106 HEAT. 



The following table is taken from Lees's paper : 



Weber's values of K 

 Liquid. K& a between 9 and 15. 



Water . . . -00136 -'0055 -00136 



Glycerine . . . -00068 -'0044 -00067 



Methyl alcohol . . -00048 -'0031 '000495 



Ethyl alcohol . . -00043 -'0058 -000423 



At first sight the results obtained by Lees appear in fair agreement with 

 those given in the last column as obtained by Weber. But while Lees's re- 

 sults for water, for example, may be put in the form K t = -00155(1 - 0048) 

 where, it must be remembered, the actual range of observation lies 

 between = 25 and tf = 45, Weber's results give K t = '0012(1 + -008) 

 where the range of observation lies between t = 4 and t = 24. They agree 

 nearly at t = 25, but have opposite signs for the temperature co-efficient 

 Further experiment is urgently needed to find whether there is any 

 reality in this change of sign. Indeed, until different methods give 

 closer agreement, all the results must be regarded as uncertain. 



Conductivity Of Gases. The investigation of the conductivity of 

 gases is complicated, not only by the ease with which convection occurs 

 but also by their transparency to radiation. It is necessary, then, to dis- 

 entangle the three effects of convection, radiation, and conduction, and 

 to find how much is due to conduction alone. Passing over earlier work, 

 which was merely qualitative, we shall describe briefly the experiments 

 of Stefan,* Winkelmann, Kundt and Warburg, and Todd. 



Maxwell calculated the conductivity of gases from the kinetic theory of 

 gases (chap, ix.), and showed that over a wide range of pressure the con- 

 ductivity should be nearly independent of the pressure. The experiments 

 to be described were made with the view of testing Maxwell's results. 



Stefan used two coaxal cylinders of thin copper. The inner one 

 served as an air thermometer, its tube passing through the end of the outer 

 and dipping into a vessel of water. The outer vessel was everywhere 

 separated by the same distance from the inner one, and the space 

 between them was occupied by the gas to be experimented on. The 

 outer cylinder was then surrounded by a mixture of snow and water, 

 and the rate of fall of temperature of the air in the inner cylinder 

 was observed. Hence, the conductivity could be calculated. For Air 

 Stefan found k =-0000558. Maxwell had already calculated the value 

 Jc = -000054. The result for Hydrogen was seven times as great, in 

 accordance with Maxwell's result The conductivity was also found to 

 be independent of the pressure, in accordance with Maxwell's prediction. 

 Stefan, however, took no account of the heat radiated. 



Winkelmann f experimented both with cylinders and spheres in a 

 method closely resembling that of Stefan, and obtained with different 

 apparatus a value of the conductivity of air very close to that of Stefan. 



Kundt and Warburg J sought to eliminate convection and radiation. 

 They noted the cooling of a thermometer in an enclosure containing 

 the gas, the heat capacity of the thermometer being known. As the 

 pressure of a gas is reduced, there is a limit beyond which the conduc- 



* Journal de Physique, ii p. 147, 1873. t Wied. Ann., xlviii., p. 181. 



J Pogg. Ann., civ. and clvi. 



