II.

The memorable investigations of Leslie and Rumford, and the subsequent
classical reasearches of Melloni, dealt, in the main, with the
properties of radiant heat; while in my investigations, radiant heat,
instead of being regarded as an end, was employed as a means of
exploring molecular condition. On this score little could be said until
the gaseous form of matter was brought under the dominion of experiment.
This was first effected in 1859, when it was proved that gases and
vapors, notwithstanding the open door which the distances between their
molecules might be supposed to offer to the heat waves, were, in many
cases, able effectually to bar their passage. It was then proved that
while the elementary gases and their mixtures, including among the
latter the earth's atmosphere, were almost as pervious as a vacuum to
ordinary radiant heat, the compound gases were one and all absorbers,
some of them taking up with intense avidity the motion of the
ether-waves.

A single illustration will here suffice. Let a mixture of hydrogen and
nitrogen, in the proportion of three to fourteen by weight, be inclosed
in a space through which are passing the heat rays from an ordinary
stove. The gaseous mixture offers no measurable impediment to the rays
of heat. Let the hydrogen and nitrogen now unite to form the compound
ammonia. A magical change instantly occurs. The number of atoms present
remains unchanged. The transparency of the compound is quite equal to
that of the mixture prior to combination. No change is perceptible to
the eye, but the keen vision of experiment soon detects the fact that
the perfectly transparent and highly attenuated ammonia resembles pitch