( 287 ) 



than the component, when it causes the quantity pc to decrease, then 



dp dp 



the general rule holds, that both -— and -— are positive when the 



dA\ a A' J 



admixture is more volatile than the pure substance and reversely. 



In general these three directions approach each other at higher 



temperature and at the critical temperature they coincide. An exception 



to the rule that with rising temperature the lines approach, must be 



allowed for the case that for certain value of T the quantity 



dpc 



^— = 0. In this case the three directions mentioned coincide at that 



da; 



value of T; as they must again coincide at 2^= Ty, and as the 



quantity -^ varies with T, they will first diverge up to a certain 



maximum amount, and finally approach each other again. 



The rule about the approaching of the lines might also be 



represented in the following manner, which would render my meaning 



dp 

 more precisely. If we write ^ under the following form, which 



follows directly from the above: 



dp Pc{Vi-Vi) dlpc 



-r— MRT~ 17 , 



dx, e — 1 



dpc Pc{v, — v^) dlpc 



Ix MRT ~dx 



dp 



PAV. — V.) dlpc , 1 ^^1 TT 



or putting ^\)'' -j-=^ ^^^ T^=^ 

 MRT dx dpc 



U 



dx 



from which follows 



k 

 dU 1 dk 



dT k dT k 



The factor ( e^ — J is always positive for k positive, and always 



negative for h negative, and is only equal to for h = 0; and h 



equal to occurs only at the critical temperature and when — 1= 0. 



dU ^ , dk . 



For all other values of k can — - only be equal to 0, when —- is 



a I (iJ- 



equal to 0. When this quantity is equal to 0, there is a maximum 



