/ | \
/ | \
/ | \
CH_{2} | CH_{2}
| | |
|CH_{3}-C-CH_{3}|
| | |
CH_{2} | C=O
\ | /
\ | /
\ | /
\ | /
\ | /
\ | /
C
|
CH_{3}

There are a considerable number of essential oils which are _phenols_.
_Thymol_, C_{6}H_{3}.(CH_{3}).(C_{3}H_{7}).OH, in oil of thyme, and
carvacrol, its isomer, in oil of hops, are familiar examples.

O-C=O
/ |
Coumarin, the anhydride of cinnamic acid C_{6}H_{4} | ,
\ |
HC=CH

is an example of an acid substance which is classed as an essential oil,
even though it is a solid at ordinary temperatures. It has an odor and
flavor similar to that of _vanillin_, the essential flavoring material of
the vanilla bean, and is often used as a substitute for the latter in the
preparation of artificial flavoring extracts.

Of the =essential oils containing sulfur=, there are two common examples;
oil of mustard, allyl isosulfocyanide, C_{3}H_{5}NCS, and oil of garlic,
allyl sulfide (C_{3}H_{5})_{2}S. The latter is present in onions, garlic,
water cress, radishes, etc., the difference in flavor of these vegetables
being due to the fact that the allyl sulfide is united with other different
groups in the glucoside arrangement, in the different plants. Similarly,
mustard oil is not present in mustard seeds as such, but as a glucoside
which, when hydrolyzed by the enzyme _myrosin_ which is always present in
other cells of the same seeds, yields C_{3}H_{5}NCS, KHSO_{4}, and
C_{6}H_{12}O_{6}.

THE RESINS

The resins were formerly supposed to be the mother substances from which
the terpenes are derived. It is now known, however, that they are the
oxidation products of the terpenes. Their exact structure is still a matter
of some u