Siliceous Genus* 10; 



Mr. Bergman has formed perfect cryftals 

 by diflblving filiceous Earth in the fparry 

 acid, and fuffering it to cryftalize flowly. 

 It is probable that nature forms them in a 

 long courfe of time from a folution or diffu- 

 fion of this Earth in pure water, or water 

 that holds a little argill or calcareous Earth 

 which probably enable it to take up more of 

 the filiceous Earth than it otherwife could. 

 Perhaps the experiment which once fucceeded 

 with Mr. Achard) was owing to fome fuch 

 caufe. 



SERIES II. 

 Opake Quartz, or Pebbles. 



Thofe are alfo cryftajized, or amorphous, 

 either white, grey, or yellowifh, or tinged 

 of other colours by metallic particles. The 

 former are lefs pure than thofe of the firft 

 feries, containing a larger proportion of argill, 

 or at leaft the argill is lefs perfectly united 

 with the filiceous part. Their fpecific gra- 

 vity is from 2,4 to 2,7 ; their furface either 

 rough, or fmooth and fhining, the laft are 

 called fat quartz : the texture either lamellar 

 or granular j they crack like the former in fire, 

 and become of a duller colour ; when rubbed 

 againft each other they emit a phofphoric 

 fmell. They are often found in round mafles 

 in the beds of rivers, 



The 



