Phenomena of some Substances and Mixtures. 197 



observed minimum critical temperature (point A). The two 

 sets agree very closely. 



Table XVIII. 









Max. p 



ressure. 





Min. a/b. 



Min. T c . : A. 

















Calculated. 



Observed : B. 



N 2 0-C 2 H 6 ... 



x =0-52 



50 



001 



020 



C 2 IL,-C 2 H 6 ... 



a?=050 



053 



036 



0-51 



C0 2 -C 2 H 6 ... 



tf=048 



045 



010 



030 



dv 



39. In Table XVIII. are also given the mixtures of maximum 

 pressure as observed and as calculated from the theory. The 

 latter calculation is simple. In coexisting phases we have the 



three conditions that ^,^S and y}r — v— i — #— ^are equal. 

 Ov ox ov ox 



Now ty may be found from the equation p v T, because 

 This leads to 



ylr=-UTl(v-b x )~^ + < f i (x). 



The two first conditions mentioned lead immediately to 



(- + l\&h _ ± B^x* 



\ v i v 2 'ox ~ a i ~d% ' 

 remembering that x is the same in the two phases ; v x and v? 

 are the volumes of the coexisting phases. At the critical 

 point : v x = v 3 = 3b x , and the equation becomes 



_2_ 



OX 



±oax 



a x o® 



There is not a very good agreement between the values 

 calculated from this formula and those observed. The points 

 of agreement are that both sets of x's are smaller than the x's 

 for the min. cr. temp., and that the smaller the x observed, 

 the smaller also is the x calculated. 



Considering how insufficient van der Waals's formula is, we 



should not expect a better agreement ; still the equation 



seems to give correct indications as regards the relative 



position of the two points A and B. This connexion may be 



* Van der Waals, I. c. p. 23. 



