2 Capt. A. C. Egerton on the 



pressures of zinc and cadmium previously investigated (Phil. 

 Mag. vol.xxxiii. p. 33, 1917) have been used to deduce their 

 chemical constants and give additional support to the validity 

 of the result, which is in close agreement with the above 

 theoretical expression. 



Method of Calculation. 



Most of the chemical constants have been calculated 

 according to the formula : 



lo ^=-WlT + 1 ' 751o S T -iSi + C ' 



where \ is the molecular latent heat at absolute zero, 



e is an individual constant (depending on change of 



spec, heat with temp.), 

 C is the " chemical constant.'' 



The constant 1*75 was derived from the assumption that 

 at the lowest temperatures the molecular heats of all gases are 

 1*5 greater at constant volume and 3*5 greater at constant 

 pressure than the molecular heats of the corresponding con- 

 densation products. Nernst's more recent work on specific 

 heats shows such an assumption to be completely unjustifiable. 

 Calculation of the constants X and therefore by means of 

 such a formula gives values which are without physical signi- 

 ficance, and further the errors introduced by the necessity for 

 combining the errors of temperature measurements in order 

 to deduce the value of the constant, might render the final 

 result liable to a serious discrepancy from the true value. 



The vapour-pressure formula, derived directly from the 



( 1 1 n ty 

 Clausius-Clapeyron relation, A T = HT 2 — -~ , and At = X +- 



,iT fT " tit 



1 C p dt— I c p dt, can be truly represented as follows : 



Jo Jo 



\q I dT | .p, . rp 

 *T ^ TTT^ 1 p Cp 



where (fi p — c p ) is the difference of molecular heats of the 

 vapour and the condensation product. As the molecular 

 heat of the vapour remains constant 4*963 to the lowest 

 temperatures, this becomes 



. . . (1) 



The molecular heat of the solid can be integrated by 



1 w = "aT'.ur 



