288 PHENOMENA DEPENDENT ON MOLECULAR PATHS 106 



duction of heat deduced from these empirical rules cannot 

 be extended without objection to vapours, nor to condensible 

 gases, nor to gases of complicated chemical structure ; for 

 these media the laws have only an approximate applicability. 



107. Observations of the Conductivity at Different 



Pressures 



That the heat-conductivity of gases is really independent 

 of the pressure, as the theory requires, has been shown by 

 Stefan's 1 experiments, in which a measurement of the con- 

 ductivity of gases was given for the first time in absolute 

 measure. 



After he had convinced himself by preliminary experi- 

 ments that the method followed by Magnus is unsuitable 

 for the purpose, by reason of the simultaneous conduction 

 of heat through the walls of the gasholder, 2 Stefan em- 

 ployed Dulong and Petit 's method, which had shortly 

 before been also used by Narr 3 in comparative experi- 

 ments on the speed of cooling in different gases. In a 

 cylindrical chamber fitted with the gas under investigation 

 was a similar cylindrical holder, which was filled with air 

 or other gas, and provided with a manometer, whereby it 

 was ready to serve as an air- thermometer. Stefan made 

 observations with this apparatus by immersing it, when 

 initially at the temperature of the room, in melting snow, 

 and measuring the speed of cooling of the air-thermometer 

 by observation of the falling condition of the manometer. 



It is by this method, or ' by similar methods which 

 have been partly improved, that most of the later measures 

 of the heat-conductivity of gases have been made. In 

 addition to a second memoir by Stefan 4 and two investiga- 

 tions by Josef Plank on gaseous mixtures 5 and a few pure 

 gases 6 which are a continuation of it, several more valuable 



1 Wien. Sitzungsber. 1872, Ixv. Abth. 2, p. 45. 



2 Buff has confirmed this. Fogg. Ann. 1876, clviii. p. 177. 



3 Ibid. 1871, cxlii. p. 123. 



4 Wien. Sitzungsber. 1875, Ixxii. Abth. 2, p. 69. 



5 Ibid. 1875, Ixxii. Abth. 2, p. 269. 



6 Ibid. 1876, Ixxiii. Abth. 2, p. 123 ; CarVs Eepert. 1877, xiii. p. 164. 



