RADIANT HEAT AND ITS EELATIONS. 83 



motion. Let us apply these facts to radiant heat. This 

 blue flame is the flame of carbonic oxide ; this trans- 

 parent 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 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 

 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, 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 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. 



To its almost total incompetence to emit white light, 

 aqueous vapour adds a similar incompetence to absorb 

 white light. It cannot, for example, absorb the lumi- 

 nous rays of the sun, though it can absorb the non-lumi- 

 nous rays of the earth. This incompetence of the vapour 

 to absorb luminous rays is shared by water and ice in 

 fact, by all really transparent substances. Their trans- 

 parency is due to their inability to absorb luminous rays. 

 The molecules of such substances are in dissonance with 

 the luminous waves ; and hence such waves pass through 

 transparent bodies without disturbing the molecular 

 rest. A purely luminous beam, however intense may 



