Radiation of Heat by Gaseous Matter, 283 



make it tantamount to a compound, and no doubt this is actually 

 the case with lampblack ; another eminent example of this kind 

 is referred to in this paper in the section on ozone. Leslie, how- 

 ever, found water to be a better radiator than lampblack, and 

 Wells found several substances which were more capable of 

 being chilled by nocturnal radiation. On reflection, moreover, 

 the following considerations arise. The lampblack of commerce 

 and the soot of a lamp or candle, that is to say, the substances 

 which have been hitherto employed in experiments on radiant 

 heat, are copiously mixed with hydrocarbons, which are the 

 most powerful absorbers and radiators in Nature. It might 

 fairly be questioned whether the reputed experiments with 

 lampblack really dealt with lampblack at all. But even the 

 impure substance is to some extent transparent to radiant heat. 



I have plates of black glass, rendered so by the solution of 

 carbon in the glass while in a state of fusion, and which, though 

 they are impervious to the rays of the most intense electric 

 light, allow of a copious transmission of the rays of obscure heat. 

 Melloni's beautiful experiments on glass of this character are 

 well known ; indeed mine are but repetitions of his. Another 

 of Melloni's experiments which I have recently verified is the 

 following. A plate of transparent rock-salt was placed over 

 a smoky camphine lamp, and soot was deposited on its surface 

 until it intercepted every ray of a brilliant jet of gas. The 

 plate was placed between a source of heat possessing a tempera- 

 ture of 100° C. and a thermo-electric pile, a polished screen 

 being placed between the salt and the source. As long as the 

 screen remained, the needle of the galvanometer connected with 

 the pile stood at zero ; but the moment the screen was removed 

 the needle promptly advanced, showing the instantaneous trans- 

 mission across the layer of soot of a portion of the heat incident 

 upon the salt. The actual numbers obtained in this experiment 

 are these : — The deflection produced by the heat transmitted 

 through the soot was 52°; which is equal to 90 units. The 

 deflection produced when the layer of soot had been carefully 

 removed, so as to leave both surfaces of the salt smooth and 

 transparent, was 71°, which is equal to 300 units. The quantity 

 transmitted through the soot is therefore to the total quantity as 



90 : 300, 

 or as 



30 ; 100; 



that is to say, the lampblack, which was perfectly opake to the 

 light of a gas-jet, was transparent to fully 30 per cent, of the 

 incident heat. On consulting Melloni's Table, I was gratified 

 to find that he made the transmission by a plate similarly pre- 



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