The bleaching of rags may be conducted like that of Esparto (see p. 31). In addition, the methods of gas-bleaching and sour-bleaching are sometimes resorted to. The former, on account of the great inconvenience due to escape of chlorine, is rarely used; the operation may be carried on in large chambers of brick and cement or stone, with a tight-fitting cover, connected by a stoneware pipe with the apparatus for generating the chlorine. This last is obtained by heating black oxide of manganese with hydrochloric (muriatic) acid, or a mixture of oxide of manganese and salt with sulphuric acid Sour-bleaching consists in alternate treatment of the rags with bleaching liquor and a weak acid, usually in large chests, or drainers. In some mills, the half-stuff is first well soaked with the bleach ing liquor, and,, then weak sulphuric or muriatic acid is run in , in others, the reverse order is observed. The most economical plan apparently would be to first bleach as much as possible with the liquor alone, and then to add an acid when this is nearly exhausted. (Spons"Encyclopaedia,' p. 1492.)