CONSTITI'TKI) OF Si'HKHU'ALLY SY.M MHTKICAI. MOLECULES. 447 



equations with those of a viscous fluid whose coefficient of viscosity is /z, viz. , 



we obtain the following expression for n : 



(33) M = 4sU!=^ l 5 



8. 7%e Coefficient of Thermal Conductivity of a Simple Gas. 

 We next substitute for A(tt a +v*+w il ) in equation (10), and so obtain 



'* 



5*7 = 4- 4" 



2ir/ 



) 8 R" n 



on substituting for a, from the equation on p. 442. This equation is simplified if we 

 recall the value of n from equation (32). Thus* 



15/u 



in the equation (l)), we get 



If we now substitute for 



If we compare this with the equation of conduction of heat in a gas at rest, whose 

 thermal conductivity is & viz., with 



1)6 i 



(where C v is the specific heat at constant volume) and remember that q is directly 

 proportional to the absolute temperature 6, we find that 



(34) = tpC.. 



D 



* We can now see why the term v =- U (U a + V 2 + W 2 ) on the left-hand side of the equation preceding 



equation (10) is negligible. The ratio of its coefficient v to the coefficient of U (U 2 + V 2 + W*) on the right 

 hand (viz., in J^u (u* + p 2 + 1^) ) is now seen to be p/p, which is an exceedingly small quantity in all gases 

 under normal conditions. 



