RADIATION IN ABSOLUTE MEASURE AT VERY LOW TEMPERATURES. 369 
gauge. Vacuum is very slowly transmitted from point to point along the connecting 
tubes which must intervene between the gauge and the space whose condition, as to 
vacuum, the gauge is to measure. In a foot-note, p. 352, this is referred to. The 
connecting tubes have been made as short as possible, but in spite of every care in 
this direction this inherent trouble has not been overcome. 
46. The third group contains three tables which show the cooling of the copper 
globe in air at ordinary atmospheric pressure. In Table XIII. the copper globe is 
finely sooted ; in Table XIV. the surface of the globe is silvered and brilliantly 
polished ; while Table XV. gives the cooling of the globe with a silvered, but dull 
white surface. # 
A comparison of these tables with the corresponding tables for the globes cooling in 
vacuum shows the part which is played by convection in the ordinary case of bodies 
cooling in air. 
47. In the following short tables, the results of such a comparison for the cases of 
the sooted globe and the highly polished silvered globe are given. The emissivitiesf 
have been taken at corresponding temperatures for the globe cooling in vacuum, and 
cooling in ah’, for the two cases mentioned. These are placed opposite the 
temperatures to which they belong, and Column 1 of each table gives the absolute 
temperatures of the cooling globe. Column 2 gives the emissivity in vacuum 
Column 3 gives the number which corresponds to the emissivity in air; while 
Column 4 gives the ratios between the numbers in Column 3 and those in Column 2. 
The following points may be noticed with regard to the numbers in these tables. In 
the first place the relatively high numbers for the emissivity in air are very remarkable, 
and it is to be noticed that the numbers are very nearly the same for the sooted globe 
and the silvered globe. The true radiation goes almost for nothing in comparison 
with the loss due to radiation and convection combined. 
1 . 
2. 
3. 
4. 
1 . 
2. 
3. 
4. 
Sooted globe. 
Silvered globe. 
Absolute 
«i 
<?2 
Ratio e ' 2 
Absolute 
«i 
«2 
temperature. 
in vacuum. 
in air. 
temperature. 
in vacuum. 
in air. 
ei 
° C. 
°C. 
284 
2-152xIO - 5 
3-842 x 10~ 4 
17-85 
270 
5-368 x 10 ® 
3-659 x 10“ 4 
68-2 
260 
1•948 x 10 -5 
3-829 x 10“ 4 
19-6 
260 
5-045 x 10 -6 
3-651x10“ 4 
72-4 
240 
1 • 714 x10“ 5 
3-783 x 10- 4 
22-1 
240 
4-115 x10“ 6 
3-637 x 10- 4 
88-4 
220 
1 •393xIQ- 5 
3-736 xIQ" 4 
26-8 
230 
3-501x10“ 6 
3-631 x10-“ 
103-4 
* This surface was caused by a fine deposit of mercury on the highly polished silvered surface, 
f It is not in accordance with strict language to use the word “ emissivity ” in the case where air is 
present; but the lapse from strictness may be pardoned here for the sake of brevity. 
VOL. CCVIII.-A. 3 B 
