The agitation being complete, chemical action takes place with the
generation of heat, and finally results in the saponification of the
fats.

At first the contents of the pan are thin, but in a few hours they
become a solid mass. As the process advances the edges of the soap
become more transparent, and when the transparency has extended to the
whole mass, the soap is ready, after perfuming, to be framed and
crutched.

The admixture of a little caustic potash with the caustic soda greatly
improves the appearance of the resultant product, which is smoother and
milder.

The glycerine liberated during the saponification is retained in the
soap.

Although it is possible, with care, to produce neutral soaps of good
appearance and firm touch by this method, cold-process soaps are very
liable to contain both free alkali and unsaponified fat, and have now
fallen considerably into disrepute.

_Saponification under Increased or Diminished Pressure._--Soaps made by
boiling fats and oils, under pressure and _in vacuo_, with the exact
quantity of caustic soda necessary for complete combination, belong also
to this class. Amongst the many attempts which have at various times
been made to shorten the process of soap-making may be mentioned
Haywood's Patent No. 759, 1901, and Jourdan's French Patent No. 339,154,
1903.

In the former, saponification is carried out in a steam-jacketed vacuum
chamber provided with an elaborate arrangement of stirrers; in the other
process fat is allowed to fall in a thin stream into the amount of lye
required for saponification, previously placed in the saponification
vessel, which is provided with stirring gear.

When the quantities have been added, steam is admitted and
saponification proceeds.

(C) _Treatment of Fat with Indefinite Amount of Alkali and no Separation
of Waste Lye._--_Soft soap_ is representative of this class. The
vegetable fluid oils (linseed, olive, cotton-seed, maize) are for the
most part used in making this soap, though occasionally bone fats and
tallow are employed. Rosin is sometimes added, the proportion ranging,
according to the grade of soap required, from 5 to 15 per cent. of the
fatty matter.

The Soft Soap Manufacturers' Convention of Holland stipulate that the
materials used in soft-soap making must not contain more than 5 per
cent. rosin; it is also interesting to note that a patent has been
granted (Eng. Pat. 17,278, 1900) for the manufacture of soft so