to light.
Now the rays of light differ from those of invisible heat merely in
point period, the former failing to affect the retina because their
periods of recurrence are too slow. Hence, in one way or other, the
transparency of our gases and vapours depends upon the periods of the
waves which impinge upon them. What is the nature of this dependence?
The admirable researches of Kirchhoff help us an answer. The atoms
and molecules of every gas e certain definite rates of oscillation,
and those waves aether are most copiously absorbed whose periods
recurrence synchronise with those of the atomic ups amongst which they
pass. Thus, when we find invisible rays absorbed and the visible ones
transmitted by a layer of gas, we conclude that the oscillating
periods of the atoms constituting the gaseous molecules coincide with
those of the invisible, and not with those of the visible spectrum.

It requires some discipline of the imagination to form a clear picture
of this process. Such a picture is, however, possible, and ought to
be obtained. When the waves of aether impinge upon molecules whose
periods of vibration coincide with the recurrence of the undulations,
the timed strokes of the waves augment the vibration of the molecules,
as a heavy pendulum is set in motion by well-timed puffs of breath.
Millions of millions of shocks are received every second from the
calorific waves; and it is not difficult to see that as every wave
arrives just in time to repeat the action of its predecessor, the
molecules must finally be caused to swing through wider spaces than if
the arrivals were not so timed. In fact, it is not difficult to see
that an assemblage of molecules, operated upon by contending waves,
might remain practically quiescent. This is actually the case when
the waves of the visible spectrum pass through a transparent gas or
vapour. There is here no sensible transference of motion from the
aether to the molecules; in other words, there is no sensible
absorption of heat.

One striking example of the influence of period may be here recorded.
Carbonic acid gas is one of the feeblest absorbers of the radiant heat
emitted by solid bodies. It is, for example, to a great extent
transparent to the rays emitted by the heated copper plate already
referred to. There are, however, certain rays, comparatively few in
number, emitted by the copper, to which the carbonic acid is
impervious; and could we obtain a source of he