EQUATION AND THE NATURE OF COHESION. 5i 



As usual the Jatent heat values give higher results than the others. 

 The mean of all is 34.04 X '0' 2 . The required value of a, if there 

 are 44 valences in the molecule and the molecular weight 100, 

 is 31.58 X 10 12 , which is a little more than 2 °/ lower than the 

 lowest of the preceding values and about 7 °/ lower than the mean. 

 I was unable to find, in a somewhat hurried search, any determi- 

 nation of the surface tension of heptane. Van Laar's value corres- 

 ponds to 27.64 X 1° 12 which is about 20 / o below the mean 

 found value. 



38. lodo-benzene . C 6 H 5 I. 



(a), a = 2.03 PT c F r /1.0S-- P,.F, 2 . 1\. = 721 ; P r = 33.012 

 mm.Hg.; d r = .5814; V r = 350.0... a = 35.88 X I0 12 



(b). a = 0.5 P,.V} ' a = 36.97 X »« 1 ' 2 



(c). a = p'M 2 j3 d; ir \ M = 203.85 ; fjc! = 43.37 a = 36.09 X 10 12 



(d). a = C'RT,.V ( .. C =-1.687 ' a= 35.48 X I0 12 



(c). a = MN^sv^T^lid— D)(T C — Tf 3 .' From Morgan and 

 Daghijan at 59.6° wo m = 925.73 from which so 2i3 is 

 calculated as 822.7 ergs. d= 1.7691; D is neglected; 

 7'. — T is 388.4 ; without correction according to Lohn- 

 stein this makes a = 36.40 X 16 12 . In making this calcu- 

 lation by Morgan's method it is assumed that the con- 

 stant for benzene is 2.121. If in place of this we use 

 the value of Richards and Coombs it would raise the 

 value of a about 1 / o . 

 The mean value of all is 36.18 X 10 12 . This mean value is between 

 9 and 10°/ o below that which we calculate for a from the mole- 

 cular weight and the number of valences, assuming that iodine is 

 monovalent, so that there are 30 valences in the molecule. The 

 value required with 30 valences is 39,34 X 16 12 - With the {b) form- 

 ula, namely where a = 6.5 P,.V, 2 , and from the surface tension 

 the found value is about 6—7 % too low. The value 36.49 X 10 12 

 would be the required value if there were 27 valences in the mole- 

 cule in place of 30. lodobenzenc is just about as divergent from 

 the theoretical as is brombenzene. Here, as in brombenzene, the 

 critical data are uncertain to some extent as they had to be deter- 

 mined by extrapolation over a long interval, over 100 degrees, in 

 fact. Decomposition occurred on heating the iodobenzene. It will be 

 noticed that we have to use the critical temperature even in cal- 

 culating the value of a from the surface tension and the other 

 critical data in the other cases. So I do not feel that we can 



l) 4* 



