90 Scientific Proceedings, Royal Dublin Society. 
VIIT.—ON THE RADIATING POWER OF SHELL-LAC FILMS 
OF VARIOUS THICKNESS, sy E. G. HULL. 
[Read March 18, 1878.] 
It seemed to be of some interest and importance to determine the 
rate at which radiation increases, for a continued uniform increase 
in the thickness of the radiating body, and also the depth below 
the surface from which radiation takes place. 
With regard to the former point, there have been no experi- 
ments yet made as far as I know. With regard to the latter, the 
only reference I have seen is a mention ky M. Jamin in his 
“Cours de Physique,” that heat is radiated from beneath the 
surface of a body. He also states that, in the case of shellac, the 
radiation ceases to increase when the thickness of the film becomes 
(025™™ This, however, is not the case; and since he makes no 
mention of having measured the thicknesses of his films, I conclude 
he only guessed at them. The radiation increases up to and 
beyond ‘05"™ or twice the thickness which he stated as the limit ; 
and since he gives no details, it seemed a matter-of interest to 
make some accurate experiments on the subject. .The following 
method was adopted, after some trials, and as it is, I think, new, 
I venture to lay the results of my determinations before you. 
The radiating body chosen was shellac, which, from its high 
radiating power, combined with the ease with which it could be 
converted into a smooth and easily applied varnish, seemed most 
suitable. A solution of known strength of the shellac in alcohol 
was made, which, when applied to the polished and heated metal, 
left a very uniform film. 
The method of caleulation of the thickness was as follows :— 
The faces of the cubes being one square cdecimetre in area; the 
specific gravity of the shellac and of the spirit being also taken ; 
and the percentage of resin in the solution, and the specific gravity 
of the solution being known; by weighing the solution, with 
suitable precaution, before and after varnishing the face of the 
cube, we get the weight of varnish applied to the surface, and 
knowing how much of this weight consists of shellac, and dividing 
by its specific gravity, the volume is obtained, which volume 
being spread over a known area gives the thickness immediately, 
In the experiments a compensating cube was used, The whole 
