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POSTSCRIPT. 
L submit the following conjectural remarks, and their ap- 
_. plication to the agricultural facts to which they refer, 
to the judgment of those who are better informed than 
L am, on chemical subjects, with which I profess to 
oh have no more than a very slight acquaintance. 
tage 
“IN 1755, Dr. Black, of Edinburgh, advanced, that 
limestone contains much air, of a different nature from com- 
-mon air. He affirmed, that the disengagement of this air 
Sih ' , 
converted it ito ime; and, that by the restoration of this 
air, calcareous stone was regenerated.” Dr. M‘Bride, Mr. 
Jacquin, and Dr. Priestly, are quoted, as having, confirmed 
this doctrine by experiment. “* This was then known by the 
name of fixed air. In 1772, Bergman proved that it was an 
acid.” It has since been distingutshed by various names ; 
‘** and, as soon as it was proved to consist of a combination 
of oxygene and carbone, or pure. charcoal, the name of car- 
bonie acid was appropriated to it.” Chaptal’s Chemistry, 
vol. 1, page 212. 
It appears then, that the crude limestone contains the air 
(in a proportion of 40-100) which is said, by Ingenhausz, to 
be the food of plants. When it becomes ime, it is deprived 
of this air, till it is slacked or effete, and then it recovers the 
fixed air sufficiently to act as a manure. 
