The Sweet Principle of Oils.
Preparation. — Take of Lead Plaster, recently prepared and yet fluid, and Boiling Water, of each, one gallon. Mix them, stir briskly for fifteen minutes, then allow them to cool, and pour off the supernatant fluid. Evaporate this until it has the specific gravity 1.15, and pass a current of sulphohydric acid slowly through it until a black precipitate is no longer produced. Filter, and boil until the sulphohydric acid is driven off. Lastly, evaporate the liquid until it ceases to lose weight. Or it may be prepared by digesting equal parts of ground litharge, (protoxide of lead), and olive oil with a little boiling water, stirring and adding water as it evaporates. When it is of the consistence of soft plaster, it is to be well washed with hot water. Decant and filter, then pass sulphureted hydrogen through the mass, in order to throw down the lead ; after which, filter, and evaporate to a syrup in a water bath. The syrupy product is glycerin, and looks a little like mucilage of gum arabic.
The following mode of preparing glycerin is by Campbell Morfit, M. D., who says : — Glycerin is generally made, on the large scale, either by directly saponifying oil with oxide of lead, or from " the waste," or spent leys of the soap-makers. The first mode of obtaining it is complex and expensive, while in the latter the difficulty of wholly separating the saline matters of the " waste," renders it impossible to obtain a perfectly pure product. In view of these obstacles, and the increasing demand for the article, both in medicine and perfumery, I submit a new process, which has been found, by actual practice, to combine the great and desirable advantages of economy of time, labor, and money.
Take one hundred pounds of oil, tallow, lard, or "stearin" (pressed lard), place it in a clean iron-bound barrel, and melt it by the direct application of a current of steam. While still fluid and hot, add fifteen pounds of lime, previously slaked and made into a milk with two and a half gallons of water, then cover the vessel, and continue the steaming for several hours, or until the completion of the saponification. This is known when a sample of the resulting and cooled soap gives a smooth and lustrous surface on being scraped with the finger-nail, and breaks with a cracking noise. By this treatment, the fat is decomposed, its acids unite with the lime to form insoluble lime soap, while the eliminated glycerin remains in solution in the water along with the excess of lime. After it has been sufficiently boiled, it is allowed to cool and settle, and it is then to be strained through a crash cloth. The soap is reserved for sale to stearic candle-makers, or else may be reconverted into a saleable fat by the process given at pp. 432, 445 Morfit's "Applied Chemistry."
" The strained liquid contains only the glycerin and excess of lime. It must be carefully concentrated by steam heat. During evaporation, a portion of the lime is deposited on account of its lesser solubility in hot than cold water. The remainder is removed by treating the evaporated liquid with a current of carbonic acid gas, boiling by steam heat, to convert any soluble 6i-carbonate of lime that may have been formed, into insoluble neutral carbonate, allowing repose, decanting or straining off the clear supernatant liquid from the precipitated carbonate of lime, and further evaporating, as before, if necessary, to drive off any excess of water. As nothing fixed or injurious is employed in the process, the glycerin thus prepared will be absolutely pure." — Silliman's Journal.
History. — Glycerin was discovered by Scheele, who called it the sweet principle of oils and fats. When perfectly pure and anhydrous, glycerin is colorless, or straw-colored, having a sweet taste and syrupy consistence, with a faint but not disagreeable odor. It combines readily with water, alcohol, or oils; dissolves many gums and resinous substances; does not crystallize, nor ferment like sugar ; will not evaporate beyond a certain point, and is destroyed by boiling. It is insoluble in fatty matter, and can only be incorporated with it mechanically, to effect which, it is necessary that the fat should have a soft consistence, which may be imparted to it by combining it with oil of sweet almonds, or some other fixed oil. It is insoluble in ether, does not evaporate on exposure to the air, and becomes decomposed by distillation. It burns with a blue flame when exposed to a full red heat. It forms sulphoglyceric acid with sulphuric acid, and in union with sulphuric and nitric acids it is converted into a liquid called Pyroglycerin, which is explosive and very poisonous. Diluted with water, the absence of lead may be determined by its affording no precipitate with hydrosulphate of ammonia; and that of iron by not precipitating with ferrocyanuret of potassium. Its formula is C6 H7, O5 + HO.
Properties and Uses. — Stimulant, antiseptic, and demulcent. Used in prurigo, psoriasis, impetigo, lichen, lepra, ptyriasis, herpes exedens, and some syphilitic and strumous eruptions. It may be added to poultices and lotions in a proportion varying from one-fourth to one-sixteenth. It acts as an emollient and soothing application, absorbing moisture from the air, and preventing the parts to which it is applied from becoming too dry. One-sixteenth of a grain, added to a few grains of borax and rose-water, furnishes one of the most elegant and efficacious washes for chapped hands, face, lips, or nipples. Pills and extracts, incorporated with a small proportion of glycerin, are preserved soft and free from moldiness. It has been highly recommended for deafness, in which there is a partial or total absence of ceruminous secretion, by protecting the tympanum, and gradually restoring the parts to their natural condition ; it is likewise said to cause hearing in cases where the tympanum is thickened and indurated, or where it is in a sound state or destroyed by ulceration, but in this last case it is not permanent ; and when there is a hardness of the cerumen, and induration of the tympanum, it has proved very successful, in several cases treated by Prof. R. S. Newton, and other practitioners. The plan is to moisten wool with the glycerin, pure or diluted with water, and pass it into the ear. The bland and unirritating character of pure glycerin, its permanence, when exposed to the atmosphere, and the completeness with which it shields the parts covered by it, render it susceptible of many important applications. Mr. J. H. Ecky has given a formula for the preparation of a glycerin ointment, especially useful for chapped hands, lips, excoriations of the skin, etc. It will also serve as a medium for applying powders, etc. to ulcers, cutaneous affections, or other difficulties, by combining them with it, in the desired proportions. The formula is as follows : Melt together spermaceti half an ounce, and white wax one drachm, with oil of almonds two fluidounces, at a moderate heat ; put these into a Wedgewood mortar, add glycerin one fluidounce, and rub together until well mixed and cold.
Dr. Goddard has given a formula for a very adhesive glycerin paste, suitable for fixing paper labels to glass and other surfaces, and which keeps well ; it is to dissolve an ounce of gum arabic in two fluidounces of boiling water, add two fluidrachms of glycerin, and strain if necessary. This forms a valuable paste for druggists, chemists and others.