370 
DR. J. T. BOTTOMLEY AND MR. F. A. KING ON THERMAL 
48. The results of experiments made at the beginning of last century by Sir John 
Leslie and others seemed to show that the total loss of heat from a body cooling in 
dry air was made up of radiation and convection in about equal proportions. 
Such a statement as this is, however, very far from representing the state of the 
case with regard to loss of heat with full air pressure at very low temperatures. 
The loss by convection is enormously increased when the air is reduced in temperature 
nearly to liquefying point. On the other hand, the loss by pure radiation from the 
sooted globe at very low temperatures is extremely small; while the loss from the 
highly polished silvered globe is, so to speak, minute. Thus convection may be from 
18 to 25 times the pure radiation from the sooted surface, instead of being about 
equal to it; while the convection is from 60 to 100 times the pure radiation from 
polished silver. We propose shortly to make some special experiments on this 
subject. 
49. A question of great interest is the comparison of the results we have obtained, 
including Dr. Bottomley’s old results, with the 4th power law, or formula of Stefan. 
According to Stefan’s law, the emission, S, of heat from a “ black ” body is propor¬ 
tional to the 4th power of the absolute temperature of the cooling surface. Taking 
this law in conjunction with the law of “heat exchanges,” the cooling of a “black” 
body at temperature 9 in a “black” enclosure at temperature 9 0 ought to be pro¬ 
portional to the product of the “ emission ” and (0 4 — 0 O 4 ); and may be represented by 
<x {9 i — d 0 4 ), where cr is a constant, sometimes called “the radiation constant.” In our 
tables it is not the “ emission ” of heat per square centimetre which is given, but the 
“ emissivity,” or the emission divided by the difference of temperatures between the 
cooling body and the surrounding envelope. If, then, e be the emissivity between 9 
and 9 0 , we have e = S/(d — 9 0 ) ; and, correspondingly, we shall have 
S e 
^ ( 0 4 - 0 o 4 ) (0+0 o )(P+6o*)' 
We have applied this formula to all the tables where the cooling of the sooted globe is 
given, and where the vacuum is sufficiently good to make the experiment suitable for 
the purpose in hand. These numbers are given in Column 5 of the respective tables. 
