Dr Turner's Analysis of a M\ca from Cornwall. 141 



another in the usual way. The peroxide, after ignition, weigh- 

 ed 7-607 grains, equivalent to 27.06 per cent., and was quite 

 pure. The alumina, after being strongly ignited, amounted 

 to 7.16 grains, or 25.47 per cent. 



After separating the iron and alumina, sub-carbonate of so- 

 da was added to the solution at a boiling temperature, and a 

 precipitate subsided, from which I obtained 0.26 grains, or 0.93 

 per cent, of lime, and 0.54 grains, or 1.92 per cent, of the red 

 oxide of manganese. 



The Mica is thus composed of — 



Silica, - 36.54 



Peroxide of iron, - 27-06 



Alumina, - 25-47 



Fluoric acid, - 2.706 



Lime, - - - .93 



Red oxide of manganese, - ■* 1-92 

 Potash, (calculated from 4.8 gr. sulphate of potash,) 5-475 



100.101 



I have examined this mica for titanium in the way recom- 

 mended by Vauquelin, without obtaining satisfactory evidence 

 of its existence ; nor did the fiuate of lime, when decomposed 

 by sulphuric acid, yield a trace of phosphoric acid. 



I have been greatly assisted in the execution of this analy- 

 sis by my able and zealous pupil Mr William Gregory, and, 

 as I still enjoy his valuable assistance, 1 expect to give the 

 analysis of several species of lithion-mica in the ensuing num- 

 ber of this Journal. That they will form several distinct 

 species, when farther examined, seems highly probable from 

 the difference in their specific gravity, which, in the brown 

 variety from Cornwall, analysed above, is far beyond the li- 

 mits of any of the other varieties of mica. The regular 

 forms of it have not yet been ascertained. Mr Haidinger 

 has found that those of a Siberian variety of lithion-mica 

 belong to the hemi-prismatic system, being oblique-rhom- 

 bic prisms, with the terminal plane inclined to the obtuse 

 edge of the prism, at an angle of about 99°. The plane 

 angle of the terminal face is nearly =119° 30 / , from which, 

 and the inclination of the axis, the transverse section of 



