Oils and fats, whether of animal or vegetable origin, are compounds of certain acids, such as the stearic, margaric, and oleic, with a base named glycerine. (See Oil.) Fatty substances may, in fact, be represented as salts of glycerine, and as such are capable of being resolved into their proximate elements like other salts. Thus, in the process of soap-making, a fat or an oil is saponified by means of potash or soda; that is, the caustic alkali unites with the stearic, margaric, or oleic acid, and glycerine is set free. If, for example, the stearate of glycerine be treated with caustic soda, the stearic acid unites with the soda, and forms stearate of soda, while the glycerine is liberated. A fatty body may also be decomposed by means of oxide of lead, as in the process for making diachylon plaster (Emplastrum plumbi). By boiling a mixture of finely pulverized or newly precipitated oxide of lead in water with any ordinary fat or oil, the lead unites with the fatty acids, and forms a solid compound, while the glycerine dissolves in the water. The solution contains a considerable portion of lead, which may be separated by passing sulphuretted hydrogen through it, and filtering; the solution is then evaporated to the consistence of syrup, and the evaporation is completed in vacuo, in the presence of sulphuric acid, until it ceases to lose weight.

The uncrystallizable inodorous syrup thus obtained has a sweet taste, and was hence termed by Scheele, who discovered it in 1789, the sweet principle of oils, or glycerine, from γλυκύς, sweet; but it was not until Chevreul undertook the investigation of fatty substances in general that the true chemical relations of this body were understood.

During many years large quantities of impure glycerine had been produced as a waste product in the preparation of lead plaster from olive oil and litharge, and in the manufacture of soap and of stearic candles. In this last example the palm oil or other fatty substance was decomposed by means of lime, and the liberated glycerine was allowed to run to waste. In the remarkable series of improvements in this manufacture, which are due to the scientific knowledge and enterprise of the managing directors of Price's Patent Candle Company at the Belmont Works, Vauxhall, London, the lime saponification was superseded by the action in the first instance of sulphuric acid, which combines with the whole of the fatty substance, and the glycerine is afterwards isolated under the action of water and of a high temperature in the form of sulphoglyceric acid, and is obtained pure by distillation. This process was partly superseded by the discovery made by Scharling in the year 1853, that neutral fats may be decomposed by the action of steam alone under great pressure. Glycerine. In this process steam, at a temperature of from 550° to 600° Fahrenheit, is introduced into a distillatory apparatus which already contains a quantity of palm oil. Under these circumstances, the fatty acids take up their equivalents of water; the glycerine also combines with an equivalent of water, and the whole of these compounds distil over, and the condensed glycerine, from its greater specific gravity, sinks below the fat acids in the receiver. In this process a sufficient quantity of steam must be supplied, and the proper temperature be kept up, or acroleine, an acid vapour, especially irritating to the eyes, will be produced by the decomposition of the glycerine. According to another process, contrived by Tilghmann, water mixed with the oil or fat which is to be decomposed, is forced through tubes at a temperature of from 500° to 600°, or even higher.

Glycerine is distinguished by remarkable solvent powers, and hence, as obtained by the former processes, it was scarcely possible to get rid of such impurities as lime or lead. Indeed, several specimens of the "pure glycerine" of the shops, which have been sold at double the price of common glycerine, have been found to contain lead—a circumstance which may have greatly interfered with the medicinal or economical application of this valuable substance. By this new process, however, glycerine is obtained in combination with water only; if it be discoloured it can be rectified by a second distillation, and it can be concentrated so as to have at the temperature of 60° a density of 1·240, in which case it contains 94 per cent. of anhydrous glycerine. It can even be concentrated to 1·260 or 98 per cent. of pure glycerine, the remaining 2 per cent. being combined water.

Since this new source of glycerine has been opened, a variety of applications have been made for it, and they are briefly stated in a paper read at the Glasgow meeting of the British Association (1853) by G. F. Wilson, Esq., F.R.S., one of the managing directors of Price's Patent Candle Company. The first suggestion of a use for glycerine which Mr Wilson has been able to trace was by Mr T. De La Rue, who, early in 1844, operating on some glycerine which he procured from Mr Warington of Apothecaries' Hall, tried the effect of it on a burn and irritation of the skin. He found it to have a soothing effect, and to keep the part moist, and he suggested the use of it to Mr Startin, through whose means it came to be extensively applied in the hospital for skin diseases. It is being introduced as an article of the toilet, as a soothing remedy for chapped hands and sun-burnt faces. In June 1849 Mr Thomas Wakley published in the Lancet the results of one year's experience on the use of glycerine in diseases of the ear, giving a number of cases in which it had proved a cure for deafness. M. Cap (Journal de Pharmacie et de Chimie, February and October 1854) suggests a number of valuable uses for glycerine as follows:—“Glycerine dissolves the vegetable acids, the deliquescent salts, the sulphates of potassa, soda, and copper, the nitrates of potassa and silver, the alkaline chlorides, potassa, soda, barvta, strontia, bromine, iodine, and even oxide of lead. It dissolves or suspends the vegetable alkaloids in the same manner as the aqueous liquids, and at the same time the resulting products may be used for the same purposes as though mixed with oil. Thus the salts of morphia dissolve in it completely, even cold, in all proportions. Sulphate of quinine, in the proportion of 1/10th, dissolves in it when hot, but when cold separates into clots, which, when triturated with the supernatant liquid, give it the consistence of a cerate very useful for frictions and embrocations. It is the same with the salts of brucine, strychnine, veratrine, and most preparations of the same order, which enables us to consider that we have now, if not medicinal oils with a vegetable alkaloid base, at least a series of new preparations which will fulfil a perfectly analogous use in therapeutics.”

M. Cap further speaks of glycerine as a solvent for sulphuret of potassium, sulphuret of lime, iodine, iodide of sulphur, iodide of potassium, iodide of mercury, for some chlorides, and for quinine and sulphate of quinine.

From the great solvent power of glycerine, Mr Wilson suggests that it might be injected into the bladder for the purpose of dissolving calcareous deposits; from its blandness it might not cause irritation, while as a solvent of urea and phosphate of lime it might answer the desired purpose. Some of the medical authorities have received glycerine for the purpose of this experiment. Glycerine is also used in the same way as cod-liver oil as a medicine. In the preparation of several medicines, glycerine has been most advantageously substituted for the syrup of sugar, with the effect of preserving the medicine in an active state free from change, and also of greatly improving its taste.

Glycerine has a remarkable preservative power on animal and vegetable substances, an application of it which formed the substance of a patent taken out by Mr Warington in 1846. A portion of a neck of mutton kept in glycerine for several months was cooked by Soyer, and eaten with great satisfaction. Glycerine has been used for mounting objects for the microscope, and also for preserving objects in natural history. Mr Wilson's first experiment was upon a brilliantly coloured two-pound trout, caught in one of the Perthshire lochs. Immediately on taking it out of the water a quantity of glycerine was poured over it, and a cloth wrapped round it. Next day it was again wrapped in a saturated cloth. Two days afterwards, in Edinburgh, the colour of the scales was unchanged. When it arrived in London, part was steeped in water, and then cooked. It was perfectly fresh and firm, but had lost nearly all its flavour. The uncooked portion, immersed in glycerine, was sent to Professor Owen, who suggested that the brilliant fishes of tropical seas might be brought home in kegs of glycerine. Specimens of trout, roach, and perch, preserved in glycerine, some of them for more than two months, were shown by Mr Wilson to have retained their natural colours.

Glycerine consists of C, H, O. The chemistry of glycerine involves a number of highly interesting points, for an account of which we must refer to a memoir by Berthelot in the Annales de Chimie for 1854.