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spectrum pass through it Without obstruction; but for the
waves of slower period, emanating from our heated plate
of copper, enormous differences of absorptive power are
manifested. These differences illustrate in the most unex-
pected manner the influence of chemical combination. Thus
the elementary gases, oxygen, hydrogen, and nitrogen, and
the mixture atmospheric air, prove to be practical vacua to
the rays of heat; for every ray, or, more strictly speaking,
for every unit of wave-motion, which any one of them is
competent to intercept, perfectly transparent ammonia in-
tercepts 5,460 units, olefiant gas 6,030 units, while sulphur-
ous acid gas absorbs 6,480 units. What becomes of the
wave-motion thus intercepted ? It is applied to the heating
of the absorbing gas. Through air, oxygen, hydrogen, and
nitrogen, on the contrary, the waves of ether pass without
absorption, and these 'gases are not sensibly changed in
temperature by the most powerful calorific rays. The po-
sition of nitrous oxide in the foregoing table is worthy of
particular notice. In this gas we have the same atoms, in
a state of chemical union, that exist uncombined in the
atmospheric air; but the absorption of the compound is
1,800 times that of air.

5. Formation of Invisible Focz'.

This extraordinary deportment of the elementary gases
naturally directed attention to elementary bodies in another
state of aggregation. Some" of Melloni’s results now at-
tained a new significance; for this celebrated experimenter
had found crystals of the element sulphur to be highly per-
vious to radiant heat; he had also proved that lamp-black
and black glass (which owes its blackness to the element
carbon) were to considerable extent transparent to calorific
rays of low refrangibility. These facts, harmonizing so
strikingly with the deportment of the simple gases, sug-

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