d as representing the quantity of water retained by
these different soils when thoroughly saturated by long continued rains.
The column immediately succeeding gives the quantity of that water which
escapes by evaporation from the same soil after exposure for four hours
to dry air at the temperature of 66 deg.. The fifth, sixth, seventh, and
eighth columns indicate the quantity of moisture absorbed, when the
soil, previously artificially dried, is exposed to moist air for
different periods. These characters are dependent principally, though
not entirely, on the porosity of the soil. The last may also be in some
measure due to the presence of particular salts, such as common salt,
which has a great affinity for moisture, but is chiefly occasioned by
their peculiar structure. It is to be remarked that clay and humus are
two of the most highly hygrometric substances known, and it is
peculiarly interesting to observe, that by a beneficent provision of
nature, they also form a principal part of all fertile soils. The
quantity of water imbibed by the soil is important to its fertility, in
so far as it prevents it becoming rapidly dry after having been
moistened by the rains. It is valuable also in another point of view,
because if the soil be incapable of absorbing much water, it becomes
saturated by a moderate fall of rain, and when a larger quantity falls,
the excess of necessity percolates through the soil, and carries off
with it a certain quantity of the soluble salts. Important as this
property is, however, it must not be possessed in too high a degree, but
must permit the _evaporation_ of the water retained with a certain
degree of rapidity. Soils which do not admit of this taking place are
the cause of much inconvenience and injury in practice. By becoming
thoroughly saturated with moisture during winter, they remain for a long
time in a wet and unworkable condition, in consequence of which they
cannot be prepared and sown until late in the season, and though
chemically unexceptionable, they are always disadvantageous, and in some
seasons greatly disappoint the hopes of the farmer.

The extent to which the imbibition and evaporation of water takes place
is very variable, but they are obviously related to one another, the
soils which absorb it least abundantly parting with it again with the
greatest, facility; for it appears that siliceous sand absorbs only
one-fourth of its weight of water, and again gives off in the co