602 JOHN JOHNSTON AND PAUL NIGGLI 
instances of the phenomena of gliding, calcite frequently is found 
twinned in the (1012) plane; diallage also frequently exhibits 
lamellae which have resulted from stress. Translational displace- 
ments are those in which the displaced portion remains parallel 
to its original position; this phenomenon is shown—though 
usually only to a small extent—by plastic crystals, such as cyanite, 
augite, mica, graphite, calcite, brucite, when they occur in meta- 
morphic rocks.* . 
On the other hand, many crystals found in crystalline schists 
are the net result of a more or less thoroughgoing destruction of the 
original configuration of the system and of more or less shattering 
of the individual crystals, followed by a healing-up of the cracks 
and spaces thus produced. To account for this we are led to con- 
sider the effect of stress on the processes of fusion and solution. 
Effects of stress on systems solid-liquid—tIt has been shown in 
two previous papers? (to which the reader desirous of a fuller dis- 
cussion of this topic is referred) that the observed effects of non- 
uniform compression can be satisfactorily correlated by ascribing 
them to the action of ‘‘unequal”’ pressure, that is, to a pressure 
which, in a system solid-liquid, acts on the solid phase but not, 
or not to the same extent, on the liquid phase. It can readily be 
shown from thermodynamic reasoning that the effect of unequal 
pressure is always to lower the melting point (or raise the vapor 
pressure, and hence the solubility) of a substance, and by an amount 
which is many times as great as the effect (raising or lowering) 
produced by the same pressure acting on both phases simul- 
taneously. 
The principle that unequal pressure lowers the melting-point 
was formulated first in 1881 by Poynting,? who applied it only to 
calculate the effect on the melting-point of ice. In 1892 the result 
was stated by Le Chatelier,4 who applied it to several problems of 
1 See papers by O. Miigge, e.g., Newes Jahrbuch f. Min., I (1898), 71; II (4899), 553 
also L. Milch, 7bid., I (1909), 60. 
2J. Johnston, Jour. Am. Chem. Soc., XXXIV (1912), 788; Z. anorg. Chem., 
LXXVI (1912), 361; J. Johnston and L. H. Adams, Am. Jour. Sci., XX XV (1913), 
205; Z. anorg. Chem., UXXX (1913), 281. 
3 Phil. Mag. (5), XII (1881), 32. 
4Z. physik Chem., 1X (1892), 335- 
