[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 water filters through charcoal, it emerges pure, having left its impurities in the pores of the charcoal.  Practically all household filters of drinking water are made of charcoal.  But such a device may be a source of disease instead of a prevention of disease, unless the filter is regularly cleaned or renewed.  This is because the pores soon become clogged with the impurities, and unless they are cleaned, the water which flows through the filter passes through a bed of impurities and becomes contaminated rather than purified.  Frequent cleansing or renewal of the filter removes this difficulty.

Commercially, charcoal is used on a large scale in the refining of sugars, sirups, and oils.  Sugar, whether it comes from the maple tree, or the sugar cane, or the beet, is dark colored.  It is whitened by passage through filters of finely pulverized charcoal.  Cider and vinegar are likewise cleared by passage through charcoal.

The value of carbon, in the form of charcoal, as a purifier is very great, whether we consider it a deodorizer, as in the case of the sewage, or a decolorizer, as in the case of the refineries, or whether we consider the service it has rendered man in the elimination of danger from drinking water.

53.  How Charcoal is Made.  Charcoal may be made by heating wood in an oven to which air does not have free access.  The absence of air prevents ordinary combustion, nevertheless the intense heat affects the wood and changes it into new substances, one of which is charcoal.