Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/722

Rh G98 GLYC analogous to distearin or monostearin substantiated with ‘ certainty, bodies of these classes being either formed syri- thetically by reversing the reactions of saponiﬁcation, or being produced by the partial saponiﬁcation of substances analogous to tristearin. Glycerin is also a product of certain kinds of fermentation, especially of the alcoholic fermentation of sugar; thus it is a constituent of many wines and other fermented liquors, l being formed together with small quantities of various other substances by reactions subsidiary only to the main change taking place, and hence varying in their nature and extent with circumstances. According to Pasteur, about 3‘5th of the sugar transformed under ordinary conditions in the fermentation of grape juice and similar saccharine liquids into alcohol and other products becomes converted into glycerin. In certain natural fatty srrbstances, e._9.. palm oil, it exists in the free state, so that it can be separated by washing with boiling water, which dissolves the glycerin but not the fatty glycerides; but how far its occurrence in this form is due to the breaking up of the glyceride by a spontaneous saporriﬁcation is open to some question. Properlr'es.—Ir1 a state of purity glycerin is a viscid, colourless liqrrid of sp. gr. 1264, possessing a somewhat mawkislr sweet taste; when exposed to a high degree of cold for a long time it sometimes solidiﬁes to a crystalline ' mass, which then melts at about 7° C. The crystals when once melted often do not resolidify again readily, even when in contact with the solid substance, although sometimes contact with a crystal of the solid at a temperature of about 0° suffices to produce solidiﬁcation of the whole. This solidiﬁcation of glycerin is, however, a very exceptional phenomenon, only occurring with extremely pure sub- stance under certain conditions not thoroughly under stood, and then only after long continued exposure to a low temperature, as during a cold winter. When corr- taining a minute quantity of water glycerin never solidi- ﬁes, and to this circumstance several of its useful applica- tions are due. A weak aqueous solution, when chilled sufficiently, allows crystals of ice to fornr, the glycerin accumulating in the unfrozen portion as alcohol does when a mixture of spirit and 'ater is partially frozen. When heated alone it partially volatilizes, brrt the greater part decomposes; by reducing the pressure to about Tlgth of an atmosphere, it can, however, be readily distilled unchanged, boiling under a pressure of 50 millimetres of mercrrry at about 20° C. In an atmosphere of steam, also, it distils without decomposition under ordinary barometric pressure. In water and alcohol it dissolves readily in all proportions; in ether it is insoluble. Under certain conditions, such as prolonged contact with poor cheese and chalk at about 35° to 40° C., it can be nrade to ferment partially, becoming changed into alcohol ; but under any circumstances, only a small fraction, at nrost a tenth, becomes thus transformed, the rest remaining unaltered. It possesses remarkable solvent powers on many substances, whence it is employed for numerous purposes in pharmacy and the arts. Its viscid character, and its non-liability to dryiand harden by exposure to air, also ﬁt it for various other uses, such as lubrication, &c., whilst its peculiar physical charac- ters, enabling it to blend with either aqueous or oily matters under certain circumstances, render it a useful ingredient in a large number of products of varied kinds. Applied to the living skin (and similarly to untanned leather) it pro- duces a remarkable softening effect, whence it is largely employed as a cosmetic, either by itself or in admixture with other substances. Taken internally it is alleged to be valuable as a substitute for cod-liver oil for phthisical patients, not possessing the disagreeable ﬁshy ﬂavour of that valuable food, and having a fattening tendency. When it is given in moderately small repeated doses to the ERIN lower animals, it does not appear to possess any nrarked injurious action peculiar to itself; when, however, large doses of glycerin are subcutaneorrsly injected into (logs, amounting to from 8 to 10 grammes per kilogranrmc of animal operated on (08 to 1'0 per cent. of the weiglrt of the dog, corresponding to from 1 lb to 1._1, lb of glycerin for tlre weight of an average man). death ensues within twenty- four hours, accompanied by symptoms analogous to those of acute alcoholism (Dnjardin, lleaumctz, and Audigt’-). Like sugar it possesses antiseptic qualities, so that meat, albumin, &c., immersed in it do not for long periods of time undergo putrefactive changes. Illanufacturc.—'l‘he simplest modes of prt-paring glycerin in :I state of purity are based on the saponifrcation of fats, either‘ by allralies or analogous basic substances, or by super-lreated .'l¢-zlm. and on the circumstance that, although Ulycerin cannot be distilled by itself under the ordinary pressure without dccornpositiorr, it can be readily volatilized in a current of superlreated steam; in this way the glycerin fornrcd is separated from the non-volatile s1rb— stances present. It was by means of saponification of olive oil or lard with lithrrrge (lead oxide), whereby a lead soap insoluble in water, or nearly so, is fornred, together with glycerin, that the existerrct of glycerin was first denronstrated by Scheele, who obtained it as :1 bye-product in the formation of the " lead-plaster" of plrmmacy rrrade from laid and lead oxide. For a long time this was the only known method of preparing glycerin, the aqueous solution obtained being treated with sulphuretted hydrogen to remove airy soluble lead compounds, ﬁltered, and evaporated rurtil almost all the water was driven off, leaving the glycerin behind as a syrupy fluid. By evaporating down the spent lcys of the soapmalcer (after the soap is separated therefrom by “suiting out,” and any excess of alkali neutralized with sulphuric acid), and treating the residue with alcohol, glycerin can also be obtained, the alcoholic solution of it thus formed being simply evaporated to drive off the alcohol; bill this process is far too costly for ordinary purposes. An inr- prorcrncnt on this method was patented in 1858 by ll. Reynolds, the concentrated leys being passed into a vessel where they are met by a stream of superheated steam at about 200° C ; the glycerin then passes over with the aqueous vapour, whilst the inorganic salts present are left behind. In the manufacture of stearie acid for candle-making (sec CAr'DLE) one of the older processes was the saponiﬁeation of tallow with lime, forming an irrsolnlnlc linre soap and Lan aqueous :solution of impure glycerin, from which the prrre substance can be readily obt:rined by distillation with super- heated steam. Less pure products were for rncrly obtained by tr'cating the crude solution with sulphuric acid to separate linre, boiling to remove small qrrarrtitics of volatile acids, craporatiorr, and ﬁltration through animal charcoal to decolorize; or by evaporat- ing, dissolving out by alcohol, and purifying by treating with lead oxide, filtering to separate an insoluble lead compound forrncd, removing lead from this ﬁltrate by sulplrnrcttcd hydrogen, filtering again, and evaporating to a syrup. In practice all these older methods have, however, been srrpersedcd by the process patented in 1854 by Vilson & Payne. This consists in hcating the fatty matter to be saponified in an appropriate still to a temperature of 290° to 315° C. (550° to 600° F:rlrr'.), and passing in heated steam in such a way that it rises up through the fatty matter in rrurnt-rou.~ streams; saponiﬁcation is thus rapidly effected, and the liberated glycerin and fatty acids are volatilizcd and carried along with the steam to the condensing arrangement. If the temperature do not exceed 310° C. there is no fear of the glycerin being deconrposed, whilst under suitable conditions even higher temperatures than this can be employed without causing its decomposition ; but there is always a great liability to destruction of glycerin when the tem- perature of 310° is exceeded. This arises from the tendency of the glycerin to char on heating, and to split up into water and acroleiu (acrylic aldehyde), thus :— C3H,,o3=2ir,0+c3ir,o. When 9. series of chambers is used as the refrigerator, the coni- partments nearest the still are found to condense little but fatty acids, the water_ and glycerin chiefly accumulating in the more dis- tant chambcrs, the last of which is usually open to the air at the end; so that there is no excess of pressure in the still and con- densers; the fatty acids readily separate from the aqueous solutions of glycerin, which only requires concentration by evaporation to be fit for the _market. Since the date of 'ilson’s patent various special forms of apparatus for effecting the transformation have been patented by Vright & Fouché, Gilbee, and others. This method of saponification is, strictly, only an improvement on former processes invented for the purpose of decomposing the glycerides and obtaining the fatty acids without the'use of alkalies, lime, &c.; inthcsc older methods thecxtraction of gl cerin was not an important feature. and they were frequently worked, in snclr a way as to decompose the whole or greater part of the glycerin paripassu with