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and the tendency of burning is to expel any other 
volatile principles that they may contain in com- 
bination. If the oxide of iron in soils is not satu- 
rated with oxygene, torrefaction tends to produce 
its further union with this principle ; and hence in 
burning, the colour of clays changes to red. The 
oxide of iron containing its full proportion of oxy- 
gene has less attraction for acids than the other 
oxide, and is consequently less likely to be dissolved 
by any fluid acids in the soil ; and it appears in 
this state to act in the same manner as the earths. 
A very ingenious author, whom I quoted at the 
end of the last Lecture, supposes that the oxide of 
iron when combined with carbonic acid is poison- 
ous to plants ; and that one use of torrefaction is 
to expel the carbonic acid from it ; but the car- 
bonate of iron is not soluble in water, and is a very 
inert substance ; and I have raised a luxuriant 
crop of cresses in a soil composed of one-fifth car- 
bonate of iron, and four-fifths carbonate of lime. 
Carbonate of iron abounds in some of the most 
fertile soils in England, particularly the red hop 
soil. And there is no theoretical ground for sup- 
posing, that carbonic acid, which is an essential 
food of plants, should in any of its combinations be 
poisonous to them ; and it is known that lime and 
magnesia are both noxious to vegetation unless 
combined with this principle. 
All soils that contain too much dead vegetable 
fibre, and which consequently lose from one-third 
to one-half of the'ir weight by incineration, and all 
such as contain their earthy constituents in an 
