duty.

I do not know that we ever apply to a plant any element which is not
a natural constituent of the vegetable structure, except perhaps
externally, for the accidental purpose of killing parasites. The whole
art of cultivation consists in learning the proper food and conditions
of plants, and supplying them. We give them water, earths, salts of
various kinds such as they are made of, with a chance to help themselves
to air and light. The farmer would be laughed at who undertook to
manure his fields or his trees with a salt of lead or of arsenic. These
elements are not constituents of healthy plants. The gardener uses the
waste of the arsenic furnaces to kill the weeds in his walks.

If the law of the animal cell, and of the animal organism, which is
built up of such cells, is like that of the vegetable, we might expect
that we should treat all morbid conditions of any of the vital unities
belonging to an animal in the same way, by increasing, diminishing, or
changing its natural food or stimuli.

That is an aliment which nourishes; whatever we find in the organism, as
a constant and integral element, either forming part of its structure,
or one of the conditions of vital processes, that and that only deserves
the name of aliment. I see no reason, therefore, why iron, phosphate of
lime, sulphur, should not be considered food for man, as much as guano
or poudrette for vegetables. Whether one or another of them is best in
any given case,--whether they shall be taken alone or in combination, in
large or small quantities, are separate questions. But they are elements
belonging to the body, and even in moderate excess will produce little
disturbance. There is no presumption against any of this class of
substances, any more than against water or salt, provided they are used
in fitting combinations, proportions, and forms.

But when it comes to substances alien to the healthy system, which never
belong to it as normal constituents, the case is very different. There
is a presumption against putting lead or arsenic into the human body, as
against putting them into plants, because they do not belong there,
any more than pounded glass, which, it is said, used to be given as a
poison. The same thing is true of mercury and silver. What becomes of
these alien substances after they get into the system we cannot always
tell. But in the case of silver, from the accident of its changing color
under the influence of light, we do kn