For this first part of the process—the generation of chlorine—various manganese ores are used. The hest comes from Devonshire. It is soft, and therefore easily dissolved by the hydrochloric acid, and contains up to 72 per cent. of hinoxide. The Spanish ores are of like strength, and sometimes as soft, but are more irregular in quality than the Devonshire. The German ores, as a rule, are weaker and smaller, 60 to 62 per cent., and often in a very finely divided state. Up to about 1857, the German qualities were chiefly used. The Spanish has now taken the lead. The following table gives the approximate composition of the various ores referred to :— Other descriptions occasionally used are imported from Virginia, California, and New Zealand. The Virginian is very irregular in quality. The best descriptions are very good, soft, and satis factory in the working ; the majority of the cargoes brought over, however, are hard, and contain a good deal of carbonate of lime. Hence they not only dissolve slowly, hut waste hydrochloric acid. The Californian, as a rule, ia very hard. The New Zealand is of recent introduction, and is well reported of. All these varieties come up to 70 per cent. —usually over.
The price of a good 70 per cent. ore is, at the present time, about 85s. per ton. Six years ago, it was 140s., hut the introduction of Weldon's recovery process has largely decreased the consumption.
The hydrochloric acid should not be too weak, in order that the manganese may he more thoroughly decomposed, and the still liquors kept as strong as possible. For this latter reason, the steam should also be as free from water as possible. With all precautions, a large loss results from undecomposed manganese. To make it as small as possible, constant supervision should be kept over the working of the stills. They should never be run off until perfectly " spent," and should be kept carefully clean by a thorough raking out before LL fresh charge of manganese is in troduced.
An efficient and economical form of still gutter is shown in Fig. 321. It will be noticed that the main body is formed of one balk of timber, to which are added sides of smaller pieces. This method of construction gives a gutter as good as if hollowed out of one large balk, costs of course muoh less, and can be made of a larger size than is readily practicable with one solid piece.
The next operation consists in bringing the chlorine gas into contact with the slaked lime in the chambers, or " boxes." Various materials and forms of construction of chamber have been from time to time adopted—wood, brick, stone, and sheet lead. The latter is now almost invariably used ; hut
the old stone and brick boxes are still occasionally met with, When built of stone, the chamber is only about 10 ft. square by 6 ft. high, the best material being hard flag, and the method of con struction somewhat similar to that of a manganese still. Sometimes a stone chamber has a lead top. A brick chamber is usually built in the shape of a long limo kik—one great arch, about 9 ft. wide and 7 ft. high. The first leaden chambers were only small—packing about 21 tons of bleaching powder each. The size has been gradually increased until the chamber of most modern and improved construction is about 60 ft. long by 30 or 35 ft. wide, and packs up to 12 tons of bleaching powder. The mode of erection is shown in Figs. 325 and 326. A wooden framework is first set up, of the size of the proposed chamber, consisting of 6 in. sq. corner posts, with stout uprights, about 7 ft. apart, of 7 in. by 3 in. deals, and the top or " crown " of 7 in. by 4 in. deals. Upon this framework, is hung a casing of 6 lb. sheet lead, the whole being burned by upright seams into one solid piece, and secured to the " crown " and uprights by straps of lead burned upon the sheet.
The top is formed of similar sheets of lead burned together and strapped to joists, 11 in. bY 3 in., set 14 in. apart. Two or three doorways are cut in the sides, and upon the top are formed two manholes—whence also samples can be drawn—and gas communication pipes. The doorways are about 5 ft.
high and 4 ft. 6 in. wide, and are closed by stout sheet-iron doors swinging upon hinges and secured by cross beams and wedges. The workmanship must be of oareful desoription to prevent any escape of chlorine. The height of a bleaching-powder chamber varies with the individual experiences of the manufacturers, but it should not average more than 7 ft. A slope of 12 in.—from 7 ft. 6 in. to 6 ft. 6 in.—is given to the top to prevent any accumulation of water. Various methods are adopted for connecting the side sheets with the bottom of the chamber and preventing any injury by the manipulation of the lime with shovels, &e. The best oonstruetion is shown iu Fig. 327. Au inner lining of lead, or skirting, 12 or 15 in. in height, is burned upon the sides and flanged 2 or 3 in. upon the ground or flooriog. Over this flange, the bottom, formed of concrete or flags, is laid. A perfectly tight joint is thereby made, and if by any mischance the lead lining is cut, the gas has still no chance of penetrating the outer sheet. The older plan is to form a skirting of 1-in. deals round the chamber.