CHAPTEK XIII. 



FREE PATH PHENOMENA (CONTINUED) 

 CONDUCTION OF HEAT. 



Elementary Theory. 



312. AN elementary theory of conduction, similar to the elementary 

 theory of viscosity arrived at in the last chapter, can be arrived at without 

 trouble. The physical principle underlying the phenomenon of conduction of 

 heat is exactly similar to that underlying the phenomenon of viscosity ; the 

 difference of detail being that instead of an excess of momentum being 

 carried in one direction, we have in the present instance, an excess of energy. 

 In equation (569), we arrived at the relation 



r = % v cl*: .............................. (585), 



and this will apply to the present problem if we now suppose /Z to be the 

 mean energy at the plane z-=z^, and F the transfer of energy across this 



rlT 1 



plane. If ^ is the coefficient of conduction of heat, the flow of heat is ^ _ t 



oz 



rlT 1 



so that the flow of energy is J^ ^- , where J is the mechanical equivalent of 



oz 



heat. 



Also /Z, the mean energy of a molecule, is identical with the quantity 

 which has previously been denoted by E, so that equation (585) can now 

 be written 



dT . .jdE 



:5- *s vd -5- 



dz 02 



dEdT 



from which it follows that the value of ^ is 



< 586 >- 



172 



