MR B. STEWART ON RADIANT HEAT. 3 
Observations were always made with as little sunlight as possible; and under 
these circumstances, it was ascertained that the stray heat reaching the cone 
was inappreciable. The needle, it was calculated, reached the limits of its swing 
about 12 seconds after the heated body had been taken out of the boiling-water 
apparatus. 
Experiments were made to ascertain if the body cooled sensibly during this 
short period of time, and it was found that its cooling was so trifling as not to 
interfere in any degree with the results of these observations. In the following 
experiments, it is therefore assumed that the body remains at its original tem- 
perature of 210° while the observation is being made. 
Four observations were generally made, and three if they agreed together ex- 
ceedingly well, but never fewer. Very often the agreement was exact. 
First Group of Experiments described. 
4. With these remarks, I proceed to describe the experiments belonging to the 
first group, or those made with the view of comparing the heat radiated from 
polished plates of different substances with that radiated from a surface of lamp- 
black at the same temperature. 
The reason why lamp-black was chosen as the standard is obvious; for, it is 
known from LEsiie’s observations, that the radiating power of a surface is pro- 
portional to its absorbing power. Lamp-black, which absorbs all the rays that 
fall upon it, and therefore possesses the greatest possible absorbing power, will 
possess also the greatest possible radiating power. The first substance compared 
with it was glass. 
A. Gilass—A piece of plate-glass, 8 inch thick, having paper coated with 
lamp-black pasted on its surface next the pile, gave a deviation of 18-1. This 
may be taken as the radiation from lamp-black. 
Three plates of crown-glass, each ‘05 inch thick, sie one behind the ee 
gave : : ree 
A single piece of erowneplass of the same » thickness ¢ gave : 3 : 165 
This difference is probably owing to the single plate cooling faster than 
the three plates. It may be argued that the radiation from glass is very nearly 
equal to that from lamp-black ; and indeed this is already well known.* 
B. Alum.—Here the boiling-water apparatus could not be used, since alum 
becomes calcined at a temperature much below 212°; but a self-regulating appa- 
ratus, invented by the late Mr Kemp, was employed instead, giving a steady tem- 
perature of 98°. 
A piece of plate-glass -18 inch in thickness gave ; , 3 : P 5:0 
A piece of alum ofthe same thickness gave. ; : : : , 5:0 
The radiation from alum may therefore be reckoned equal to that from glass. 
* See Lustix’s “ Inquiry into the Nature and Propagation of Heat,” 
