214 PHENOMENA DEPENDENT ON MOLECULAR PATHS 84 



air; according to Kirchhoff's 1 law concerning the emis- 

 sion and absorption of radiant heat, they have therefore 

 equal coefficients of absorption also. Hence the conclusion 

 that the radiation received by the blackened side of the vane 

 from the glass envelope is equal to that which the blackened 

 side of the vane would send out to the glass wall if it were 

 as much the warmer as the glass envelope is. 



I have calculated 2 in absolute measure the amount of 

 heat which a warmed black surface radiates to the re- 

 ceiver of an air-pump under which it is placed from the 

 observations made by Stewart and Tait, 3 on the heating 

 of a blackened disc of aluminium in a vacuum of 7 '6 mm. 

 pressure. I have found the value 



h = 0-0017 



for the constant of radiation, referred to a millimetre, second, 

 and degree C. as units ; with centimetres substituted for 

 millimetres, we have 



h = 0-00017, 



and this number simply means that the heat radiated in one 

 second by a square centimetre of a blackened surface of 

 aluminium at a temperature 1 degree C. above its environ- 

 ment would suffice to raise 0-00017 gram of water through 

 1 degree C. that is, is equal to 0*00017 calorie. From 

 observations by Dulong and Petit, and also by F. 

 Neumann, I found & = 0-00013. Later on Lehnebach 

 found a value which, reduced to 1 degree C., is 

 li = 0-00015 ; and, lastly, Kundt and Warburg 4 have 

 obtained h = 0'00014 in the same units. The perfect 

 agreement between these numbers vouches for their correct- 

 ness. 



A vane of the radiometer of 1 sq. cm. area would, 

 therefore, receive on its blackened side a radiation of about 

 0-00015 calorie in a second if the glass envelope were the 



1 Pogg. Ann. 1860, cix. p. 275 ; Untersuchungen iiber das Sonnenspectrum, 

 2nd ed. Berlin 1862, p. 22. 



2 Ibid. 1868, cxxxv. p. 285. 



3 Proc. Roy. Soc. 1865, xiv. p. 339 ; Phil. Mag. 1865 [4] xxx. p. 314. 



4 Pogg. Ann. 1875, clvi. p. 208. 



