( 288 ) 



dp 



dtV^ 

 or a minimum value for - — and the variation of this quantity can 



da 



dk 

 reverse its sign with rising temperature. Reversely v^hen — ; cannot 



dT 



be equal to 0, reversal of sign cannot take place in the variation 



of this quantity. 



/\(i>3 — v^ dlpc dk 



From k = — 777:7;; 7- follows as condition of -— ■ = 0. 



MKl dx dT 



d 

 dlpc 



MRT 





da dT ' MRT dTdx 



T,—T dip,: dlpy, f dTy_ dHpc f dTy, 



Now Ipc = Ipy. — / — — — , = 7 , 7, = • 



T dx dx T dx dxdT T^ dx 



dlpx f dTx 

 If we put —— = —-——, in which T^ can have all possible values 

 dx I J dx 



dk 

 irom '■ — 00 to -j- 00 , then — = may also be written : 



L MRT J_ 2- _ 1 

 L MRT J T^ T T, 



The tirst member of this equation is always positive, at lower 

 temperatures nearly equal to 0, and at the critical temperature in- 

 finitely great. So the second member must also be positive. Or, if 

 this equation is to be satisfied T^ <^ T, but positive. In all the cases, 



dk 

 therefore, in which 7\ is negative, -—; cannot become equal to 0, 



di 



and no reversal of sign takes therefore place in the course of 



dp 



dx, . 



— W'lth the temperature. 



dpc 



dx 



So the reversal of sign only occurs, w'hen in the equation 



dlpy. ƒ dT,i 



dx 7'j dx 



Ti lies between and Ty. The two extreme values give for Ty = 



