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numerous processes in Nature which entirely elude the eye
of the body, and must be figured by the eye of the mind.
The processes of chemistry are examples of these. Long
thinking and experimenting on the materials which compose
our world have led philosophers to conclude that matter is
composed of atoms from which, whether separate or in com-
bination, the whole material world is built up. The air we
breathe, for example, is mainly a mixture of the atoms of
two distinct substances, called oxygen and nitrogen. The
water we drink is also composed of two distinct substances,
called oxygen and hydrogen. But it differs from the air in
this particular, that in water the oxygen and hydrogen are
not mechanically mixed, but chemically combined. In fact,
the atoms of oxygen and those of hydrogen exert enormous
attractions on each other, .so that when brought into sufficient
proximity they rush together with an almost incredible
fofiefitomform a chemical compound. But powerful as is the
‘force with which these atoms lock themselves together, we
have the means of tearing them'asunder, and the agent by
which we accomplish this may here receive a few moments’

Into a vessel containing acidulated water I dip these
two strips of metal, the one being zinc and the other plati-j
num, not permitting them to touch each other in the liquid.
I now connect the two upper ends of the strips by a piece
of copper wire. The wire is apparently unchanged, but it
is not so in reality. It is now the channel of what, for
want of a better name, we call an electric current—a power
generated and maintained by the chemical action going on
in the vessel of acidulated water. What the inner change
of the wire is we do not know, but we do know that a change
has occurred, by the external efl‘ects produced by the wire.
Let me show you one or two of these effects. And here it
is convenient to Operate with greater power than can be ob-
tained from a single small pair of strips of metal, and a

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