( 24.1 ) 



and minimum discussed above) tiie two />, 7' curves touch. This point 



of contact yields of coui-se an element for (he three-pliase-pressure. 



Tiie ditferential equation for Ihe section of tiie two />, 7', ,/; surfaces, 



is found from the two relations which hold both at the same time : 



and 



^1 ^^/' = (''•2— '''i) I ^7^ J '^'"'i + ^ '^^ 

 f ^% \ "'m 



We find then : 



d'^C A dT 



dp y^'f^i'/pT T 



(.''6— t-i)'<'21 — (.*'"2— .t'l)'<'.Si "M"'2 1-f'2l"'M {'^'S — 'V^)V^^-{^C^—.C.^)VS^ 



We shall shortly mention some obvious cojisequences. (1) Tf 

 = 0, the I), .1' and the J\ ./' figure show a mininnnn oi' a 



maximum. »So they exist for a plaitpoint. (2). Fur a maximum or 



• • i' "'21 ^ , "'m 



nnnunum ot ,r, — must be — . 



''21 ''"21 



Now : 



and : 



"Vi =/'f'>'i + s, — f 1 — (.*', — .''i) 



0^ 



(See Cont. II p. 110). From this we derive : 



^. f. /of, \ fs— f, /Ö5, ^ f,— f, e.—e. 



'^■9— '^'i K^'vJpT A-s— 'i'l vö'^'iy/'7' •'■2— ■''■1 '^'.s— A'l 



'^■2— '^'i y^'hJpT .'■«— .'"i V^'''i//'V' ''"2— '^'i .<'s— .'"i 



TJ • 1 If /'"'/'■iA M'M^ "'/'i.;.' . , „ r 



Ihis leads to -— = — — = —— ; or m words, the du-ec- 

 \<U J J \dl J J- dl 



tion of the {[).,T)x curve for licpiid and \apoiir, and that of the 

 {l),1)x curve for solid and fluid state are the same in the point of 

 maximum and minimum value of ./' and the same as that of the 

 p/r curve for the three-phase-pressure. The p,T curve of the three- 

 phase-pressure descending- with the teuiperature in the case of miiiiumui 

 .t' and vice versa, we conclude concerning the point of contact that 

 in the first case it lies between critical point <tf contact and maximum 

 pressure of the liipud \aponr curve, in the second case on the vapour 

 branch of the curve. 



