unless we
are acquainted with the causes of good health, it will be impossible
for us to form any estimate of those variations from that state,
called diseases: hence it is that a number of diseases, which have
been brought on merely by the undue action of the exciting powers,
such as gout, rheumatism, and the numerous trains of nervous
complaints, which were by no means understood, may be easily and
satisfactorily explained, and as easily cured, by restoring the
proper action of these powers, and bringing the excitability to its
proper state. As this theory, therefore, is so important, not only in
respect to the preservation of health, which nearly concerns every
individual, but to the cure of diseases, which is the province of the
physician, I have endeavoured to explain it as fully and minutely as
possible; to make it still plainer we may perhaps make use of the
following illustration.

Suppose a fire to be made in a grate or furnace, filled with a kind
of fuel not very combustible, and which could only be kept burning by
means of a machine, containing several tubes placed before it, and
constantly pouring streams of air into it. Suppose also a pipe to be
fixed in the back of the chimney, through which a constant supply of
fresh fuel is gradually let down into the grate, to repair the waste
occasioned by the combustion kept up by the air machine.

The grate will represent the human body; the fuel in it the life or
excitability, and the tube behind, supplying fresh fuel, will denote
the power of all living systems, constantly to regenerate or produce
excitability; the air machine, consisting of several tubes, may
denote the various stimuli applied to the excitability of the body;
the flame produced in consequence of that application, represents
life; the product of the exciting powers acting upon the
excitability.

Here we see, that flame, like life, is drawn forth from fuel by the
constant application of streams of air, poured into it from the
different tubes of the machine. When the quantity of air poured in
through these different tubes is sufficient to consume the fuel as it
is supplied, a constant and regular flame will be produced: but if we
suppose that some of them are stopped, or that they do not supply a
sufficient quantity of air, then the fuel will accumulate, and the
flame will be languid and smothered, but liable to break out with
violence, when the usual quantity of air is supplied.

On the contra