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RADIATION. 179

gested further inquiry. Sulphur dissolved in bisulphate of
carbon was found almost perfectly transparent. The dense
and deeply-colored element bromine was examined, and
found competent to cut off the light of our most brilliant
flames, while it transmitted the invisible calorific rays with
extreme freedom. Iodine, the companion-element of bro-
mine, was next thought of, but it was found impracticable
to examine the substance in its usual solid condition. It
however dissolves freely in bisulphide of carbon. There is
no chemical union between the liquid and the iodine; it is
simply a case of solution, in which the uncombined atoms
of the element can act upon the radiant heat. When per-
mitted to do so, it was found that 'a layer of dissolved
iodine, sufficiently opaque to cut off the light of the mid-
day sun, was almost absolutely transparent to the invisible
calorific rays.

By prismatic analysis Sir William Herschel separated
the luminous from the non-luminous rays of the sun, and
he also sought to render the obscure rays visible by con-
centration. Intercepting the luminous portion of his spec-
trum he brought, by a converging lens, the ultra-red rays
to a focus, but by this condensation he obtained no light.
The solution of iodine offers a means of- filtering the solar
beam, or, failing it, the beam of the electric lamp, which
renders attainable far more powerful foci of invisible rays
than could possibly be obtained by the method of Sir Wil-
liam Herschel. For to form his spectrum he was obliged
to operate upon solar light which had passed through
a narrow slit or through a small aperture, the amount of
the obscure heat being limited by this circumstance. But
with our Opaque solution we may employ the entire surface
of the largest lens, and having thus converged the rays,
luminous and non-luminous, we can intercept the former by
the iodine, and do what we please with the latter. Ex-
periments of this character, not only with the iodine solu-

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