e used as lubricants. The
so-called "drying oils" (see page 132), such as linseed oil, etc., are used
in the manufacture of paints and varnishes. Some cheap vegetable oils are
used as the basis for the manufacture of soaps, etc. Hence, industrial
plants and processes for the extraction of oils from plant tissues are of
very great economic importance.

CHEMICAL CONSTITUTION

The fats (of either plant or animal origin) are glycerides, i.e., glycerol
esters of organic acids. As has been pointed out, esters are derived from
organic acids and alcohols in exactly the same way that mineral salts are
derived from inorganic acids and metallic bases.

--------
Thus, Na|OH + H|ONO_2(HNO_{3}) = NaNO_{3} + H_{2}O
--------
Base Acid Salt

-----------
and, C_{2}H_{5}|OH + H|OOC.H = C_{2}H_{5}OOC.H + H_{2}O
-----------
Alcohol Acid Ester

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or, R.|OH + H|OOC.R = R.OOC.R + H_{2}O
---------------
Any alcohol Any acid Any ester

Glycerol is, however, a trihydric alcohol, i.e., it contains three
replaceable (OH) groups. Its formula is C_{3}H_{5}(OH)_{3}, or
CH_{2}OH.CHOH.CH_{2}OH. Hence, three molecules of a monobasic acid are
required to replace all of its (OH) groups.

For example,

C_{2}H_{5}OH + HOOC.C_{17}H_{35} = CH_{2}OOC.C_{17}H_{35}
| |
CHOH + HOOC.C_{17}H_{35} = CHOOC.C_{17}H_{35} + 3H_{2}O
| |
C_{2}H_{5}OH + HOOC.C_{17}H_{35} = CH_{2}OOC.C_{17}H_{35}

It is theoretically possible, of course, to replace either one, two, or
three of the (OH) groups in the glycerol with acid radicals, thus
producing either mono-, di-, or triglycerides. If the primary alcohol
groups in the glycerine molecule are designated by (1) and the secondary
one by (2), thus, CH_{2}OH(1).CHOH(2).CH_{2}OH(1), it is conceivable
that there may be either (1) or (2) monoglycerides, either (1, 1) or (1,
2) diglycerides, or a triglyceride, depending upon which of the (OH)
groups are replaced. Compounds of all of these types have been produced
by combinations of glycerol with varying proportions of organic acids
under carefully controlled conditions; and all of them found to possess
fat-like properties. All natural fats are triglycerides, h