ids, without any
change of their chemical nature. We do not yet know how the
transmission of radiant heat may be affected by the entanglement due
to cohesion; and, as our object now is to examine the influence of
chemical union alone, we shall render our experiments more pure by
liberating the atoms and molecules entirely from the bonds of
cohesion, and employing them in the gaseous or vaporous form.

Let us endeavour to obtain a perfectly clear mental image of the
problem now before us. Limiting in the first place our enquiries to
the phenomena of absorption, we have to picture a succession of waves
issuing from a radiant source and passing through a gas; some of them
striking against the gaseous molecules and yielding up their motion to
the latter; others gliding round the molecules, or passing through the
intermolecular spaces without apparent hindrance. The problem before
us is to determine whether such free molecules have any power whatever
to stop the waves of heat; and if so, whether different molecules
possess this power in different degrees.

In examining the problem let us fall back upon an actual piece of
work, choosing as the source of our heat-waves a plate of copper,
against the back of which a steady sheet of flame is permitted to
play. On emerging from the copper, the waves, in the first instance,
pass through a space devoid of air, and then enter a hollow glass
cylinder, three feet long and three inches wide. The two ends of this
cylinder are stopped by two plates of rock-salt, a solid substance
which offers a scarcely sensible obstacle to the passage of the
calorific waves. After passing through the tube, the radiant heat
falls upon the anterior face of a thermo-electric pile, [Footnote: In
the Appendix to the first chapter of 'Heat as a Mode of 'Motion,' the
construction of the thermo-electric pile is fully explained.] which
instantly converts the heat into an electric current. This current
conducted round a magnetic needle deflects it, and the magnitude of
the deflection is a measure of the heat falling upon the pile. This
famous instrument, and not an ordinary thermometer, is what we shall
use in these enquiries, but we shall use it in a somewhat novel way.
As long as the two opposite faces of the thermo-electric pile are kept
at the same temperature, no matter how high that may be, there is no
current generated. The current is a consequence of a difference of
temperature between the two