758 



1 dri 

 also possible that even if formula (5) is valid, tiie value of — — — must 



be negative, in consequence of the factor e ^'^ , or because o\ hence 



also f) depend on the temperature. The experimental data are not 



sufficiently accurate to decide this question. 



More satisfactory results are furnished by another test, which can 



be applied on a more extensive scale. It consists in this that we 



1 d^i 

 compare the experimental values ot — ^^77') with the values foUow- 



ing from equation (5). 



For this purpose we write: 



1 dïj 1 / b^ dl] dv 



and in this we put: 



/ a 



1 ö>^ 1 1 èvi 11 -b V ^ w» 



1] dT„ 27' ij dv-j V V — h v(v — b) v.RT. 



a 

 We shall neglect p by the side of and roughly assume 



8 rt \ a 



Rl\=z and pT,=z— --■, we then get: 



27 b 27 6' ^ 



1 dti 1 f^YR'Tk' 



1 

 V is the volume per gram-molecule, hence v = m - , so that we 



9 



finally find to test: 



'1 dïA 1 3'R*q'Tjc' n dv 



1 dv 



(7) 



Borrowing the values of o, ])k, Tk and — r-— from the "Recueil 



' V dip 



etc.", we find: (See Table p. 754). 



The agreement is on the whole as satisfactory as could be expected 



in view of the many approximations. Generally the experimental 



value is somewhat smaller than the theoretical one, for ether more 



than for other substances, benzene and orlhoxj'lene deviating in the 



opposite sense. For acetic acid and for the alcohols the agreement 



is much less than for the normal substances. 



^) See note 1 on pag. 751. 



49 

 Proceedings Royal Acad. Amsterdam. Vol. XXI. 



