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LECTURE IV. 
ON SOILS : THEIR CONSTITUENT PARTS. ON THE ANA- 
LYSIS OF SOILS. OF THE USES OF THE SOIL. — OF THE 
ROCKS AND STRATA FOUND BENEATH SOILS. OF THE 
IMPROVEMENT OF SOIL. 
No subjects are of more importance to the farmer 
than the nature and improvement of soils ; and no 
parts of the doctrines of agriculture are more ca- 
pable of being illustrated by chemical enquiries. 
Soils are extremely diversified in appearance and 
quality ; yet, as it was stated in the Introductory 
Lecture, they consist of different proportions of 
the same elements ; which are in various states of 
chemical combination, or mechanical mixture. 
The substances which constitute soils have been 
already mentioned. They are certain compounds 
of the earths, silica, lime, alumina, magnesia, and 
of the oxides of iron and manganesuin ; animal and 
vegetable matters in a decomposing state, and 
saline, acid or alkaline combinations. 
In all chemical experiments on the composition 
of soils connected with agriculture, the constituent 
parts obtained are compounds; and they act as 
compounds in nature : it is in this state, therefore, 
that I shall describe their characteristic properties. 
1. Silica , or the earth of Jlints , in its pure and 
crystallized form, is the substance known by the 
name of rock crystal, or Cornish diamond. As 
