﻿ran der Waals Formula. 595 



The modified van der Waals' formula 



(p+^)(v-b-ySl)=Rt, 



where 7= -57 = constant, 



leads to results very close to the measured results. In many 

 ways it is to be preferred to the Biot's formula 



logp=A + Ba' + C/3*. 



(1) It contains three adjustable constants, whereas Biot's 

 formula contains five. 



(2) Each o£ these three constants has a physical signi- 

 ficance. 



(3) The agreement with the measured values of p f v and 

 t is very close — the discrepancies being of the same order of 

 magnitude as when Biot's formula is used. 



(4) The derived equation (B) gives values of the latent 

 heat of vaporization in satisfactory agreement with the 

 measured values (e. g. at the boiling-point under one atmo- 

 sphere pressure — see Table), whereas Bakker, using the 

 unmodified van der Waals' equation, obtained results 20 per 

 cent, too low*. 



(5) The tables in Applebey and Chapman's paper (pp. 739— 

 742) show that the agreement between the two series of 

 calculated latent heats is still better at higher temperatures. 



The fact that ~r is found to be constant, suggests that the 



molecule expands uniformly with increase of temperature. 



[A uniformly expanding atom has been assumed by Schott 

 in the Adams' Prize Essay for 1908 1, aud by Richards in 

 some researches published in the Zeits. Phys. Chem. xlii. 

 and subsequent vols.] 



Applebey and Chapman remark (p. 742-3) " for halogen 

 derivatives " of benzene " the calculated values are all much 



too high, the mean percentage differences for chloro- 



benzene, bromobenzene, and iodobenzene being respectively 

 3*22, 5*54, 5*32." There are no experimental results with 

 which to compare these two series of calculated values. It 

 seemed desirable, therefore, to obtain Mill's constant J, fi\ 

 for each of these substances. 



* Bakker, Zeits. Phys. Chem. 1895, p. 519. 



t Electromagnetic Radiation (Schott), Camb. Univ. Press, 1912. 



% See 'Journal of Physical Chemistry,' 1905 and 1909. 



2 Q2 " 



