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and silica more absorbent than carbonates of lime 
and magnesia : these differences may, however, 
possibly depend upon the differences in their state 
of division, and upon the surface exposed. 
The power of soils to absorb water from air is 
much connected with fertility. When this power 
is great, the plant is supplied with moisture in 
dry seasons ; and the effect of evaporation in the 
day is counteracted by the absorption of aqueous 
vapour from the atmosphere, by the interior parts 
of the soil during the day, and by both the exterior 
and interior during night. 
The stiff clays approaching to pipe clays in their 
nature, which take up the greatest quantity of 
water when it is poured upon them in a fluid form, 
are not the soils which absorb most moisture from 
the atmosphere in dry weather. They cake, and 
present only a small surface to the air ; and the 
vegetation on them is generally burnt up almost as 
readily as on sands. 
The soils that are most efficient in supplying the 
plant with water by atmospheric absorption, are 
those in which there is a due mixture of sand, 
finely divided clay, and carbonate of lime, with 
some animal or vegetable matter ; and which are 
so loose and light as to be freely permeable to the 
atmosphere. With respect to this quality, carbo- 
nate of lime and animal and vegetable matter are 
of great use in soils : they give absorbent power 
to the soil without giving it likewise tenacity : 
sand, which also destroys tenacity, on the contrary, 
gives little absorbent power. 
I have compared the absorbent powers of many 
U 
