Fats and oils, from
which soaps are manufactured, are a kind of _quasi_ salts, composed of a
fatty acid and a chemical constant, if I may use the term, in the shape
of base, namely, glycerin. When these fats and oils, often called
glycerides, are heated with alkali, soda, a true salt of the fatty acid
and soda is formed, and this is the soap, whilst the glycerin remains
behind in the "spent soap lye." Now glycerin is soluble in water
containing dissolved salt (brine), whilst soap is insoluble, though
soluble in pure water. The mixture of soap and glycerin produced from
the fat and soda is therefore treated with brine, a process called
"cutting the soap." The soap separates out in the solid form as a curdy
mass, which can be easily separated. Certain soaps are able to absorb a
large quantity of water, and yet appear quite solid, and in purchasing
large quantities of soap it is necessary, therefore, to determine the
amount of water present. This can be easily done by weighing out ten or
twenty grams of the soap, cut in small pieces, into a porcelain dish and
heating over a gas flame, whilst keeping the soap continually stirred,
until a glass held over the dish no longer becomes blurred by escaping
steam.
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