1. Dr. Brewster^ s Memoirs. 366 



The Brazilian Topazes differ from the blue and colourless 

 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 New^ton's scale and are produced 

 by a polarising force of less intensity. The Brazilian 

 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 tesselae. 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 laminae and in long stripes parallel to the axis of 

 the prism. Its surface has a high metallic lustre aiid 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 Topazes 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, along with Magn. 

 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 laminae, scratches talc ; several specimens 

 crystallized in a reg. hexahed. prism. Spec. Grav. 2.336. 

 Composition, Magn. 69.75. water 30 25= It has one pos- 

 itive axis of double refraction perpendicular to the lam 

 inse. 



