tt A SYSTEM OF 



SECT. XIII. 



B. Satiated or united with the Acid of Vitriol, 

 Terra calcarea acido vitrioli faturata. Gyp- 

 fum, Plafter-ftone or Parget, 



This is 



I. Loofer and more friable than a pure cal- 

 careous earth. 



2 ? Either crude or burnt, it does not excite 

 any effervefcence with acids, or at mofl it 

 effervefces but in a very flight degree, and 

 then only in proportion as it wants fome 

 of the vitriolic acid to compleat the fatu- 

 ration. 



3. It readily falls into a powder in the fire. 



4. If burnt, without being red-hot, its pow- 

 der readily concretes with water into a 

 mafs, which foon hardens ; and then 



5. No heat is perceived in the operation. 



6. It is nearly as difficult to be melted by 

 itfelf as the limeftone *, and fhews moilly 

 the fame effects, with other bodies, as 

 the limeftone : the acid of vitriol feems, 

 however, to promote its vitrification. 



7. When melted in the fire with borax, it 

 puffs and bubbles very much, and for a 

 long while, during the fufion, owing to 

 the nature of both the falts -f*. 



* I have found mofl: of the gypfeous kind, and particularly 

 the fibrous, to melt pretty eaiily by thcmfelves in the fire. 



t When a fmall quantity of any gypfum is melted to- 

 gether with borax, the glafs becomes colourlefs and tranfpa- 

 rent ; but I have found fome forts of a'abafter and fparry gyp- 

 fa that, when melted in fome quantity with borax, yield a 

 fin* yellow tranfparent colour, refembling that of the beft 

 topafes. This phcenometiOn might probably happen with 

 every one of the gypfeous hind. But it is to be obferved, 

 |ijat if too much of fuch gypfum is> ufed in proportion to the 



borax, 



