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the latter description of heat ; but the difference of behaviour of the 
two kinds of heat with regard to this substance is not so marked as 
in the case of ordinary mica. 
So far as tested by mica, and mica split by heat, it was shown 
that, — - 
Lamp-black heat of 700° F. bears to 
Lamp-black „ 212° F., the same relation as 
Lamp-black ,, 212° F. bears to 
Rock-salt „ 212° F. 
that is to say, rock-salt heat possesses greater average wave-length 
than lamp-black heat. 
With regard to the third group of experiments, it was shown that 
glass or mica, by being heated, does not change in any measure its 
capacity for transmitting a given description of heat ; for instance, 
cold glass transmits heat of 700° F. just as well as glass heated to 
700° F. does. Proceeding, then, to the theoretical part of the 
paper, it was then shown that, owing to the following facts : — 
1st, That the absorptive power of a thin plate of any substance 
equals its radiative power,, 
2d, That (by the third group of experiments) the absorptive 
power of cold glass for heat of 700° F., is the same as that of glass 
heated to 700° F. 
3 d, That cold glass has a greater transmissive, or less absorptive 
power for heat of 700° F., than for heat of 212° F. 
we must conclude that “ the radiation of a thin plate of glass, or 
other substance, at 700°, bears a less proportion to the total radia- 
tion of 700° F., than its radiation at 212° does to the total radia- 
tion of 212° F. It was also shown that this difference is more 
marked for a thin plate than for a thick one ; and it was argued, 
that Dulong and Petit’s law does not express the law of radiation 
of a material particle, but that this law, whatever it be, increases 
(for all bodies) less rapidly with the temperature than Dulong and 
Petit’s law. 
