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 structural formula what sort of odor the compound would
have or whether it would have any, yet we can divide odoriferous
substances into classes according to their constitution.
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