MR B. STEWART ON RADIANT HEAT, 7 
(G.) The middle-sized piece of rock-salt was bound tightly to the thickest 
piece, with a slip of tin-foil between, so that the whole might cool as one piece, 
and thus obviate any objection that might be brought against the results, 
founded on the unequal cooling of the plates, owing to their thicknesses being 
different. 
The surface of the middle-sized piece facing the a Bey : j F 6°3 
That of the thickest piece, gave - F : 8:1 
The plates, therefore, still retained their tegiealie? of radiation; but the 
amount from each was increased, owing, no doubt, to reflection and radiation 
from the tin-foil. The radiation from the tin-foil may be estimated at 1:0, de- 
ducting which, we have 53 and 7:1; the increase now being due to reflection 
from the tin-foil. 
7. It thus appears, that while the difference between the radiating power of 
thick and thin glass is so small as not to be capable of being directly observed, 
there is a perceptible difference between the radiation from thick and thin mica, 
and a still more marked difference between the radiation from plates of rock-salt 
of unequal thickness. 
But (at least with the thicknesses used) the greatest radiations from mica 
and rock-salt were still below that from lamp-black, and the radiation from rock- 
salt greatly so. 
The following table exhibits the results of the second group of experiments :— 
TaB_e II. 
Radiation from Radiation from 
Substance, thick plate. thin plate. 



Glass, 4 ; : 4 : 100 100 
Mica, > ‘ : ; 100 89 
; Middle) 81 
Rock-salt, CRA ads Srnegiir ery: 100 rh } Q 

Third Group of Experiments described. 
8. I now proceed to consider the third group of experiments, or those made 
with the view of comparing the radiations from various polished surfaces with 
that from lamp-black, as regards the quality of the heat; its quality being tested 
by its capability of transmission through a screen of the same material as the 
radiating plate. 
A. Glass.—In an experiment already described, where a plate of crown-glass 
05 inch thick was used as a screen, and a similar plate of crown-glass as the 
source of heat— 
We had, . ; : : 0:95 
A similar plate :1 inch thick a as the source e of heat, gave ink , 1°45 
Blackened paper attached to a similar surface of plate gia 3 inch thick, 
the blackened side being next the pile, : ' 1:95 
