/ 
266 G. H. Williams—Pyroxene and Hornblende. 
state of perfectly stable equilibrium. Were they free to move, 
they must at once rearrange themselves to suit the altered con- 
ditions, but if they have passed into the solid state, this 
tendency to rearrangement may not be sufficient to overcome 
the force of cohesion. Sometimes indeed, as in the case of 
ance with existent circumstances. More frequently, however, 
the molecular tendency can only manifest itself in an optical 
disturbance. illard” has recently show 
temperatures. It is not probable, however, that this tendency 
alone would be sufficient to effect a complete change of crystal: _ 
line structure. If, on the other hand, the assistance of some 
external agency could be introduced, which would render the 
a rea 
ment naturally takes place to suit the altered conditions. 
out rupture, just as heat does. Of course this pressure 
produces heat but acting as slowly as it does in the e evation 
*3 Bul. Soc. Min. de France, v, p. 144, 1882. 
ra Neues Jahrbuch fiir Min., etc., 1884, i, p. 185 ref., ib., p. 237. 
Ib., 1884, i. 
°6 Nachrichten der kin. Ges. d. Wiss. Géttingen, May, 1884. 
