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is called carbonic acid gas. Two atoms of oxygen and one
of carbon unite to form the molecule of carbonic acid which,
as I have-said, is the material from which wood and vege-
table tissues are mainly derived. The carbonic acid of the
air being subjected to an action somewhat analogous to
that of the electric current in the case of our lead and silver
solutions, has its carbon liberated and deposited as woody
fibre. The watery vapor of the air is subjected to similar
action; its hydrogen is liberated from its oxygen, and lies
down side by side with the carbon in the tissues of the
tree. The oxygen in both cases is permitted to wander
away into the atmosphere. But What is it which thus tears
the carbon and the hydrogen from the strong embrace of
the oxygen ? What is it in Nature that plays the part of
the*electric current in our experiments ? The rays of the
sun. The leaves of the plants absorb both the carbonic
acid and the aqueous vapor of the air; these leaves an-
swer to the cells in which our decompositions by the electric
current took place. In the leaves the solar rays decompose
both the carbonic acid and the water, permitting the oxygen
in both cases to escape into the air, and allowing the carbon
and the hydrogen to follow the bent of their own forces.
And just as the molecular attractions of the silver and the
lead found expression in the production of those beautiful
branching formsseen in our experiments, so do the molecular
attractions of the liberated carbon and hydrogen find ex-
pression in the architecture of grasses, plants, and trees.
In the fall of a cataract and the rush of the wind we
have examples of mechanical power. In the combinations
of chemistry and in the formation of crystals and vegetables
we have examples of molecular power. But before pro-
ceeding further I should like to make clear to you the
present condition of the surface of our globe with reference
to power generally. You have learned how the atoms of
oxygen and hydrogen rush together to form water. I have

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