igc>4\ Mills — Molecular Attraction. 153 



mal pentane, normal hexane, normal heptane, normal octane, 

 benzene, hexamethylene, fluo-benzene, carbon tetrachloride, 

 methyl alcohol, ethyl alcohol, propyl alcohol, and acetic acid, 

 sixteen substances in all, the observations are practically com- 

 plete, extending- from near the freezing point of the liquid 

 nearly to the critical temperature. Among- these sixteen sub 

 stances the only divergences appearing- marked to the eye are 

 normal octane and ethyl alcohol at 0° C, di-isobutyl, those 

 for the alcohols as the lines approach the origin, and acetic 

 acid. 



The curves for chlor-benzene, brom-benzene, iodo-benzene, 

 stannic chloride, and water, are not complete. All of these 

 incomplete curves show irregularities and yet it is made most 

 evident by the diagrams, as well as by what has already been 

 said, that with the exception of stannic chloride, the diver- 

 gences are not so pronounced as to be considered weighty 

 evidence against the theory. 



In conclusion therefore we point out that, of the twenty- 

 one substances examined, stannic chloride and the associated 

 substances (methyl alcohol, ethyl alcohol, propyl alcohol, 

 and acetic acid) are the only substances that show variation 

 in fx r without at the same temperature exhibiting irregulari- 

 ties in the data used. That these irregularities in the data 

 are due to the measurements is much to be doubted. But if 

 not so produced they are significant of unknown changes tak- 

 ing place in the substance under examination— changes which 

 were not taken into account in the theory of molecular attrac- 

 tion under discussion and to which that theory as outlined 

 would not be strictly applicable. 



THE LATENT HEAT OF VAPORIZATION. 



It is interesting to examine more closely and to compare 

 the heats of vaporization calculated by use of the following 

 equations: 



