PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 603 
geological interest—to the regelation of ice and the consolidation 
of beds of gypsum. 
In a paper" first published in 1894, E. Riecke discussed the effect 
of a one-sided pressure (a stress) on the melting-point of ice, and 
quite recently he has restated his conclusions in a second paper.? 
Riecke’s formula for the lowering of melting-point of ice effected 
by the stress Z (tensile or compressive) is 
AT,=—aZ? 
where (for ice) a has the value 0.00036 when Z is expressed in kg. 
per sq. cm. He appears to consider this formula, which was 
developed for the case of a single crystal only, valid only for small 
values of Z; so that its range of applicability is somewhat limited. 
It has, under the name of the Riecke principle, been adduced 
occasionally by geologists* to aid in correlating observations con- 
nected with metamorphic processes and results. 
The basic principle underlying Riecke’s reasoning is identical 
with that used in the derivation of the equation used in the present 
paper, which, as mentioned above, was derived first by Poynting; 
but the latter, which is derived from general thermodynamic 
principles without specific assumptions (other than those common 
to all thermodynamic reasoning), is much more generally applicable, 
holding as it does for solid aggregates, and for any range of pressures, 
subject always of course to the condition that the pressure acts on 
the solid phase, but not, or to a less extent, on the liquid phase 
produced by melting or by solution. 
Derivation of the formula: The derivation of the formula by 
means of which the effect of unequal pressure on a substance is 
computed is based on the thermodynamical fact that unequal 
pressure acting on any phase increases the “activity” of that phase, 
or its tendency to pass over into another phase; in other terms, 
unequal pressure acting only on the solid phase increases its vapor 
pressure and its solubility (in any particular solvent) and lowers 
t Nachr. Ges. Wiss. Gottingen, 1894, 278; Ann. Physik, LIV (1895), 731. 
2 Centralblatt Min. Geol., 1912, p. 97- 
3 1 atmosphere= 1.033 kg. per sq. cm. 
4For instance by Grubenmann, Die kristallinen Schiefer, 2 Aufl. (1910), and by 
Becke, Denkschriften Wien Akad., 7 Mai, 1903. 
