Constituents of various Minerals. 253 



It had previously been considered as a stilbite, and as an apo- 

 phylite. Mr BROOKE, in the paper just referred to, has describ- 

 ed the primary form and modifications of its crystals. 



Its colour is white ; its lustre vitreous. It is transparent, and 

 has a specific gravity of 2.628. It fuses with great facility be- 

 fore the blowpipe, like the zeolites, to which it has obviously an 

 affinity. This mineral was analysed with much care by Mr WIL- 

 LIAM MUIR, but he obtained an excess of about 3^ per cent. I 

 therefore requested Mr RICHARD MITCHELL, who was a much 

 more experienced analyst, being in my laboratory for the second 

 year, to repeat the analysis, with every attention to accuracy. 

 The result of his analysis almost coincided with that of Mr 

 MUIR, and there was the same excess of 3^ per cent. Upon exa- 

 mining the silica, I found that it cohered strongly before the 

 blowpipe. It had, therefore, retained a little soda : for the ana- 

 lysis had been conducted by igniting the powdered Brewsterite 

 with carbonate of soda, and dissolving the fused mass in muria- 

 tic acid. It was obvious from this, that the excess was chiefly 

 owing to the state of the silica. 



The constituents are as follows : 







Silica, .... 58.800 

 Alumina, . . . 18.912 

 Lime, .... 12.384 

 Potash, .... 1.500 

 Water, .... 11.700 



103.296 



If we admit a slight excess in the silica, from the cause above 

 specified, the constituents of the mineral seem to be 



2A/S 2 +(C, K)S 3 +3Aq; 



K k 2 



