from our heated plate of copper, enormous differences of absorptive
power are manifested. These differences illustrate in the most
unexpected 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 intercepts, perfectly transparent ammonia
intercepts 5,460 units, olefiant gas 6,030 units, while sulphurous
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, the waves of aether pass
without absorption, and these gases are not sensibly changed in
temperature by the most powerful calorific rays. The position 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 atmosphere; but the absorption of the compound
is 1,800 times that of air.

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5. Formation of Invisible Foci.

This extraordinary deportment of the elementary gases naturally
directed attention to elementary bodies 'in other states of
aggregation. Some of Melloni's results now attained a new
significance. This celebrated experimenter had found crystals of
sulphur to be highly pervious to radiant heat; he had also proved that
lamp-black, and black glass, (which owes its blackness to the element
carbon) were to a considerable extent transparent to calorific rays of
low refrangibility. These facts, harmonising so strikingly with the
deportment of the simple gases, suggested further enquiry. Sulphur
dissolved in bisulphide of carbon was found almost perfectly
diathermic. The dense and deeply-coloured element bromine was
examined, and found competent to cut off the light of our most
brilliant flames, while it transmitted the invisible calorific rays
with extreme freedom. Iodine, the companion element of bromine, was
next thought of, but it was found impracticable to examine the
substance in its usual solid condition. It however dissolves freely
in bisulphide of carbon. There is no chemical union between the
liquid and the iodine; it is simply a case of solution, in which the
uncombined atoms of the element can act upon the radiant heat. When
permitted to do so, it was found that