450 Dr. Clarke's Observations on Gehlenite. [Dec. 



to a few bodies, I have always supposed to be owing to the 

 presence either of an alkali, or of an alkaline .earth, in stones 

 containing silica. Very often an attention to this property alone 

 is sufficient to make- known the presence of potass or of soda in a 

 mineral. If there be exceptions, it is only where zincor /iweispre- 

 sent with silica; but in the instances of needle-stone and datolite, 

 which both exhibit the most perfect aid transparent jelly upon 

 the action of acids, and both contain lime, 1 have detected also the 

 presence of soda. In a case of minerals which I lately received 

 from the Tirol there were some specimens of the substance 

 called Gehlenite, whose properties and constituents have not, 1 

 think, been either adequately or accurately stated in the accounts 

 published of this mineral. In the fifth edition of your Chemistry 

 you have placed it at the head of the feldspar family ; and that 

 this is its proper situation will, perhaps, appear evident from the 

 discovery I have now made of an alkali in the stone; when added 

 to the nature of its other constituents, and also to its crystalliza- 

 tion, which, in the examples I possess, does not exhibit rectan- 

 gular, but oblique-angled parallelopipeds. 



The purest crystals which I have been able to detach from my 

 specimens of Gehlenite were imbedded in a matrix containing 

 carbonate of lime. The same limestone is also imbedded as 

 extraneous matter in the crystals, together with oxide of 

 iron. And to the presence of these foreign bodies, as in the 

 example afforded by Fontainbleau carbonate of time (accord- 

 ing to a well-known theory of the celebrated Bowiion), may 

 be owing the simplicity observable in the form of the crystals 

 themselves; which is nearly that of the primary form of feldspar; 

 but improperly called cubes, in the accounts published of this 

 mineral ; owing, perhaps, to the minuteness of the crystals, 

 which prevented a more accurate observation. Observing that 

 these crystals, when pulverized and exposed to the action of 

 muriatic acid, were converted into a perfectly transparent jelly, 

 I suspected that an alkali might be present, which proved to be 

 true. For the manner of detecting it, I am indebted entirely to 

 the instruction which I received from Dr. Wol/aston, whose 

 valuable experiment, applicable at all times to stones acted on by 

 acids, I will endeavour to describe, previously to giving any 

 further observations of my own. The gelatinous substance, 

 contained in a watch-glass, was exposed to a temperature not 

 exceeding that of boiling water, until it became perfectly dry. In 

 this state, if there be present potass or soda, small cubic crystals 

 may sometimes be discerned with a lens ; and such crystals were 

 visible in the present instance. Distilled water being then added, 

 the silica was separated, and collected upon a filter. The solu^ 

 tion which passed the filter, and which contained the muriates, 

 was then evaporated to dryness ; and the dry mass being exposed 

 to a high temperature, the muriates of alumina and iron were 

 decomposed ; and distilled water being again added, their bases 



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