[Footnote 1: A paper read before the Royal Society, April 5, 1883.]
By JOHN TYNDALL, F.R.S.
Chemists are ever on the alert to notice analogies and resemblances in
the atomic structure of different bodies. They long ago indicated
points of resemblance between bisulphide of carbon and carbonic acid.
In the case of the latter we have one atom of carbon united to two of
oxygen, and in the case of the former one atom of carbon united to two
of sulphur. Attempts have been made to push the analogy still further
by the discovery of a compound of carbon and sulphur analogous to
carbonic oxide, but hitherto, I believe, without success. I have now
to note a resemblance of some interest to the physicist, and of a more
settled character than any hitherto observed.
When, by means of an electric current, a metal is volatilized and
subjected to spectrum analysis, the "reversal" of the bright band of
the incandescent vapor is commonly observed. This is known to be due
to the absorption of the rays emitted by the vapor by the partially
cooled envelope of its own substance which surrounds it. The effect is
the same in kind as the absorption by cold carbonic acid of the heat
emitted by a carbonic oxide flame. For most sources of radiation
carbonic acid is one of the most transparent of gases; for the
radiation from the hot carbonic acid produced in the carbonic oxide
flame it is the most opaque of all.
Again, for all ordinary sources of radiant heat, bisulphide of carbon,
both in the liquid and vaporous form, is one of the most diathermanous
bodies ever known. I thought it worth while to try whether a body
reputed to be analogous to carbonic acid, and so pervious to most
kinds of heat, would show any change of deportment when presented to
the radiation from hot carbonic acid. Does the analogy between the two
substances extend to the vibrating periods of their atoms? If it does,
then the bisulphide, like the carbonic acid, will abandon its usually
transparent character, and play the part of an opaque body when
presented to the radiation from the carbonic oxide flame. This proved
to be the case. Of the radiation from hydrogen, a thin layer of
bisulphide transmits 90 per cent., absorbing only 10. For the
radiation from carbonic acid, the same layer of bisulphide transmits
only 25 per cent., 75 per cent. being absorbed. For this source of
rays, indeed, the bisulphide transcends, as an absorbent, many
substances which, for all other sources, far transcend _it_.
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