(U5) 



with my formula for the substances examined by him (except for 

 naphtaline and cliloro-nitrobenzol). 



Now BtJCHNER thinks the assumption of a (COj)^ bimolecular at 

 the critical temperature very doubtful, and Kuenen too recently called 

 my attention to the fact that according to his measurements ^) of the 

 vapour pressures of liquid CO^ at different temperatures, the vapour 

 pressure factor ƒ presents a perfectly normal course, in opposition 

 to what the measurements of Regnault at 0° and 10° C, and those 

 of Cailletet at — 50° lo —80° give for it ^). 



Nor has the assumption of a bimolecular (SOj)^ really any foundation. 



Now, just recently ') I have examined the accurate course of the 

 plaitpoint curves for binary mixtures of normal substances, so that 

 it is now possible to derive a more accurate expression than the 

 above, in which the critical (plaitpoint) temperatures of the mixture 

 were identified by approximation with the temperatures of Ihe coin- 

 cidence of the inflection points of the successive ip-curves. That this 

 was, of course, not true, was sufKiciently known, and that the difference 

 can be considerable has been more than once emphatically stated by 

 VAN DER Waals. One look at the plate adjoined to my paper mentioned 

 above shows at once how perfectly different the course of the plait- 

 point line — also at the beginning, at T^ — can be. 



It will appear from the following derivation that the values found 

 from the above approximated formula should be more than douhhd 

 in many cases. 



Keesom has already derived *) a general expression for the moleculai- 



rise — f — - I , but as he used the law of the corresponding states, 

 and as in his final expression, viz. 



2\ V ^'^' yo 



\? 



-m 



c." 



da)d\ 



there occur all kinds of quantities, which have either to be determined 

 experimentally, or have to be calculated from the equation of state, 

 I preferred to derive the required expression directly from the relation 

 found by me for the course of the plaitpoint line for mixtures o 

 normal substances. 



1) Phil. Mag. 61, Vol. 2. 



2) See my paper in the Arch. Teyler (2) 9, 3^ Partie, p. 54. 



3) These Proc. of June 1905, p. 33 et. seq. 

 *) These Proc; Conini. Leiden N". 75, p. 6. 



