5 S Plaifter of Paris. [Book VI. 



and lies at the bottom of the veffel as before heating. It 

 is then prepared for plaifter of Paris, for if a quantity 

 of it is put into fo much water as gives it the confift- 

 ence of cream, it will foon become folid, and ring like 

 avefiel of earthen-ware, or metal. It lofes its tranf- 

 parency by heat, and becomes white. The fofter kinds 

 are beft for the plaifter of Paris. 



The explanation of all thefe phenomena is not dif- 

 ficult, when we confider the nature of gypfum, which 

 being a faline compound, has all the qualities of a fait. 

 In its natural ftate it is cryftallized in confequence of its 

 containing a quantity of water. Heat expels the water ; 

 which being again added, is attracted by the gypfum, 

 and occafions a cryftallization. 



A more violent heat produces very little change, as 

 the vitriolic acid adheres very clofely to the earth. If 

 thefe fubftances, however, are mixed in powder with 

 a quantity of charcoal-duft, the whole of the acid may 

 be diffipated, and the calcareous earth only left behind, 

 and thus a particular phofphorus is formed. In this 

 experiment the charcoal attracts the oxygen of the 

 acid, which is by that lofs rendered volatile, and is dif-r 

 perfed in fumes. 



7. Calcareous earth is alfo found faturated with mu^ 

 riatic acid iii fea-water and in falt-pits. 



8. Calcareous earth, united with the fluoric acid, 

 forms the fufible fpar or fluor, which is commonly 

 known by the name of DERBYSHIRE SPAR. The 

 texture of this compound is either fparry, or irregularly 

 mattered or cracked. It is either tranfparent or 

 opake j and the fpecimcns are of a cubic, rhomboidal, 

 polygonal, or irregular figure. The coloured fpars 

 have the property of emitting light when laid on a 

 hot iron, or otherwife heated ; but they lofe this pro- 

 perty by being made red hot. The green fpars are 



the 



