Constituents of various Minerals. 249 



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3. Corundum. 





The specimen chosen for the analysis was a beautiful white 

 semitransparent crystal from Madras, constituting a six-sided 

 prism of considerable size. It was evidently very pure, and had 

 a distinctly foliated structure. The faces of cleavage were so 

 brilliant, that the specimen approached pretty closely to a sap- 

 phire. The specific gravity was 3.9511. This mineral was 

 analysed with great care by Mr THOMAS Mum. During the 

 trituration in the agate mortar, there was a quantity of silica 

 abraded from the mortar, and mixed with the pounded corun- 

 dum. The constituents were found to be, 



Alumina, . . . 98.46 

 Silica, .... 1.54 



100.00 



But the quantity of silica abraded from the mortar was 1.56. 

 Hence it is clear that the corundum is composed of alumina 

 alone, without any silica whatever. The whole of the alumina, 

 to make sure of its purity, was converted into alum. 



4. Hyacinth from Expailly. 



For the analysis of this mineral, very pure crystals were pick- 

 ed out. I requested Mr THOMAS Mum, who made the analysis, 

 to heat the crystals to redness, and select those that had become 

 colourless. When pounded in the agate mortar, no loss of 

 weight was sustained by the mortar, showing that the hyacinth 



