( '-^0 ) 



Hence ;v iiiinimum critical temperature and a uiaximnni xapoiir 

 pressure are two phenomena wliicli as a rule occur together, but 

 not necessarily ; this is only the case in field 3. 



From fig. 1 it appears that, according to our reasoning, only 

 a maximum vapour pressure is possible ; yet we know that there 

 are mixtures which show a minimum \'apour pressure, and it has 

 been proved by Kuenen ^) that this phenomenon occurs even under 

 the critical circumstances. Here it seems that there is a fundamental 

 deviation from the observation. Nevertheless it is remarkable that 

 the mixtures which show a minimum vapour pressure are always 

 of such kind that at least one of the components is an anomalous 

 substance^); so that there is reason to suppose that with mixtures 

 of normal substances a minimum \ apour pressure never occurs ; and 

 our speculations, Avhich are based on the law of corresponding states, 

 are applicable to normal substances only. 



8. Starting from the same suppositions as set fortli here, van Laar ') 

 has found an accurate expression for the projection of the plaitpoint 

 line on the ni'-plane. I have tried to derive from this the equation 

 of the plaitpoint line in the pT — hence also in the ct — diagram^;, 

 but without success. Without therefore occupying myself further with 

 the general form which the plaitpoint line in our case takes in the zr- 

 diagram, 1 shall investigate a few points, namely the occurrence of 

 a maximum or a minimum [)laitpoint temperature, and that of a maxi- 

 mum or a minimum plaitpoint pressure. 



According to Keesom's ') formulae (2^/) and (2/y) we have 



1 /(WruA Ti , 1 



Pok\ d.v J„ JTj- 



d]\j,i (ITj-pi 



Hence follows that the boundarv between — --~>0 and — ^<'Ois 



(Lc ax 



formed bv the cur\e : 



1) Arch. Need., (2), 5, .300, 1900 (Livrc jubilairo de H. A. Lorentz). 



2) For the bibliography of this cf. Hartman, Thesis for the doctorate, page 84, 

 Leiden 1899. 



') Proceedings April 2:2, 1905. ' 



■*) By eliminating x and V between the equation of the rx projection of the 

 plaitpoint line, the equation of the spinodal surface (van Laar's formula Ö, loc. 

 i:it.), and the equation of stale, 



''i Uommuuicalion no, 75, Proceedings Dec. 28, 190], 



