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 in
nature that plays the part of the electric current in our experiments,
tearing asunder the locked atoms of carbon, oxygen, and hydrogen? The
rays of the sun. The leaves of plants which absorb both the carbonic
acid and the aqueous vapour of the air, answer to the cells in which
our decompositions took place. And just as the molecular attractions
of the silver and the lead found expression in those beautiful
branching forms seen in our experiments, so do the molecular
attractions of the liberated carbon and hydrogen find expression 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. You have learned how the atoms of oxygen and hydrogen rush
together to form water. I have not thought it necessary to dwell upon
the mighty mechanical energy of their act of combination; but it may
be said, in passing, that the clashing together of 1 lb. of hydrogen
and 8 lbs. of oxygen to form 9 lbs. of aqueous vapour, is greater than
the shock of a weight of 1,000 tons falling from a height of 20 feet
against the earth. Now, in order that the atoms of oxygen and
hydrogen should rise by their mutual attractions to the velocity
corresponding to this enormous mechanical effect, a certain distance
must exist between the particles. It is in rushing over this that the
velocity is attained.

*****

This idea of distance between the attracting atoms is of the highest
importance in our conception of the system of the world. For the
matter of the world may be classified under two distinct heads: atoms
and molecules which have already combined and thus satisfied their
mutual attractions, and atoms and molecules which have not yet
combined, and whose mutual attractions are, therefore, unsatisfied.
Now, as regards motive power, we are entirely dependent on atoms and
molecules of the latter kind. Their attractions can produce motion,
because sufficient distance intervenes between the attracting atoms,
and it is this atomic motion that we utilise in our machines. Thus we
can get power out of oxygen and hydrogen by the act of their union;
but once they are combined, and once the vibratory motion consequ