A,t,J,aat RoclCnfial 

 it ertervefced, and depar'tcd Hlex wlien treated with acids. The cruft was viGbly compofed 

 of two kinds of cryftals. When boiled with water, a great part of thefe were difTolved and 

 afforded, after tlie evaporation, carboiint? of pot-afli. Tiie remaining cryftals were tetra 

 hedral pyramids in groups. Acids had no aclion neon them. They nvcrc perfeBh tranf 

 parent, and fa hard as to ^Ive fire ivlthjlecl. T]-.ey diflblved i:: four parts of deliquefce'nt pot- 

 aft, and afforded a mafs of foluble glafs, from which filex could be pr£C^nitated. Tbe fluoric 

 acid attacked them, and diffolved a confiderable part. They were therefore perf-a rock 

 cryftals, produced by art. Part of the cruft. which was lefs tranfpnrent, fell down durin. the 

 boding. It was filiceous earth, fo hard and firm that it aded as a grinding powder on glafs. 

 Though I am convinced that this obfervation affords us no light with regard to the pro- 

 cefs of nature in the produftion of rock cryftal, it has not given me lefs fatisfadion, from the 

 proof that It IS poffible at leaft for us to imitate this natural produQion, though perhaps by 

 different means. ° * f o uy 



I explain the origin of rock cryftal in this glafs nearly in the following manner- The 

 liquor of flints was very weak, and fuper-faturated with alkali. This difengaged alkali in- 

 fenfibly attraaed the carbonic acidfrom the atmofphere, and formed in this manner a cruft 

 upon the hquor, which was at reft, and was not therefore fo much expofed to the adion of 

 the carbonic acid The alkali could not attraft this laft principle but with difiiculty. and 

 confequently the filex was feparated very (lowly in the form of a fine powder. Now as the 

 moft perfea repofe obtained in this operation, the fine particles of the filex were not difturbed 

 m their attradion, which they exerted without impediment, and formed cryftals. As foon 

 as a double cruft was formed of carbonate of pot-afh and rock cryftal, the carbonic acid could 

 no longerpenetrate to the Hquid ; whence followed a cryftallization of part of the difengaged 



An admirer of hypothefes might apply this obfervation to the formation of natural rock 

 cryftal. It would be fufficlent for this purpofe to fuppofe that a foluble mafs of filex and 

 foda was formed by any fuoterraneous revolution; that, by the prefence of moifture, this mafs 

 v,as converted into the liquor of flints, from which afterwards, by a flow attradion of car- 

 bonic acid, the rock cryftal was depofited. 



But it will reafonably be alked, What has become of the foda in this operation .' Perhaps 

 It may have been neutrahfed in the courfe of time by other acids, or elfe (for it is eafy to 



ment of Oft^urg and fome other authors would prefent themfelves to be quoted. FSr my 

 part, I prefer an avowal of my ignorance, and am difpofed to fay with a certain poet, thalj 

 no mortal can penetrate the depths of nature, and few even the fuiface." I am never- 

 thelefs defirou, that ,1ns obfervation may engage other chemifts to repeat the experimenn 

 and examine more particularly into the fads. Circumftances, perhaps, in part to me unknown 

 may liave a marked influence on the fuccefs of the experiment. 



^ ^ 2 VI. Geo/c'sica/ 



