purely as the recipients or the donors of this
motion, that we ourselves become conscious of the action of heat and
cold.

********************

3. The Atomic Theory in reference to the Aether.

The word 'atoms' has been more than once employed in this discourse.
Chemists have taught us that all matter is reducible to certain
elementary forms to which they give this name. These atoms are
endowed with powers of mutual attraction, and under suitable
circumstances they coalesce to form compounds. Thus oxygen and
hydrogen are elements when separate, or merely _mixed_, but they may be
made to _combine_ so as to form molecules, each consisting of two atoms
of hydrogen and one of oxygen. In this condition they constitute
water. So also chlorine and sodium are elements, the former a pungent
gas, the latter a soft metal; and they unite together to form chloride
of sodium or common salt. In the same way the element nitrogen
combines with hydrogen, in the proportion of one atom of the former to
three of the latter, to form ammonia. Picturing in imagination the
atoms of elementary bodies as little spheres, the molecules of
compound bodies must be pictured as groups of such spheres. This is
the atomic theory as Dalton conceived it. Now if this theory have any
foundation in fact, and if the theory of an aether pervading space,
and constituting the vehicle of atomic motion, be founded in fact, it
is surely of interest to examine whether the vibrations of elementary
bodies are modified by the act of combination--whether as regards
radiation and absorption, or, in other words, whether as regards the
communication of motion to the aether, and the acceptance of motion
from it, the deportment of the uncombined atoms will be different from
that of the combined.

********************

4. Absorption of Radiant Heat by Gases.

We have now to submit these considerations to the only test by which
they can be tried, namely, that of experiment. 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
necessarily 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. Gases and vapours aggregate to liquids and sol