RADIANT HEAT AND ITS RELATIONS. 221 



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. Here is an instrument called a siren, by which 

 a powerful musical note can be produced. Beginning with 

 a note of low pitch, and ascending gradually to a higher 

 one, I finally reach the note of the flame in the longest tube. 

 The moment it is. reached, the flame bursts into song. But 

 the other flames are still silent within their tubes. I urge 

 the instrument on to higher notes ; the second flame has 

 now started, and the third alone remains. But a still higher 

 note starts it also. Thus, as the sound of the siren rises 

 gradually in pitch, it awakens every flame in passing, by 

 striking it with a series of waves whose periods of recur- 

 rence are similar to its own. 



Now the wave-motion from the siren is in part taken up 

 by the flame which synchronizes with the waves ; and had 

 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-motion. 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 intense vibration. 

 It thus resembles the sounding tuning-fork, while this cold 

 carbonic acid resembles the silent one. What is the con- 

 sequence ? Through the synchronism of the hot and cold 

 gas transmission of motion through the gas is prevented ; 

 it is all transferred. 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 

 scientific men to determine the substances of which the sun, 



