Energy Increment and Trouton' 's Rule. 159 



would be contiguous and the conditions obtaining would 

 then coincide with those assumed by Planck for equilibrium 

 in a "Hohlraum." The plausibility of this hypothesis 

 is indicated from a consideration of the phenomenon of 



evaporation. According to Trouton, the molecular latent 

 heat of vaporization divided by the boiling-point is a 

 constant and approximately equal to 22. Perrin (Joe, 

 cit. p. 103) points out that, on the quantum hypothesis, 

 this rule can be expressed in the form that the frequency 

 of the radiation which determines the boiling-point is 

 proportional to the boiling-point. The boiling-point under 

 normal atmospheric pressure is approximately tw T o thirds 

 of the critical temperature, and Trouton's rule holds more 

 correctly when the critical temperature is taken as the 



reference temperature, or ^=2/3x22 = 14-66. The value 



of Trouton's constant, calculated from van der Waals' 

 equation of state, is 2/3 x 10 8 = 7*2. 



At the critical temperature T c the maximum number of 

 molecules in a substance are activated to the extent necessary 

 for them to effect combination with one another to form a 

 non-polymerized liquid, and on the above hypothesis this 

 fraction is indicated by the value of the ordinate at A max .. 

 Applying Wien's displacement law, A max T 6 . = <?, where \ max . 

 is the wave-length corresponding to the energy maximum, 



T 



or — = constant where v is the respective frequency. The 



v v 



latent heat of evaporation for a non-associated liquid is 

 given by the expression L = NAv or r „ = constant. 



