290 PHENOMENA DEPENDENT ON MOLECULAR PATHS 107 



comes out plainly from these numbers, at least within the 

 given limits of the pressure. Under greater pressure the 

 regularity was disturbed by currents of the gas. 



In the opposite case also, when the pressure was made 

 very small, Kundt and Warburg observed deviations from 

 constancy ; there is here, however, no question of currents 

 in the gas but of a phenomenon of a similar kind to that 

 noticed by them in their experiments on viscosity, which 

 have been described in 81. Just as the slip of a gas on the 

 wall of the containing vessel becomes the more appreciable 

 the further the rarefaction is carried, so a difference between 

 the temperature of the enclosure and that of the gas touch- 

 ing it becomes the more considerable the less the pressure. 

 If the pressure is high and the gas. dense, many particles 

 strike against the wall and cause so complete an interchange 

 of heat that only a small difference between their tempera- 

 tures can arise. But if the pressure becomes smaller, and 

 therefore the number of particles fewer, the difference of 

 temperature increases and may become so great as to 

 amount to several degrees. 



This behaviour is quite analogous to that observed in con- 

 nection with viscosity ; for in viscosity the internal friction 

 alone comes into account in dense gases, while in rarefied 

 gases the phenomenon depends, not only on the internal 

 friction, but also on the external friction as well ; and, just 

 in the same way, the conduction of heat in dense gases 

 practically depends only on the internal conductivity of the 

 gas, while in rarefied gases it is conditioned by the external 

 conductivity as well. The internal conductivity, like the 

 internal friction, is independent of the density and pressure ; 

 the external conductivity, however, alters, like the external 

 friction, with the pressure, and, indeed, according to the 

 same law. In both cases there is a simple proportionality to 

 the pressure, and for the same reason in both cases ; for both 

 the external friction and the external conduction increase in 

 the ratio of the number of the particles which meet the wall. 



For the clear recognition of this behaviour, first noted by 

 Kundt and Warburg, the later memoirs of Crookes, 1 



1 Proc. Roy. Soc. 1881, xxxi. p. 239. 



