, produce a
very perceptible smell. Metals which appear nearly inodorous, excite
a sensation of smell by friction, particularly lead, tin, iron, and
copper. Even gold, antimony, bismuth, and arsenic, under particular
circumstances, give out peculiar and powerful odours. The odour of
arsenic in its metallic state, and in a state of vapour, resembles
that of garlic. The chief means of developing the odorous principles
are friction, heat, electricity, fermentation, solution, and mixture.
The effect of mixture is very remarkable in the case of lime and
muriate of ammoniac, neither of which, before mixture, has any
perceptible odour.

There is perhaps then no body which is perfectly inodorous, or
entirely destitute of smell: for those which have been generally
accounted such, may be rendered odorous by some of the methods I have
mentioned.

Several naturalists and physiologists, such as Haller, Linneus, and
Lorri, have attempted to reduce the different kinds of odours to
classes, but without any great success; for we are by no means so
well acquainted with the physical nature of the odorous particles, as
we are with that of light, sound, and the objects of touch; and till
we do obtain a knowledge of these circumstances, which perhaps we
never shall, it will be in vain to attempt any accurate
classification. The division of them into odours peculiar to the
different kingdoms, is very inaccurate; for the odour of musk, which
is thought to be peculiarly an animal odour, is developed in the
solution of gold by some mineral solvents; it is perceptible in the
leaves of the geranium moschatum, and some other vegetables. The
smell of garlic is possessed by many vegetables, by arsenic, and by
toads. The violet smell is perceived in some salts, and in the urine
of persons who have taken turpentine. The same may be observed with
respect to several other odours.

As taste keeps guard, or watches over the passage by which food
enters the body, so smell is placed as a sentinel at the entrance of
the air passage, and prevents any thing noxious from being received
into the lungs by this passage, which is always open. Besides, by
this sense, we are invited or induced, to eat salutary food, and to
avoid such as is corrupted, putrid, or rancid. The influence of the
sense of smell on the animal machine is still more extensive: when a
substance which powerfully affects the olfactory nerves is applied to
the nostrils, it excites, in a wonder