disturbing the surrounding aether. By special modes of experiment the
same was proved to hold good for the vapours of volatile liquids, the
radiative power of every vapour being found proportional to its
absorptive power.

The method of experiment here pursued, though not of the simplest
character, is still easy to grasp. When air is permitted to rush into
an exhausted tube, the temperature of the air is raised to a degree
equivalent to the _vis viva_ extinguished. [Footnote: See above for a
definition of _vis viva_.] Such air is said to be dynamically heated,
and, if pure, it shows itself incompetent to radiate, even when a
rock-salt window is provided for the passage of its rays. But if
instead of being empty the tube contain a small quantity of vapour,
the warmed air communicates its heat by contact to the vapour, the
molecules of which convert into the radiant form the heat imparted to
them by the atoms of the air. By this process also, which I have
called Dynamic Radiation, the reciprocity of radiation and absorption
has been conclusively proved. [Footnote: When heated air imparts its
motion to another gas or vapour, the transference of heat is
accompanied by a change of vibrating period. The Dynamic Radiation of
vapours is rendered possible by this transmutation of vibrations.]

In the excellent researches of Leslie, De la Provostaye and Detains,
and Balfour Stewart, the same reciprocity, as regards solid bodies,
has been variously illustrated; while the labours, theoretical and
experimental, of Kirchhoff have given this subject a wonderful
expansion, and enriched it by applications of the highest kind. To
their results are now to be added the foregoing, whereby gases and
vapours, which have been hitherto thought inaccessible to experiments
with the thermo-electric pile, are proved by it to exhibit the
indissoluble duality of radiation and absorption, the influence of
chemical combination on both being exhibited in the most decisive and
extraordinary way.

********************

15. Influence of Vibrating Period and Molecular Form. Physical
Analysis of the Human Breath.

In the foregoing experiments with gases and vapours have employed
throughout invisible rays, and found e of these bodies so impervious
to radiant heat, that lengths of a few feet they intercept every ray
as actually as a layer of pitch. The substances, however, which show
themselves thus opaque to radiant heat perfectly transparent