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visible if we could burn the gas in a clean atmosphere.
But the atmOSphore, even at the summit of Mont Blanc,
is dirty; in London it is more than dirty; and the burning
dirt gives to this flame the greater portion of its present
light. But the heat of the flame is enormous. Cast-iron
fuses at a temperature of 2,000° Fahr. A piece of platinum
is heated to vivid redness at a distance of two inches be-
yond the visible termination of the flame. The vapor
which produces incandescence is here absolutely dark. In
the flame itself the platinum is raised to dazzling white-
ness, and is finally pierced by the flame. When this flame
impinges on a piece of lime, we have the dazzling Drum-
mond light. But the light is here due to the fact that
when it impinges upon the solid body, the vibrations ex-
cited in that body by the flame are of periods different from
its own.

Thus far we have fixed our attention on atoms and
molecules in—-- a state of vibration, and. surrounded by a
medium which accepts their vibrations, and transmits them
through space. But suppose the waves generated by one
system of molecules to impinge upon another system, how
will the waves be affected ? Will they be stopped, or will
they be permitted to pass? Will they transfer their mo-
tion to the molecules on which they impinge, or will they
glide round the molecules, through the intermolecular
spaces, and thus escape ?

The answer to this question depends upon a condition
which may be beautifully exemplified by an experiment on
sound. These two tuning-forks are tuned absolutely alike.
They vibrate with the same rapidity, and mounted thus
upon their resonant stands, you hear them loudly sounding
the same musical note. I stop one of the forks, and throw
the other into strong vibration. I now bring that other
near the silent fork, but not into contact with it. Allow-
ing them to continue in this position for four orfive seconds,

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