Atomic Laws of Thermochemistry, 9 



identical with (11), which was established for the metals; 

 and I have found this same equation to hold for compounds 

 of the types RS 2 , RS 3 , RS 4 , such as CaCl 2 , A1C1 3 , and SnCl 4 . 

 When S instead of being an atom is a compound radical such 

 as ]ST0 3 , the equation (13) does not become so simple, but in it 

 we must put the value of Mc and take k as we have already 

 implied that it is, namely the reciprocal of the number of 

 radicals in the molecule. Thus in equation (13) a second 

 method of finding I has been established, depending only on 

 density and melting-point. 



There is still another approximate method which comes in 

 useful for a number of compounds which are liquid at ordinary 

 temperatures and for which only density and boiling-point 

 are known ; it is 



M 2 Z = 1190xlO- 6 (M/ / o)T 6 (14) 



We have now to determine in what way we ought to pass 

 from the values of I given by the three equations (7), (11), 

 and (13) to the total latent heat of vaporization of the solid 

 at ordinary temperatures, say 15° C. We have seen that for 

 the latent heat of vaporization of a liquid at its ordinary 

 boiling-point (4) holds when the unit of force is 10 6 dynes ; 

 with 10 12 dynes as unit it becomes 



M/M = 61'3xlO- 6 MX. 



But in thermochemical experiments we have, as a rule, to do 

 with the heat given out when the reagents are taken at about 

 15° G., and the products are brought back to the same tem- 

 perature, and in most cases the latent heat at 15° G. will be 

 larger than at the boiling-point. Under these circumstances 

 it seems to me best to regard the matter in the following: 

 way : l/v is the potential energy of the molecules of a gramme 

 occupying volume v, due to their mutual attractions ; hence 

 if v is the volume of the gramme when solid, and v B is a large 

 volume into which it is supposed to be vaporized, the change 

 of potential energy, due to the separation of the molecules in 

 a gramme-molecule, is ~M.l/v — Mljv^ Now the second term is 

 so small compared to the first that it can be neglected, when 

 we have M//r , which when expressed in calories becomes 

 M//»oJ. 



It is this potential energy which constitutes the main part 

 of the latent heat of vaporization ; and it is this M.l/v J, which 

 can be written M.lp/J ; that I propose to use in place of the 

 actual heat of vaporization at 15° C. The manner in which 

 the latent heat is to be used in connexion with thermochemical 

 data is as follows : — Suppose a solid element It to combine 



