162 BLEACHING, BLUEING, STARCHING 



coloring matter but the fiber itself, and leaves it shrunken 

 and inferior. Cotton and linen fibers, apart from the small 

 amount of coloring matter present in them, contain nothing 

 but carbon, oxygen, and hydrogen, while animal fibers con- 

 tain in addition to these elements some compounds of nitro- 

 gen. The presence of these nitrogen compounds influences 

 the action of chlorine and produces unsatisfactory results. 

 For animal fibers it is necessary to discard chlorine as a 

 bleaching agent, and to substitute a substance which will 

 have a less disastrous action upon the fibers. Such a sub- 

 stance is to be had in sulphurous acid. When sulphur burns, 

 as in a match, it gives off disagreeable fumes, and if these 

 are made to bubble into a vessel containing water, they dis- 

 solve and form with the water a substance known as sul- 

 phurous acid. That this solution has bleaching properties is 

 shown by the fact that a colored cloth dipped into it loses its 

 color, and that unbleached fabrics immersed in it are whitened. 

 The harmless nature of sulphurous acid makes it very desir- 

 able as a bleaching agent, especially in the home. 



Silk, lace, and wool when bleached with chlorine become 

 hard and brittle, but when whitened with sulphurous acid, 

 they retain their natural characteristics. 



This mild form of bleaching substance has been put to 

 uses which are now prohibited by the pure food laws. In 

 some canneries common corn is whitened with sulphurous 

 acid, and is then sold under false representations. Cherries 

 are sometimes bleached and then colored with the bright shades 

 which under natural conditions indicate freshness. 



Bleaching with chlorine is permanent, the dyestuff being 

 destroyed by the chlorine. But bleaching with sulphurous acid 

 is temporary, because the milder bleach does not actually de- 

 stroy the dyestuff, but merely modifies it, and in time the natu- 

 ral vellow color of straw, cotton, and linen reappears. The 



