368 
DR J. T. BOTTOMLET AND MR F. A. KING ON THERMAL 
experiments on this subject. The presence of the liquid air surrounding the enclosure 
causes condensation of any kind of collapsible vapour which might exist in the 
so-called vacuous space, e.g., vapour of mercury, or any trace of vapour of water, &c., 
the existence of which would not be indicated by the M‘Leod gauge, but which, should 
it exist, would certainly play its part in causing heat transference by convection. 
42. Tables III., IV., Y. give the emissivity from a sooted globe at a good vacuum, 
but less perfect than in the cases of Tables I. and II., and at gradually increasing 
pressure up to 60 M., or about half-a-tenth of a millimetre. 
43. The construction of the tables is clearly indicated by the headings. For each 
experiment there is given the condition of the surface, the vacuum pressure, and the 
temperature of the enclosure. In the second column is given the difference of tem¬ 
peratures between the globe and its surroundings at the time of the galvanometer 
reading. This difference, as has been stated, is obtained from the calibration 
curves of the thermojunctions. Column 3 gives the emissivity, calculated as has 
been explained and exemplified in Section 5 of the paper. Column 4 gives the 
absolute temperature of the globe at the moment of obtaining the galvanometer 
reading. Column 5 will be explained below. It contains what has been called “ the 
radiation constant,” calculated in accordance with Stefan’s law. 
, 44. Coming now to the tables of the second group, these give the emissivity for the 
highly polished silvered globe. Tables VII. and VIII. show the loss of heat at the 
highest vacuum we could obtain, with Sprengel pumps and cooled charcoal; and it will 
be seen that the loss from the sooted surface under these circumstances is four times 
as much as from the highly polished silvered surface at the same absolute temperatures 
of the cooling body and surroundings. This result quite agrees with those obtained 
in the earlier experiments of Dr. J. T. Bottomley. 
Tables IX. and X. give the loss from a silvered surface in a less perfect vacuum. 
These, however, are of comparatively small interest. 
45. Tables VI., XL, and XII., give the case of the silvered globe and of the sooted 
globe cooled to a temperature below that of the enclosure, and receiving heat by 
radiation from warmer surrounding’s. 
Not many experiments of this kind have been made, so far as we are aware ; but 
the results are interesting when considered in connection with the principle of “ Heat 
Exchanges.” 
•To 
It may be noticed that in every case, both sooted and silvered, where the globe 
has been cooled below the temperature of its surroundings, and is allowed to rise in 
temperature by receiving heat from the walls of the enclosure, the calculated 
emissivity has turned out to be higher than in the reverse, and more ordinary, 
experiment. It is probable that this is due to deterioration of the vacuum, on 
account of the escape of gases or vapours from the surrounding walls on the removal 
of the liquid air. Such deterioration of the vacuum would not be indicated by the 
M‘Leod gauge, as there is always a very considerable time lag in the readings of the 
