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soil during the fermentation of vegetable matter, 
which will disengage it from the earths, may 
be decomposed : but the earths themselves cannot 
be supposed convertible into other substances, by 
any process taking place in the soil. 
In all cases, the ashes of plants contain some of 
the earths of the soil im which they grow ; but 
these earths, as may be seen from the table of the 
ashes afforded by different plants given in the last 
Lecture, never equal more than 5V of the weight 
of the plant consumed. 
If they be considered as necessary to the vege- 
table, it is as giving hardness and firmness to its 
organization. Thus, it has been mentioned that 
wheat, oats, and many of the hollow grasses, have 
an epidermis principally of siliceous earth , the 
use of which seems to be to strengthen them, and 
defend them from the attacks of insects and para- 
sitical plants. 
Many soils are popularly distinguished as cold ; 
and the distinction, though at first view it may 
appear to be founded on prejudice, is really just. 
Some soils are much more heated by the rays 
of the sun, all other circumstances being equal 
than others ; and soils brought to the same degree 
of heat cool in different times, i. e . some cool much 
faster than others. 
This property has been very little attended to 
in a philosophical point of view ; yet it is of the 
highest importance in agriculture. In general, 
soils that consist principally of a stiff white clay 
are difficultly heated ; and being usually very 
moist, they retain their heat only for a short 
M 2 
