MR B. STEWART ON RADIANT HEAT—SECOND SERIES. 61 
boiling-water apparatus, being used as the source of heat, and no diaphragm 
used,— 
fo} 
The deviation was, : . : ; : f 2 : ; : a kO 
When the source of heat was a similar plate, +10 inch thick, the deviation became, 1:5 
And, lastly, when the source of heat was a plate covered with lamp-black, the 
deviation was, ; : ‘ , , ° : é é : ald 
With the same sources of heat, only the glass polished instead of roughened, 
these numbers were 0°:95, 1°45, 1°95. From the correspondence between these 
two sets of results, we may infer that the quality of the heat radiated by glass 
(at least in so far as transmission through a plate of glass can test it) is not altered 
by roughening the surface of the glass. 
7. And from all these experiments, we may infer (what has indeed been 
already remarked by Professor Forsss), that although roughening its surface with 
sand or emery-paper renders a body dim for light, yet it still remains specular for 
heat rays, which, possessing a greater wave-length than those of light, are less 
liable than the latter to be influenced by scratches or furrows. 
Second Group of Hxperiments described. 
8. The second group of experiments has reference to the nature of the heat 
which is radiated by rock-salt at 212°. Its quality being tested by transmission 
through 
a. A screen of mica, 
(3. One of mica split by heat. 
ry. One of glass, 
9. a. Mica Screen.—By the mean of three sets of experiments, a mica screen 
(thickness="003 inch nearly) passed about 31 per cent. of ordinary lamp-black 
heat, while it only passed 18 per cent. of rock-salt heat. Or if we call the propor- 
tion of black heat passed by the mica 100, that of rock-salt heat passed will be 58. 
10. 6. Split Mica Screen.—Two sets of experiments agreed in giving 20 per 
cent. as the proportion of lamp-black heat of 212°, transmitted through a screen 
of mica split by heat, while the proportion of rock-salt heat transmitted by the 
same screen was only 154 per cent. These numbers are to one another as 100 to 76. 
11. y. Glass Screen.—In order to avoid secondary radiation from the screen, 
which, in this case, absorbs nearly all the heat, two screens of microscopic glass 
were used, the one behind the other, with an interval between. 
Moreover, as in this case, the proportion of heat transmitted is exceedingly 
small, the following arrangement was adopted to make it measurable. 
The experiment consisted of four parts,— 
1st, The effect of the rock-salt heat upon the pile without a screen was ob- 
served by the ordinary galvanometer. 
2d, The effect of lamp-black heat, also without a screen, was observed by the 
same galvanometer. 
