_Lime._--The use of lime for the saponification of oils and fats was first adopted on the technical scale for the production of candle-making material, by De Milly in 1831. The insoluble lime soap formed is decomposed by sulphuric acid, and the fatty acids steam distilled. The amount of lime theoretically necessary to hydrolyse a given quantity of a triglyceride, ignoring for the moment any catalytic influence, can be readily calculated; thus with stearin the reaction may be represented by the equation:-- CH_{2}OOC_{18}H_{35} CH_{2}OH | | 2CHOOC_{18}H_{35} + 3Ca(OH)_{2} = 3Ca(OOC_{18}H_{35})_{2} + 2CHOH | | CH_{2}OOC_{18}H_{35} CH_{2}OH stearin milk of lime calcium stearate glycerol In this instance, since the molecular weight of stearin is 890 and that of milk of lime is 74, it is at once apparent that for every 1,780 parts of stearin, 222 parts of milk of lime or 168 parts of quick-lime, CaO, would be required. It is found in practice, however, that an excess of 3-5 per cent. above the theoretical quantity of lime is necessary to complete the hydrolysis of a fat when carried on in an open vessel at 100 deg.-105 deg. C., but that if the saponification be conducted under pressure in autoclaves the amount of lime necessary to secure almost perfect hydrolysis is reduced to 2-3 per cent. on the fat, the treatment of fats with 3 per cent. of lime under a pressure of 10 atmospheres producing a yield of 95 per cent. of fatty acids in seven hours. The lower the pressure in the autoclave, the lighter will be the colour of the resultant fatty acids. _Magnesia._--It has been proposed to substitute magnesia for lime in the process of saponification under pressure, but comparative experiments with lime and magnesia, using 3 per cent. of lime and 2.7 per cent. of magnesia (_Journ. Soc. Chem. Ind._, xii., 163), show that saponification by means of magnesia is less complete than with lime, and, moreover, the reaction requires a higher temperature and therefore tends to 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.