"Those who are interested in the preparation of coak would do well to
remember that every 96 ounces of coal would furnish four ounces at the
least of oil, probably six ounces might be obtained; but if we put the
product so low as five ounces from 100, and suppose a coak oven to work
off only 100 tons of coal in a year, there would be a saving of five tons
of oil, which would yield above four tons of tar; the requisite alteration
in the structure of the coak ovens, so as to make them a kind of
distilling vessels, might be made at a very trifling expense."--5th ed.,
1789, vol. ii. p. 351.

We have yet to chronicle another chapter in the history of coal philosophy
before finishing with this part of the subject. There is a branch of
manufacture carried on, especially in Scotland, which results in the
production of burning and lubricating oils, and solid paraffin, a wax-like
substance which is used for candle-making. The manufacture of candles out
of coal will perhaps be a new revelation to many readers of this book. It
must be admitted, however, that the term "coal" is here being extended to
only partially fossilized vegetation of younger geological age than true
coal, and to bituminous shales of various ages. Shale, geologically
considered, is hardened mud; it may be looked upon as clay altered by time
and pressure. Now if the mud, at the period of its deposition, was much
mixed up with vegetable matter, we should have in course of time a mixture
of more or less carbonized woody fibre with mineral matter, and this would
be called a carbonaceous or bituminous shale. Shales of this kind often
contain as much as 80 to 90 per cent. of mineral matter, and seldom more
than 20 per cent. of volatile matter, _i.e._ the portion lost on ignition,
and consisting chiefly of the carbonaceous constituents.

The story of the shale-oil industry is soon told. About the year 1847 oil
was "struck" in a coal mine at Alfreton in Derbyshire, and in the hands of
Mr. James Young this supply furnished the market with burning-oil for
nearly three years. Then the spring became exhausted, and Mr. Young and
his associates had to look out for another source of oil. Be it remembered
that this happened some nine years before the utilization of the great
American petroleum deposits. Many kinds of vegetable matter were submitted
to destructive distillation before a substance was found which could be
profitably worked, but at length Mr. Young tried a kind o