598 General Notes. [June, 
lie parallel to the face R, which plays here the same ré/e as “ Glei- 
tungsflache” as —14R does in calcite. A similar structure is 
frequent in the Graves Mt. rutile parallel oP. In these cases 
also, as with the silicates, the nature of the material prevented the 
artificial production of the lamella. In some minerals where no 
alteration of molecular arrangement could be brought about by 
pressure, this result was accomplished by heat. By a sufficient 
increase of temperature twinning lamella were seen to be devel- 
oped in anhydrite, cryolite’ and leadhillite,t while thenardite was 
rendered optically uniaxial, as Mallard had showed was the case 
ing it as a separate species. 
J. Beckenkamp’ describes a new and delicate apparatus for the 
measurement of the constants of elasticity in crystals. The results 
of numerous determinations are given whereby the conclusion is 
reached that the elasticity, like all the other physical properties of 
crystals, is in exact accord with the geometrical properties. The 
known substance, which accounts for its anomalous double refrac- 
tion, inasmuch as very slight pressure can produce a molecular 
disturbance. The elasticity of any crystal was found steadily to 
decrease with repeated compression. 
R. Brauns? considers that the much discussed optical anomalies 
1 Neues Jahrbuch fiir Min., etc., 1884, 1, 216. 
* Ib., 1883, 11, p. 258. 
* Jahresbuch der Wissensch. Anstalten zu Hamburg, 1883, p. 67. 
* Neues Jahrbuch fiir Min., etc., 1884, 1, 63 
5 Ib., 1884, 11, p. 1. p 
° Zeitschrift für Krystallographie, 1x, 1884, pp. 38-72. 
-7 Iba X, 1885, 41-57. 
, Neues Jahrbuch für Min., etc., 1885, 1, 96-118 
. 
