from which 



Further 



( 188 ) 



r rf ' 



y - 7 V _ ± P— Po 



a y 



In this last equation the sign -\- must be chosen, if, as is the 

 case, T and p have at the same time either maximum or minimum 

 value. We find then : 



dp__ J_ 

 dT~ a 



and 



dv 

 dx 



-*1/t-*|/* 



dv (dv \ 

 That this value of — = — follows from the derivation. 



dx \dxJ Pt T 



dp 

 So we find a definite value for — , and as no lower value of T 



dT 



exists for minimum value of T, and no higher value for maximum 



value, the ^ ) 7 1 -projection of the plaitpoint curve must possess "cusps". 



dp 

 That this value of — is positive, and so p and T are at the same 

 dT l l 



time maximum or minimum, follows inter alia from the equation : 



dp = fè) dv + (f) dx + (Q) dT. 

 \dvJxT \dxJ T \dljvx 



(±) 



dv dv \dxJuT 



For, as — = = , this equation reduces to 



dx dXf,T fdp^ 



JxT 

 T \dT) vx 



Other special points of the plaitpoint curve. 



It appears from the form for — (Versl. Kon. Ak. Deel IV p. 20) 



dp 

 that also the case that — = is possible, and tor some mixtures a 

 dT ' 



maximum value of p in the //.^'-projection has been experimentally 



