ndescence. Thus, the invisible waves of our filtered
electric beam may be regarded as generating synchronous vibrations
among the atoms of the platinum on which they impinge; but, once these
vibrations have attained a certain amplitude, the mutual jostling of
the atoms produces quicker tremors, and the light-giving waves follow
as the necessary product of the heat-giving ones.

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11. Absorption of Radiant Heat by Vapours and Odours.

We commenced the demonstrations brought forward in this lecture by
experiments on permanent gases, and we have now to turn our attention
to the vapours of volatile liquids. Here, as in the case of the
gases, vast differences have been proved to exist between various
kinds of molecules, as regards their power of intercepting the
calorific waves. While some vapours allow the waves a comparatively
free passage, the faintest mixture of other vapours causes a
deflection of the magnetic needle. Assuming the absorption effected
by air, at a pressure of one atmosphere, to be unity, the following
are the absorptions effected by a series of vapours at a pressure of
1/60th of an atmosphere:

Name of vapour Absorption

Bisulphide of carbon 47

Iodide of methyl 115

Benzol 136

Amylene 321

Sulphuric ether 440

Formic ether 548

Acetic ether 612

Bisulphide of carbon is the most transparent vapour in this list; and
acetic ether the most opaque; 1/60th of an atmosphere of the former,
however, produces 47 times the effect of a whole atmosphere of air,
while 1/60th of an atmosphere of the latter produces 612 times the
effect of a whole atmosphere of air. Reducing dry air to the pressure
of the acetic ether here employed, and comparing them then together,
the quantity of wave-motion intercepted by the ether would be many
thousand times that intercepted by the air.

Any one of these vapours discharged into the free atmosphere, in front
of a body emitting obscure rays, intercepts more or less of the
radiation. A similar effect is produced by perfumes diffused in the
air, though their attenuation is known to be almost infinite.
Carrying, for example, a current of dry air over bibulous paper,
moistened by patchouli, the scent taken up by the current absorbs