States, and the Thermodynamic State-Equation. 161 



It will be of great interest to put these two relations to 

 the test of experiment at low reduced temperatures. Under 

 ordinary atmospheric conditions, the value of M?*/RT lies 

 between 10*0 and 11*5 for most substances ; for some, such 

 as the alcohols, it is about 13 or 14. For these values, van 

 der Waals' equation already gives practically equal values of 

 dlp/dlT ; JBerthelot's and Clausius' equations still give rather 

 higher values of dlp/dlT. For most substances the value of 

 dlp/dlT is rather greater than that of Mr/RT, but there is no 

 exact agreement with either equation. Accordingly, at atmo- 

 spheric conditions, p(v 1 — v 2 )/T is generally less than R/M. 

 The following examples are taken from substances which give 

 almost equal values of M?-/RT and dlpjdlT. The quantities are 

 calculated from data in Landolt-Bornstein, Tabellen, 1912. 





Mr 



cllp 





T. 



P(«i-w a ) + 





ET' 



dlT' 





T 



Water, H.,0. 



131 - 



13-0 



_ 



373 



- 00443 



E/M = 0-00456 t. 









303 



- 00456 



0^=638. 









263 

 253 



00472 

 - C0481 



Ethyl ether, C t H 10 O. 



10-3* 1 

 11 J - 







373 



- 000091 



E/M = 0001 11. 



102 



— 



308 





T, =468. 



k 









273 



252 



- 00107 

 00114 



Carbon Bisulphide, CS„. 



10-1* 1 



104 J- 



10-1* 



}- 



373 



- o-ooioo 



E/M =000108. 



103 



320 





T,=546. 









273 

 259 



- 00105 



- 00104 



These results are not conclusive. Such as they are, they 

 indicate that at low reduced temperature p(r, — t , 2 )/T does not 

 always tend to the limit R/M, but passes that value and 

 becomes greater. Conclusive experiments on this point will 

 be very interesting from a thermodynamic point of view. 



The following Table shows the relation of Trouton's 

 constant and Crafts' constant as experimentally determined 

 for about forty substances. The values are calculated from 

 Landolt-Bornstein's data, extreme values of different experi- 

 menters being included. 



Column I. gives the experimental values of M?*/RT, 

 Trouton's constant divided by R = 1*985, at atmospheric 

 pressure. Column II. gives the corresponding experimental 

 values of d\ogp/d log T. The values of dlogp/diogT 

 calculated from the experimental values of M?*/RT by means 

 of Clausius' and Berthelot's equations are given in column 

 III.; the values in column I. are identical with the values of 

 this constant obtained by van der Waals' equation. 



* In cases where there are considerable discrepancies between the 

 results of different experimenters, both results are given. 



t Here the unit of energy chosen is the Litre -Atmosphere. 

 R^8-207.10- 2 . 



Phil. Maq. S. 6. Vol. 30. No. 175. July 1915. M 



