446 
MR. J. T. BOTTOMLEY OX THERMAL 
March 30, at 11 a.m., I commenced running the five-fall Sprexgel pnmp con¬ 
tinuously, making frequent observations on radiation and with the McLeod gauge. 
Starting with a pressure of '0071 mm. (= 10 M), the pressure was reduced in an hour 
and a half to 0T M, as indicated by the McLeod gauge, though the actual pressure 
surrounding the wire may have been considerably different from this. As this point 
was reached the radiation of heat seemed to become steady, so that further working 
of the pump did not make any diminution in the amount of current required to 
maintain the fixed temperature of the ware. The pumping was continued for another 
half-hour, with a vacuum gradually improving, but with no change as to rate of loss 
of heat from the wire. I am therefore, I think, entitled to conclude that the limiting 
value of the radiation reached in this asymptotic experiment is the absolute radiation 
of the platinum wire for the difference of temperatures stated in a space freed from 
everything that the mercurial pump will remove, and that is not collapsible in the 
McLeod gauge. There was no doubt vapour of mercury present, and possibly, seeing 
that it is so difficult to remove, some minute trace of vapour of water. There seems, 
however, very little doubt, from the concurrent testimony of recent experimenters, 
that the numbers commonly set down from Regnault s observations as representing 
the pressure of mercury vapour at ordinary temperatures are very much in excess of 
the truth. 
The numbers which I take, for the present at least, as representing the radiation 
from my particular polished platinum wire contained in a non-reflecting envelope of 
copper, at the temperatures 16° C. and 17° C. respectively, are as follow :— 
At 408° C. 378'8 X 10 -4 gramme-water-degree Centigrade units 
per square centim. per sec. 
At 505° C. ..... 726-1 X 10" 4 
Two sets of experiments, made at an interval of a fortnight, gave me precisely the 
same number for the radiation at the lower temperature. That at the higher tempera¬ 
ture I have only determined on one occasion. I hope, with experiments to be shortlv 
undertaken, and with perhaps improved arrangements, to confirm or possibly modif'v 
these numbers, and to obtain radiations at other temperatures. 
