186i.J Prof. Tyndall — Contributions to Molecular Physics. 165 



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 platinum wire 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 water ; hence we should infer that the trans- 

 mission through water will be more copious when the wire is in the flame 

 than when it is absent. Experiment proves this conclusion to be true. 

 Water, from being opaque, opens a passage to 6 percent, of the radiations 

 from the flame and spiral. A thin plate of colourless glass, moreover, 

 transmitted 58 per cent, of the radiation from the hydrogen flame ; but 

 when the flame and spiral were employed 78 per cent, of the heat was 

 transmitted. For an alcohol fliame Knoblauch and Melloni found glass to 

 be less transparent than for the same flame with platinum spiral immersed 

 in it ; but Melloni afterwards showed that this result was not general, 

 that black glass and black mica were decidedly more diathermic to the 

 radiation from the pure flame. The reason of this is now obvious. Black 

 mica and black glass owe their blackness to the carbon diffused through 

 them. The carbon, as proved by Melloni, is in some measure transparent 

 to the extra-red rays, and the author had in fact succeeded in transmitting 

 between 40 and 50 per cent, of the radiation from a hydrogen flame 

 through a layer of carbon sufficient to intercept the light of the most bril- 

 liant flames. The products of combustion of the alcoliol flame are carbonic 

 acid and aqueous vapour, the heat of which is almost wholly extra-red. 

 For this radiation the carbon is in a considerable degree transparent, while 

 for the radiation from the platinum spiral it is in a great measure opaque. 

 By the introduction of the platinum wire, therefore, the transparency of 

 the pure glass and the opacity of its carbon were simultaneously aug- 

 mented ; but the augmentation of opacity exceeded that of transparency, 

 and a difference in favour of opacity remained. 



No more striking or instructive illustration of the influence of coinci- 

 dence could be adduced than that furnished by the radiation from a car- 

 bonic oxide flame. Here the product of combustion is carbonic acid ; and 

 on the radiation from this flame even the ordinary carbonic acid of the 

 atmosphere exerts a powerful effect. A quantity of the gas, only one- 

 thirtieth of an atmosphere in density, contained in a polished brass tube 

 four feet long, intercepted 50 per cent, of the radiation from the carbonic 

 oxide flame. For the heat emitted by solid sources, olefiant gas is an in- 

 comparably more powerful absorber than carbonic acid ; in fact, for such 

 heat the latter substance, with one exception, is the most feeble absorber to 

 be found among the compound gases. For the radiation from the hydro- 

 gen flame, moreover, olefiant gas possesses twice the absorbent power of 



