The gas from the still is carried for a short distance along earthenware pipes, on accotmt of the heat and softening power of the steam. It is afterwards con veyed to the chamber in lead pipes of from 3 in. to 10 in. in diameter, depending upou the number of " mains" employed.
Sometimes three or four stills are connected together ; some times a whole range of stills plays into one large main pipe.
By the former plan—separated mains, a more perfe,ct command is obtained over the quality of gas served to the chamber ; but the cost of repairs and the waste of gas are greater than when only one or two mains are used, and the strong and weak gases pass on to the lime indiscriminately. In any case the pipes frorn the stills must have a considerable fall, so as to keep all condensed steam out of the chambers, and allow it a free course back into the still. The best method of connecting the gas mains with the pipes leading finally to the individual chambers is shown in Fig. 328, and consists of two water lutes and a movable bend, or " elbow." The small vertical pipe is to carry off into any suitstble cistern or drain the last products of condensation.
The lime employed is of two ldnds—French " cliff," obtained from the banks of the Seine, and somewhat akin to the Dover chalk cliffs, and the pure limestone found in various parts of England and Ireland, more particularly in Derbyshire and the neighbourhood of Belfast. Cliff varies considerably in its composition, from pure limestone to impure " chalk." Some English descrip tions of the latter have indeed been often worked vp into bleaching powder, the temptation being the lowness of cost ; but in the long run, the operation does not pay, owing to the exceedingly uncertain composition of the chalk. Good limestone—the lumps that are usually burned for bleaching powder—has about the following composition :— Carbonate of lime 98 • 25 Silica .. . • .. 0 • 50 ff magnesia .. 0•50 Iron and alumina .. 0.40 manganese .. .. trace 1) Phosphate of lime .. .. trace 99 • 65 The stone is thoroughly calcined in any convenient form of kiln, care being taken to reject all portions not completely burned, and then slaked with water. This slaking requires considerable care and experience. Only a very slight excess of water should be used. If too great, a pasty mass is formed which resists the action of the chlorine ; if, however, the whole is not thoroughly slaked, the absorption of the gas is incomplete, and raw lime goes through into the bleaching powder. The plan usually adopted is to spread the lumps of calcined stone in.a layer about 12 in. deep, and to add the water through a rose pipe until the pieces fall into a fine powder. The portions of unburnt lime can be readily discovered and thrown out during the slaking process. The lime is next passed through a sieve of about sixteen meshes to the inch, and is spread upon the floor of the bleaching-powder chamber to a depth of about 8 in., the surface being slightly
furrowed. If the depth of lime is too great, it is impossible to bring the chamber up to strength without frequent turnings, and consequent loss of gas. When the charging is completed, the manhole doors are luted on, the sheet-iron doors wedged up, and plastered round with lime or loam. The chlorine gas is now admitted from the stills, and the chamber " pressed " until a sample drawn from the top shows about 22 per cent. of chlorine. The gas is then turned on to another chamber, and the accumulated gas in the first box allowed to be thoroughly absorbed by the lime. When the green colour in the chamber has subsided, the doors are opened, and the charge is carefully and systematically turned over. The gassing operation is then repeated, occasionally a second turning is resorted to, and when a sample tests 37 per cent., the supply of chlorine is finally turned off, and the chamber is left to stand for six or seven hours. The doors are then opened, the manholes are removed, and the finished bleaching powder is packed into casks. Instead of allowing the unabsorbed chlorine to escape into the air when a chamber is opened, and to save the time necessary for the slow final absorption, it is very usual to have a communication with a freshly charged chamber, or a small antechamber, into which the gas is drawn when the bleach is finished, or when the box requires " turning." Various precautions should be observed in gassing the lime. A due proportion of " maiden "—i. pure—chlorine, and " spent " gas—gas mixed with steam—should be used. If the weak gas be brought into contact with the lime in too great a proportion—especially with fresh lime, a coating will be formed upon the sur face, which resists the penetration of the chlorine. If a chamber he too rapidly pressed, a large pro portion of the chlorine will remain in the powder only mechanically held, and will fly off when the bleach is packed. The common belief that the temperature of a chamber should not exceed about 80° or 90° F., while right in practice is probably wrong in theory, the mischief of a higher temperature arising not from the actual heat of combination, but from the presence of steam, of which the tem perature is an indicator. Perhaps the best admixture of strong and walk gas is obtained by passing the product of a large number of stills into one main. The more usual plan is to have separate mains for about every three stills, and, with careful management, the gassing of a chamber can be more perfectly regulated by this method. The loss of chlorine during the operation of packing is about 0.75 per cent.