ON RADIANT HE A T. 61 



flames are inserted into three glass tubes of different 

 lengths. Each of these flames can be caused to emit a 

 musical note, the pitch of which is determined by the 

 length of the tube surrounding the flame. The shorter 

 the tube the higher is the pitch. The flames are now 

 silent within their respective tubes, but each of them can 

 be caused to respond to a proper note sounded anywhere 

 in this room. With an instrument called a syren, a pow- 

 erful musical note, of gradually increasing pitch, can be 

 produced. Beginning with a low note, and ascending 

 gradually to a higher one, we finally attain the pitch of 

 the flame in the longest tube. The moment it is reached 

 the flame bursts into song. The other flames are still 

 silent within their tubes. But by urging the instrument 

 on to higher notes, the second flame is started, and the 

 third alone remains. A still higher note starts it also. 

 Thus, as the sound of the syren rises gradually in 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 takeu 

 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 orignal wave motion. 

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

 is the flame of carbonic oxide: this transparent gas is car- 

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

 intensely heated, or, in other words, in a state of intense 

 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 arid 

 cold gas, tne waves emitted by the former are intercepted 

 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 trans- 

 parent 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 scientific mea jbo 

 determine the substances of which the sun, the stars, and 

 even the nebulae are composed; the principle, namely, that 

 a body which is competent to emit, any ray, whether of 

 heat or light, is competent in the same degree to absorb 



