72 FRAGMENTS OF SCIENCE 



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 dim- 

 cult 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 act- 

 ually 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 

 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 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 ab- 

 sorbed by carbonic acid. Of the rays emitted by a car- 

 bonic oxide flame, carbonic acid absorbs twice as much 

 as olefiant gas. This wonderful change in the power of 



