112 Elements of Mineralogy. 



With regard to ferruginous ftones in which 

 the calx of iron is not much dephlogifti^ 

 Cated, this decompofition is eafily under- 

 flood, for this calx gradually becomes more 

 dephlogifticated by the aftion of water and 

 air, attracts water and fixed air, and lofes its 

 adherence with the filiceous or other ftony 

 particles : this is feen to happen to bafaltes, 

 toadftone, ferruginous limeftone, &c. In other 

 ftones this decompofition may arife from their 

 containing calcareous Earth in a cauftic ftate, 

 or manganefe, for thefe will gradually attract 

 water and fixed air, and then fwell, burft and 

 loofen the whole texture of the ftone, as we fee 

 happen to bricks that contain lime. Thus alfo 

 glafs is decompofed by long expofure to the 

 air, the alkali attracting water and aerial acid. 

 Mortar, on the contrary hardens by long ex- 

 pofure to the air, becaufe, though the aerial 

 acid be attracted, yet a great part of the wa- 

 ter exhales. 



SPECIES V. 



The finer Flints mixed with various Propor* 

 tions of other Earths and Iron. 



Precious Stones of the fecund Order. 



I. VARIETY, 



Agates. 



The oriental agate is almoft tranfparent, 

 and of a glafly appearance, but whitiih, with 



inward 



