On the Radiating Power of Shell-lac Filme. 91 
radiation from a clean polished metal surface of the cube being 
first determined, this was then exactly neutralised by the com- 
pensating cube, which was coated with lamp-black. The first 
cube was then turned round, and the additional radiation due to 
one known thickness of varnish determined (which, added to the 
radiation from the metal, gave the whole radiation); this was 
again counterpoised, and a thicker varnished surface substituted 
and so on: and then the radiation from lamp-black was determined. 
The results were reduced to percentages of the total lamp-black 
radiation. From the results a curve was constructed, which 
finally appeared to become asymptotic. 
The following are some of the determinations :— 
Average radiation from polished metal, 3 : 13°3 
a + shellac, ‘ 0001™™ thick, 16-1 
re FP rf ; COG p rr a oe 
- te 5 ’ QUOTE ec 
” ” ” . 029": ” 42-6 
” ” ” : "0041™™ np 46:1 
” ” ” : °0149™-™- ” 71:2 
” ” ” . ‘OZ0R 99 91°8 
fe vs es : gO hanes eg ey" 
se a 3. : 5 51) ates ere a! (oC 
« P lamp black, . : : . 100-0 
On further increasing the thickness of the shellac, the radiation 
from it became equal to and finally swperior to the radiation from 
the lamp-black, attaining a maximum of abeut 102 (lamp-black 
= 100), after which no further increase of thickness affected the 
radiation. 
To summarise— 
1. The radiation increases rapidly up to ‘05"™, but slightly 
beyond that thickness. 
2. The radiation increases much more quickly at first than 
subsequently, 
3. Shellac appears to have a higher radiating power than 
lamp-black. * 
* I have to express my thanks to Professor Barrett for the opportunity of conducting 
part of the foregoing investigation in the Laboratory of the Royal College of Science. 
