On Plaister of Paris, 103 



It should seem, that it is for this reason, that we find plaister 

 to operate favourably on limed land. The sulphuric acid in 

 the gyps, finds the carbonic or fixed air in the lime, which 

 it disengages ; and puts in a state to act, with increased vi- 

 gour, on the grass. 



Although the chemists do not allow that gyps^ like lime- 

 stone, contains ^xed air^ yet it may have the faculty of 

 communicating to the plant, by operating on other sub- 

 stances, the carbonic acidy or whatever be its food. In 

 Chaptal, page 186, it appears that 100 parts of gypsum 

 contain 30 of sidphuric acid, 32 oi pure earthy and 38 water. 

 It loses 20 per cent by calcinatio7i. In other experiments, 

 a greater quantity of sulphuric acid is found, according 

 to the plaister assayed. This analysis excludes ^xed airy 

 from this substance. It could not reside with the sulphu- 

 ric (vitriolic) acid, but would be expelled in a state of g-as. 



If Ingenhausz's ideas, of the almost magical powers of the 

 oil of vitriol (sulphuric acid) on vegetation, be just, in any 

 important degree, the sulphuric acid 7}iay be considered^ 

 either in itself or as it sets other agents at worky the 



MAIN SPRING or OPERATION IN PLAISTER* It is COmmOulv 



used, by chemists, to separate the carbonicy and all other 

 acids, from their combinations, wherever they are found. The 

 earth, according to the theories before stated, is constantly 

 filled with the carbonic acidy by furnishing carbone to the 

 air it inhales. It is found in calcareous substances, with 

 which, in great varieties, the earth abounds : it exists in, or 

 is produced by, the roots of decaying or decayed vegetables, 

 trees, and all animal or vegetable manures. I therefore think 

 it a corollary fairly to be drawn from this theory, and the 

 actual analysis of the gypSy that it is this sulphuric or 



VITRIOLIC ACID WHICH CONSTITUTES ITS OPERATIVE PRIN- 

 CIPLE; and that, though it may in itself (nor is lime) be no ma- 

 nure, yet, when scattered on the earth, it decomposes all sub- 



