these relations, and by once more combining the constituents of fuel
with the oxygen of the air, reverse the action which caused the growth
of the plants, that is to say, by destroying the plant reproduce the
heat and light which fostered it. The energy which can be set free by
this process cannot be greater than that derived originally from the
sun, and which, acting through the frail mechanism of green leaves,
tore asunder the strong bonds of chemical affinity wherein the carbon
and oxygen were hound, converting the former into the ligneous
portions of the plants and setting the latter free for other uses. The
power thus silently exerted is enormous; for every ton of carbon
separated in twelve hours necessitates an expenditure of energy
represented by at least 1,058 horse power, but the action is spread
over an enormous area of leaf surface, rendered necessary by the small
proportion of carbonic acid contained in the air, by measure only
1/2000 part, and hence the action is silent and imperceptible. It is
now conceded on all hands that what is termed heat is the energy of
molecular motion, and that this motion is convertible into various
kinds and obeys the general laws relating to motion. Two substances
brought within the range of chemical affinity unite with more or less
violence; the motion of transition of the particles is transformed,
wholly or in part, into a vibratory or rotary motion, either of the
particles themselves or the interatomic ether; and according to the
quality of the motions we are as a rule, besides other effects, made
conscious of heat or light, or of both. When these emanations come to
be examined they are found to be complex in the extreme, intimately
bound up together, and yet capable of being separated and analyzed.

As soon as the law of definite chemical combination was firmly
established, the circumstance that changes of temperature accompanied
most chemical combinations was noticed, and chemists were not long in