period, though of the utmost importance, is not
competent to account for the whole of the observed facts. The aether,
as far as we know, accepts vibrations of all periods with the same
readiness. To it the oscillations of an atom of free oxygen are just
as acceptable as those of the atoms in a molecule of olefiant gas;
that the vibrating oxygen then stands so far below the olefiant gas in
radiant power must be referred not to period, but to some other
peculiarity. The atomic group which constitutes the molecule of
olefiant gas, produces many thousand times the disturbance caused by
the oxygen, it may be because the group is able to lay a vastly more
powerful hold upon the aether than single atoms can. Another, and
probably very potent cause of the difference may be, that the
vibrations, being those of the constituent atoms of the molecule,
[Footnote: See 'Physical Considerations,' Art. iv.] are generated
in highly condensed aether, which acts like: condensed air upon sound.
But whatever may be the fate of these attempts to visualise the physics
of the process, it will still remain true, that to account for the
phenomena of radiation and absorption we must take into consideration
the shape, size, and condition of the aether within the molecules, by
which the external aether is disturbed.

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16. Summary and Conclusion.

Let us now cast a momentary glance over the ground that we have left
behind. The general nature of light and heat was first briefly
described: the compounding of matter from elementary atoms, and the
influence of the act of combination on radiation and absorption, were
considered and experimentally illustrated. Through the transparent
elementary gases radiant heat was found to pass as through a vacuum,
while many of the compound gases presented almost impassable obstacles
to the calorific-waves. This deportment of the simple gases directed
our attention to other elementary bodies, the examination of which led
to the discovery that the element iodine, dissolved in bisulphide of
carbon, possesses the power detaching, with extraordinary sharpness,
the light of the spectrum from its heat, intercepting all luminous
rays up to the extreme red, and permitting the calorific rays beyond
the red to pass freely through it. This substance was then employed
to filter the beams of the electric light, and to form foci of
invisible rays so intense as to produce almost all the effects
obtai