CONTRIBUTIONS TO MOLECULAR PHYSICS. 393 



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 

 m?*y 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 radiation 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 carbonic 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 great measure 



