It is in fact what may be termed an all-round purifier. It is a
deodoriser, a disinfectant, and a decoloriser. It is an absorbent of bad
odours, and partially removes the smell from tainted meat. It has been
used when offensive manures have been spread over soils, with the same
object in view, and its use for the purification of water is well known
to all users of filters. Some idea of its power as a disinfectant may be
gained by the fact that one volume of wood-charcoal will absorb no less
than 90 volumes of ammonia, 35 volumes of carbonic anhydride, and 65
volumes of sulphurous anhydride.

Other forms of carbon which are well-known are (1) coke, the residue left
when coal has been subjected to a great heat in a closed retort, but from
which all the bye-products of coal have been allowed to escape; (2) soot
and lamp-black, the former of which is useful as a manure in consequence
of ammonia being present in it, whilst the latter is a specially prepared
soot, and is used in the manufacture of Indian ink and printers' ink.

CHAPTER IV.

THE COAL-MINE AND ITS DANGERS.

It is somewhat strange to think that where once existed the solitudes of
an ancient carboniferous forest now is the site of a busy underground
town. For a town it really is. The various roads and passages which are
cut through the solid coal as excavation of a coal-mine proceeds,
represent to a stranger all the intricacies of a well-planned town. Nor
is the extent of these underground towns a thing to be despised. There is
an old pit near Newcastle which contains not less than fifty miles of
passages. Other pits there are whose main thoroughfares in a direct line
are not less than four or five miles in length, and this, it must be
borne in mind, is the result of excavation wrought by human hands and
human labour.

So great an extent of passages necessarily requires some special means of
keeping the air within it in a pure state, such as will render it fit for
the workers to breathe. The further one would go from the main
thoroughfare in such a mine, the less likely one would be to find air of
sufficient purity for the purpose. It is as a consequence necessary to
take some special steps to provide an efficient system of ventilation
throughout the mine. This is effectually done by two shafts, called
respectively the downcast and the upcast shaft. A shaft is in reality a
very deep well, and may be circular, rectangular or oval in form. In
order t