40 



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 



