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LECTURE VIII. 
ON THE IMPROVEMENT OE LANDS BY BURNING; CHEMICAL 
PRINCIPLES OF THIS OPERATION. ON IRRIGATION AND 
ITS EFFECTS. — ON FALLOWING ; ITS DISADVANTAGES AND 
USES. ON THE CONVERTIBLE HUSBANDRY FOUNDED ON 
REGULAR ROTATIONS OF DIFFERENT CROPS. ON PAS- 
TURE ; VIEWS CONNECTED WITH ITS APPLICATION. ON 
VARIOUS AGRICULTURAL OBJECTS CONNECTED WITH CHE- 
MISTRY. CONCLUSION. 
The improvement of sterile lands by burning was 
known to the Romans. It is mentioned by Virgil 
in the first book of the Georgies : “ Saepe etiam 
steriles incendere profuit agros.” It is a practice 
still much in use in many parts of these islands ; 
the theory of its operation has occasioned much 
discussion, both among scientific men and farmers. 
It rests entirely upon chemical doctrines ; and 
I trust I shall be able to offer you satisfactory 
elucidations on the subject. 
The bases of all common soils, as I stated in the 
Fourth Lecture, are mixtures of the primitive 
earths and oxide of iron ; and these earths have a 
certain degree of attraction for each other. To 
regard this attraction in its proper point of view, 
it is only necessary to consider the composition of 
any common siliceous stone. Feldspar, for in- 
stance, contains siliceous, aluminous, calcareous 
earths, fixed alkali, and oxide of iron, which exist 
in one compound, in consequence of their chemical 
attractions for each other. Let this stone be 
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