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LTX. The van der Waals Formula (and the Latent Heat of 

 Vaporization). By T. Carlton Sutton, B.Sc, Government 

 Research Scholar in the University of Melbourne* . 



IN the Journal of the Chemical Society, 1914 (p. 734), 

 Applebey and Chapman derive a " formula for the latent 

 heat of vaporization" which gives results in good agreement 

 with those of Mills and Young f. 



The impression given by their paper is that the Mills- 

 Young values are " observed," and may be used as experi- 

 mental data confirming theoretical results. This is so, only 

 in the sense that the Mills- Young values are " calculated 

 from observed " values of pressure, temperature, and volume 

 by a process very similar to that which Applebey and 

 Chapman themselves employ (see Appendix). 



This considerably modifies the inferences that should be 

 drawn from their work. 



The chief difference in the processes is that Mills % uses 

 the Biot's formula 



logp=A + B** + C#* 



to express the relation between vapour-pressure and temper- 

 ature, where ABC«j8 are five arbitrary constants chosen to 

 suit the measured values of the vapour-pressure, t is the 

 temperature, and p the saturation vapour-pressure ; whereas 

 Applebey and Chapman prefer to use a form of van der 

 Waals' equation 



(p+ £)(*-»)-»*. 



in which b is to be treated as a variable. They find after- 

 wards that b varies uniformly with the temperature. Con- 

 sequently, the equation may be put in the form 



(p+^fa-k-yfc-O-Hf. 



where t c is the critical temperature, 7 =---== constant, and 



at 



b c is the value of b at the critical temperature. 



* Communicated by Prof. Sydney Young:. 



t Sci. Proc. R. Dublin Soc. 1910, p. 412. 



X Journal of Physical Chemistry, 1902, 1905 et sqq. 



Phil. Mag. S. 6. Vol. 29. No. 172. April 1915. % Q 



