n originally applied to
the product obtained by treating tallow with ashes. In its strictly
chemical sense it refers to combinations of fatty acids with metallic
bases, a definition which includes not only sodium stearate, oleate and
palmitate, which form the bulk of the soaps of commerce, but also the
linoleates of lead, manganese, etc., used as driers, and various
pharmaceutical preparations, _e.g._, mercury oleate (_Hydrargyri
oleatum_), zinc oleate and lead plaster, together with a number of other
metallic salts of fatty acids. Technically speaking, however, the
meaning of the term soap is considerably restricted, being generally
limited to the combinations of fatty acids and alkalies, obtained by
treating various animal or vegetable fatty matters, or the fatty acids
derived therefrom, with soda or potash, the former giving hard soaps,
the latter soft soaps.

The use of ammonia as an alkali for soap-making purposes has often been
attempted, but owing to the ease with which the resultant soap is
decomposed, it can scarcely be looked upon as a product of much
commercial value.

H. Jackson has, however, recently patented (Eng. Pat. 6,712, 1906) the
use of ammonium oleate for laundry work. This detergent is prepared in
the wash-tub at the time of use, and it is claimed that goods are
cleansed by merely immersing them in this solution for a short time and
rinsing in fresh water.

Neither of the definitions given above includes the sodium and potassium
salts of rosin, commonly called rosin soap, for the acid constituents of
rosin have been shown to be aromatic, but in view of the analogous
properties of these resinates to true soap, they are generally regarded
as legitimate constituents of soap, having been used in Great Britain
since 1827, and receiving legislative sanction in Holland in 1875.

Other definitions of soap have been given, based not upon its
composition, but upon its properties, among which may be mentioned that
of Kingzett, who says that "Soap, considered commercially, is a body
which on treatment with water liberates alkali," and that of Nuttall,
who defines soap as "an alkaline or unctuous substance used in washing
and cleansing".

_Properties of Soap._--Both soda and potash soaps are readily soluble in
either alcohol or hot water. In cold water they dissolve more slowly,
and owing to slight decomposition, due to hydrolysis (_vide infra_), the
solution becomes distinctly turbid. Sodium oleate is pe