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 know what happens. It is thrown out, in
part at least, under the epidermis, and there it remains to the patient's
dying day. This is a striking illustration of the difficulty which the
system finds in dealing with non-assimilable elements, and justifies in
some measure the vulgar prejudice against mineral poisons.

I trust the youngest student on these benches will not commit the
childish error of confounding a presumption against a particular class of
agents with a condemnation of them. Mercury, for instance, is alien to
the system, and eminently disturbing in its influence. Yet its efficacy
in certain forms of specific disease is acknowledged by all but the most
sceptical theorists. Even the esprit moqueur of Ricord, the Voltaire of
pelvic literature, submits t