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, anti 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 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 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 and 
cold gas, the 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 men to 
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 
