RADIANT HEAT AND ITS RELATIONS. 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 pre- 

 vented. The cold gas is intensely opaque to the radia- 

 tion from this particular flame, though highly trans- 

 parent to heat of every other kind. We are here mani- 

 festly 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 vibra- 

 tions of the atoms from which .the rays, or more cor- 

 rectly 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 va- 

 pour to absorb luminous rays is shared by water and ice 

 in fact, by all really transparent substances. Their 

 transparency is due to their inability to absorb lumi- 

 nous rays. The molecules of such substances are in 

 dissonance with the luminous waves; and hence such 

 waves pass through transparent bodies without disturb- 



