| Name of Oil. | Reagents. | ||
| Sulphuric Acid. | Solution of Bichromate of Potash. | ||
| Not stirred. | Stirred. | Stirred. | |
| Rape oil with whale-train oil | More red ground than with rape oil | Brownish-olive coloured | Small reddish lumps on a grey ground. |
| Rape oil with linseed oil | No perceptible difference from the rape oil | Olive coloured | Small and more numerous red lumps on a very dark-green ground. |
| Rape oil with olein or oleic acid | No perceptible difference from the rape oil | Greenish brown | Small brownish lumps on an olive-coloured ground. |
“The adulteration being ascertained as far as is possible, the oil is then tested by endeavouring to discover the adulterating oil, either by reagents or by its odour when gently heated, as before described. This having been found out, small quantities of the suspected oil are added to a perfectly pure oil of the kind under examination. Every mixture is then tested by the reagents, until precisely similar results are obtained as those yielded by the oil under examination. Thus, the proportions of the two mixed oils will be discovered by approximation.” (M. Penot.) A simpler method of finding the proportions of the so mixed oils is referred to above.
4. (From increase of temperature.) M. Maumené proposed the increase of temperature arising from the admixture of monohydrated sulphuric acid (oil of vitriol, sp. gr. 1·845) with the fatty oils as a test of their purity, but a sufficient number of observations have not yet been made to furnish data for a general application of this method. According to MM. Faisst and Knauss, who have re-examined the subject, the following are the results when 15 grammes of oil are mixed with 5 grammes of the acid:
| Rise of Temperature. | |
| Almond oil | 72·5° |
| Olive oil | 68·0° |
| Poppy oil | 127·0° |
| Rape or colza oil | 100·0° |
| Linseed oil (with Nordhausen or fuming acid only) | 133·0° |
The above method is less liable to error when a larger quantity of the substances are thrown together.
5. The presence of FISH OIL in the vegetable oils may be readily detected by passing a stream of chlorine through them; the pure vegetable oils are not materially altered, but a mixture of the two turns dark brown or black.
6. Mr Coleman states that the presence of mineral oils in animal or vegetable oils can be easily detected by two characteristic tests—(1) the fluorescent properties they impart to all animal or vegetable oils. (2) The
strongly marked aromatic burning flavour they communicate to mixtures containing them. The first-mentioned property is brought out by smearing a metallic surface, such as tin plate or steel, with the oil, and then viewing it at different angles in the open air or sunlight. Mr Coleman suggests that, in examining a dark-coloured oil, it may first be necessary to refine the sample by successive treatments with concentrated sulphuric acid and weak soda solution or lime water, so small a quantity as 21⁄2 per cent. may then be detected by the bluish colour noticed on viewing the oil at certain angles and by tasting it.
The absence of resin oil must also be proved. Nitric acid is said to be a good test, as the colour developed is much greater than in pure oils. Sometimes it may be detected by the smell. The presence of 10 per cent. of resin or mineral oil in non-drying oils delays their solidification with the nitrate of mercury test.[50]
[50] ‘Journal of Applied Chemistry,’ Dec., 1874.