304 ILLINOIS STATE ACADEMY OF SCIENCE 



tiire T, and Cs and Ss are constants having the same 

 significance as in equation (8). 



The constant Ss is related to the heat of sublimation 

 by the expression, 



Ss = Ls / 4.58 : (10) 



Now the latent heat of sublimation is equal to the sum 

 of the heats of fusion and of vaporization, or, 



Ls = Lf + Lv (11) 



Equation (4) furnishes a method for calculating Lv, 

 leaving then only the latent heat of fusion to be obtained. 

 A careful search has revealed the fact that there are 

 relatively very few reliable measurements of the latent 

 heat of fusion recorded in the literature, and almost no 

 data are available for substances melting below 0°. Sev- 

 eral empirical rules for calculating the latent heat of 

 fusion have been proposed. Probably the simplest and 

 most generally useful is that proposed by Walden", by 

 which the molecular latent heat of fusion is equal to the 

 absolute melting point times a constant, or, 



Lf = T,n K (12) 



Walden has shown that the constant K has a value 

 equal to about 13.5 for many organic compounds. This 

 value is, however, too high for many classes of substances 

 After considering all of the available data upon the latent 

 heats of fusion, we have concluded that the following 

 tentative values for Walden 's constant ma}^ be used for 

 calculating the latent heats of fusion of these classes of 

 substances here represented. 



No. of 

 sub- 

 stances 

 investi- 

 Substance. gated. 



Metals 18 



Aromatic Hydrocarbons 10 



Halogen Compounds 15 



Nitro Compounds 6 



Nitro Halides 6 



Amino Compounds 7 



Aromatic Hydroxy Substances... 5 



Aromatic Acids, Anhydrides and 



Ketones 7 12 . 1 1.0 



