( 189 ) 



The difFerential equation of the spinodal curve is 



C^) d. + (^) dp-C^) dT = 0. . . . (1) 

 The ditTerential equation of the plaitpoint curve is 



From (1) follows : 



'dh] 



dTjspin ~ rd^V\ 



dp 

 If we substitute this value of — - in (2), we find 



and 



(3) 



(4) 



, fdT\ 

 From this equation (3) follows that -- can become when 



Kd^-cjpl 



d^v\ fdp\ 



= 0. In this case — ] is not equal to 0. A similar case is 



dx'^JpT \dxy,ji 



found for substances, for which no three phase pressure occurs when 



there exists a minimum critical temperature. It is wellknown that 



in this case the binodal curve splits up, and that there is a point of 



inflection for the isopiest in this point. There is a double plaitpoint 



also then, vkiiich originates or disappears at a certain temperature ; 



but though we can speak of a double plaitpoint, the value of 



/"d'^\ 



— I is not = then. 

 ydx'JpT 



In the case under consideration the value of ( — - ) is equal to 



\dx'JpT 



in the point at which a double plaitpoint appears or disap- 



13* 



