titution of that secretion which is formed to
facilitate their passage. With the solid excrements, the phosphates
of lime and magnesia, which were contained in the food and not
assimilated, are carried off, these salts being insoluble in water,
and therefore not entering the urine.

We may obtain a clear insight into the chemical constitution of the
solid excrements without further investigation, by comparing the
faeces of a dog with his food. We give that animal flesh and
bones--substances rich in azotised matter--and we obtain, as the
last product of its digestion, a perfectly white excrement, solid
while moist, but becoming in dry air a powder. This is the phosphate
of lime of the bones, with scarcely one per cent. of foreign organic
matter.

Thus we see that in the solid and fluid excrements of man and
animals, all the nitrogen--in short, all the constituent ingredients
of the consumed food, soluble and insoluble, are returned; and as
food is primarily derived from the fields, we possess in those
excrements all the ingredients which we have taken from it in the
form of seeds, roots, or herbs.

One part of the crops employed for fattening sheep and cattle is
consumed by man as animal food; another part is taken directly--as
flour, potatoes, green vegetables, &c.; a third portion consists of
vegetable refuse, and straw employed as litter. None of the
materials of the soil need be lost. We can, it is obvious, get back
all its constituent parts which have been withdrawn therefrom, as
fruits, grain and animals, in the fluid and solid excrements of man,
and the bones, blood and skins of the slaughtered animals. It
depends upon ourselves to collect carefully all these scattered
elements, and to restore the disturbed equilibrium of composition in
the soil. We can calculate exactly how much and which of the
component parts of the soil we export in a sheep or an ox, in a
quarter of barley, wheat or potatoes, and we can discover, from the
known composition of the excrements of man and animals, how much we
have to supply to restore what is lost to our fields.

If, however, we could procure from other sources the substances
which give to the exuviae of man and animals their value in
agriculture, we should not need the latter. It is quite indifferent
for our purpose whether we supply the ammonia (the source of
nitrogen) in the form of urine, or in that of a salt derived from
coal-tar; whether we derive the phosphate of lime