bbed lightly over them.
The matter added to facilitate spinning and weaving generally detracts
from the appearance of the uncolored fabric, and also interferes with
successful dyeing. Thus it is easy to see that the natural coloring
matter and the added foreign matter must be entirely removed from
fabrics destined for commercial use. Exceptions to this general fact
are sometimes made, because unbleached material is cheaper and more
durable than the bleached product, and for some purposes is entirely
satisfactory; unbleached cheesecloth and sheeting are frequently
purchased in place of the more expensive bleached material. Formerly,
the only bleaching agent known was the sun's rays, and linen and
cotton were put out to sun for a week; that is, the unbleached
fabrics were spread on the grass and exposed to the bleaching action
of sun and dew.

[Illustration: FIG. 158.--Preparing chlorine from hydrochloric acid
and manganese dioxide.]

218. An Artificial Bleaching Agent. While the sun's rays are
effective as a bleaching agent, the process is slow; moreover, it
would be impossible to expose to the sun's rays the vast quantity of
fabrics used in the civilized world of to-day, and the huge and
numerous bolts of material which daily come from our looms and
factories must therefore be whitened by artificial means. The
substance almost universally used as a rapid artificial bleaching
agent is chlorine, best known to us as a constituent of common salt.
Chlorine is never free in nature, but is found in combination with
other substances, as, for example, in combination with sodium in salt,
or with hydrogen in hydrochloric acid.

The best laboratory method of securing free chlorine is to heat in a
water bath a mixture of hydrochloric acid and manganese dioxide, a
compound containing one part of manganese and two parts of oxygen. The
heat causes the manganese dioxide to give up its oxygen, which
immediately combines with the hydrogen of the hydrochloric acid and
forms water. The manganese itself combines with part of the chlorine
originally in the acid, but not with all. There is thus some free
chlorine left over from the acid, and this passes off as a gas and can
be collected, as in Figure 158. Free chlorine is heavier than air, and
hence when it leaves the exit tube it settles at the bottom of the
jar, displacing the air, and finally filling the bottle.

Chlorine is a very active substance and combines readily with most
subst