DETERMINATION OF ATOMIC WEIGHTS 523 



From the values of M given in the last two tables, the 

 following atomic weights are readily obtained : 



Table X 



Carbon. 



12*003 from CO. 

 1 2*004 „ CH 4 . 



These results are discussed later. 



It is convenient here to refer to another method of calculating 

 molecular weights, due to Berthelot, which also requires a 

 knowledge of L, T c and p c . In the course of an elaborate 

 discussion of the compressibilities of gases between o and 3 

 atmospheres, based largely upon the experimental results 

 obtained by Chappuis, Berthelot (6) was led to propose a 

 characteristic equation for gases which is of the same form 

 as that given by van der Waals but in which a and b are not 

 constants. The values at N.T.P. according to Berthelot, are 

 a = io" 8 x 2-071 x T3/p c , b = 10- 4 x 2*575 x T c /p c , 



the units of pressure and volume being those already explained 

 (p. 513) in connexion with Berthelot's other method. Denoting 

 (a — b) by e, we have as before (p. 514) 



A = e/i - e 



and the calculation of molecular weights is made by the 



"limiting-density," formula (5) on p. 511. Berthelot calls this 



the "indirect" method of limiting densities. 



The following values of io 5 . A^ are obtained from the critical 



data given in Table I. : 



Table XI 



