Evaporation is continued to 26° B., when the glycerine is of a browuish colour, awl known ae " raw," in which state it is sold for many purposes. At Price's Caudle Cornpany'e works the fur ther purification is conducted as follows. The raw glycerine, sp. gr. 1.245-1.250, is heated in a jacket pan with that kind of animal charcoal known as ivory-black, and is then distilled ; this alternate treatment is repeated as often ae may be necessary. The distillation is performed with superheated etc= in a copper still provided with copper fractional condensers (the same as shown in Fig. 463, p. 58], but omitting the right half of the apparatus, including the tanks G), the kill being also heated externally ; the operation is performed at as low a temperature as is consistent with distillation, usually about 227° (440° F.). The number of distillations depends upon the quality of the raw glycerine and the purity of the product demanded. Of the six runs, Nos. 1, 2, and 3 usually give pure glycerine, while the dilute condense-products from Nos. 4, 5, and 6 are generally retained tu the still, thnugh occasionally concentrated in an enTeritear unieersel, or in a acuum-pau. Some stills hold ae much as 3 tone, but they are usually smaller, and in all eases the process is conducted very slowly. A form of still and condenser much used on the Continent of Europe is outlined in Fig. 1281. External heat and injected superheated steam are used to effect distillation. The still A has an unusually large head B, and the goose-neck C is provided with a catch-box D, in case the still-contents should, as sometimes happens, boil over ; the fractional con densers E are upright cylinders with longitudinal partitions F running nearly their whole length ; the condensed products run out through G into re ceptacles H. The whole apparatus is of iron, and usually made to distil ten at a time; in some cases, the process is conducted continuously, with a pro perly arranged feed.
Enormous quantities of glycerine are run to waste in the spent leys (p. 1779) of the soap-maker.
One of the earliest attempts to extract it was a patent by H. Reynolds, June 10, 1858, for concentrating the spent leys, and distilling off the glycerine by super heated steam between 193i° and 204i° (380°-400° F.); the large quantity of sodium salts, especially sodium chloride, were found, however, to be an almost insuperable difficulty. On March 31, 1879, a patent was taken out by C. Thomas, W. J. Fuller, and S. A. King, of Broad Plain Soap Works, Bristol, by which process the first successful pro duction of crude glycerine from spent soap leys was introduced into commerce, and several tons per week are now manufactured. The specification states :" We evaporate the spent or partially spent leys until the boiling-point of the liquid rapidly rises, when nearly all the salts that can be thrown down by simple evaporation are deposited in the pan. The resulting liquor is chiefly com posed of raw or impure glycerine. This we draw off into a second pan, and boil it with excess of fatty acid, which, readily combining with some of the salts in solution, separates them from the liquor, and at the same time removes from it the fine crystals of salt formed during this operation.
After this treatment, wo skim off the saponified fatty matter, allow the liquid to cool, and filter it to remove the gelatinous, albuminous, and other impurities. The clear liquid may then be refined, distilled, or concentrated, as desired." The recent extremely rapid rise in the price of glycerine has caused much attention to be directed to this abundant source of it. 'Victor Clolus, of Billancourt, near Paris, has patented a process for its recovery from soap-boiler's waste, which will (Oct. 1881) shortly be worked in England also ; and H. Flemming, of Kalk, has patented in Germany (No. 12,209) a process for removing glycerine from spent leys by dialysis, a membrane of parchment-paper serving as the diaphragm through which the crystalline salts present in the glycerine diffuse themselves. Another process has been proposed, according to which, the salts are said to be removed from spent leys by saturating the latter first with carbonic acid gas, and then with hydrochloric acid gas.
Although evaporation and distillation are the usual methods of purifying glycerine, the action of cold upon more or less dilute glycerine is sometimes employed in conjunction with them, especially by Sarg, at 'Vienna. When au aqueous solution of glycerine partially freezes, the frozen rnass contains more water than the remaining liquid, hence some amount of concentration may be thus effected. The following table gives the freezing-points of such mixtures : In January 1867, some glycerine sent in tin cans from Germany to England froze into pea sized octahedral crystals ; these, while melting, had a constant temperature of 7°2 (45° F.), but would not freeze again even when cooled to 18° (0° F.). According to Werner, commercial glycerine will freeze more readily if chlorine gas be passed into it. In purifying glycerine by cold, the whole mass is cooled to nearly 0° (32° F.), and some crystals of solid glycerine are added ; almost the whole mass solidifies on agitation, and a centrifugal machine is used to separate the solid from tbe liquid parte. Treated in this way, glycerine at 28° B. yields crystals which, when melted, aro 30°8 B.
Pure glycerine is a viscid, colourless, and traneparent liquid, with an intensely sweet taste, soluble in water in all proportions, in alcohol, chloroform, and carbon bisulphide, but not in ether ; its sp. gr. is 1.267; it solidifies at 40° ( 32° F.) to an amorphous mass. When distilled, it decomposes, unless ste,am be present, hence its boiling-point cannot be accurately determined at atmospheric pressure. According to Bolas, at 12.5 mm. pressure, it boils at 179°5 (355° F.), and at 50 mm., at 210° (410° F.), while Hemiuger gives 179' (354° F.) as its boiling-point under 20 mm.