the
action of the chlorine and produces unsatisfactory results. For animal
fibers it is therefore 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 substance 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 dissolve and form with the water a substance known as sulphurous
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 desirable 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 destroy the dyestuff, but merely
modifies it, and in time the natural yellow color of straw, cotton,
and linen reappears. The yellowing of straw hats during the summer is
familiar to everyone; 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 gradually decomposed by
sunlight, and in consequence the original color is in time partially
restored. The portion of a hat protected by the band retains its
fresh appearance because the light has not had access to it. Silks and
other fine fabrics bleached in this way fade with age, and assume an
unnatural color. One reason for this is that the dye used to color the
fabric requires a clear white background, and loses its characteristic
hues when its foundation is yellow instead of white. Then, too,
dyestuffs are themselves more or less affected by light, and fade
s