40 THE TRUE VALUE OF a OF VAN DER WALLS' 



14. Bromine. Br.,. 



(ft), a = 3 Yl/iV 1/:i 7 7 c 6'/ft" . I have only a single determination for 

 Br 2 made from the surface tension. C is equal to 2.115 (Morgan 

 and Daghlian, corrected after Lohnstein method.) 7',. = 575; 1\ 

 not given. M =159.84. d , calculated from d at 0° C. of 3.187 

 by formula d = (T r j(T r — 7'))'" 4 is 3.95 a = 12.49 X 10 2 



This value is to be regarded, however, as only an approximation, 

 but it is probably not very far wrong. If we calculate a from the 

 molecular weight and valences with 6 valences to the molecule we 

 have 11.44 X10 12 ; if, like Cl 2 , it has 7 valences, then the calcu- - 

 lation would give 12.68 which is in good agreement with the fore- 

 going computation from the surface tension. This would mean 7 

 valence electrons in the molecule. 



15. Carbon bisulphide. CS 2 . 



(a), a =2MBT c r c /l.0S — P C V C 2 . P c =72.87; 7;.= 546; d r = 

 .3924 (Cited from Goldhammer who gives it as Ba- 

 telli's figure). M= 70. These make JS= 3.176 which is 

 clearly too low. There is evidently some error in the critical 

 data and it is probably in the critical density which should 

 be higher. However using these data a = 1 1.56 X 10 12 



(<5). a = 0.5 B r V, 2 a = 18.00 X N> 12 



This great variation between the («) 

 and (b) formulae shows that the critical 

 density is not high enough. If S were 

 3.6, which is probably nearer its true 

 value, the first formula would have given 

 13.14 and the second 14.00. 

 ( C ). a = i u'Jf 2 4.1S5 X 10 7 /3 d; 11 ". p' = 82.4; 



d r = . 3924 a = 12.35 X 10'- 



(d). a = CRTJ,.. C' = 1.516 a= 13.33 X 10 12 



(e). a=3MN i l' 3 T c C/d . d = 1.6776 (Körber, 

 Ann. Physik, 37) C = 2,08 (-Ramsay 



and Aston) a = 13.07 X 10 12 



In view of the uncertainty in the critical density, which enters 

 into each of the formulae except the surface tension, we shall have 

 to depend chiefly on the latter. The mean of all the determinations, 

 except the very aberrant one of 18.00, is 13.33 and as this is 

 not very different and a little larger than the surface tension value, 

 the surface tension having been determined by the capillary rise 

 method, it is probably close to the true value. At any rate it is 



