_Classification of Soaps--Direct Combination of Fatty Acids with Alkali--Cold Process Soaps--Saponification under Increased or Diminished Pressure--Soft Soap--Marine Soap--Hydrated Soaps, Smooth and Marbled--Pasting or Saponification--Graining Out--Boiling on Strength--Fitting--Curd Soaps--Curd Mottled--Blue and Grey Mottled Soaps--Milling Base--Yellow Household Soaps--Resting of Pans and Settling of Soap--Utilisation of Nigres--Transparent Soaps--Saponifying Mineral Oil--Electrical Production of Soap._ Soaps are generally divided into two classes and designated "hard," and "soft," the former being the soda salts, and the latter potash salts, of the fatty acids contained in the material used. According to their methods of manufacture, soaps may, however, be more conveniently classified, thus:-- (A) Direct combination of fatty acids with alkali. (B) Treatment of fat with definite amount of alkali and no separation of waste lye. (C) Treatment of fat with indefinite amount of alkali and no separation of waste lye. (D) Treatment of fat with indefinite amount of alkali and separation of waste lye. (A) _Direct Combination of Fatty Acids with Alkali._--This method consists in the complete saturation of fatty acids with alkali, and permits of the use of the deglycerised products mentioned in chapter ii., section 2, and of carbonated alkalies or caustic soda or potash. Fatty acids are readily saponified with caustic soda or caustic potash of all strengths. The saponification by means of carbonated alkali may be performed in an open vat containing a steam coil, or in a pan provided with a removable agitator. It is usual to take soda ash (58 deg.), amounting to 19 per cent. of the weight of fatty acids to be saponified, and dissolve it in water by the aid of steam until the density of the solution is 53 deg. Tw. (30 deg. B.); then bring to the boil, and, whilst boiling, add the molten fatty acids slowly, but not continuously. Combination takes place immediately with evolution of carbonic acid gas, which causes the contents of the vat or pan to swell, and frequently to boil over. The use of the agitator, or the cessation of the flow of fatty acids, will sometimes tend to prevent the boiling over. It is imperative that the steam should not be checked but boiling continued as vigorously as possible until all the alkali has been absorbed and the gas driven off. The use of air to replace steam in expelling the carbonic acid gas has been patented (Fr. Pat. 333,974, 1903). A better method of procedure, however, is to commence with a solution of 64 deg. Tw. (35 deg. B.) density, made from half the requisite soda ash (9-1/2 per cent.), and when this amount of alkali has all been taken up by the fatty acids (which have been added gradually and with continuous boiling), the remaining quantity of soda ash is added in a dry state, being sprinkled over each further addition of fatty acid.