486 JOHN JOHNSTON AND PAUL NIGGLI 
morphism were the rule rather than the exception, at least with 
inorganic substances, especially with those of geologic interest. 
Let us consider what happens when we heat very slowly a substance 
capable of existing in more than one polymorphic form; and for 
the sake of simplicity, let us assume that the substance exists only 
in two forms, which we may designate as a and 8. This assump- 
tion of only two forms really implies no limitation of generality, 
for even with a substance which may be obtained in more than two 
forms we are dealing with only two of them in the neighborhood 
of any single transformation temperature. Now as we slowly heat 
the a form, we find a slow but continuous change in its physical 
properties:* e.g., in density, refringence, and (in general) axial 
ratios; this gradual change goes on, until finally we observe a much 
more marked change of properties—which may take place either 
at a definite temperature or within a small range? of temperature— 
denoting that the transformation into the 6 form has been effected. 
Further heating now is accompanied anew by a gradual change of 
properties, a change characteristic of the 6 form and bearing no 
necessary relation to the corresponding changes shown by the a 
form. 
If now we cool again through the transformation temperature, 
the a form may or may not appear. Failure to reappear may be 
due to one of two causes: (1) that under the particular conditions 
the rate of transformation of B into a is extremely slow, or, in other 
words, the attainment of equilibrium requires a long time; (2) that 
the equilibrium between and a is such that £ is the stable form 
throughout the temperature range in question; consequently a can 
then not appear no matter how much time is allowed. 
Transformations belonging to the first class—that is, where 
equilibrium can be attained from either side—are known as enan- 
tiotro pic; changes which are irreversible are called monotropic. We 
shall discuss these separately; but before doing so, we wish to 
direct attention to a point which, though important, is often lost 
sight of. 
A much more profound, though continuous, change is of course possible, e.g.,, 
on heating zeolites and other hydrous minerals (see F. Rinne, Fortschritte Min., 1913. 
(3), 159-83). 
2 At least, if the rate of heating has been small. 
