RADIATION IN ABSOLUTE MEASURE AT VERY LOW TEMPERATURES. 
373 
loss of heat from the warm radiator, particularly when it is.considered how minute a 
proportion of the heat radiated in all directions falls upon the bolometer and is 
utilised for the measurement. 
Kurlbaum also quotes in his paper the results of earlier investigators, but these 
experiments also are mostly indirect. Thus the results of Leitnebacii are deduced 
with the help of a comparison between the radiating power of glass and that of the 
theoretical black body; and the same applies to those of Graetz. A result deduced 
from the work of Kundt and Warburg depends on the conducting power of gases for 
heat, while numbers quoted from Christiansen are founded on considerations still 
more complicated. 
In each case Kurlbaum deduces the emission (not emissivity) of heat by a body at 
100° C. to a body at 0° C., or rather the heat received by a body at 0° C. from a 
“ black ” surface at 100° C. placed opposite to it. This is taken as the “ emission” of 
heat between the absolute temperatures 273° C. and 373° C., and Stefan’s coefficient 
is deduced from these numbers. 
The following table is taken from Kurlbaitm’s paper, except that I have interpolated 
Column 3. The results are in Cm. Cal./cm 2 x sec. :— 
Observer. 
o 
o 
1 
Average emissivity 
between 
Stefan’s 
coefficient. 
0° C. and 100° C. 
cr. 
Lehnebach, 1874 
0-0152 
1 • 52 x10~ 4 
11-Ox 10- 13 
Kundt and Warburg, 1875 
0-014 
1-4 x 10“ 4 
r io-i x io- 13 * 
\ 11-1 x io- 13 
Graetz, 1880 
0-0150 
1-5 x10 -4 
" 10 ■ 8 x 10~ 13 
Christiansen, 1883 
0-0167 
1-67 x10~ 4 
12-1 x 10~ 13 
Kurlbaum, 1898 
0-0176 
1 • 76 x 10~ 4 
12-8 x 10" 13 t 
In calculating the results of my experiments, following my original plan, I am not 
led to tabulate “ emissions ” but “ emissivities.” Hence my figures do not compare 
directly with those placed in the column of this table under the heading h m —h 0 . For 
this reason I have interpolated Column 3, which shows what may be taken to be the 
average emissivity between the temperatures 100° C. and 0° C. For comparison with 
these numbers I may quote the following table, compiled from my paper in the ‘ Phil. 
Trans.’ for 1893. The numbers are taken out of tables giving emissivities at different, 
but extremely low, pressures. 
* The number 10'1 is reckoned by using the value 0‘014 as the emission between /uoo-^o. as given 
above ; 11 • 1 is a number obtained by Graetz from the data given by Kundt and WARBURG. 
f This number seems to be considered as final by Kurlbaum. I find that he makes use of it in subse¬ 
quent papers as an accepted figure, e.g., ‘Wied. Ann.’, Band 2, 1900, p. 550; and others quote it as 
authoritative. See, e.g., Poynting, ‘ Phil. Trans.’, 1904, p. 526, and Poynting and Thomson, ‘ Heat.’ 
