MR B. STEWART ON RADIANT HEAT. 5 


TaBLeE I. 
Temperature. 
Radiating Substance. 
PAWS 98° 
Lamp-black, . : : . 100 
Glass, : é : ‘ ; 98 27 
Alum, : ; 2 : < 27 
Selenite, . : : : : 98 27 
Thick mica, : ; 2 ‘ 92 
Thin mica, ; : : 81 
Rock-salt, : : : : 15 
Second Group of Experiments described. 
6. I now proceed to the second group of experiments, or those designed to 
compare together the quantities of heat radiated at the same temperature from 
polished plates of the same substance, but of different thicknesses. 
A. Glass.—No direct experiment of this kind was made on glass; for although 
a thick plate gave a somewhat greater radiation than a thin plate, it was ima- 
gined that this was due to the unequal cooling of the two plates. Indirectly, 
however, we niay gather that thick glass radiates somewhat more than thin 
glass, from the following experiment, which belongs more properly to the fourth 
group :— 
__ A plate of crown-glass -05 inch thick, being placed before the cone as a screen, and a 
similar plate -05 inch thick, and 3°75 inches square, being used as the source of 
heat at a distance of 6 inches, and no diaphragm used, the deviation was 0-95* 
But when the source of heat was a similar plate *10 inch thick, the deviation 
became . 2 . : - : : : : . - . 1:45 
Such a difference cannot be accounted for by the unequal cooling of the plates ; 
and it would seem to indicate that a small quantity of heat from the interior of 
the thick plate reached the surface; which heat, having already been sifted by 
its passage through glass, was easily able to pierce the screen. 
In another similar experiment, 
One plate of crown-glass ‘05 inch thick, gave a deviation of : : 11 
Two plates 05 inch thick, the one behind the other, . : : ; ‘ 1:55 
Three such plates, 2 : ‘ . 1:9 
B. and C.—No tte of this kind were attempted with alum or selenite. 
* Without any screen, it was calculated that the intensity of effect would have been equal to 
about 150°. 
VOL. XXII. PART I. B 
