heir presence as esters in all plant and animal fats
indicates that they must have some biological function.

_Phytosterol_ is not a single alcohol, but a mixture of at least two, which
have been separated and studied as _sitosterol_, C_{27}H_{43}OH, and
_stigmasterol_, C_{30}H_{49}OH. As has been said, these are found in small
proportions in all vegetable fats, being present in largest amounts in oily
seeds, especially those of the legumes.

The saponification of esters of cholesterol and phytosterol is a difficult
and unsatisfactory process; but since this affords the only known means to
distinguish between fats of plants and of animal origin, its technique has
been fairly well worked out, and the process used in the study of the
changes which take place in plant fats when they are used by animals as
food.

HYDROLYSIS AND SYNTHESIS OF FATS

The reaction for the hydrolysis of fats has been discussed in connection
with the process for the manufacture of glycerine. This reaction takes
place very slowly with cold water alone, can be easily brought about by the
action of superheated steam, and much more easily and rapidly in the
presence of some catalyst (sulfuric acid is an especially effective
catalyst for this purpose).

Fats can be artificially synthetized by heating mixtures of glycerol and
fatty acids, under considerable pressure, for some time at temperatures of
200 deg. to 240 deg. C.; or by heating a mixture of the disulfuric ester
of glycerol with a fatty acid dissolved in sulfuric acid. Recently, fatty
acids have been prepared from carbohydrates, by first breaking the hexoses
down into three-carbon compounds, then carefully oxidizing these to
pyruvic acid, CH_{3}.CO.COOH, which can then be condensed into acids
having longer chains. The violent reagents and long-continued processes
which must be employed for the artificial hydrolysis or synthesis of the
fats are in sharp contrast with the easy and rapid transition of
carbohydrates to fats, and _vice versa_, which take place in both plant
and animal nutrition.

THE EXTRACTION OF OILS FROM PLANT TISSUES

There are three types of methods which are employed for the extraction of
oil from oil-bearing seeds, etc., either as a commercial industry or for
the purposes of scientific study. These are (1) by pressure; (2) extraction
with volatile solvents; and (3) boiling the crushed seeds or fruits with
water.

By the firs