Most common oils and fats are mixtures of ethereal salts, termed glycerides or glyceric ethers, formed from glycerine and acids of the fatty, the oleic, and other allied series.

Of the methods employed for the decomposition of oils and fats, the following are the most usual:—

1. 1. Distillation of the oil or fat; those which yield volatile acids, such as the acetins, butyrins, &c., may be more or less distilled without decomposition; but those which yield fixed acids, e.g., palmitin, olein, &c., are almost wholly decomposed by a heat of 300° C., yielding acrolein, numerous hydrocarbons, &c.

2. 2. Distillation with dilute sulphuric acid, by which the volatile acids are separated from the non-volatile.

3. 3. Distillation in a current of superheated steam, when the glycerides are decomposed, the fatty acids being liberated.

4. 4. Saponification; oils and fats are decomposed by caustic alkalies into glycerine and a soap, the soap, when boiled with dilute sulphuric or hydrochloric acid, being decomposed into a sulphate or chloride of the alkali employed and the fatty acid set free.