dioxide and water falls upon a fire, it acts like a
blanket, covering the flames and extinguishing them. The value of a
fire extinguisher depends upon the amount of carbon dioxide and water
which it can furnish. A fire extinguisher is a metal case containing a
solution of bicarbonate of soda, and a glass vessel full of strong
sulphuric acid. As long as the extinguisher is in an upright position,
these substances are kept separate, but when the extinguisher is
inverted, the acid escapes from the bottle, and mixes with the soda
solution. The mingling liquids interact and liberate carbon dioxide.
A part of the gas thus liberated dissolves in the water of the soda
solution and escapes from the tube with the outflowing liquid, while a
portion remains undissolved and escapes as a stream of gas. The fire
extinguisher is therefore the source of a liquid containing the
fire-extinguishing substance and further the source of a stream of
carbon dioxide gas.

[Illustration: FIG. 25.--Inside view of a fire extinguisher.]

51. Carbon. Although carbon dioxide is very injurious to health,
both of the substances of which it is composed are necessary to life.
We ourselves, our bones and flesh in particular, are partly carbon,
and every animal, no matter how small or insignificant, contains some
carbon; while the plants around us, the trees, the grass, the flowers,
contain a by no means meager quantity of carbon.

Carbon plays an important and varied role in our life, and, in some
one of its many forms, enters into the composition of most of the
substances which are of service and value to man. The food we eat, the
clothes we wear, the wood and coal we burn, the marble we employ in
building, the indispensable soap, and the ornamental diamond, all
contain carbon in some form.

52. Charcoal. One of the most valuable forms of carbon is charcoal;
valuable not in the sense that it costs hundreds of dollars, but in
the more vital sense, that its use adds to the cleanliness, comfort,
and health of man.

The foul, bad-smelling gases which arise from sewers can be prevented
from escaping and passing to streets and buildings by placing charcoal
filters at the sewer exits. Charcoal is porous and absorbs foul gases,
and thus keeps the region surrounding sewers sweet and clean and free
of odor. Good housekeepers drop small bits of charcoal into vases of
flowers to prevent discoloration of the water and the odor of decaying
stems.

If impure wa