19 
fortune to decompose. They consist of pure air, 
united to highly inflammable metallic substances ; 
but there is no reason to suppose, that they are 
reduced into their elements in any of the processes 
of vegetation. 
In this part of the Course, I shall dwell at consi- 
derable length on the important subject of Lime, 
and I shall be able to offer some novel views. 
Slacked lime was used by the Romans for ma- 
nuring the soil in which fruit-trees grew. This 
we are informed by Pliny. Marie had been em- 
ployed by the Britons and the Gauls from the 
earliest times, as a top-dressing for land. But 
the precise period in which burnt lime first came 
into general use in the cultivation of land, is, 
I believe, unknown. The origin of the application 
from the early practices, is sufficiently obvious ; a 
substance, which had been used with success in 
gardening, must have been soon tried in farming ; 
and in countries, where marie was not to be found, 
calcined limestone would be naturally employed 
as a substitute. 
The elder writers on agriculture had no correct 
notions of the nature of lime, limestone, and marie, 
or of their effects ; and this was the necessary 
consequence of the imperfection of the chemistry 
of the age. Calcareous matter was considered by 
the alchemists as a peculiar earth, which, in the 
fire, became combined with inflammable acid ; and 
Evelyn and Hartlib, and, still later, Lisle, in their 
works on husbandry, have characterised it merely 
as a hot manure of use in cold lands. It is to 
Dr. Black, of Edinburgh, that our first distinct 
c 2 
