CONTRIBUTIONS TO MOLECULAR PHYSICS. 393 



same, whether we consider the lime to be struck by 

 a particle of aqueous vapour oscillating at a certain rate, 

 or by a particle of ether oscillating at the same rate. 



By plunging a platinum wire into a hydrogen flame 

 we cause it to glow, and thus introduce shorter periods 

 into the radiation. These, as already stated, are in 

 discord with the atomic vibrations of water; hence we 

 may infer that the transmission through water will be 

 rendered more copious by the introduction of the wire 

 into the flame. Experiment proves this conclusion to 

 be true. Water, from being opaque, opens a passage to 

 6 per cent, of the radiation from the spiral. A thin 

 plate of colourless glass, moreover, transmits 58 per 

 cent, of the radiation from the hydrogen flame; but 

 when the flame and spiral are employed, 78 per cent, 

 of the heat is transmitted. 



For an alcohol flame Knoblauch and Melloni found 

 glass to be less transparent than for the same flame 

 with a platinum spiral immersed in it; but Melloni 

 afterwards showed that the result was not general 

 that black glass and black mica were decidedly more 

 diathermic to the radiation from the pure alcohol 

 flame. Melloni did not explain this, but the reason is 

 now obvious. The mica and glass owe their blackness 

 to the carbon diffused through them. This carbon, as 

 first proved by Melloni, is in some measure transparent 

 to the ultra-red rays, and I have myself succeeded in 

 transmitting between 40 and 50 per cent, of the radia- 

 tion from a hydrogen flame through a layer of carbon 

 which intercepted the light of an intensely brilliant 

 flame. The products of combustion of alcohol are car- 

 bonic acid and aqueous vapour, the heat of which is 

 almost wholly ultra-red. For this radiation, then, the 

 carbon is in a considerable degree transparent, while 

 for the radiation from the platinum spiral, it is in a 



