of different kinds are
distilled at a low red heat in iron retorts, and from the volatile
portions there are separated those valuable products which have already
been alluded to, viz. burning and lubricating oils, solvent mineral oil,
paraffin wax for candles, and ammonia. We may fairly claim these as coal
products, although the shales used contain much mineral matter, the carbon
averaging about 20 per cent., the hydrogen three per cent., the nitrogen
0.7, and the ash about 67 per cent. The shales worked are approximately of
the same age as true coal, _i.e._ Carboniferous. The Scotch companies are
distilling about two million tons of shale per annum, this quantity
producing about sixty million gallons of crude oil, and giving employment
to over 10,000 hands.

It is not the province of the present work to enter into the chemical
nature of the products of destructive distillation in any greater detail
than is necessary to enable the general reader to know something of the
recent discoveries in the utilization of these products. We shall,
however, have occasion later on to make ourselves acquainted with the
names of some of the more important raw materials which are derived from
this source, and certain preliminary explanations are indispensable. In
the first place then, let us start from the fact that coal--including
carbonaceous shale and lignite--when heated in a closed vessel gives gas,
tar, coke, and a watery liquor. A clear understanding must be arrived at
concerning the manner in which these products arise.

There is a widely-spread notion that the substances derived from coal and
utilized for industrial purposes are present in the mineral itself, and
that the art of the chemist has been exercised in separating the said
substances by various processes. This idea must be at once dispelled. It
is true that there is a small quantity of water and a certain amount of
gas already present in most coals, but these are quite insignificant as
compared with the total yield of gas and watery liquor. So also with
respect to the tar; it is possible that in some highly bituminous minerals
we might dissolve out a small quantity of tarry matter by the use of
appropriate solvents, but in the coals mostly used for gas-making not a
trace of tar exists ready formed, and still less can it be said that the
coal contains coke. All these products are formed _by the chemical
decomposition of the coal_ under the influence of heat, and their