PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 489 
have the use of sodium tungstate in accelerating the reciprocal 
transformations of the forms of silica, and the effect of water on 
mixtures of calcite and aragonite—and still more, of water contain- 
ing carbon dioxide, in which the solubility is greater—in converting 
aragonite into calcite at 100° or lower, whereas in the dry state the 
rate of conversion is not appreciable until temperatures around 400°. 
The action of many catalytic agents is known to depend upon the 
formation of unstable intermediate products, but for a very large 
number the mechanism of their action is still unknown. 
It is essential, then, that we bear in mind, on the one hand, the 
distinction between the theoretical state of equilibrium and, on the 
other hand, that actually attained under a given set of conditions 
together with its dependence upon the rate of reaction under those 
conditions. Indeed, we may say that in a very large number of 
cases the reaction velocity is the decisive factor in determining the 
final state, though of course the direction in which the reaction 
proceeds is determined by the energy relations, under the specific 
conditions of the experiment, of the various substances involved. 
Enantiotropic transformations —Enantiotropic transformations 
are those in which equilibrium can be attained from either side, and 
mark the transition of one perfectly stable form into another. The 
rate of transformation varies enormously from one substance to 
another, or for a single substance even from one transformation to 
another. As an illustration take the substance silica:’ quartz, 
when heated to 575° goes over quite sharply into another form 
(6-quartz), but both its transformation to tridymite at or near the 
inversion point (870°), and the change of the latter to cristobalite 
at 1,470, proceed very slowly. There is as yet no means of prog- 
nosticating the rate of transformation; except that one may hazard 
the general statement that, for any single substance, those inver- 
sions which are accompanied by the slightest changes of crystal 
form tend to proceed most rapidly, while the rate of transformation 
is small wherever the change of crystal form is large. 
The temperature at which an enantiotropic transformation takes 
place is always perfectly definite, although it may appear to lack 
definiteness when equilibrium is established only slowly; it is 
tC. N. Fenner, Jour. Wash. Acad. Sci., II (1912), 471. 
