BLEACHING POWDER. It has long been known that chlorine and certain chlorine compounds possess the property of bleaching. A solution of chlorine in water was prepared for this purpose in 1785; a solution made by passing chlorine into so dium or potassium carbonate, and known as `'eau de Javelle," was put on the market in 1789; and in 1 798 Charles Tennant erected a works in Glasgow to manufacture, as a bleaching agent, the product obtained by acting upon lime with chlorine. Under the name "chloride of lime" bleaching powder will be familiar to many as a disinfectant in common use. It is manufactured by the direct action of chlorine gas upon slaked lime ; and although its chemical constitution is still somewhat in dispute, its composition when pure is usually represented by formula As manufac tured it always contains a certain amount of chemically com bined water and an excess of lime. In manufacture the selection of the lime used is a matter that requires care ; it must be of good quality and as free as possible from iron and magnesium ; it must also be fairly dense to assist packing, and it must have the prop erty of settling quickly in water to save time when the bleach is actually used. The lime is treated with the proper quantities of water and steam to convert it to hydrated (or slaked) lime and is stored some days to cool and mature. It is then, after sieving, ready to receive the chlorine.
The older method, still largely used, is to spread the slaked lime in layers of 2 or 3 inches depth on the floor of large rectangular chambers made of lead, or of concrete protected with a coating inside of tar; these chambers are from 6 to 7 f t. high, io to 2oft. wide, and up to 1 oof t. long. The lime is raked into ridges in order to increase the surface exposed to the gas. The chamber, or series of chambers, is then closed and chlorine admitted. The gas is readily absorbed, and if it be concentrated so much heat may be generated by the reaction that in hot climates cooling coils are .necessary in the floor of the chambers to prevent decomposition of the bleaching powder. A more recent method dispenses with chambers and employs mechanical means to propel the lime through a horizontal tube or series of tubes. The gas is fed into the plant in a direction opposite to that in which the lime travels.
Bleaching powder is a somewhat unstable substance ; moisture, sunlight and undue temperature all cause it to decompose rapidly; and this tends to limit its usefulness, especially in tropical coun tries where, as a disinfectant, it is so valuable. Attempts to pro duce a more stable form have engaged the attention of chemists for many years and no doubt the problem will be eventually solved. In spite of the increasing competition of liquid chlorine, which can replace bleaching powder in many respects, the world output of the latter reaches many hundreds of thousands of tons yearly. Most of it is consumed in the paper and textile indus tries for the purpose denoted by its name. As an alternative to the powder there are manufactured solutions of sodium and cal cium hypochlorites which have advantages under certain condi tions. Sodium hypochlorite, for example, is particularly suited for the bleaching of fine cotton goods, and is, incidentally, a most powerful germicide.
For the method of using bleaching powder see article BLEACHING.
(A. E. H.)