1815 
takes place. The liquid is, therefore, formed by the fusion of a part 
of each of the solid substances. We will call this a congruent or 
mutual fusion of #4 FL”. 
2. If the point #”" is situated between f and JZ the reaction 
FF L oceurs. Hence, the liquid is formed because a part of 
fF" melts with separation of #. The formation of liquid is thus 
accompanied with a conversion of /” into #. We will eall that an 
incongruent or transformation fusion. 
3. If the point # lies between #” and Z the reaction FZ’ + L 
occurs. This case is quite analogous to the previous one. 
If we change the temperatur2 we must, of course, also change 
the pressure in order to keep together the three phases /’, 7’, and L. 
The liquid Z then also changes its composition. In a P, 7-diagram 
we thus obtain a curve like d’’S in fig. 1, which we will call the 
melting point line of #4 HY, 
If between the three phases occurs the reaction mentioned sub 1 
we call d’’S a congruent or mutual melting point Ene of “+ F’; 
if the reaction sub 2 or sub 3 occurs we call d'S an incongruent 
melting point line or the transformation melting point line of /’-++ /”’. 
We now allow the system 4 + /’ + L to traverse the melting 
point line d’S in such a direction that the pressure diminishes; at 
a definite pressure Py and its correlated temperature 7'p an infinitely 
small quantity of vapour will form so that the four-phase equili- 
brium / + #” + L + G again appears. The complex therefore 
passes from the melting point line on to the four-phase line DS. 
The melting point line therefore terminates in the point S and, as 
we shall see presently, comes into contact with the four-phase line 
in this point. We will call S the-minimum melting point, or the 
melting point of the complex /’-+ /” under its own vapour pressure. 
The vapour G forming in the point S will as a rule, not be repre- 
sented by a point of the line #4”, but the liquid Z wili, of course, 
still be represented by such a point. 
The sublimation line a"D and the melting point line Sd" of the 
complex # + #” are therefore connected with each other by the 
part DS of the four-phase curve. The fact that the points D and S 
will not, as a rule coincide may be perceived in the following manner. - 
In the maximum sublimation point the points /, #’, and G, in the 
minimum melting point /’, #”, and Z are situated on a straight line. 
Hence, both points will coincide only then when incidentally the 
four phases of the system “+ JF’ + 1+ G lie on a straight line. 
The course of the sublimation curve, of the four-phase curve and 
of the melting point line is, as we will see presently, determined 
