THERMAL CONDUCTIVITY AND VISCOSITY OF GASES. 117 



The values calculated by this equation agree very well 



with the values obtained from equation (1) if Eucken's 



original results for the conductivities are used. When, 



however, the conductivity of air is taken as 5*22 x 10 -5 



instead of 5*66 x 10 -5 , the expression is no longer accurate, 



and Hercus and Laby suggest, on empirical grounds, another 



linear form, 



f = 2-816 y - 2*2 (2) 



which gives numbers closely agreeing, in the majority of 



cases, with the results they obtain from equation (1). 



I had previously mentioned 1 that the values of f, calcu- 

 lated by equation (1) with Eucken's original numbers for 

 the conductivities, are approximately represented by an 

 equation of the form, f = a (y- l)/y, where a is a constant. 

 I have now found a value of a appropriate to the new values 

 of f, and the expression becomes 



f= 615jp0 (3) 



In Table I the values of f obtained from equations (2) 

 and (3) are compared with those given by the defining 

 equation (1). An inspection of the differences, 1 — 2 and 

 1 -3 in the fifth and sixth columns of the table, shows, that 

 although the differences 1-3 are on the whole smaller, 

 there is little to choose between the two expressions, but 

 equation (3) has an advantage in that it indicates for per- 

 fect gases a simple approximate relation between the 

 conductivity and the viscosity which may be put in the 

 form myk Q _ ^ a 



where m is the molecular mass and a the constant of equa- 

 tion (3). How this relation fares in the case of actual 

 gases may be seen by an inspection of the calculated values 

 of 2a given in the last column of the table. 



1 Pollock, Jour, and Proc. Roy Soc. N.S.W., xnx.p. 249, 19L5. I now 

 learn that Hercus and Laby's paper was set down to be read before the 

 meeting of the British Association in Australia in 1914, but owing to 

 pressure of other business was not reached. The paper was, therefore, 

 completed for publication before my previous note was written. 



