in mind that this gas is one of the products of the respiration of
animals, so that the animal kingdom is one of the sources of plant carbon.

The transition from carbon dioxide to woody fibre is brought about in the
plant by a series of chemical processes, and through the formation of a
number of intermediate products in a manner which is not yet thoroughly
understood; but since carbon dioxide consists of carbon and oxygen, and
since plants feed upon carbon dioxide, appropriating the carbon and giving
off the oxygen as a waste product, it is certain that work of some kind
must be performed. This is evident, because it has been explained that
when carbon combines with oxygen a great deal of heat is given out, and as
this heat is the equivalent of the energy stored in the carbon, it follows
from the doctrine of the Conservation of Energy, that in order to separate
the carbon from the oxygen again, just the same amount of energy must be
supplied as is evolved during the combustion of the carbon. If a pound of
carbon in burning to carbon dioxide gives out heat equivalent to eleven
million foot-pounds of work, we must apply the same amount of work to the
carbon dioxide produced to separate it into its constituents. Neither a
plant nor any living thing can create energy any more than it can create
matter, and just as the matter composing a living organism is assimilated
from external sources, so must we look to an external source for the
energy which enables the plant to do this large amount of chemical work.

The separation of carbon from oxygen in the plant is effected by means of
energy supplied by the sun. The great white hot globe which is the centre
of our system, and round which this earth and the planets are moving, is a
reservoir from which there is constantly pouring forth into space a
prodigious quantity of energy. It must be remembered that the sun is more
than a million times greater in bulk than our earth. It has been
calculated by Sir William Thomson that every square foot of the sun's
surface is radiating energy equivalent to 7000 horse-power in work. On a
clear summer day the earth receives from the sun in our latitude energy
equal to about 1450 horse-power per acre. To keep up this supply by the
combustion of coal, we should have to burn for every square foot of the
sun's surface between three and four pounds per second. A small fraction
of this solar energy reaches our earth in the form of radiant heat and