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RADIATION. 1 7 1

irresistibly infers that the appearance of the colors corre-
sponds to certain contemporaneous changes in the wire.
What is the nature of these changes ? In virtue of what
condition does the wire radiate at all? We must now
look from the wire as a whole to its constituent atoms.
Could we see those atoms, even before the electric current
has begun to act upon them, we should find them in a
state of vibration. In this vibration, indeed, consists such
warmth as the wire then possesses. Locke enunciated this
idea with great precision, and it seems placed. beyond the
pale of doubt by the excellent quantitative researches of
Mr. Joule. “ Heat,” says Locke, “is a very brisk agitation
of the insensible parts of the object, which produce in us
that sensation from which we denominate the object hot:
so what in our sensation is heat in the object is nothing
but motion.” When the electric current, still feeble, begins
to pass through the wire, its first act is to intensify the
vibrations already existing, by causing the atoms to swing
through wider ranges. Technically speaking, the ampli-
tudes of the oscillations are increased. The current does
this, hoWever, without altering the periods of the old vi-
brations, or the times in which they were executed. But
besides intensifying the old vibrations the current gener-
ates new and more rapid ones, and When a certain definite
rapidity has been attained the wire begins to glow. The
color first exhibited is red, which correSponds to the lowest
rate of vibration of which the eye is able to take cognizance.
By augmenting the strength of the electric current more
rapid vibrations are introduced, and orange rays appear.
A quicker rate of vibration produces yellow, a still quicker
green; and by further augmenting the rapidity, we pass
through blue, indigo, and violet, to the extreme ultra-violet
rays.

Such are the changes which science recognizes in the
wire itself, as concurrent with the Visual changes taking

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