When packed, the loss is at the rate of about 1 per cent. per month in hot weather, and 0.5 per cent. in cold.
Instead of building the bleaching powder chambers upon the ground, a,fter the manner shown in Fig. 329, a very usual and excellent plan is to arrange th em at a considerable elevation, upon suitable supports, to form a warehouse below.. The packing is then performed by raking the bleaching powder into wooden spouts, countersunk in tbe floor, from which it falls into casks placed beneath. A closed connection between spout and cask is made by sheet indiarubber. This method of packing is less laborious and hurtful to the men employed ; it is more quickly performed, and, although a seemingly greater loss of chlorine is appa rent between the test in the chamber and the test in the cask, the bleach is more stable aftervvards, the free chlorine being well shaken out of it by tumbling down the spout.
The usual bleach cask holds from 5 to 7 cwt. The powder should be packed as tightly as possible, both to preserve the quality and to economize the cost of casks. Owing to the disagreeable nature of the operation, this is a point which requires constant care and supervision on the part of the manufacturer.
The variations of the actual process of bleaching-powder manufacture are very few. The only important one has already been noticed—the preparation of liquor, by passing chlorine gas into lime water. Bleaching liquor is more esteemed upon the Continent than in this country, and is usually prepared by the bleachers themselves. The use of chalk, or carbonate of lime, has been proposed in place of the hydrate.
The question of yields is an important one, as there are many loopholes for loss. Upon an average, and in round figures,.13 cwt. of good 70 per cent. manganese ore, or 17 cwt. of 60 per cent., should yield 1 ton of bleaching powder. These figures refer of course to cases where the manganese is not recovered by Weldon's cr Borne other process. A very great toss is incurred by the more finely divided portions of the ore being swept away with the waste liquors from the still, untouehed by the acid. Further loss is sustained by the ore getting coated over, the amid being thereby prevented from penetrating to the centre. Sometimes an arrangement of washing and settling pits is made, and the waste imanganese is restored to the still with a fresh charge ; but it is doubtful if the result pays for the expense and trouble.
One ton of lime makes about 11 ton of bleach ; but the quality varies considerably with the quality of limestone employed. About 2 tons of stone, or cliff, go to 1 ton of calcined lime. An experienced
manufacturer can readily tell, from the appearance of the limo in the kiln when the fire has burned low, when the calcining operation is completed ; and from the feeling and texture of the bleaching powder in the chamber, when the absorption of chlorine has been sufficient.
The chief uses of bleaching powder are, as its name denotes, for various bleaching processes, for the raising of metallic colours, and steam blues, and for the discharging of Turkey reds in calico printing. It is also used, but to a smaller extent, as a disinfectant. Its use in the latter capacity has been somewhat circumscribed of late years by the introduction of various other substances, such as carbolic acid, &c. It is, however, one of the best agents that can be employed for this pur pose, a great merit being its harmlessness—the safety with which it can be handled and treated. The chief seats of the industry in this country are the districts of the Tyne, Lancashire, and Glasgow ; the total output is about 70,000 tons per annum. The price fluctuates constantly and rapidlY. In 1805, it was about 115/. a ton. From this puint, with an increased output and better methods of manufacture, it gradually declined, until, in 1866, it ruled about 11/. a ton. Since then, it has risen to 17/., fallen again to 5/., recovered to 91., and, at the present time, remains steady at 5/. The cost price, when the recovery of the manganese is carefully carried out by Weldon's process—to be pre sently described, is about 4/. 15s. a ton. Under the old proems it is fully 6/. a ton. Of course this variation in selling price by no means represents variation in profit. Owing to the different methods from time to time adopted in manufacturing sulphuric acid, it is difficult to give any reliable indication of the varying cost of this material ; but the following table shows the approxi mate values of salt, since the commencement of the present century :— It remains to notice the most important of the many processes devised for the recovery of the manganese, and for avoiding the use of manganese altogether. The consideration of these points has been reserved because the actual manufacture of the bleaching powder—the action upon lime with chlorine gas—is in all cases the same.