364 PEOFESSOE TYNDALL’S CONTEIBUTIONS TO MOLECULAE PHYSICS. 
spiral would augment the penetrative power of the heat through the glass. Melloni, 
with two plates of glass of different thicknesses, found the following transmissions for 
the flame 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 sub- 
stance is highly opaque 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 trans- 
mission. Thus, with two specimens of selenite, of different thicknesses, Melloni found 
the transmission to be as follows : — 
Flame. Platinum. 
4-4 19-5 
1-7 8-5 
So far the results of Melloni correspond with those of M. Knoblauch ; hut the Italian 
philosopher pursues the matter further, and shows that M. Knoblauch’s results, though 
true for the particular substances examined by him, are far from being applicable 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 
substances the radiation from the flame is more copiously transmitted than the radiation 
from the platinum spiral. For two pieces of black glass of different thicknesses, he 
found the following transmissions : — 
From the flame. From the platinum. 
52-6 42*8 
29-9 27T 
And for two plates of black mica the following transmissions were found ■ 
From the flame. From the platinum. 
62-8 52-5 
43-3 28-9 
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 emitted by a flame of alcohol. The case of the carbon is there- 
fore precisely antithetical to that of the transparent glass — the former transmitting the 
heat of long period most freely, and the latter transmitting the heat of short period 
most freely. Hence it follows that the introduction of the platinum wire, by converting 
