GLYCERINE, or GLYCEROL. In 1783 Scheele showed that by acting upon olive oil by oxide of lead a substance may be obtained which has a sweetish taste; and in the follow ing year he showed that the same substance may be had by acting in a similar manner upon other oils and fats, such as butter. He also ob served that the substance in question may be obtained in the form of a syrupy fluid; that al though it has a sweetish taste like sugar, it cannot be fermented; and that although it gives oxalic acid by oxidation, it differs from sugar in many respects. He failed, however, to as certain its true relation to the oils which fur nish it, and to the lead plaster (or °lead soaps) which accompanies its formation. The true explanation of the reactions was given some 30 years later by Chevreul, as a result of his fa mous researches upon the animal fats, which were begun about 1811, and were concluded about 1823. In the course of these researches Chevreul showed that an animal fat consists, in general, of a mixture of several definite chemical substances, each of which is itself a fat, and each of which consists of Scheere's sweetish substance (which is now called ceri0),combined with an organic acid. When i the fat is treated with an alkali, or with lime or oxide of lead, the organic acid that is present combines with the alkali, or the lime, or the lead, to produce a new substance called a "'soap," the organic base (glycerin) which was previously combined with the. acid being there by set free. Since the time of Scheele aid Chevreul much attention has been paid to gly cerin and its compounds, and it is now univer agreed that glycerin is a trihydric or tria tomic alcohol(see ALconot.), having the for mula that is, containing the radicle Cal in combination with three OH groups. Hence glycerol bears the same relation to ordi nary ethyl alcohol as orthophosphoric add bears to nitric acid. And just as tribasic phos phoric acid forms three distinct classes of salts with three different proportions of the same base, so does glycerol form three dis tinct classes of esters: monoglycerides, digly cerides and triglycerides, and. that it forms an
acid and an oxide, and various substitution com pounds and esters, of which latter class the fats (q.v,) are the most important mem bers, and are distinguished by the name of "'glycerides? Glycerin does not exist as such in the fats and fatty oils, but is formed by the assimila tion of three molecules of water. However, glycerin does occur in nature in the uncombined form, notably as a constituent of palm-oil, and it is also a product of the alcoholic fermenta tion of sugar, and is therefore a normal con stituent of beer, wine, etc., 100 parts of sugar yielding in fermentation 3.5 parts of glycerin. On the large scale, however, glycerin is pre , pared by the decomposition of fats. In com merce, five varieties of crude glycerin are rec ognized: (1) crude saponification glycerin; (2) crude distillation glycerin; (3) Twitchell crude glycerin; (4) fermentation glycerin; (5) soap lye glycerin. The purest form results from the saponification of fats with lime in open vessels: that from soap lye processes may be equally pure if from a good class of fats. The other three sorts contain characteristic organic impurities which are not eliminated by the best known refining processes. The largest output is the soap lye grade.
In soap making the fat is decomposed by heating with an alkali, the soap which is formed by the combination of the alkali with the or ganic acid of the fat remaining in solution until it is precipitated by the addition of com mon salt. The fluid that remains after the soap has been so precipitated contains the liber ated glycerin, which can be separated by dis tilling in a partially exhausted boiler, the gly cerin passing over with the water vapor, from which it may be subsequently separated by re evaporation in a vacuum. In its commercial form it contains from 80 to 86 per cent of pure glycerin, 10 per cent of salts and 4 to 10 per cent of water.