But the question may be asked, what does this'
mean, how can mechanical work be really transformed
into heat or vice versd ? The answer is, the energy
which was supplied by chemical action to the muscles
of the man or horse, or to the water converted' into
steam by combustion of coal, which originated the



70 MODERN SCIENCK AND MODERN THOUGHT.
mechanical work, was first transformed into its equiva-
lent amount of mechanical energy of motion, and then,
when that motion was arrested, was transformed into
heat, which is simply the same energy transformed into
increased molecular motion.
If we wish to carry our inquiry a step further back
and ask where the original energy came from which
has undergone these transformations, the answer must
be, mainly from the sun. The sun's rays, acting on the
chlorophyl or green matter of the plants of the coal era,
tore asunder the atoms of carbon and oxygen which
formed the carbonic acid in the atmosphere, and locked
up a store of energy in the form of carbon in the coal
which is burned to produce the steam. In like manner
it- stored up the energy in the form of carbc)n in the
vegetable products which, either directly, or indirectly
after having passed through the body of some animal,
supplied the food, whose slow combustion in the man or
horse supplied the energy which did the work.
