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244 FRAGMEN TS OF SCIENCE.

mechanically the same as that due to the timed impulses
of a boy upon a swing. The single tick of a clock has no
appreciable effect upon the unvibrating and equally long
pendulum of a distant clock; but a succession of ticks, each
of which adds, at the proper moment, its infinitesimal push
to the sum of the pushes preceding it, will, as a matter of
fact, set the second clock going. So likewise a single puff
of air against the prong of a heavy tuning-fork produces
no sensible motion, and, consequently, no audible sound;
but a succession of puffs, which follow each other in periods
identical with the tuning-fork’s period of vibration, will
render the fork sonorous. I think the chemical action of
light is to be regarded in this way. Fact and reason point
to the conclusion that it is the heaping up of motion on the
atoms, in consequence of their synchronism with the shorter
waves, that causes them to part company. This I take to
be the mechanical cause of these decompositions which are
effected by the waves of ether.

And now let us return to that faint cloudiness already
mentioned, from which, as from a germ, these considerations
and speculations have sprung. It has been long known that
light effected the decomposition of a certain number of
bodies. The transparent iodide of ethyl, or of methyl, for
example, becomes brown and opaque on exposure to light,
through the discharge of its iodine. The art of photography
is founded on the chemical actions of light; so that it is
well known that the effects for which the foregoing theoretic
considerations would have prepared us, are not only proba-
ble, but actual.

But the method employed in the experiments in which
the cloudiness above referred to was observed, and which
consists simply in offering the vapors of volatile substances
to the action of light, enables us not only to give such ex-
periments a beautfiul form, but also to give a great exten-
sion to the operations of light, or rather of radiant force, as

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