56 On Plaisfer of Paris. 



workmen and the French burr mill-stone manufac- 

 turers prefer the American, as having a more binding 

 quality ; whether that makes it a better manure I can- 

 not say. 



You remark that " it is said in an English work, that 

 plaister is composed of a mineral acid and a calcareous 

 earth, and that it is good or bad according to the pre- 

 valence or deficiency of the latter." I think it is of the 

 former and not of the latter, because it would require a 

 much greater quantity of the latter (perhaps twenty 

 five or thirty cart loads) to bring about the wonderful 

 effects of three or four bushels of plaister. 



You will find by Dr. Bergman, who has analyzed 

 this fossil, that it contains twenty two parts water, 

 tiiirty three parts calcareous earth, and forty five parts 

 vitriolic acid.^ And you will also find in a small work 

 of Dr. Home of Edinburgh, upon the principles of 

 vegetation a variety of accurate experiments conti- 

 nued for the space of four years, in order if possible, to 

 discover the food of plants, the result of which was, 

 that it is a compound of oils, salts and acids. 



If these gentlemen are right, we may conclude, that 

 the wonderful effects of the plaister are occasioned by 

 the great quantity of acid it contains y and that clover, 

 above all other plants, requires the most acid in its food, 



* See hereafter Ingenhausz's theory of the supposed effects 

 ©f oil of vitriol on vegetation. 



See also the new theory of the carbonic acid being chiefly 

 the food of plants. 



