nt.

The circumstances under which absorption takes place modify, in a manner
which cannot well be explained, the amount absorbed by the same soil. It
is found generally to be most complete with very dilute solutions, and
if a soil be agitated with a quantity of ammonia larger than it can
take up, it will absorb only a certain amount of that substance, but by
a further increase of the amount of ammonia a still larger quantity will
be absorbed.

It is important to observe that when a salt is used, the base only is
absorbed, and the acid escapes in combination with lime; even nitric
acid, notwithstanding its importance as a food of plants, being in this
predicament. From this it may be gathered that lime is not readily
absorbed from solutions of its salts; indeed, it would appear that the
only salt of that substance liable to absorption is the bicarbonate,
from which it is taken to the extent of 1.4 per cent by the soil. The
absorption of lime from this salt, and that of phosphoric acid, which
takes place to a considerable extent, probably occurs, however, quite
independently of the clay present in the soil, and is occasioned by its
_lime_, which forms an insoluble compound with phosphoric acid, and by
removing half the carbonic acid of the bicarbonate of lime converts it
also into an insoluble state.

In addition to these mineral substances, organic matters are also
removed from solution. This is conspicuously seen in the case of putrid
urine, which not only loses its ammonia, but also its smell and colour,
when allowed to percolate through soil; and an equally marked result was
obtained with flax water, from which the organic matter was entirely
abstracted.

The cause of this absorptive power is still very imperfectly known. Mr.
Way having observed that sand has no such property, while clay, even
when obtained from a considerable depth, always possesses it, supposed
that the absorption was entirely due to that substance. A difficulty,
however, presents itself in explaining how it should happen that while a
pure clay absorbs only 0.2847 of ammonia, a loamy soil, of which
one-half probably is sand, should absorb a larger quantity. The
inference is, that the effect cannot be due to the clay as a whole, and
Mr. Way has sought to explain it by supposing that there exist in the
soil particular double silicates of alumina and lime. He has shown that
felspar and the other minerals from which the soil is produced have no
ab