1. Dr. Brewster’s Memoirs. 365 
The Brazilian Topazes differ from the blue and colourlese 
Topazes of Aberdeensh. and New-Holland in their opti- 
cal properties. The resultant axis of the latter is about 
65°, that of the former varies from 50° 5’ to 43°. The tints 
of the former deviate more than those of the latter 
from the colours of Newton’s scale and are produce 
Topazes are generally phosphorescent on a _ heated 
iron. The tesselated crystals exhibit it in a peculiar 
manner. The light of the nucleus is less intense than 
that of the border, sometimes it is entirely wanting. 
Sometimes the greatest intensity is at the boundary of the 
nucleus and the outer tessele. Many of the crystals con- 
tain a white powder, resembling in its analysis, the zeolites. 
Others contain a bright red substance in thin plates be- 
tween the laminz and in long stripes parallel to the axis of 
the prism. Its surface has a high metallic lustre and its 
structure appears crystallized. Berzelius has analysed the 
Brazilian Topaz and has found its composition exactly the 
“same as in the other Topazes, except a little iron as col- 
ouring matter. Dr. B. suspects an error in this analysis 
from the great difference in optical properties, and he 
quotes the authority of Mr. Gregor, who found it to contain 
a portion of lime and potash. The white powder in some 
of these T'opazes was found by Berzelius to consist of si- 
lex, alumine, lime and water. Dr. B. thinks it may be 
the topaz not crystallized. 
4. Account of the native Hydrate of Magnesia, dicovered 
by Dr. Hibbert of Shetland.—It was found (1817) in Unst, 
traversing serpentine in all directions, | $ 
Carb. Lime, forming veins from ; in., to 8 in. broad. 
Structure lamellar, colour white slightly tinged.with green 
on the edges, transparent, opaque by exposure from disin- 
tegration of the lamine, scratches talc; several specimens 
crystallized in a reg. hexahed. prism. Spec. Gray. 2.336. 
Composition, Magn. 69.75. water 30.25. It has one pos~ 
itive axis of double refraction perpendicular to the lam 
ine. 
