ALLOTROPY. 33
ALLOTROPY.^1
Many solid substances appear, under different conditions, in two or more
distinct forms (known as allotropic modifications), which are distinguished
from one another by color, density, crystalline form, solubility, and other
physical properties. The best known instances of this are shown by carbon
and phosphorus.
In the same way that different states of aggregation are separated by a
temperature boundary which is dependent on the pressure, so also for the
mutual transformation of numerous allotropic forms, there is a definite tem-
perature, dependent only on the pressure, below which the one, above which
the other form is stable. This is known as the transition temperature.
In addition to this sort of allotropy, which, because the two forms can
change simultaneously into one another and can exist in equilibrium at the
transition temperature, is known as enantiotropy, there is also a second sort
known as monotropy. Two modifications of a substance are monotropic
when the one changes into the other, but the latter cannot change back
directly into the first. They possess no transition temperature; one form is
under all conditions less stable than the other, and therefore is only to be
observed in virtue of the extreme slowness with which the transformation
takes place. i
An example of enantiotropy is shown by sulphur, which above 96° is
monoclinic, below 96° rhombic. An example of monotropy is furnished by
iodine chloride, IC1, the labile form of which melts at 14° and the stable
form at 27°. (Cf. No. 55.)
The energy difference between allotropic modifications corresponds entirely
with that existing between the solid and the liquid states and, like the latter,
is measured by the heat of transformation.
For determining the transition point of allotropic forms, the following
methods are chiefly used:
(1) The THERMIC METHOD. The point on the heating or cooling curve is
determined at which, in consequence of the absorption or setting free of the
heat of transition, a retardation in the otherwise steady rise or fall of tem-
perature occurs; compare the determination of the melting-point of tin,
No. 6, alao Nos. 15 and 16.
(2) The DILATOMETRIC METHOD, which depends on a comparison of the
densities (or of the volumes) on both sides of the transition point.
(3) The OPTICAL METHODS, which frequently permit a very sharp observa-
tion of a change in crystalline form, or in color, at the transition point.
(No. 17.)
(4) ELECTRICAL METHODS, (a) The electrical conductivity of a substance
is plotted graphically against the temperature and the point of inflection of
the curve determined.(^1) Concerning allotropy, see also B. Roozeboom, Heterogene Gleichge-
wichte, Vol. 1, p. 109 (1901).