10 MR B. STEWART ON RADIANT HEAT. 
since the temperature of the rock-salt remains the same, it must radiate as much 
as it absorbs. But a thicker plate of rock-salt, placed under the same circum- 
stances, would absorb more of the heat radiated from 
the lamp-black, because each ray would have to pass 
through a greater depth of the substance of the salt ; 
hence a thick plate of rock-salt must radiate more 
than a thin plate. Wesee, likewise, the reason for 
the small radiative capacity of rock-salt to be its 
small absorptive capacity. In order to prove this 
deduction from Prevost’s theory experimentally true, the following experiment 
was devised :— | 
A boiling-water canister, coated with lamp-black, was put behind the dia- 
phragm, filling up the field of view, and the three pieces of rock-salt heretofore 
used as sources of heat, were now separately used as screens, being put before 
the diaphragm, so that the heat from the canister had to pass through their sub- 
tance before reaching the cone. The following was the result :— 

Without any Screen of Rock- Screen of Rock- Screen of Rock- 
Screen. salt, -18 inch thick. salt, ‘36 inch thick. salt, -77 inch thick. 
Radiation from Canister, . . 21°3 17-6 16°8 15°8 
The difference between heat absorbed by plate, thickness=°18 inch, Is 1-2 
And that absorbed by plate, : : thickness = ‘36 inch, Mean 1:1 
Another similar experiment gives 0:9 
The difference between heat absorbed by plate, thickness = 36 aa Is 1-0 
And that absorbed by plate, : ; thickness =°77 inch, Mean 1:1 
Another similar experiment gives. : 1:3 
These should nearly correspond with the differences between the radiations 
from the same plates, under their ordinary circumstances of position (if the theory 
be true which asserts that the absorption of such a plate equals its radiation) ; 
accordingly we find that 
The difference between heat radiated by Plate, thickness =°18 inch 
And that radiated by plate, : thickness = 36 inch, Hoe 
While the difference between radiation of plate, thickness =-36 inch, Is 1:0 
And that Pe Re of plate, ... =-°77 inch, 
(Art. 6, mean of four sets of experiments). 
We see, therefore, that there is an agreement between the two sets of differ- 
ences, as near as can be reasonably expected. 
13. If we now suppose a plate of glass, and not a plate of rock-salt, placed be- 
tween surfaces of lamp-black, the plate, whether thin or thick, will allow scarcely 
any heat to pass through it ; and, consequently, plates of different thicknesses will 
all absorb very nearly the same amount,—that is, nearly all that enters them. 
In this case, therefore, the radiation (which is equal to the absorption) will be 
very slightly increased by an increase of thickness of the plate. Also the amount 
