1821.] Scientific Intelligence. 235 



Fig. 10 differs from fig. 9 only by the addition of the little planes n : 

 won T 131° 30'. 



Fig. 11, P on / 129° 11'. The plane P is at right angles to the axis 

 of the prism. 



Fig. 12. In this, the planes n of fig. 10 are seen in combination. 

 P and / of fig. 11. w on P 138° 30'. 



The crystals seldom occur perfect; they vary ';: ^•ze from very 

 imall to the length of 1 and 2 inches, 3-lOths to 4-lOths of an inch in 

 treadth. The colour is sometimes greenish. By long exposure to 

 air and light, the coloured crystals become perfectly white. Their 

 surface is dull. The lustre greasy. Fracture, dull-earthy. Trans- 

 lucent only when in thin lamina). The crystals are more or less cohe- 

 rent : some readily crumble, and are then unctuous to the touch ; 

 others are harder, yielding to the knife but not to the nail, and feel 

 then harsh in the fracture. The same crystal is sometimes hard in one 

 end, and not at the other. It seems to harden in the air. By fracture 

 three-sided prisms are very easily obtained. Its specific gravity is 

 2-555 to 2-594-. 



The powder put upon a red-hot spoon phosphoresces with a bright 

 bluish light. 



When heated with the blow-pipe at a little below a red heat, it 

 becomes blackish, and by continued exposure to this heat, it is ren- 

 dered white, swells, and melts at the edges into a white enamel. 



It melts with borax readily into a clear glass, which, by adding a 

 little nitre shows traces of manganese. A little of the phosphate of 

 Boda, or ammonia added to a button of borax, saturated with the 

 mineral, renders it after cooling an opaque and white enamel. A little 

 piece of the mineral heated with glass of phosphorus effervesces 

 slightly at first, but they eventually combine. 



With soda it melts to a clear glass; with a yellowish-green tinge, 

 the colour is most easily distinguished upon white paper. 



The analysis, which was conducted in the usual mode, viz. by heat- 

 ing the fine powdered mineral with three times its weight carbonate of 

 potash, dissolving it in muriatic acid, gave 



Silica 56-62 



Magnesia 23-38 



Alumine 338 



Lime 5*58 



Protoxide of manganese 99 



Peroxide of iron 009 



Water 3 58 



Bituminous matter and loss 638 



To attempt a formula for this mineral before it is determined whe- 

 ther the bituminous matter belongs to the chemical composition or not 

 would certainly be premature. But it may be observed that as the 

 oxygen of the alumine and lime is equal, and that of the silica, 1 

 times, of the magnesia six times, of the water twice, the quantity of 

 oxygen in the alumine, the formula is likely to be A S* -f C S* -f-- 



