n the soil, without which no nitrogen can be
assimilated even when there is a most abundant supply. The ammonia
contained in animal excrements exercises a favourable effect,
inasmuch as it is accompanied by the other substances necessary to
accomplish its transition into the elements of the blood. If we
supply ammonia associated with all the conditions necessary to its
assimilation, it ministers to the nourishment of the plants; but if
this artificial supply is not given they can derive all the needed
nitrogen from the atmosphere--a source, every loss from which is
restored by the decomposition of the bodies of dead animals and the
decay of plants. Ammonia certainly favours, and accelerates, the
growth of plants in all soils, wherein all the conditions of its
assimilation are united; but it is altogether without effect, as
respects the production of the elements of blood where any of these
conditions are wanting. We can suppose that asparagin, the active
constituent of asparagus, the mucilaginous root of the marsh-mallow,
the nitrogenised and sulphurous ingredients of mustard-seed, and of
all cruciferous plants, may originate without the aid of the mineral
elements of the soil. But if the principles of those vegetables,
which serve as food, could be generated without the co-operation of
the mineral elements of blood, without potash, soda, phosphate of
soda, phosphate of lime, they would be useless to us and to
herbivorous animals as food; they would not fulfil the purpose for
which the wisdom of the Creator has destined them. In the absence of
alkalies and the phosphates, no blood, no milk, no muscular fibre
can be formed. Without phosphate of lime our horses, sheep and
cattle, would be without bones.

In the urine and in the solid excrements of animals we carry
ammonia, and, consequently, nitrogen, to our cultivated plants, and
this nitrogen is accompanied by all the mineral elements of food
exactly in the same proportions, in which both are contained in the
plants which served as food to the animals, or what is the same, in
those proportions in which both can serve as nourishment to a new
generation of plants, to which both are essential.

The effect of an artificial supply of ammonia, as a source of
nitrogen, is, therefore, precisely analogous to that of humus as a
source of carbonic acid--it is limited to a gain of time; that is,
it accelerates the development of plants. This is of great
importance, and should al