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It should seem, that it is for this reason, that we find plaister 
to operate iavourably on limed land. The su/phuric acid in F 
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 ile 
stone, contains fixed air, yet it may have the faculty of 
communicating to the plant, by operating on other sub- 
stances, the carbonic acid, or whatever be its food. In 
Chaptal, page 186, it appears that 100 parts of gypsum 
contain 30 of sulphuric acid, 32 of pure earth, and 38 water. 
It loses 20 per cent by calcination. In other experiments, 
a greater quantity of sulphuric acid is found, according - 
to the plaister assayed. This analysis excludes fixed air, 
from this substance. It could not reside with the sulphu- 
ric (vitriolic) acid, but would be expelled in a state of gas. 
Ii 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 may be considered, 
either in itself, or as it sets other agents at work, THE 
MAIN SPRING OF OPERATION IN PLAISTER. It is commonly 
used, by chemists, to separate the carbonic, 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 acid, 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 gyps, 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- 
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