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X. Thermal Radiation in Absolute Measure at Very Low Temperatures. 
By J. T. Bottomley, M.A., LL.D ., TJ.Sc., F.R.S., and F. A. King. 
Received June 15,—Read June 27, 1907. 
Contents. 
Page 
Introduction. ... 349 
Description of the apparatus employed.350 
Calibration of the thermo junctions.354 
Method of experimenting.356 
Calculation of the results.357 
Discussion of the results and Stefan’s coefficient .367 
Note added July 11, 1907 . 372 
Introduction. 
1. The experiments described in the following paper form part of an investigation 
with which one of the present authors has been engaged for many years past. The 
object of this investigation is the direct determination, in absolute measure, of the 
loss of energy, from a heated body to cooler surroundings, under differing conditions 
as to (l) the dimensions of the cooling body; (2) the nature of the surfaces of the 
cooling body and of the enclosure ; (3) the mean absolute temperature of the cooling 
body and the envelope ; (4) the nature of the atmosphere surrounding the cooling body. 
Several papers have already been published on this subject in the 1 Philosophical 
Transactions of the Royal Society ’ and in the ‘ Proceedings,’ and have been 
communicated to the British Association and elsewhere.* In all of these the lowest 
temperature available was the atmospheric temperature, or rather the temperature of 
the water supply of the laboratory; and the body from which radiation was to take 
place was heated to a temperature as much above the atmospheric temperature as 
the circumstances connected with the experiment would, from time to time, permit. 
Thus the upper and lower limits of these experiments have been something below 
1000° C. as the highest temperature of the cooling body, and, say, 10° C. as the 
lowest temperature of the enclosure. 
2. Some time ago, through the kindness of Lord Blythswood, F.R.S., an unlimited 
supply of liquid air was placed at our disposal, to be followed later by a supply of 
liquid hydrogen. It seemed therefore most desirable to extend the research in the 
* ‘Phil. Trans.,’ 1887 and 1893; ‘Brit. Assoc.,’ 1884, 1901, 1905, &c. 
VOL. CCVIII.—A 436. 27.7.08 
