P26 
ZFZ, therefore comes into contact with the three-phase line KF of 
fig. 4 and is situated further above and to the left of this three- 
phase line. 
In order to deduce something more from the P,7-curves, we take 
a temperature 7; lower than the minimum melting point of 
the compound /. The saturation line of F under its own vapour 
pressure has at this temperature 7’; a form as in fig. 7 (I) or 11 (Es 
the minimum vapour pressure in the point in of this saturation line under 
its own pressure we call P,,. the maximum pressure Pr. Of all the 
equilibria of # + liquid + gas appearing at the temperature 
Tz, the highest vapour pressure is, therefore, Py, and the lowest 
P,. If, in fig. 4, we represent both pressures by the points M 
and m, one P.7-curve passes through the point Jf and one through 
the point m, whereas ail the others must intersect the perpendicular 
line placed in 5 between Jf and m. One obtains the P, 7-curve passing at 
Tr through the point J/ when the moving line ZFZ, of fig. 1 
coincides with the line / Jf, and the one passing through the point 
m when the line ZFZ, coincides with the line /, of fiere teer 
11 (1). In fig. 4 two P,7-curves must pass through each point 
between M and mm. For if we choose a pressure P between Py; and 
Pn we notice from fig. 7 (D and 11 (I) that at the temperature 
Tr two different systems: solid /” + liquid + gas have a vapour 
pressure P, from which it follows at once, that in fig. 4 two P,7- 
curves must pass through each point between J/ and m. 
If on the curve Mamh of fig. 7 (I) or 11 (I) we imagine two 
points of equal pressure connected by a straight line, we notice 
that there must be a definite-pressure 7, at which this conjugation 
line passes through the point /”. If now, the straight line ZFZ of fig. 1 
passes through this conjugation line, the corresponding P, 7-curve at 
the temperature 7’, and the pressure P, must exhibit a double point. This 
curve is represented in fig. 4 by Z6/4Z,. All the other P, 7-curves 
as a rule intersect the line Afm in two points of which one is 
situated above and the other below the point 5. 
If the temperature 7’; is changed, then in fig. 7 (I) or 11 (1) the 
saturation line under its own vapour pressure changes its position 
and form, while Pj. P, and /% also change. The points M, 
m and hin fig. 4 then proceed along a curve; the curve through 
which the points M and m go, is represented by MM, M,M,Fm,Km; 
we will call this curve the boundary curve of the system: solid 
F + liquid + gas. 
The equilibrium between solid /’, liquid, and gas is determined by 
(6) Il and 7 (ID. To the point JZ and m also applies the relations 
