light, and it is the latter which enables the plant to perform the work of
separating the carbon from the oxygen with which it is chemically
combined. It is, in fact, well known that the growth of plants--that is,
the assimilation of carbon and the liberation of oxygen--only takes place
under the influence of light. This function is performed by the leaves
which contain the green colouring-matter known as chlorophyll, the
presence of which is essential to the course of the chemical changes.

If we now sum up the results to which we have been led, it will be seen--

(1) That the chief source of the energy contained in coal is the carbon.

(2) That this carbon formed part of the plants which grew during the
Carboniferous period.

(3) That the carbon thus accumulated was supplied to the plants by the
carbon dioxide existing in the atmosphere at that time.

(4) That the separation of the carbon from the oxygen was effected in the
presence of chlorophyll, by means of the solar energy transmitted to the
earth during the Carboniferous period.

We thus arrive at the interesting conclusion, that the heat which we get
from coal is sunlight in another form. For every pound of coal that we now
burn, and for every unit of heat or work that we get from it, an
equivalent quantity of sunlight was converted into the latent energy of
chemical separation during the time that the coal plant grew. This energy
has remained stored up in the earth ever since, and reappears in the form
of heat when we cause the coal to undergo combustion. It is related that
George Stephenson when asked what force drove his locomotive, replied that
it was "bottled-up sunshine," and we now see that he was much nearer the
truth in making this answer than he could have been aware of at the time.

Before passing on to the consideration of the different products which we
get from coal, it will be desirable to discuss a little more fully the
nature of the change which occurs during the transformation of wood into
coal. Pure woody fibre consists of a substance known to chemists as
cellulose, which contains fifty per cent. of carbon, the remainder of the
compound being made up of hydrogen and oxygen. It is thus obvious that
during the fossilization of the wood some of the other constituents are
lost, and the percentage of carbon by this means raised. We can trace this
change from wood, through peat, lignite, and the different varieties of
coal up to graphite, which