On Gypsum. 239 



the English acquire more experience in the use of 

 plaster, we shall, I fear, derive little benefit from the 

 speculations of their most scientific writers on this 

 subject ; however instructive they may be, and certain- 

 ly are ; on other agricultural topics. 



The sulphuric ackl^ or oil of vitriol^ is known to pos- 

 sess the faculty of attracting, and being diluted by, 

 water ^ in an uncommon degree : and the solvent pow- 

 er of water is well established. What other parts of 

 the gypsum has, so peculiarly, this property ? If the 

 sulphuric acid does not produce the effect on vegeta- 

 tion, as stated in my conjecture ; there is no other 

 part likely thus to operate. 



All substances containing this acid, operate alike on 

 vegetation ; and that^ as I have reason to believe, ac- 

 cording to the proportions of the acid included in 

 them : and, although these substances be totally vari- 

 ant in their natures. — To instance only a few ; — 



1. The Plaster of Paris, we use, consists of 

 calcareous earth; mixed, irregularly, with other earths: 

 and the sulphuric acid^ combined with them, is in, the 

 greatest proportion of any other part of tl>e mass. 



2. The Pyrites are a genus of inflamable substan- 

 ces, composed chiefly of sulphur ; which has dissolv- 

 ed, or saturated itself with, metals; oftenest with 

 IRON. It is the principal ore oi sulphur. The use of 

 this, when calcined, or burnt, as a manure^ occasioned 

 Dr. Priestly to mention to me, what I relate in Vol. 

 I. page 158. And see appendix to Vol. I. page 32. 



3. Barytes, is an alkaline earth. It posses- 

 ses strong and peculiar attraction for the sulphuric acid. 

 It will always detect its presence, in any substance ; 



