252 Dr T. THOMSON'S Account of the 



of iron. Mr THOMAS MUIR repeated my analysis last summer 

 and I give his in preference to mine, because I consider it as 

 more carefully made. 



Good crystals were selected for the analysis, the specific gravi- 

 ty of which was found to be 3.7112. A portion of silica was 

 abraded from the agate mortar. This silica was detected in the 

 powder ; but if it be abstracted, then chrysoberyl contains no si- 

 lica whatever. Its constituents were found to be 



Alumina, 76.752 



Glucina, 17.791 



Protoxide of iron, . . 4.494 



Driven off by heat, . . 0.480 



99-517 



The last portion, which is so difficult of decomposition, was 

 found to yield to ignition, with a sufficient quantity of carbonate 

 of soda. A good deal of the success depends upon reducing the 

 mineral to a very fine powder. Neither Mr MUIR nor myself 

 found any oxide of titanium, though we looked for it carefully. 



If we consider all the constituents found in the chrysoberyl to 

 be chemically combined, it will be a compound of 



6 atoms sexaluminate of glucina. 

 1 atom sexaluminate of iron. 



The alumina in this mineral seems to act the part of an acid. 



6. Brewsterite. 



It is known to mineralogists that Mr BROOKE first constituted 

 this mineral (Edin. Phil. Journ.vi. 112) a peculiar species, and 

 named it in honour of Dr BREWSTER, Secretary to this Society. 



