oup of substances all closely allied, but
distinguished by minor differences in properties and constitution. Some
of them are very widely distributed throughout the vegetable kingdom,
but others are almost peculiar to individual plants. They are all
compounds of carbon, hydrogen, and oxygen, and are at once distinguished
from the preceding class, by containing much less oxygen than is
required to form water with their hydrogen. The principal constituents
of the fatty matters and oils of plants are three substances, called
stearine, margarine, and oleine, the two former solids, the latter a
fluid; and they rarely, if ever, occur alone, but are mixed together in
variable proportions, and the fluidity of the oils is due principally to
the quantity of the last which they contain. If olive oil be exposed to
cold, it is seen to become partially solid; and if it be then pressed, a
fluid flows out, and a crystalline substance remains; the former is
oleine, though not absolutely pure, and the latter margarine. The
perfect separation of these substances involves a variety of troublesome
chemical processes; and when it has been effected, it is found that each
of them is a compound of a peculiar acid, with another substance having
a sweet taste, and which has received the name of glycerine, or the
sweet principle of oil. Glycerine, as it exists in the fats, appears to
be a compound of C_{3}H_{2}O, and its properties are the same from
whatever source it is obtained. The acids separated from it are known by
the names of margaric, stearic, and oleic acids.

_Margaric Acid_ is best obtained pure by boiling olive oil with an
alkali until it is saponified, and decomposing the soap with an acid,
expressing the margaric acid, which separates, and crystallising it from
alcohol. It is a white crystalline fusible solid, insoluble in water,
but soluble in alcohol and in solutions of the alkalies. Its composition
is--

Carbon 75.56
Hydrogen 12.59
Oxygen 11.85
------
100.00

and its formula C_{34}H_{34}O_{4}.

_Stearic Acid._--Although this acid exists in many plants, it is most
conveniently extracted from lard. It is a crystalline solid less fusible
than margaric acid, but closely resembling it in its other properties.
Its formula is C_{36}H_{36}O_{4}.

_Oleic Acid._--Under this name two different substances appear to be
included. It has been applied generally to the fluid acids of