t together. These twin senses stand like
sentries at the portals of the body, where they closely scrutinize
everything that enters. Sounds and sights may be disagreeable, but they
are never fatal. A man can live in a boiler factory or in a cubist art
gallery, but he cannot live in a room containing hydrogen sulfide. Since
it is more important to be warned of danger than guided to delights our
senses are made more sensitive to pain than pleasure. We can detect by
the smell one two-millionth of a milligram of oil of roses or musk, but
we can detect one two-billionth of a milligram of mercaptan, which is
the vilest smelling compound that man has so far invented. If you do not
know how much a milligram is consider a drop picked up by the point of
a needle and imagine that divided into two billion parts. Also try to
estimate the weight of the odorous particles that guide a dog to the fox
or warn a deer of the presence of man. The unaided nostril can rival the
spectroscope in the detection and analysis of unweighable amounts of
matter.

What we call flavor or savor is a joint effect of taste and odor in
which the latter predominates. There are only four tastes of importance,
acid, alkaline, bitter and sweet. The acid, or sour taste, is the
perception of hydrogen atoms charged with positive electricity. The
alkaline, or soapy taste, is the perception of hydroxyl radicles charged
with negative electricity. The bitter and sweet tastes and all the odors
depend upon the chemical constitution of the compound, but the laws of
the relation have not yet been worked out. Since these sense organs, the
taste and smell buds, are sunk in the moist mucous membrane they can
only be touched by substances soluble in water, and to reach the sense
of smell they must also be volatile so as to be diffused in the air
inhaled by the nose. The "taste" of food is mostly due to the volatile
odors of it that creep up the back-stairs into the olfactory chamber.

A chemist given an unknown substance would have to make an elementary
analysis and some tedious tests to determine whether it contained methyl
or ethyl groups, whether it was an aldehyde or an ester, whether the
carbon atoms were singly or doubly linked and whether it was an open
chain or closed. But let him get a whiff of it and he can give instantly
a pretty shrewd guess as to these points. His nose knows.

Although the chemist does not yet know enough to tell for certain from
looking at the