RADIATION IN ABSOLUTE MEASURE. 
603 
globe was polished to the highest degree possible. The polishing was done very 
frequently, and with special care, by the kindness of Mr. Thomas Smith, Electro¬ 
plater, Glasgow. After polishing the globe was wrapped in tissue paper and kept, 
surrounded by a very large quantity of cotton wool, in a box until tlie last moment 
before it was screwed on to the plug by whiich it was suspended. It was never 
touclied by the bare hands, being held in its tissue-paper cover while the op^ei’ation 
of screwing on was performed. The surface was so smooth and slippery that it was 
difficult to keep fast hold of the globe while it was being screwed on; and when 
suspended it presented the most beautiful mirror surface I have ever seen. 
I am supposing that the minimum of radiation is had from such a silver surface a^ 
has just been described ; and it was extremely interesting to hnd that a marked 
effect, in reducing the amount of radiation in vacuum from the silvered globe, was 
produced Ijy effbrts to attain the highest possible degree of polish. 
In calculating the emissivity from the recorded deflections of the galvanometer, 
the first process was to obtain the “corrected deflection”'^ of the galvanometer. 
Secondly, a correction is applied to the numbers for the fact that the deflection 
produced by'the thermo-junction is not in simple proportion to the difierence of 
temperatures (defined by the ah' thermometer) between the two junctions.! Thus a 
series of numbers is obtained corresponding to the successive times noted, the 
intervals being 5 minutes or 2^ minutes, as has already been mentioned. From these 
numbers I have calculated the value of the “ emissivity,” or quantity of heat lost per 
second, per square centimetre of surface, per degree centigrade of difference of 
temperatures of surface and surroundings. 
Let c be the capacity for heat of the cooling globe, S the surface, and {v — ly) tlie 
difference of temperatures of source and surroundings at any time t. Then in the 
formula 
- c I' = eS (r - i\), 
the coefficient e corresponds with the definition of “emissivity ’’just given ; and this 
formula is commonly taken to be a representation of the “ law of cooling,” whether in 
air or in any other gas, or in vacuum, the range of temperatures dealt with being 
moderate. The numerical value of e, however, depends on the circumstances under 
which the cooling takes pfiace; and, when air is present, on the dimensions and 
shape of the cooling body. Important theories, to which I have already alluded, 
have been from time to time put forward as to the form, practically .speaking, of e as 
a function of v for the case of pure radiation. 
One way, hovmver, of dealing with the matter, from an experimental point of view, 
* See note p. o99. 
t Eor example, one of the formulas, with numerical coefficients actually used, was as follows : — 
t = G 8 _ -000086 
S'So 
where t is the difference of temperatures, and c the corrected deflection ” of the galvanometer. 
4 H 2 
