492 JOHN JOHNSTON AND PAUL NIGGLI 
simple phases and not mix-crystals (solid solutions). This same 
reservation holds for melting-temperatures, and is there well 
known. When a mix-crystal of the a-form undergoes enantiotropic 
change into a mix-crystal of the 6-form, there is in general a tem- 
perature-interval in which the a and 6 forms may coexist in real 
equilibrium. As the temperature changes within this region, there 
is a continuous change in the composition of the mix-crystals, 
which are in equilibrium with one another, precisely as in the melt- 
ing of mix-crystals or the distillation of binary liquid mixtures, and 
for the same reason. The position of this temperature interval is 
dependent upon the composition of the possible solid solutions, and 
may be displaced considerably if the corresponding inversion tem- 
peratures of the end-members of the series of mix-crystals are far 
apart. 
As an illustration consider the system Hgl.-HgBr. and its 
inversion tetragonal—orthorhombic.? At 127° red tetragonal Hel, 
changes into yellow orthorhombic HgI.; HgBr. is known only as 
the white orthorhombic form, so that the inversion point of the 
tetragonal form, if it exist at all, must be at a low temperature. 
From melts containing both components there separate out a 
continuous series of orthorhombic yellowish-white mix-crystals 
(n HgI.:m HgBr.); but the mix-crystals containing little HgBr, 
invert at low temperatures into a tetragonal modification. The 
temperature at which this inversion begins is lower the richer the 
mix-crystals are in HgBr,; hence the reason that mixtures rich in 
HgBr, exhibit no inversion point is in all probability that the 
velocity of transformation at the equilibrium temperature—which 
then must be below o°—is inappreciable. When a mix-crystal of 
the composition 9.5 mol. per cent HgBr., 90.5 mol. per cent HgL,, 
is cooled slowly (care being taken that the rate of transformation 
is sufficiently great), it begins about 80° to change to the tetragonal 
form, producing tetragonal crystals approximately of the compo- 
sition 2.6 mol. per cent HgBr., 97.4 mol. per cent Hgl.. Further 
t See papers by N. L. Bowen, Am. Jour. Sci., XX XIII (1912), 561; “‘The Melting 
Phenomena of the Plagioclase Feldspars,” ibid., XX XV (1913), 577- 
2W. Reinders, Z. physik. Chem., XXXII (1900), 494; P. Niggli, Z. anorg. Chem., 
LXXV (1912), 161. 
