The term oil is of very wide significance, being applied to substances of vastly different natures, both organic and inorganic, but so far as soap-making materials are concerned, it may be restricted almost entirely to the products derived from animal and vegetable sources, though many attempts have been made during the last few years to also utilise mineral oils for the preparation of soap. Fats readily become oils on heating beyond their melting points, and may be regarded as frozen oils. Although Scheele in 1779 discovered that in the preparation of lead plaster glycerol is liberated, soap at that time was regarded as a mere mechanical mixture, and the constitution of oils and fats was not properly understood. It was Chevreul who showed that the manufacture of soap involved a definite chemical decomposition of the oil or fat into fatty acid and glycerol, the fatty acid combining with soda, potash, or other base, to form the soap, and the glycerol remaining free. The reactions with stearin and palmitin (of which tallow chiefly consists) and with olein (found largely in olive and cotton-seed oils) are as follows:-- CH_{2}OOC_{18}H_{35} CH_{2}OH | | CHOOC_{18}H_{35} + 3NaOH = 3NaOOC_{18}H_{35} + CHOH | | CH_{2}OOC_{18}H_{35} CH_{2}OH stearin sodium sodium glycerol hydroxide stearate CH_{2}OOC_{16}H_{31} CH_{2}OH | | CHOOC_{16}H_{31} + 3NaOH = 3NaOOC_{16}H_{31} + CHOH | | CH_{2}OOC_{16}H_{31} CH_{2}OH palmitin sodium sodium glycerol hydroxide palmitate CH_{2}OOC_{18}H_{33} CH_{2}OH | | CHOOC_{18}H_{33} + 3NaOH = 3NaOOC_{18}H_{33} + CHOH | | CH_{2}OOC_{18}H_{33} CH_{2}OH olein sodium sodium glycerol hydroxide oleate Berthelot subsequently confirmed Chevreul's investigations by directly synthesising the fats from fatty acids and glycerol, the method he adopted consisting in heating the fatty acids with glycerol in sealed tubes. Thus, for example:-- 3C_{18}H_{35}O_{2}H + C_{3}H_{5}(OH)_{3} = C_{3}H_{5}(C_{18}H_{35}O_{2})_{3} stearic acid glycerol tristearin