BLEACHING. 173 



is then taken out and rinsed, to remove the acid ; 

 and afterwards washed with soap, to give a degree 

 of softness. 



This mode of bleaching by the combustion of 

 sulphur is also used for other substances, as straw 

 in the manufacture of hats, &c. 



A superior method of employing the sulphureous 

 acid in bleaching is the following. Water is im- 

 pregnated with the sulphureous acid, and tubs are 

 filled with this; then the stuffs are drawn through 

 it upon reels, till they are whitened. The sulphur- 

 eous acid is made by decomposing the sulphuric 

 acid by the addition of any combustible matter 

 capable of taking away a part of its oxygen. A 

 cheap method of effecting this is by putting chopped 

 straw, or saw-dust, into a mattrass, and then pour- 

 ing over it sulphuric acid, and afterwards applying 

 heat. The sulphureous acid gas will be formed, 

 but will be combined with the water in the vessel. 



The stuffs are then taken out, and left to drain 

 upon a bench covered with cloth ; a precaution 

 which is necessary, because the wood might be 

 decomposed by the sulphuric acid, and would stain 

 the goods. They are afterwards washed in clear 

 water. It is generally necessary to sulphur them 

 twice before the white is sufficiently bright. Some- 

 times Spanish white is put into the water used for 

 washing them; and they are also azured or blued 

 by disolving some Prussian blue in the water. 

 Nothing then remains to be done but drying, 

 stretching, and pressing. 



Bleaching of Silk. 



Silk is an animal substance, and is prepared by 

 a caterpillar, usually called the silk-worm. This 



