BOILING POINT AND SYMMETRY 57 



Similar results have been noted for alcohols, ethers, amines, 

 mercaptans, haloids, in short for all compounds of the 

 formula 



that is, the boiling point is lowest when the group X oc- 

 cupies the most central position. 



4. CALORIMETRIC RELATIONS. 



We will bring together under this heading all that refers 

 to the quantities of heat absorbed or given out in physical 

 changes. Of these the latent heat of evaporation will be 

 taken first, because it can be brought into connexion with 

 the characteristic equation; next comes the specific heat, 

 with regard to which the connexion with theory is not so 

 far advanced. 



A. Latent Heat of Evaporation. 



In applying the second law of thermodynamics in the 

 well-known form (Part I, p. 19) 



to the act of evaporation, the increase of volume T 7 , when 

 the liquid volume is small, as is the case for small values of 

 P, may be set, as a first approximation, equal to the volume 

 of the vapour itself ; then from the equation 



we get 



dP dT 



in which q refers to the molecular quantity. The law of 

 corresponding states may be applied to this relation when 

 transformed into 



dP m dT q 



~P '' zT " f r ' 



The first term is obviously constant for corresponding 

 states. If e. g. two substances have critical pressures and 



