e of magnesia, and oxide of iron.

These mineral ingredients of the blood--without the presence of
which in the food the formation of blood is impossible--both man and
animals derive either immediately, or mediately through other
animals, from vegetable substances used as food; they had been
constituents of vegetables, they had been parts of the soil upon
which the vegetable substances were developed.

If we compare the amount of the phosphates in different vegetable
substances with each other, we discover a great variety, whilst
there is scarcely any ashes of plants altogether devoid of them, and
those parts of plants which experience has taught us are the most
nutritious, contain the largest proportion. To these belong all
seeds and grain, especially the varieties of bread-corn, peas,
beans, and lentils.

It is a most curious fact that if we incinerate grain or its flour,
peas, beans, and lentils, we obtain ashes, which are distinguished
from the ashes of all other parts of vegetables by the absence of
alkaline carbonates. The ashes of these seeds when recently
prepared, do not effervesce with acids; their soluble ingredients
consist solely of alkaline phosphates, the insoluble parts of
phosphate of lime, phosphate of magnesia, and oxide of iron:
consequently, of the very same salts which are contained in blood,
and which are absolutely indispensable to its formation. We are thus
brought to the further indisputable conclusion that no seed suitable
to become food for man and animals can be formed in any plant
without the presence and co-operation of the phosphates. A field in
which phosphate of lime, or the alkaline phosphates, form no part of
the soil, is totally incapable of producing grain, peas, or beans.

An enormous quantity of these substances indispensable to the
nourishment of plants, is annually withdrawn from the soil and
carried into great towns, in the shape of flour, cattle, et cetera.
It is certain that this incessant removal of the phosphates must
tend to exhaust the land and diminish its capability of producing
grain. The fields of Great Britain are in a state of progressive
exhaustion from this cause, as is proved by the rapid extension of
the cultivation of turnips and mangel wurzel--plants which contain
the least amount of the phosphates, and therefore require the
smallest quantity for their development. These roots contain 80 to
92 per cent. of water. Their great bulk makes the amount of produ