166 
powers of the soil are not only connected with its 
temperature, but likewise with its relations to 
moisture. 
The moisture in the soil influences its temper- 
ature ; and the manner in which it is distributed 
through, or combined with, the earthy materials, 
is of great importance in relation to the nutriment 
of the plant. If water is too strongly attracted 
by the earths, it will not be absorbed by the roots 
of the plants ; if it is in too great quantity, or too 
loosely united to them, it tends to injure or destroy 
the fibrous parts of the roots. 
There are two states in which water seems to 
exist in the earths, and in animal and vegetable 
substances : in the first state it is united by che- 
mical, in the other by cohesive, attraction. 
If pure solution of* ammonia or potassa be poured 
into a solution of alum, alumina falls down com- 
bined with water ; and the powder dried by ex- 
posure to air will afford more than half its weight 
of water by distillation ; in this instance the water 
is united by chemical attraction. The moisture 
which wood, or muscular fibre, or gum, that have 
been heated to 212 *, afford by distillation at a red 
heat, is likewise water, the elements of which were 
united in the substance by chemical combination. 
When pipe-clay dried at the temperature of the 
atmosphere is brought in contact with water, the 
fluid is rapidly absorbed : this is owing to cohesive 
attraction. Soils in general, vegetable, and animal 
substances, that have been dried at a heat below 
that of boiling water, increase in weight by expo- 
sure to air, owing to their absorbing water existing 
