ae On Plaister of Paris. 
eee EES 
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, 
thirty 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 itis a compound of oils, salts and acids. 
If these gentlemen are right, we may conclude, that 
the wonderful eflects of the plaister are occasioned by 
the great quantity of acid it contains, and. that clover, 
above all other plants, requires the most acid in its food, 
* See hereafter Ingenhausz’s theory of the supposed effects 
of oil of vitrial on vegetation. 
See also the new theory of the carbonic acid being chiefly 
the food-of plants. 
