POSTSCRIPT 



I 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 

 I am^ on chemical subjects^ with which I profess to 

 have no more than a very slight acquaintance. 



JR. jP. 



" 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 

 converted it into litne ; 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 oi fixed air. In 1772, Bergman proved that it was an 

 acid^"* It has since been distinguished 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- 

 home acid was appropriated to it." Chaptal's Chemistry, 

 yol. 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 t\vt food of plants. When it becomes lime, it is deprived 

 of this air, till it is slacked or ejfete, and then it recovers the 

 fixed air sufficiently to act as a manure. 



