302 ILLINOIS STATE ACADEMY OF SCIENCE 



the log P vs 1/T curves for normal liquids with a degree 

 of precision approaching that often found in the record- 

 ed experimental results. 



In column 6, Table 1, are given the molecular latent 

 heats of vaporization of some of these liquids. These 

 results have been calorimetrically determined. Column 7 

 shows the values of the latent heats calculated as shown 

 in equation (2). These values have been obtained by 

 multiplying the observed slope (column 3) by 4.58. Col- 

 umn 8 shows the ratio of the calculated to the observed 

 latent heats. It will be observed that in all cases below 

 hydrogen the calculated result is larger than the ob- 

 served. This difference averages about 8%. This, then, 

 is equivalent to changing the constant 4.58 to 4.23. Ac- 

 cordingly, a new empirical equation for calculating lat- 

 ent heats of vaporization may be developed by combining 

 equations (2) and (3) and using the constant 4.23 in- 

 stead of 4.58, viz., 



Lv = 4.23 (— 68 -I- 4.877 Tb + .0005 Tb=) (4) 



In columns 9, 10, 11 and 12, Table I, are given, first, 

 the latent heats of vaporization calculated from equation 

 (4), second, from Trouton's^ equation, Lv = 21.5 . Tb : — 



(5) third, from Bingham's* equation, Lv= (17 + 



.011 Tb) Tb, (6) and fourth, from Nernst's' 



equation, L. =^ (9.5 . log Tb — .007 Tb) Tb (7) 



Comparing the results obtained by these various equa- 

 tions with those obtained by direct measurement, it will 

 be observed that at the very lowest temperatures, 

 Nernst's equation gives the best results. At all other 

 temperatures, except for isolated cases, equation (4) 

 gives as good, if not better, results than any of the others. 

 At high temperatures it is very evident that equation (4) 

 gives much the best results. Nernst's equation actually 

 goes through a maximum and finally to negative results. 

 The results obtained by Trouton's rule are also much too 

 low at high temperatures, while those from Bingham's 

 equation are much too high. 



Finally, Table I contains the values for the constant, 

 C, in equation (1). In column 13 are given the values of 

 the constant which should be used when the pressure is 



