192 Dr. J. P. Kuenen on the Condensation and Critical 



the index indicating the quantity which is kept constant in 

 the differentiation. Van der Waals* has discovered that for 



T dr> 

 single substances - jr is approximately equal to 7 at the 



critical point. It is an interesting confirmation of the theory 

 that my maximum-curves also give a value about equal to 7 

 at point B. 



31. An important question in connexion with this is, What 

 is the direction of the plaitpoint-curve at B ? In my former 

 paper this point was left undecided. In the mean time Prof, 

 van der Waals, to whom I showed my results, has published 

 two papers f in which the properties of both points B and A 

 are completely discussed. I will deduce here some of his 

 results, sometimes using his equations in a different manner. 



32. For -£■ of the plaitpoint-curve the following may be 



at 



written : — 



/dp v = /§£\ /B£\ /do\ + rdp\ (dx\ _ 

 \dt )p \dt )v* \dv/t x \dt/p \ox/vt\dt /p' 



At B both |£ and ^ will be seen to be =0. This follows 



immediately from the conditions of the v-x diagram, =r- being 



= 0, because the line of equal pressure always touches the 

 connodal curve at the plaitpoint, and is therefore parallel to 



the t)-axis in this particular case ; while ^ =0, because the 

 1 ox 



for the maximum mixture at B; for other mixtures it would hold at the 

 hypothetical critical point for the homogeneous mixture. The hypo- 

 thetical vapour-pressure curve for a mixture lies in hetween the two 

 branches of the loop in the p-t diagram, and comes to an end at this 

 hypothetical critical point at some distance from the plaitpoint-curve. 

 The equation consequently does not hold for a mixture at its plaitpoint. 



It seems therefore impossible to identify the expression f-^rr) "with 



the — of the hypothetical vapour- pressure curve at the plaitpoint of 



an arbitrary mixture, although this is done by van der Waals in his 

 interpretation of the equation which gives the difference of the value of 



l^\ and l-2\ (Eon. Ah. Amsterdam, Mei 25, 1895, pp. 13, 30 



Jnni 29, 1895, p. 3, form. (5)). 



* "Continuity" &c, Supplementary Notes; Kon. Ah. Amsterdam, 

 Mbj 25, 1895, p. 27. 



T Kon. Ah. Amsterdam, Mei 25, 1895, pp. 20-30; Juni 29, pp. 1-12. 



