( 524 ) 



The conductivity of the pure fï-acid has heen determined previously 

 In Loven (Ph. Ch. 19, 458) who found : 



100 h — 0,0078. 

 which agrees well with the value found by myself. 



The influence of the position of the sulphur atom in regard to 

 the earboxyl group is very marked. 



From these observations it appears that the acids in aequeous 

 solutions exert but very little influence on their mutual conductivity 

 power, as the conductivity power of the mixtures agrees fairly well 

 with the calculated result. A condensation of their molecules in such 

 a solution cannot therefore be supposed t<> take place. 



Physics. — "A remark on tin theory of the ^-surface r'<>r binary 

 mixtures." By Prof. .1. I). van der Waals. 



(Communicated in the meeting of December _".», L906). 



Kamèrlingh Onnes' startling experiment, in which a gas was 

 obtained that sinks in a liquid, lias drawn the attention more closely 



to the direction of the tangent in the plaitpoint of a binary mixture. 

 Leaving the further particulars required for the realisation of such a 

 mixture to the investigations of Kamerlingh ( >NNES and his collaborators, 

 I will make a remark of general significance, in close connection 

 with this experiment. 



In my Theorie moleculaire and more fully in (out. II I have 

 examined the condition, on which the tangent in the plaitpoint runs 



dv 



parallel to the y-axis, or in Other words I — J— oo. The problems 



related to this may be reduced to 3. All three refer to the inter- 



A//A fdp\ 



section of the two curves — - 1 = and — = 0. 



\dvJ xT \dxJvT 



As first problem I should like to regard the principal one. namely 



S, tl 



s if' d> 



that where - — = and —-- = 0. The point considered lies, there 



fore, on the spinodal curve, and at the same time the curve —j == 



Of 



d 3 tf> 

 has for constant value of x two equal values for v and so also — — =0. 



ov j 



Then the point considered is the critical point of the mixture 



taken as homogeneous. The value of T is that of 7\ for such 



a mixture and the value of x is then found, when the approximate 



equation of state with b constant is applied, from : 



