Value of "a" of -van der Waals' Equation 185 



It will be seen in Table III that C has a mean value of 2.98 X 

 io~ 37 for the twenty- six non-associating, or nearly non-asso- 

 ciating, substances studied by Young. The greatest devia- 

 tion from, the mean is octane on the one side, which deviates 

 a little more than four percent; and on the other, fluorben- 

 zene, which deviates 6 percent, methyl isobutyrate deviating 

 a little over 3 percent, and brom- and iodobenzene. The 

 deviation of the last two substances, about 7 percent, may, I 

 think, be disregarded since the critical temperature and pres- 

 sure were estimated and not directly determined; the critical 

 data are therefore less certain. The other twenty-one sub- 

 stances do not deviate more than 3 percent from the mean. 

 The cause of the deviation of fluorbenzene is uncertain, but 

 there is a regularity in the other deviations which suggests 

 that the size, shape, or compressibility of the molecule may 

 play a slight role in determining the critical data, or the co- 

 efficients of our formulas. For it appears that M 2 K, and 

 hence C, is always lower in the iso-compounds than in the cor- 

 responding normal compounds. Diisobutyl has C of the mean 

 value, whereas normal octane has a high value for C; a similar 

 relationship exists between diisopropyl and hexane; and be- 

 tween iso- and normal pentane. Some of this difference 

 would disappear if, instead of taking v equal to v c /4, I had 

 used the accurate ratio computed from the law of rectilinear 

 diameter. Thus V of isopentane is only V c /3-92; while 

 that of pentane is V c /3-96 and octane is V c /4.4- The sub- 

 stitution of these values for V in computing M 2 K would have 

 made C of isopentane 2.98; of pentane, 3 . 03 . The difference 

 is not entirely due then to this factor. It seems more proba- 

 ble to me that b c is not always exactly the same fraction of 

 V c and that consequently the coefficients of the formulas used 

 in computing M 2 K are not exactly the same in all substances. 



Notwithstanding x these slight deviations, the constancy 

 of C is certainly remarkably good and shows beyond ques- 

 tion, I think, that a close connection exists between molecu- 

 lar cohesion and. the molecular weight and number of valences 

 in the molecule. 



