530 Prof. TyndalPs Contributions to Molecular Physics. 



tive power of the heat through the glass. Melloni, with two 

 plates of glass of different thicknesses, found the following trans- 

 missions for the name and the spiral : — 



For the flame. For the platinum. 



41-2 52-8 



5-7 26-2 



The same remarks apply to the transparent selenite examined 

 by Melloni. This substance is highly opake to the extra-red 

 undulations ; but the radiation from an alcohol flame is almost 

 wholly extra-red, and hence the opacity of the selenite to this 

 radiation. The introduction of the platinum spiral shortens the 

 periods and augments the transmission. Thus, with two speci- 

 mens of selenite, of different thicknesses, Melloni found the 

 transmission to be as follows : — 



lame. 



Platinum. 



4'4 



19-5 



i-7 



35 



So far the results of Melloni correspond with those of M. Knob- 

 lauch ; but the Italian philosopher pursues the matter further, 

 and shows that M. Knoblauch's results, though true for the par- 

 ticular substances examined by him, are far from being appli- 

 cable to diathermic media generally. Melloni shows that in 

 the case of black glass and black mica, a striking inversion of 

 the effect is observed ; that is to say, that through these sub- 

 stances the radiation from the flame is more copiously trans- 

 mitted than the radiation from the platinum spiral. For two 

 pieces of black glass of different thicknesses, he found the fol- 

 lowing transmissions : — 



From the flame. From the platinum. 



52-6 42-8 



29-9 27-1 



And for two plates of black mica the following transmissions 

 were found : — 



From the flame. From the platinum. 



62-8 52-5 



43-3 289 



These results were left unexplained by Melloni ; but the solution 

 is now easy. The black glass and the black mica owe their 

 blackness to the carbon incorporated in them, and the blackness 

 of this substance, as already remarked, proves the accord of its 

 vibrating-periods with those of the visible spectrum. But it 

 has been proved that carbon is in a considerable degree pervious 

 to the waves of long period — that is to say, to such waves as are 



