BLEACHING

(Ger. bleichen.) This proce consists in a series of operations, by which the natural colours of various substances are discharged, so as to whiten them. It is effected either by the action of various solvents, aided by exposure to light, air, and moisture, upon the bleaching ground; or by the aid of chlorine. Cotton is more easily bleached than linen, in consequence of its being originally whiter, and having a le powerful attraction for the colouring matter. In bleaching these goods upon the old principle, warm water is first liberally applied to remove the weaver's paste or dre ing f they are then bucked, or boiled in a weak alkaline ley; and after having been well washed, are spread out upon the gra , so as to be freely exposed to the joint agencies of light, air, and moisture; the bucking and exposure are alternately repeated as often as nece ary; the goods are soured, that is, immersed in water slightly acidulated by sulphuric acid; lastly, they are very thoroughly washed and dried. By these operations the texture of the goods is to a certain extent impaired, and much time is required to complete the proce , which also cannot be carried on in the winter months. But the exposure upon the t)leaching ground is now to a great extent discontinued; and the same effect is obtained, after the proce of bucking, by the action of weak solutions of chlorine, or of chloride of lime, which, if skilfully used, can scarcely be said to injure the goods more than the long-continued exposure. The theory of bleaching has not been satisfactorily developed; but, from such experiments as have been made in reference to it, it appears to be a proce of oxidize ment, and to depend upon some peculiar influence of nascent oxygen upon the colouring matter. The colour of manufactured wool depends partly upon its own oil, and partly upon the applications made to it in the loom. These are got rid of in the fulling mill by the joint action of fullers' earth and soap; the cloth is then well washed and dried, and is tolerably white; if the slight yellow tint which it retains is objectionable, it is prevented by adding a little stone-blue to the washing water, or by exposure to the fumes of burning sulphur; this latter method, however, gives it a harsh feel, and if afterwards soaped its yellowishne returns. The colour of raw silk depends upon a natural yellow varnish, which is got rid of by boiling it in white soap and water, and by repeated rincings. Certain articles of wove cotton, such as stockings, are bleached as usual, and finished by the action of sulphurous acid, or the fimes of burning sulphur. Straw is also whitened by a similar operation; and hence bleached straw hats are apt to have a disagreeable sulphurous smell. A good account of bleaching will be found in Parkcs" Chemical fl ays. BLE' [s. 163]