MR B. STEWART ON RADIANT HEAT. 9 
Without any screen, the same pieces gave— 
Thickest, 3 3 ; : 3 ; : : : : : 4:9 
Middle-sized, : ; F ‘ . : : : F ‘ 4:1 
Thinnest, ; 4 é ; : : : : : j = 33 
Proportion of heat from thickest piece passed, s : : ‘51 
middle-sized_... } : : ; , “41 
thinnest he ‘ , : : 5 33 
A similar experiment, with a screen :29 inch thick, gave— 
With screen. Without screen. Proportion passed. 
Thickest piece, ; : ; ; ; 
Middle-sized, ‘ ‘ ; ; ; 1:8 4:5 ‘40 
Thinnest, . 4 : ; - ‘ ite 3:5 33 
It follows from this, that a screen of rock-salt passes heat from thick, more 
easily than heat from thin rock-salt. 
11. From this fourth group of experiments, we learn that heat from thick 
plates of glass, mica, or rock-salt, is more easily transmitted by screens of the 
same nature as the heated plate than heat from thin plates of these materials. 
The following table exhibits the results of the third and fourth group of ex- 
periments :— 
PWAB IE ie 
No. of Rays out of every 100 
that pass through a screen No. of Rays of Lamp-black 
of the same material as the | Heat, out of every 100 that 
source of Heat in 1st column, pass through the same 
the screen being of only one screen. 
thickness for each material. 
Source of Heat. 


Glass (crown ,j;thinchthick), . . 0°66 } 1:33 
Glass (crown jj5thinch thick), . . 1-0 
Mica (thickness ‘0009 inch), . . 22 30 
Mica (thickness ‘02 inch), . . . 26 
Rock-salt (thickness ‘18 inch), .  . 33 
Rock-salt (thickness ‘36 inch), . . 4] aes 82 (Art. 12) 
Rock-salt (thickness *‘77 inch), . . 50 

Results deducible from the foregoing Experiments. * 
12. These experiments, as well as others yet to be described, may be ex- 
plained by Prevost’s theory of exchanges, somewhat modified. 
In the first place, it would seem to be a consequence of this theory, that radia- 
tion must take place from the interior as well as from the surfaces of bodies. 
For suppose that we have two indefinitely extended surfaces of lamp-black, as 
in the figure, and between them a plate of rock-salt of a certain thickness, also 
indefinitely extended; and let the whole be kept at the same temperature. Then, 
VOL. XXII. PART I. c 
