But its greatest, because it is its most useful property, is undoubtedly the power which it has of absorbing great quantities of gas into itself.  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 to keep out water which may be struck in passing through the various strata, it is protected by plank or wood tubbing, or the shaft is bricked over, or sometimes even cast-iron segments are sunk.  In many shafts which, owing to their great depth, pass through strata of every degree of looseness or viscosity, all three methods