3. _Elaidin and colour tests._--Pure olive oil is converted into a solid mass when treated with various oxidising agents, the change being retarded by the presence of some of its adulterants. The test may be made in several ways:-- (_a_) Ten grms. of the sample are shaken with 5 grms. of nitric acid (sp. gr. 1·40) and 1 grm. of mercury, and the colour produced and time required for solidification noticed. In this manner the following results have been obtained:-- ------------+----------------------+----------------- Oil. | Coloration. | Minutes for | | Solidification. ------------+----------------------+----------------- Olive | Pale yellowish green | 60 Almond | White | 90 Arachis | Pale reddish | 105 Rape | Orange | 200 Cotton-seed | Orange red | 105 Sesamé | Yellowish orange | 150 Beech-nut | Reddish orange | 360 Poppy | Red | Remains fluid. ------------+----------------------+----------------- (_b_) Or a few pieces of copper foil are added to a mixture of equal parts of the oil and nitric acid, the liquor occasionally stirred, and then set aside. If the oil be pure, it will be converted into a nearly white buttery mass in from three to six hours; sesamé oil yields a red, cotton and rape-seed a brown, and beech-nut a reddish-yellow colour, the solidification being delayed from 10 to 20 hours, while poppy oil fails to solidify at all. (_c_) Nine parts of the sample are oxidised by heating with one part of concentrated nitric acid, the mixture being well stirred; pure olive oil forms a hard, pale-yellow mass in the course of two hours; seed oils (including cotton-seed) turn orange-red in colour and do not become solid in the same time or manner. (_d_) A portion of the sample is well mixed with one-fourth of its weight of chromic acid; if pure, the oil will be converted into an opaque mass. (_e_) Introduce 2 c.c. of the sample into a narrow graduated glass cylinder, add 0·1 gramme potassium dichromate, next 2 c.c. of a mixture of sulphuric and nitric acids, shake well, and then add 1 c.c. of ether; shake again and allow the mixture to stand at rest. Lively effervescence and evolution of nitrous fumes soon take place, and the oil rises to the surface, showing a characteristic coloration. Olive oil exhibits a green colour, whereas in presence of 5 per cent. of sesamé, arachis, cotton-seed, or poppy oil, the colours will vary from greenish-yellow to yellow or yellowish red. The coloration is more readily observed upon agitating the mixture with water and, setting it aside for a short time.