Sulphur, Lime, and Charcoal.—John McGregor, of Tobago and Trinidad, has recently introduced a plan called the " arvation " process. It consists in burning sulphur, lime, and charcoal in a furnace, and conducting the fumes into the liquor ; its advantages are nil.
Yellow Crystals.—Tbe beautiful Demerara " yellow crystals" owe much of their brilliant colour and transparency to delicacy of tempering. The temper need ie lime-water rather than cream of lime, the density being only 10° instead of 17° B., and preference is given to rain over trench water. The clarifier is filled with sulphured juice, tested repeatedly, while it is entering and while lhne is being added, to ascertain the exact quantity of lime necessary : when it is known, the whole quantity is for the future introduced before the clarifier is one-quarter The exact proportion of temper is decided (1) by the neutral reaction on test-paper, and (2) by the appearance of the limed and agitated juice when filled into a foot-glass plaeed in the light and allowed to subside for 5 minutes. The appearance wished for is brilliant transpareney oom bined with a golden colour. The right quantity of lime is that which will give this result, though the liquor may be a trifle alkaline. With inferior juice, colour miud be sacrificed for transpareucy, and lime added till transparency is attained, even though the colour be intensified to light-red. Too light a colour, which is sometimes compatible with good transparency in the case of superior juice, will result in a green-coloured sugar. Over-tempering causes the suga.r to turn greyish brown when cured. For the subsequent treatment of the liquor ia the vacuum-pan, see pp. 1894-5.
Fidratien. —Filtration of the juice is a necessary adjunct to the defecation by heat and chemicals, its object being the removal of the matters rendered insoluble by these operations. The chief kinds used are bag-, charcoal-, and capillary filters.
Bsg-filtere.—The construotion and arrangement of these are shown in Figs. 1358 and 1359. The filter consists of a wrought-iron ease a, with openings at b, and an internal flange at top to carry a cast-iron box c, having holes in the bottom, for the reception of gun-metal bells d, to which are attached the cotton t will filter.bags Fig. 1360 shows au
enlarged section of the gun-metal bell 01.
The bags e fastened to these bells are 3-6 ft. oirc. and 6-10 ft. long, woven without a seam. They are crumpled up inside "sheaths" of strong open webbing, about 18 in. cire., which restrict their expansion. They aro arranged ia series of 100 or more.
Charcoal-Mon.—These are large, slightly tapering, cylindrical vessels, generally of wrought iron, with per forated false bottom about in. from the bottom. A blanket covers this false bottom, to prevent the charcoal from being carried through with the liquor. Some charcoal, however, always accompanies the first liquor, which is caught in a separate receiver, to be filtered over again.. Iu filling these vessels, the first fow inches of charcoal should be pressed compactly down, after which, it is packed lightly but evenly as near to the top ELS leaVe a convenient space for the liquor. The object of these filters is to remove the vegetable e,olouring matter from the liquor, together with the fine suspended impurities that have escaped the bag-filters. Use is made of both animal charcoal (bone-black) aud wood charcoal, but the former is in most general favour. (See also Beet-sugar, pp. 1851-4 ; Refining.) Capillary fllters.—A representative filter of this class is that invented by F. A. Bonnefin, of Guadeloupe, and made by Corcoran & Witt, 30 Mark Lane, London. It is intended to be used in eonjunotion with his " continuous preparator " (see p. 1898), which effects the removal of the coarser impurities prior to the tempered juice entering the filter proper. This latter separates solid matters from liquid by capillary action taking place through fibres held between surfaces of a yielding material, and aided by pressure or suction. The bundles of fibres are usually, for convenience, woven into au exceedingly loose &brie, preferably of pure cotton. At one end, they are in contact with the mixture to be filtered ; the capillary action of the fibres draws the clear liquid past the yielding surface, whilst the solid matters are left behind.