354-356] Maxwell's Theory of Conduction of Heat 297 



355. We canuot use this equation for numerical calculation of K because 

 it involves the unknown quantity 77. We can, however, use the equation to 

 obtain a value for 77, by utilising an observed value for K. 



Let us take the case of air at 0C. We have tc= '0001 72, and the 

 general formula is 



RT 



if . 



V 



Taking T=273, and using the value J R = 9'3xlO- 17 from p. 113, we 

 obtain 



?7=l-5xlO- 10 . 



The value of t]v is accordingly 6 x 10 a , so that the time of relaxation is 



1 



6 x 10 9 



seconds. 



It is, as we should expect, comparable with the time of describing a 

 free path. 



Conduction of Heat. 



356. Multiplying equation (655) throughout by m, we obtain the 

 relation 



W 2 ) 



_ du _du /*I>TV 



uv ^ ^uw ^ ...... (717). 



This is the equation of transfer of Q, when Q is taken to be [p + v~ 2 + W 5 . 

 It is therefore the equation of transfer of energy, and this, in the Kinetic 

 Theory, is the transfer of heat. 



Writing w + v* + \w = 3q, the left-hand member becomes 3p -. . As 

 regards the first term on the right-hand side, we have from equation (706), 



2 171; 

 by equation (709). The remaining terms on the right hand, containing 



