4 MR B. STEWART ON RADIANT HEAT. 
C. Selenite.—At the temperature of 98°— 
A piece of selenite -125 inch in thickness gave ; : . ‘ 51 
Under the same circumstances, glass -18 inch thick gave. : . 5:0 
In the boiling-water apparatus, 
The same piece of selenite gave : : ; : ‘ ; . Se 
While blackened glass gave. : : : : : 5 . 185 
The radiation from selenite may therefore be reckoned equal to that from 
alum or glass. 
D. Mica.—A small box was constructed, having two windows of mica, the 
thickness of the mica in the one being ‘0009 inch, and of that in the other -02 inch. 
This box was filled with mercury (Professor Forzes having suggested the use of 
that metal, to keep up the temperature, while interfering very little with the radia- 
tion). The whole was then set on a glass dish in the boiling-water apparatus. 
The radiation from the thin window was, . ; ‘ . 12 
While that from the thick window was, . 2 : : 127 
As it would have been manifestly erroneous to compare these with the radia- 
tion from blackened glass lying in contact with the bottom of the apparatus, the 
thin window was removed, and blackened paper substituted in place of it. 
While the thick mica window gave . ; : : : 12-7 
The blackened paper gave ; : ‘ : : : 13-8 
In comparing the radiations from the two windows, they were observed 
alternately. We see, therefore, that the radiation from mica, especially thin mica, 
is less than that from lamp-black in the proportion of 11-2 to 13°8, or the heat 
from thin mica is 80 per cent. of that from lamp-black. 
E. fock-Salt.—As in the experiments with rock-salt, it was desirable to ob- 
tain results of the greatest possible accuracy, the radiation from the rock-salt 
was not compared with that from blackened glass; for it was found that glass 
cooled more rapidly than rock-salt. The following plan was adopted :— 
A piece of rock-salt -18 inch thick (the temperature as in all the previous examples 
being about 210°), gave. ; : : 3:2 
A canister with water kept boiling, coated with ‘lamp-black, - : x 220 
In order to estimate how much the rock-salt had cooled during the observa- 
tion, the following experiment was made, without any diaphragm :— 
Rock-salt +18 inch thick taken to the cone at once, gave. : ; 5:1 
After cooling for 15 seconds, it gave : : ; 4:9 
It will be seen from this, that were the rock-salt, instead of cooling during 
the 12 seconds necessary for the observation, kept at the temperature of 212", it 
would not have given more than 3°3, while the hot-water canister gave 22°0. 
5. From these experiments, it appears that glass, alum, and selenite, at low 
temperatures, have an intensity of radiation very nearly equal to that from lamp- 
black; while mica radiates somewhat less, and rock-salt greatly less. This is 
shown by the following table :— 
