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l 74 FRAGMEN TS OF SCIENCE.

of heat are in this way reducible to interchanges of motion;
and it is purely as the recipients or the donors of this mo-
tion, that we ourselves become conscious of the action of
heat and cold.

3. The Atomic Theory in reference to the Ether.

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. con-
dition 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 or spirit
of hartshorn. Picturing in imagination the atoms 'of ele-
mentary 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
ether pervading space and constituting the vehicle of atomic
motion be founded in fact, we may assuredly expect the
vibrations of elementary bodies to be profoundly modified
by the act of combination. It is on the face of it almost
certain that both as regards radiation and absorption, that
is to say, both as regards the communication of motion to
the ether and theacceptance of motion from it, the deport-
ment of the uncombined will be different from that of the
combined atoms.

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