64 PHYSICAL PROPERTIES AND COMPOSITION 

 vapour at constant volume may be calculated from the 



q 



kinetic theory as 3 per kilogram-molecule, i.e. = 0-015 



per kilogram. In liquid mercury the work of expansion 

 must be added to this. 



To calculate this we will assume that the internal mole- 

 cular attraction is a force proportional to the square of the 

 density, and so inversely as the square of the volume of 

 a kilogram the so-called specific volume or 



a 

 = ' 



Then the work of expansion is 



ra 

 y 



or in expanding from V l to V 



, TT a 



= ^dV = =+ const., 



in which a may be calculated from the internal latent heat 

 of evaporation L. The latter represents the work done in 

 expanding to the very great volume of the vapour l . So 

 we get 



in which V L V d are the volume of the liquid and vapour 

 respectively, D t and D d their densities, so that mostly the 

 second term is negligible, and we have simply 



L = aD h 

 in which for mercury at 350 



L = 56- 



The work of expansion per degree at that temperature, 

 since the coefficient of expansion is then 0-00026, is 



A = a ID, -- 1 7 \ 0-00026&A = 0-00026 x 56 = 0-015, 



V I-OOO2O/ 



1 Bakker, ' Inaugural dissertation,' 1888. 



