ter 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.

The wood which smolders on the hearth and in the stove is charcoal in
the making. Formerly wood was piled in heaps, covered with sod or sand
to prevent access of oxygen, and then was set fire to; the smoldering
wood, cut off from an adequate supply of air, was slowly transformed
into charcoal. Scattered over the country one still finds isolated
charcoal kilns, crude earthen receptacles, in which wood thus deprived
of air was allowed to smolder and form charcoal. To-day charcoal is
made commercially by piling wood on steel cars and then pushing the
cars into strong walled chambers. The chambers are closed to prevent
access of air, and heated to a high temperature. The intense heat
transforms the wood into charcoal in a few hours. A student can make
in the laboratory sufficient charcoal for art lessons by heating in an
earthen vessel wood buried in sand. The process will be slow, however,
because the heat furnished by a Bunsen burner is not great,