( '-^9 ) 



tciiiponitiires of tho roiiipniieiits difrer little and the ciilical pressures 

 dilfer relatively much. It is known that experience confirms this 

 conclusion. 



7. Now 1 shall try to find how the snhstances must be chosen 

 in order tliat one of the mixtures near the critical circumstances may 

 sliow a maximum — or a minimum — vapour pressure. At the critical 

 l)oint (at the same time plaitpoint) of that mixture we then have, 



l\k dp J I: /op" 



alou"- the critical line, -— - = ^- 



As we ha\'e based our speculations on the original equation of 



state of VAN DF.R Waat.s, we must, strictlv taken, use for ( -- ) 



VötA 

 the value which follows from this eipiatiou, i.e. 4. Thus we find') 

 that the area within which the seconil component must lie, is 

 i)Ouuded by the curves : 



T = A (3.' — ;») and r = -^ — , ') 



represented by OAl and KAL respectively in fig. J. KAL is again 

 the critical line which sliows at the point .1 itself the property 



mentioned above, while (>A[ combines all the points where :^ 4 



n (It 



or rz— = '2. The second component must be situated between these 

 at 



two lines, namely in field 3 or 4. 



We may repeat that in order to observe the property under con- 

 sideration, the pure substances must be chosen so that the critical 

 temperatures differ little, but the values of the critical pressures 

 differ relatively miu-h ; ho\vevei-, the component with the higher 

 critical temperature must also have the higher critical pressure'). 



1) The elements of tlie mixture, uf which tiie vapour pressure is maximum 

 or minimum, are given by 



_ 2jt, t/jTi + T, t/.T,— 3t,jr^ _ Jr,—T , (/jTj 



_ 9 (1/. T.-1)K-T. l/^,) 



^) The general conditions for the existence of a maximum or a minimum vapour 

 pressure have been derived by van der Waals (Versl. Kon. Ak. 1895/9G). 



My quotation of van Laar in the Dutch edition (same note) resulted from a 

 misunderstanding. 



3) The latter does not always hold good, as for instance with mixtures of 

 UOj and C^Ho (Kuenen, Zeilschr, f. physik, Ghem., 24, 081, 1897). 



