PAPERS OX CHEMISTRY AND PHYSICS 305 



There is insufficient data to indicate what would be the 

 value of this constant for mixed derivitives, but in gen- 

 eral, it may be said that the higher the molecular weight 

 and the higher the melting point, the larger will the con- 

 stant tend to become in any of the series of compounds 

 investigated. 



Assuming, then, that Ls, the latent heat of sublimation, 

 is known or may be calculated from equations (11) and 

 (12) there still remains the constant Cs to be detennined. 

 This is accomplished readily when it is remembered that, 

 at the melting point, the vapor pressure is equal to the 

 sublimation pressure. Hence, 



Cj — S> T,, = C — S Trn ri3) 



from which Cs may be calculated. 



It should be remembered that there is an independent 

 method of calculating the latent heat of fusion of solids 

 from their solubilities and freezing point lowering. In 

 order for this method to yield accurate results, however, 

 the substance chosen as solvent must be one which will 

 foiTU a thennodynamieally ideal mixture, or, otherwise 

 there must be introduced a factor which will correct for 

 the non-ideality of the given mixture. In a recent paper 

 the Author' has described a method for determining this 

 factor, and hence for finding the latent heat of fusion, 

 for several classes of organic substances. It is probable 

 that the pi-inciples there discussed would apply to other 

 classes of compounds as well, but at present there is too 

 little data to enable one to make a comparison. 



In conclusion, it may be said that, from the principles 

 outlined above, it should be possible to determine the 

 vapor pressures, the sublimation pressures and the heats 

 of vaporization, of sublimation and of fusion for normal 

 substances when only the freezing point and normal boil- 

 ing points are known. 



SUMMABY 



Empirical methods for evaluating the constants in the 

 vapor pressure and sublimation pressure equations, loe 

 P = C— S. / T and log P = Cs— Ss / T have been given 

 and their significance discussed. 



