170 On Gypsum. 



I remember, objected at first to the jw/y&/zwnc caof being 

 the agent ; because gypsum was insokible, and the acid 

 remained in combination. And of "this opinion are 

 some chymists, with whom I have lately conversed. 

 But the Doctor finally told me, that by some process 

 in nature (which I do not correctly recollect) it M^as set 

 free ; and was at liberty to perform its ofiice. Nature 

 provides means to effect her designs, superior to our 

 artificial substitutes. It is incredible, that while the 

 gypsum is performing astonishing operations, it should 

 remain an inert, insoluble compound. 



Much weight has been given by some, to the opinion 

 that the calcareous matter of the plaister, is the princi- 

 pal cause of its utility. But this is a subject operated 

 upon and not an actor: — a place of deposit for the 

 acids. If it were otherwise, the small portion of it in 

 the plaister, applied to a large surface, would not prac- 

 tically justify the conclusion. I know by experience, 

 that it requires, of calcareous substances alone, a very 

 great quantity indeed, to produce important effects on 

 vegetation. But the oil of vitriol, without calcareous 

 matter, will operate powerfully. The pyrites have no 

 calcareous matter, being compounded of metals and 

 sulphur. And yet they operate on plants, by the vi- 

 triolic acid contained in them. For the burning the 

 pyrites by the Brabmiters^ I cannot account. I have 

 been informed, that " it is converted to a sulphat by 

 moisture, and exposure to the atmosphere ; and then 

 becomes soluble by lime." But the combustion of 

 plaister is not beneficial. I have often failed in the ap- 

 plication of calcined plaister ; and yet the chymists say, 

 that it is not the sulphuric add, but the water of chrys- 



