is the case with the impure, especially if a few drops of dilute sulphuric acid be introduced. {Roller.) {See No. 1151 (To Test the Purity of Glycerine).)

4365. Gelatine

4365.    Gelatine. Animal jelly obtained by heat from the organic tissue of the bones, tendons, and ligaments, the cellular tissue, the skin, and the serous membranes in contact with water. Glue and size are coarse varieties of this substance, prepared from hoofs, hides, skins, etc.; and isinglass is a purer kind, prepared from the air-bladders and some other membranes of fish. Gelatine is insoluble in cold water, but dissolves with greater or less readiness on the application of heat, forming a tremulous and transparent jelly on cooling. It is insoluble in alcohol or ether, and is decomposed by strong alkali or acid.