CONTRIBUTIONS TO MOLECULAR PHYSICS. 99 



secondly, in the circumstance that the lime is heated hy 

 the collision of the molecules of aqueous vapor, before 

 their heat has assumed the radiant form. But it cannot 

 be doubted that the same effect would be produced by 

 radiant heat of the same periods, provided the motion of 

 the ether could be rendered sufficiently intense.* The 

 effect in principle is the same, whether we consider the 

 lime to be struck by a particle of aqueous vapor 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 praves 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 colorless 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 plati- 

 num spiral immersed in it; but Melloni afterward 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 car- 

 bon, as first proved by Melloni, is in some measure trans- 

 parent 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 carbonic acid and 

 aqueous vapor, the heat of \vhichisalmostwhollyultra-red. 

 For this radiation, then, the carbon is in a considerable 

 degree transparent, while for the radiation from the plati- 

 num spiral, it is in a great measure opaque. The platinum 

 wire, therefore, which augmented the radiation through 

 the pure glass, augmented the absorption of the black glass 

 and 



* This was soon afterward accomplished. See pp. 35, 36. 



