, which is called
saliva. This liquor acts an important part in the production of
taste; it does not differ much from water, excepting by containing a
quantity of mucilage; and nothing is sapid, or capable of affecting
the sense of taste, unless it is in some degree soluble in this
liquor. Hence earthy substances, which are nearly insoluble, have
little or no taste.

It is not, however, sufficient that the substance be possessed of
solubility alone; it is necessary likewise that it should be
possessed of saline properties, or, at least, of a kind of acrimony,
which renders it capable of stimulating the nervous papillae. Hence
it is that those substances which are less saline, and less acrid
than the saliva, have no taste.

We are capable of distinguishing various kinds of taste, but some of
them with less accuracy than others. Among the different kinds of
taste, the following have been considered by Haller, and some other
physiologists, as primitive: sweet, sour, bitter, and saline. The
others have been thought to be compounded of these; for the sense of
taste, as well as sight and hearing, is capable of perceiving
compound impressions. To these primitive tastes, Boerhaave added
alkaline, spirituous, aromatic, and some others. Of these, in
different proportions, all the varieties of tastes, which are
extremely numerous, are composed.

Some tastes are pleasant and agreeable, others disagreeable, and
scarcely tolerable: there is, however, a great diversity in this
respect experienced by different persons; for the same taste, which
is highly grateful to some, is extremely unpleasant to others.

But the most pleasant tastes, agreeably to the general laws of
sensation, which I described in the last lecture, become gradually
less pleasant, and at last disgusting; while, on the contrary, the
most disagreeable savours, such as tobacco, opium, and assafoetida,
become, by custom, not only tolerable, but highly agreeable.

Nature designed this difference of tastes that we might know and
distinguish such foods as are salutary; for we may in general
observe, that no kind of food which is healthy, and affords proper
nutriment to the body, is disagreeable to the taste; nor are any that
are ill tasted proper for our nourishment. Those substances,
therefore, which possess strong or disagreeable savours, and which,
in general, possess a power of producing great changes on our
constitution, are to be ranked as medicines, and only t