107 CONDUCTION OF HEAT 289 



memoirs have been published by Winkelmann, 1 Kundt 

 and Warburg, 2 Janssen, 3 Graetz, 4 Eichhorn, 5 

 and others. Schleiermacher 6 made his observations by 

 Andrews' method. 



I first bring forward some of Stefan's results for the 

 experimental proof of the theoretical law that the conduc- 

 tivity of a gas is independent of its pressure. From four 

 observations on air under the ordinary pressure of about 

 750 mm. of mercury Stefan found the following values, 

 referred to the centimetre and second as units, for 10 7 ? ? 

 viz. 554, 560, 552, 554, and thus the mean value 



* = 0-0000555 ; 



and when he rarefied the air to the pressure 428 mm. he 

 found in the same way f _ Q-0000552 



If, then, there is a diminution of the conductivity with the 

 density, this is certainly so small as to elude observation. 



Kundt and Warburg obtained similar results by ob- 

 serving the cooling of a mercury thermometer in a closed 

 space. They found the following values for the interval t 

 which was necessary for the thermometer to cool from 

 59-3 C. to 19-6 C. when the apparatus was filled with the 

 gases named at the pressures specified. 



Air . . . p = 19-5 mm. t = 277 sees. 



9 277 



4 278 



0-5 280 



Carbonic acid . p = 7- 7 t = 349 



1-5 350 



Hydrogen . . p = 154 t = 66 



8-8 68 



The constancy of the conduction and radiation of heat 



1 Fogg. Ann. 1875, clvi. p. 497 ; 1876, clvii. p. 497; 1876, clix. p. 177 ; Wied. 

 Ann. 1877, i. p. 63 ; 1880, xi. p. 474 ; 1883, xix. p. 649 ; 1886, xxix. p. 68 ; 1891, 

 xliv. p. 177 ; 1893, xlviii. p. 180. 



2 Pogg. Ann. 1875, clvi. p. 177 ; previously in abstract in Berl. Monatsber. 

 1875, p. 160. 3 Wied. Beibl. 1879, iii. p. 701. 



1 Wied. Ann. 1881, xiv. p. 232 ; Munch. Habilitationsschrift, 1881. 

 5 Ibid. 1890, xl. p. 697. 6 'Ibid. 1888, xxxiv. p. 623 ; 1889, xxxvi. p. 346. 



U 



