o darken the
product.

_Zinc Oxide._--The use of zinc oxide as accelerating agent has been
suggested by two or three observers. Poullain and Michaud, in 1882, were
granted a patent for this process, the quantity of zinc oxide
recommended to be added to the oil or fat being 0.2 to 0.5 per cent.
Rost, in 1903, obtained a French patent for the saponification of oils
and fats by steam under pressure in the presence of finely divided
metals or metallic oxides, and specially mentions zinc oxide for the
purpose.

It has also been proposed to use zinc oxide in conjunction with lime in
the autoclave to obviate to some extent the discoloration of the fatty
acids.

Other catalytic agents have been recommended from time to time,
including strontianite, lead oxide, caustic baryta, aluminium hydrate,
but none of these is of any practical importance.

_Soda and Potash._--Unlike the preceding bases, the soaps formed by soda
and potash are soluble in water, and constitute the soap of commerce.
These reagents are always used in sufficient quantity to combine with
the whole of the fatty acids contained in an oil or fat, though
doubtless, by the use of considerably smaller quantities, under
pressure, complete resolution of the fatty matter into fatty acids and
glycerol could be accomplished. They are, by far, the most important
saponifying agents from the point of view of the present work, and their
practical use is fully described in Chapter V.

CHAPTER III.

RAW MATERIALS USED IN SOAP-MAKING.

_Fats and Oils--Waste Fats--Fatty Acids--Less-known Oils and
Fats of Limited Use--Various New Fats and Oils Suggested for
Soap-making--Rosin--Alkali (Caustic and
Carbonated)--Water--Salt--Soap-stock._

_Fats and Oils._--All animal and vegetable oils and fats intended for
soap-making should be as free as possible from unsaponifiable matter, of
a good colour and appearance, and in a sweet, fresh condition. The
unsaponifiable matter naturally present as cholesterol, or phytosterol,
ranges in the various oils and fats from 0.2 to 2.0 per cent. All oils
and fats contain more or less free acidity; but excess of acidity,
though it may be due to the decomposition of the glyceride, and does not
always denote rancidity, is undesirable in soap-making material.
Rancidity of fats and oils is entirely due to oxidation, in addition to
free acid, aldehydes and ketones being formed, and it has been proposed
to estimate rancid