64 Internal Pressure and Latent Heat of Liquids. 



liquids, the differences are greater owing to the work involved 

 in breaking down the complexes almost completely on 

 vaporization. The following table allows o£ a rough com- 

 parison. 



Substance. 



I atmos./cin. 2 . 



Temperature 0° C. 



1 eals./c.n. 



L vaporization 

 of 1 gram. 



Methyl alcohol 



4095 

 3453 

 2348 

 3000 



97 

 82 

 55 

 71 



290 

 225 

 165 

 161 



n Propyl alcohol 



Isonronvl alcohol 



Jr x »* 



It is rather remarkable that the values of the heat of 

 vaporization and of expansion should be as alike as they are 

 in the case of normal liquids. One would have expected, 

 I think, that the numerical values would have been of a 

 different order of magnitude. In the process of vaporization 

 the molecules are pulled so far apart that they must be 

 nearly independent of one another compared with their relation 

 in the liquid. In expanding a liquid at constant tempera- 

 ture — a process the results of which are realized in experi- 

 ments in which a completely filled and closed vessel is cooled 

 without corresponding contraction of the liquid — the relative 

 separation of the molecules from one another, even in the 

 expanded state, must be small compared with that of the 

 vapour molecules. The fact that both heat effects are nearly 

 the same suggests that the range of attraction must be ex- 

 tremely small, of the same order as the mean distance apart 

 of the molecules of the liquid in its normal state (about 

 10" 8 cm.). 



With the values thus obtained for I (latent heat of ex- 

 pansion) we can calculate the values of K by means of the 

 expression 



/ 



K = 



1 + «T 



the results being as follows :■ 



