167 
in the state of vapour in the air, in consequence 
of cohesive attraction. 
The water chemically combined amongst the 
elements of soils, unless in the case of the decom- 
position of animal or vegetable substances, cannot 
be absorbed by the roots of plants ; but that ad- 
hering to the parts of the soil is in constant use in 
vegetation. Indeed, there are few mixtures of 
the earths found in soils, that contain any che- 
mically combined water ; water is expelled from 
the earths by most substances that combine with 
them. Thus, if a combination of lime and water 
be exposed to carbonic acid, the carbonic acid 
takes the place of water ; and compounds of 
alumina and silica, or other compounds of the 
earths, do not chemically unite with water : and 
soils, as it has been stated, are formed either by 
earthy carbonates, or compounds of the pure 
earths and metallic oxides. 
When saline substances exist in soils, they may be 
united to water both chemically and mechanically ; 
but they are always in too small a quantity to in- 
fluence materially the relations of the soil to water. 
The power of the soil to absorb water by cohe- 
sive attraction depends in great measure upon the 
state of division of its parts ; the more divided 
they are, the greater is their absorbent power. The 
different constituent parts of soils likewise appear 
to act, even by cohesive attraction, with different 
degrees of energy. Thus vegetable substances 
seem to be more absorbent than animal substances ; 
animal substances more so than compounds of 
alumina and silica; and compounds of alumina 
m 4 
