igo/f\ Mills — Molecular Attraction. 171 



We would here call attention to the fact that for bodies of 

 similar constitution and closely related chemically the values 

 of n may be the same, though the observations are not suffici- 

 ently extended to permit of any definite conclusion. Thus: 



Normal pentane, 457.3 



Normal hexane, 454.2 



Normal heptane, 458.6 



Normal octane, 451.1 



But isopentane, ----- 438.6 



It is well also to note that for the associated substances, 

 water, the alcohols, and to a less degree for acetic acid where 

 it is less associated, the values for /* are greatly larger than 

 for the other substances examined, and this very large attrac- 

 tion is suggestive in view of the conclusion drawn in the sec- 

 ond paper that quite possibly the molecular association of 

 these substances was caused by the molecular attraction. 



Resuming a line of argument followed in the first paper 

 (p. 228) we can test our conclusions further. In a gas indef- 

 inite expansion takes place as the pressure is decreased. This 

 shows that the attraction between the molecules cannot be 

 great enough to make the paths of the molecules closed 

 curves. In a liquid, while undoubtedly many molecules 

 whose velocity is above the average molecular velocity, are 

 .continually flying away from the surface, j T et it must cer- 

 tainly be the case that most of the molecules are drawn back 

 by the molecular attraction. There must be for each sub- 

 stance a certain temperature at which the molecular attrac- 

 tion, without outside pressure, is just strong enough to over- 

 balance the kinetic translational energy of the average parti- 

 cle and cause it to return to the liquid or solid substance. 

 At this point, if the attraction varies inversely as the square 

 of of the distance between the molecules, we will have from 

 mechanics, 



[23] V ' 8==2 ^' 



where V is the molecular velocity and R is the distance apart 



