CAPTATION. 49 
periods of recurrence synchronize with those of the atomic 
groups among which they pass. Thus, when we find the 
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 
ether 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 mole- 
cules, 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 vapor. There is here no 
sensible transference of motion from the ether to the mole- 
cules; 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 heat emit- 
ting such rays only, we should find carbonic acid more 
opaque to the radiation from that source than any other 
gas. Such a source is actually found in the flame of car- 
bonic oxide, where hot carbonic acid constitutes the main 
radiating body. Of the rays emitted by our heated plate 
of copper, olefiant gas absorbs ten times the quantity 
absorbed by carbonic acid. Of the rays emitted by a car- 
bonic oxide flame, carbonic acid absorbs twice as much as 
