PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 4097 
carbon dioxide; for carbon dioxide increases the solubility of both 
forms, and hence the rate of transformation.* 
The occurrence of a real monotropic change implies that one 
modification of the substance involved is altogether metastable 
throughout its range of existence.2 But we may have inversions 
which appear to be monotropic, but are in reality delayed enan- 
tiotropic changes; in such cases both forms of the substance have 
stable fields of existence, the apparent monotropy being due entirely 
to the sluggishness of transformation. A good example of this is 
shown in the silica diagram, as determined by Fenner.’ Quartz, 
when heated in presence of a suitable flux, goes over slowly into 
tridymite at about 870°,4 and the change is reversible; but when 
the quartz is heated without a flux, it goes over into @-cristobalite, 
the temperature at which this transformation occurs (about 1,.400° 
or higher) varying with the rate of heating. -cristobalite on cool- 
ing, in absence of a flux, does not return to quartz (or go over into 
tridymite), but at a temperature around 200° is transformed into 
a-cristobalite, which therefore at ordinary pressure must be alto- 
gether metastable. Thus transformations which are sluggish may 
appear to be monotropic until by the choice of an appropriate 
solvent medium their rate is so increased that their true character 
can be ascertained. 
The temperature at which a monotropic change takes place can 
be influenced only by those factors which affect the rate of trans- 
formation; it will therefore be affected by the presence of impurities 
when the latter can act as fluxes and in this way increase the rate, 
but it will be unaffected by uniform pressure, because uniform 
pressure apparently has little or no influence on the rate of reaction, 
even where the reaction is attended by a large volume change. In 
«It is to be observed that this argument of an increased solubility being attended 
by an increased rate of reaction must be used with caution; it will in general hold only 
when the original solubility was comparatively small. 
2 At least, at ordinary pressure. At higher pressures such a modification may have 
a stable field of existence; though up to the present there is no direct experimental 
evidence bearing on this point. 
3 Jour. Wash. Acad. Sci., IL (1912), 471. The system silica has also been investi- 
gated by Endell and Rieke, Z. anorg. chem., LXXIX (1912), 239, and by Endell and 
Smits, ibid., LX XX (1913), 176. 
4 This is the lowest temperature at which this transformation will take place. 
It occurs at all temperatures up to 1470°, at which point tridymite ceases to be the 
stable phase and is replaced by (-cristobalite. 
