250 Proceedings of the Royal Physical Society. 
and not in others, as when a piece of glass is rendered 
doubly-refracting by bending, the inner strata on the con- 
cave side of the bend being compressed, and those on the 
outside dilated. 
The optical properties of doubly-refracting crystals are 
also changed by pressure, as was also shown by Sir David 
Brewster, and also by the action of heat. Now crystals, ex- 
cept those of the cubic system, are, in general, dilated dif- 
ferently in different directions by the action of heat. Thus, 
in a rhombohedron of calcite increase of temperature alters 
the angles between the faces, making them approach more 
nearly to a cube ; and, at the same time, the extraordinary 
refractive index is increased. A similar property has been 
recently observed by Fizeau in quartz, wdio finds that the 
extraordinary refractive index is dimiDished by rise of tem- 
perature, the rhombohedron in this case departing further 
from the form of a cube. In gypsum, to take the case of a 
biaxal crystal, Mitscherlich found that change of tempera- 
ture alters in a remarkable manner the angle between the 
optic axes. More recently, Des Cloizeaux has show^n that 
this is the case in a number of minerals, such as brookite, 
chrysoberyl, and certain species of felspar. In general, when 
the heat is removed, and the crystal regains its original 
temperature, the angle between the optic axes becomes the 
same as before, the molecules having then regained their 
original positions. Des Cloizeaux also found that, in certain 
minerals, the angle between the optic axes became perma- 
nently changed by exposing them to a high temperature. 
In these cases we may suppose a permanent change to have 
taken place in the position of the molecules, as when a solid 
is distorted beyond the limits of elasticity, or the limits 
within which Hooke's law holds, in which case a permanent 
change in the position of the molecules takes place. 
In the above and similar cases, therefore, a change in the 
position of the material molecules produced by heat, causes a 
change in optical properties. 
It is well known that we may render a cylinder of glass 
transiently doubly refracting by immersing it in heated oil, 
a circular cylinder giving the same rings as a uniaxal, and 
