Glycerine, GLYCEROL, or PROPENYL ALCOHOL, , was discovered by Scheele in 1779, who obtained it in the preparation of lead-plaster, and named it 'the sweet principle of fats.' It is a colorless, viscous, neutral, inodorous fluid, of an intensely sweet taste, is soluble in water and alcohol in all proportions, but is insoluble in ether and in chloroform. Its specific gravity is 1.27. If quickly cooled down, glycerine does not crystallise, but solidifies at 40° C. into a gnm-like mass. In the winter of 1867 it was discovered that some glycerine which was being shipped to England had frozen into a solid crystalline mass; till then glycerine was believed to be uncrystallisable. At 100° it is slightly volatile, but if distilled alone the greater part of it becomes decomposed; it may, however, be distilled without alteration in a current of superheated steam. By this means Wilson succeeded in 1854 in separating heated fats into glycerine and the acid with which it was previously in combination; the glycerine is thus obtained in a high state of concentration as a colourless, syrupy liquid, which can be thus prepared in unlimited quantity.
Glycerine occurs ready formed in a few fats (as, e.g., old palm-oil), and, according to Pasteur, is contained in all fermented liquors, and especially in wine. It is a product of the saponification of the various fats. See SOAP.
Glycerine is a triatomic alcohol—i.e. it is derived from three molecules of water by replacing three atoms of hydrogen by the triatomic radical ; or it may be considered a compound of with three molecules of hydroxyl, —and may be represented by the formula ; and in the animal and many vegetable fats, the three molecules of hydroxyl are replaced by three molecules of the anhydrous fatty acid. In the saponification of these fats—that is to say, when they are treated with potash, soda, or oxide of lead, or under the influence of superheated steam—the fatty acid separates from , which assimilates three molecules of hydroxyl and becomes glycerine. Glycerine forms soluble compounds with baryta, strontia, and lime; and it dissolves oxide of lead and numerous salts. It is found that glycerine is convertible into a true fermentable sugar when treated with a mixture of potassium bichromate and sulphuric acid, or with potassium permanganate in presence of sunlight.
We have already referred to the best mode (Wilson's process) of obtaining glycerine on a large scale; the usual method of obtaining it on a small scale is from olive-oil, which is saponified by treating it with an equal weight of litharge (lead oxide). This is mixed with water, and added to the oil, with which it is boiled till the saponification is complete. The glycerine is dissolved by the water, and is easily separated from the insoluble lead-plaster (a mixture of oleate and palmitate of lead). Any traces of lead are removed by sulphuretted hydrogen, and the water is expelled in vacuo, as the glycerine would turn brown in the open air.
The uses of glycerine are numerous. In medicine it is employed as a local application in diseases of the skin and of the ear; it is used as a solvent for many drugs; and is taken internally for the same purposes as cod-liver oil. It is a valuable preservative fluid for small and delicate anatomical preparations, and it has been applied to the preservation of meat. It is used in perfumery, in calico-printing, and in the preparation of leather. It is used by the wine-dealer to 'improve' the quality of wine, and by the brewer, as it is said, to impart keeping power to beer. Very large quantities of glycerine are required for the production of Nitro-glycerine (q.v.) and other explosives. It has been added to the water in gas-meters with the view of preventing it from freezing. It is used in the manufacture of copying-ink, and is of general application where a lubricating agent is required.
Like the alcohols in general, to which class glycerine belongs, it forms several classes or series of derivatives, the most important of which are its combinations with acids, which are analogous in their composition to the various fats and oils. See Roscoe and Schorlemmer's Treatise on Chemistry, and Schorlemmer's Manual of the Chemistry of the Carbon Compounds.