242 BLEACHING 



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 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 a 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 yellow color of straw, cotton, and linen reappears. The 

 yellowing of straw hats during the summer is familiar to every 

 one ; the straw is merely resuming its natural color which had 

 been modified by the sulphurous acid solution applied to the 

 straw when woven. 



222. Why the Color Returns. Some of the compounds 

 formed by the sulphurous acid bleaching process are gradu- 

 ally decomposed by sunlight, and in consequence the original 

 color is in time partially restored. The portion of a hat pro- 



