4 Capt. A. C. Egerton on the 



For the determination of the chemical constants, therefore,. 

 ftv, a, and two values of p must be known or one value of p 

 and the value of X . 



X can be obtained from X T and the molecular heat 



accord in o- to X T = X + \ 2 jdt— 1 c p dt, or : — 



Jo Jo 



X =X I -4-963T + 3/2ET [^ + _ggL] + 2/5aT^ 



= X T -4-963 T + T p UT ~ U QJ + 0'4aT 5 / 2 . . . . (3) 



Unless Xt can be measured with accuracy directly, it has 

 to be obtained from two or more measurements of the vapour 

 pressure. Values of X,t can be obtained at various tempe- 

 ratures from the vapour pressure by the Clausius-Clapeyron 

 relation, and an average value of \ can be so derived with 

 fair accuracy. In dealing with a liquid, the latent heat of 

 fusion (fm) must be added to the molecular heat (c p ) term, 

 the latter would be integrated to the melting-point according 

 to the formula for the solid, and thereafter measurements of 

 the change of atomic heat of the liquid (c p ) must be obtained 

 and integrated to the desired temperature at which the vapour 

 pressure is measured. 



Discussion of Errors. 

 On differentiating formula (2), 



if X and (3v are known with accuracy, it is clear the higher 

 the temperature the less the error in the determination of 

 the constant C, provided the gas laws are applicable to the 

 vapour. 



On the other hand, as it may more often be convenient to 

 eliminate X , and determine C from two values of p at two 

 temperatures 1\ and T 2 , it will become of great importance 

 to choose the two temperatures as widely separated as possible, 

 otherwise the error may become great. Starting with two- 

 equations of the above form, the subtraction would lead to: 



-A^ T 4-iii[^] +o ' 0583a [^f] * 



