RADIATION. 203 



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, and this is act- 

 ually the case when the waves of the visible spectrum pass 

 through a transparent gas or vapor. There is here no sen- 

 sible 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 here 

 be recorded. Carbonic-acid gas is one of the feeblest of 

 absorbers of the radiant heat emitted by solid sources. 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 num- 

 ber, emitted by the copper, to which the carbonic acid is 

 impervious ; and could, we obtain a source of heat emitting 

 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 carbonic 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 carbonic-oxide flame, car- 

 bonic acid absorbs twice as much as olefiant gas. This won- 

 derful change in the power of the former as an absorber is 

 simply due to the fact that the periods of the hot and cold 

 carbonic acid are identical, and that the waves from the 

 flame freely transfer their motion to the molecules which 

 synchronize with them. Thus it is that the tenth of an at- 

 mosphere of carbonic acid, enclosed in a tube four feet long, 

 absorbs 60 per cent, of the radiation from a carbonic-oxide 

 flame, while one-thirtieth of an atmosphere absorbs 48 per 

 cent, of the heat from the same origin. 



