MR B. STEWART ON RADIANT HEAT—SECOND SERIES. 65 
22. These experiments, which are not, perhaps, individually susceptible of very 
great exactness, agreed, however, in rendering it probable that glass, owing to its 
being heated up to about 700° F., does not change its diathermancy for heat of 
700° F. 
23. B. Mica.—The experiments on mica were made with the ordinary galvano- 
meter. <A piece of mica, thickness about ‘008 inch, being used as a screen, and 
a diaphragm, ‘65 inch square, at the distance of 3 inches from the mouth of the 
pile, being employed, the mean of two sets of experiments made the proportion of 
black heat of 200° F. passed by the mica to be 13 per cent. Placing an additional 
diaphragm of same size 34 inches beyond the first, and using as a source the tem- 
perature of 400° F., the mean of two sets of experiments made the proportion of 
heat passed by the mica screen to be 21 per cent. 
In order to test whether the apparently greater diathermancy of the screen for 
heat of 400° F. was owing to the difference in the nature of the heat, or to the 
heat at 400° F. striking the screen more nearly at a perpendicular incidence, 
and thus experiencing less reflection as well as passing through a smaller thick- 
ness of mica, an experiment was made on heat at 200° F., with the arrange- 
ment and distance used for heat of 400° F., which seemed to show that this 
difference of distance does not affect sensibly the proportion transmitted. We 
may therefore conclude that the difference in the proportions transmitted is owing 
to a difference of quality in the two descriptions of heat. 
24. A cast-iron box was next constructed, having this same plate of mica 
inserted as a window, so that, while one side of the box consisted merely of a 
moderately thin plate of cast-iron, the other, except round the edges, was com- 
posed of mica. The cast-iron side was then blackened, and the box filled with mer- 
cury. A thermometer inserted in the box measured the temperature. At 200°F., 
with the usual diaphragm 3 inches from the mouth of the pile, the proportion 
between the radiation of the blackened side and the mica window was, by the 
mean of 3 sets of experiments, as 100 to 87°8, while at 400° F., with the usual 
arrangement of 2 diaphragms, the same proportion was 100: 84:1. 
25. Let us endeavour to discuss these results. The radiation from the mica 
window consists of three portions :— 
a, The proper radiation of the mica plate. 
G6. That portion of the radiation of the mercury which has been able to pene- 
trate the mica plate. 
.That portion of the radiation of the mica which, striking upon the mer- 
cury, is reflected back by it and has penetrated the mica plate. 
Now, supposing there was no mercury behind the mica, and that mica be- 
tween 200° and 400° does not alter its diathermancy as a screen in any respect, 
let us inquire what ought to have been the result obtained. Then, since the radi- 
ation of a thin plate equals its absorption (First Series, Art 19), and since the 
