42 THE TRUE VALUE OF a OF VAN DEE WAALS' 



from the valences and molecular weight is, as before, 16.28, which 

 is in good agreement. 



18. Ether. ('JJ n) 0. 



(a), a = 2.03 BT c F c /l.0B~P c F 2 . P c = 27.060 mm. Hg. ; d c = 



202 5; 7;. = 400.8; F c = 281.9 a=I8.70X10' 2 



According to Galitzine and Willis, 



P,. is 36.28. With this number a = 19.24 X 10 12 



(Ù). a = 0.5 P, ,V e with Young's value of P, a = 18.64 X 10 12 



(<?). a = /iJ/ 2 /3^-' :! . // = 10:'>. 8 ; J/ = 74. . . a = 19.34 X 10 ia 



(,[). a = C'RT C V C C' = 1.724 a = 18.87 X 10' 2 



0). a = 3 J/iA ,;! r,.r/V/ ; ^ = 1.0000; C= 2.10 (IUmsay and As- 

 ton); 2.27 (Eötvös). a = 18.82 (R. and A.); 19-78 (Eötvös). 

 (/). We have also, in this case, the determination from the 

 Ramsay and Young equation of the pressure under con- 

 stant volume of 18.52 X H) 12 . 

 There is considerable variation in these numbers. The mean of 

 the two surface tension measurements is 19.30. Ramsay and Aston's 

 determination was by the capillary rise method and is probably a 

 little low; Eötvös was by a reflection method and may be a little high. 

 The mean of all is 19.02. I think this is probably not far from the 

 true value, a computed from the molecular weight and valences, 

 with 28 valences to the molecule, is 19.12 X *0 12 , which is in very 

 good agreement with the mean value. Van Laak gives for ether 

 the value corresponding to 17.10 X 10 12 , which is obviously too low. 



10. n-Pentane. C b H l2 . 



(a). a = 2.0dBT c F c /\.0S--P c F c 2 . P c = 25.100 mmHg.; d c = 



.2323: 'J\ . = .1/0.2: / , =30<).!>4. . . a = 20.6SXI0' 2 



(/,). a = 0.5 P C V 2 a = 20.90 X»0 12 



(c). a = fjt,'M 2 /3 d? !Z . ft! = i 10.0. M = 72 . . . a = 21.05 X JO 12 



(d). a = C'BT C V C . C' = 1.707 a = 20.69 X 10' 2 



( 6 '). a—a'V 2 in formula P = b' T — a' a' = 



162,890 mm. Hg a = 20.88 X I0 1 - 



I could not find the surface tension of pentane. 

 These figures agree very well among themselves. The mean is 

 20.81 XI*) 1 -. which is probably very close to the true value. The 

 calculation of a from the molecular weight and valences, with 3.2 

 valences in the molecule, gives the value 20.52, which is in good 

 agreement with these values. Van Laar gives the value equivalent 

 to 19.12 X 10 12 which is 8°/ below 20.84. 



