is nearly pure carbon. It is in fact possible
to construct a series showing the conversion of wood into coal, this
series comprising the varieties given in the table on p. 23, as well as
younger and older vegetable deposits. The series will be--

I. Woody fibre (cellulose).
II. Peat from Dartmoor.
III. Lignite, or brown coal, an imperfectly carbonized vegetable
deposit of more recent geological age than true coal.
IV. Average bituminous coal.
V. Cannel coal from Wigan.
VI. Anthracite from Wales.
VII. Graphite, the oldest carbonaceous mineral.

The percentage of the chief elements in the members of this series is--

Carbon. Hydrogen. Oxygen.

I. 50.0 6.0 44.0
II. 54.0 5.2 28.2
III. 66.3 5.6 22.8
IV. 77.0 5.0 11.2
V. 81.2 5.6 7.9
VI. 90.1 3.2 2.5
VII. 94-99.5, the remainder being ash.

In the above table the increase of carbon and the decrease of oxygen is
well brought out; the hydrogen also on the whole decreases, although with
some irregularity. The exact course of the chemical change which occurs
during the passage of wood into coal is at present involved in obscurity.
The oxygen may be eliminated in the form of water or of carbon dioxide or
both; some of the carbon is got rid of in the form of marsh gas, a
compound of carbon and hydrogen, which forms the chief constituent of the
dangerous "fire-damp" of coal mines.

Marsh gas is an inflammable gas which becomes explosive when mixed with
air and ignited; it often escapes with great violence during the working
of coal seams, the jets blowing out from the coal or underclay with a
rushing noise, indicative of the high pressure under which the hydrocarbon
gas has accumulated. These jets of escaping gas are known amongst miners
as "blowers." If the air of a mine contains a sufficient quantity of the
gas, and a flame accidentally fires the mixture, there results one of
those disastrous explosions with which the history of coal mining has
unfortunately only made us too familiar.

From the account of coal which has thus far been rendered, it will be seen
that as a source of mechanical power, we are far from using it as
economically as could be desired; and when we look at our open grates with
clouds of unburnt carbon particles escaping up the chimney, and so
constructed that only a small fraction of the total heat warms our ro