Chap. io.] Piwjler of Paris, 57 



feous fpar is of a platey texture. Gypfeous fpar varies 

 in compactnefs and tranfparency, is fometimes white, 

 fometimes reddifh, and is mixed with a greater or 

 lefs quantity of other matter: fometimes we meet wi-.h 

 it in cryftals. The ores of metals are often found in 

 this fpar. 



Sixthly,, gypfum is often met with in waters of 

 fprings in a difiblved ftate ; for we find by experiment, 

 that it is foluble in water, though but in fmall quan- 

 tity. When the water is evaporated, it is depofited 

 in cryftals very final!, and which appear like a white 

 powder ; but by the microfcope are found to be ob- 

 long regular concretions. 



MargrafF firft mewed that all thefe fpecies were 

 compofed of calcareous earth and vitriolic acid. He 

 took a quantity of gypfeous earth in a fubtile' powder, 

 and boiled it for fome time in a folution of fixed vege- 

 table alkali j and upon examination, he found the cal- 

 careous earth at the bottom in an uncombined ftate, 

 and in the folution a vitriolated tartar. He alfo com- 

 pofed an artificial gypfum of calcareous earth and the 

 vitriolic acid, which had all the properties of a natural 

 gypfum. From knowing the compofition of thefc 

 fubftances, we may fufficiently underftand their pro- 

 pertiesj particularly that of being converted by a mo- 

 derate heat into plaifter of Paris. If the purer kinds 

 are reduced to fine powder, and put into an iron veffel, 

 by the time the veflel is heated the powder grows 

 light, and is thrown into motion like a boiling fluid. 

 There are alfo a vaiiety of other earthy powders, which 

 when moift give the fame appearance; for the vapour 

 fifing up, makes its way through the powder, keeps 

 it afloat, and agitates it like a liquid in a boiling ftate. 

 This appearance continues till moil of the water is 

 evaporated. After this the powder becomes heavy, 



and 



