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power must be referred not to period, but to some other
peculiarity of the elementary gas. The atomic group which
constitutes the molecule of olefiant gas, produces many
thousand times the disturbance caused by the oxygen, be-
cause the group is able to lay a Vastly more powerful hold
upon the ether than the single atomscan. The cavities and
indentations of a molecule composed of spherical atoms
may be one cause of this augmented hold. Another, and
probably very potent one may be, that the ether itself, con-
densed and entangled among the constituent atoms of a
compound, Virtually increases the magnitude of the group,
and hence augments the disturbance. But whatever may
be the fate of these attempts to visualize the physics of the
process, it will still remain true that, to account fer the
phenomena of radiation and absorption we must take into
consideration the shape, size, and complexity of the mole-
cules by which the ether is disturbed.

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 com-
bination on radiation and absorption were considered and
experimentally illustrated. Through the transparent ele-
mentary 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 bisul-
phide of carbon, possesses the power of detaching, with
extraordinary sharpness, the light of the spectrum from its
heat, intercepting all luminous rays up to the extreme red,

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