298 ILLINOIS STATE ACADEMY OF SCIENCE 



tion constant, C, is the value which log P would have 

 when the temperature is some hypothetical maximum. 

 Numerically, the value of C, assuming the pressure to 

 be measured in mm. of mercury, varies from about 4.2 for 

 helium to about 11.2 for tungsten. The exact value for 

 any substance may be calculated when the latent heat and 

 the boiling point at some particular pressure are known. 

 The integration constant may be eliminated if the vapor 

 pressures at two different temperatures are known. 

 Equation (1) may then be transformed into: 



log p, — log Pi A log p — L 



• =- = = Slope = S (2) 



1/T, — 1/T, A (1/T) 4.58 



Thus it is evident that if the slope of the logaritlunic 

 vapor pressure curve be determined, the molecular latent 

 heat of vaporization may be calculated, or vice versa. 



Having shown that there is a simple expression for 

 vapor pressures involving only two constants, the ques- 

 tion naturally arises, Can these constants be evaluated 

 from existing data ? Since the slope of the straight line 

 vapor pressure equation is directly related to the latent 

 heat of vaporization (equation 2), and since the heat of 

 vaporization is closely related to the normal boiling point 

 as shown by the well known rules of Trouton% Nernst^ 

 and Bingham'', it is probable that the slope, S, is a func- 

 tion of the normal boiling point, a constant which is 

 known for many liquids. 



In order to show the nature of this function we shall 

 make use of the data collected in Table I. The data 

 from which this table was constructed have been taken 

 mainly from the Landolt und Boernstein, ''Physickalisch 

 Chemisch Tabellen" and the French Tables. For the 

 vapor pressures of the metals, the most recent deter- 

 minations of Ruff and BergdahP and of Langmuir^ have 

 been consulted as well. The vapor pressures of tungsten 

 and platinum have not been directly determined above 

 the melting point. Langmuir has determined the subli- 

 mation pressures, however, throughout wide ranges of 

 temperatures. The values here given for the vapor pres- 

 sures of these two metals have been calculated from the 

 thermodynamic relations existing between the vapor 



