RADIANT HEAT AND ITS RELATIONS 89 



alone remains. A still higher note starts it also. Thus, 

 as the sound of the syren rises gradually m pitch, it 

 awakens every flame in passing, by striking it with a 

 series of waves whose periods of recurrence are similar 

 to its own. 



Now the wave- motion from the syren is in part taken 

 up by the flame which synchronizes with the waves; and 

 were these waves to impinge upon a multitude of flames, 

 instead of upon one flame only, the transference might be 

 so great as to absorb the whole of the original wave-mo- 

 tion. Let us apply these facts to radiant heat. This blue 

 flame is the flame of carbonic oxide; this transparent gas 

 is carbonic acid gas. In the blue flame we have carbonic 

 acid intensely heated, or, in other words, in a state of in- 

 tense vibration. It thus resembles the sounding fork, while 

 this cold carbonic acid resembles the silent one. What is 

 the consequence ? Through the synchronism of the hot 

 and cold gas, the waves emitted by the former are inter- 

 cepted by the latter, the transmission of the radiant heat 

 being thus prevented. The cold gas is intensely opaque 

 to the radiation from this particular flame, though highly 

 transparent to heat of every other kind. We are here 

 manifestly dealing with that great principle which lies at 

 the basis of spectrum analysis, and which has enabled sci- 

 entific men to determine the substances of which the sun, 

 the stars, and even the nebulae, are composed; the princi- 

 ple, namely, that a body which is competent to emit any 

 ray, whether of heat or light, is competent in the same de- 

 gree to absorb that ray. The absorption depends on the 

 synchronism existing between the vibrations of the atoms 

 from which the rays, or more correctly the waves, issue, 

 and those of the atoms on which they impinge. 



