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4. Absorption of Radiant Héat by Gases.

We have now to submit these considerations to the
only test by which they can be tried, namely,'that of ex-
periment. An experiment is well defined as a question put
to Nature; but to avoid the risk of asking amiss we ought
to purify the question from all adjuncts WhiCh do not neces-
sarily belong to it. Matter has been shown to be composed
of elementary constituents, by the compounding of which
all its varieties are produced. But besides the chemical
unions which they form, both elementary and compound
bodies can unite in another and less intimate Way. By the
attraction of cohesion gases and vapors aggregate to liquids
and solids, without any change of their chemical nature.
We do not yet know how the transmission of radiant heat
may be afl’ected by the entanglement due to cohesion, and
as our object now is to examine the influence of chemical
union alone, we shall render our experiments more pure by
liberating the atoms and molecules entirely from the bonds
of cohesion, and employing them in the gaseous or vapor-
ous form.

Let us endeavor to obtain a perfectly clear mental image
of the problem now before us. Limiting in the first place
our inquiries to the phenomena of absorption, we have to
picture a succession of waves issuing from a radiant source
and passing through a gas; some of them striking against
the gaseous molecules and yielding up their motion to
the latter; others gliding round the molecules or passing
through the inter-molecular spaces Without apparent hinder-
ance. The problem before us is to determine whether such
free molecules have any power whatever to stop the waves
of heat, and, if so, whether difierent molecules possess this
power in different degrees.

The source of waves which I shall choose for these

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