If we assume that oleic acid predominates, which in most cases is the fact, 1° of acidity correspond to 0·28 per cent. by weight of oleic acid. The olive oil of commerce has an acidity ranging from 0·4° to 12°. The first passes as very fine, and is called free from acid or salad oil, while the latter is known by smell and taste as very rancid. Oil that has 4° to 6° of acidity has been found to answer very well as a lubricator.
What relation there exists between the degree of acidity and any injurious effect upon metals is shown by the following experiments:—Four shallow vessels of sheet brass, having a surface of 40 square centimètres each, at the bottom, were filled to the depth of 2 millimètres, with oils of different acidity, and exposed to the air at the ordinary temperature. The vessels were soon more or less covered with green fatty salts, and the oil too acquired a green colour. Oil and vessel No. 1 were the only ones in which no change could be perceived. At the end of 3 days the vessels were cleaned with ether and weighed. The following table shows the amounts of action:
| Vessel | No. 1, | filled with oil of | 0·8° | lost | 0·03 | gr. |
| ” | No. 2, | ” | 4·6° | ” | 0·22 | ” |
| ” | No. 3, | ” | 7·8° | ” | 0·36 | ” |
| ” | No. 4, | ” | 8·8° | ” | 0·04 | ” |
The quantity of metal destroyed, in equal times and under equal conditions, increases with the acidity of the oil.
The table on page 1128, by Mr Bottome, describes the most striking physical properties of some of the principal fixed oils.
⁂ The following are the principal fixed oils met with in commerce, or which are objects of interest or utility:
Oil of Al′monds. Syn. Oleum amygdalæ
(B. P., Ph. L), O. amygdalarum (Ph. D.), O. amygdali communis (Ph. E.), L. “The oil expressed from the kernels.” (Ph. L.) “Bruise the fresh almonds in a stone mortar, then put them into a hempen sack, and express the oil, without heat.” (Ph. E.) The oil of almonds B. P. and of commerce is obtained from either the bitter or sweet almond, but chiefly from the first, on account of their less value, and the marc being employed in the manufacture of essential oil.
Prop., &c. Oil of almonds is black, demulcent, emollient, and nutritious; possesses a purely oleaginous taste, and is one of the most agreeable of the fixed oils; when taken in quantity it is mildly laxative; it is little affected by cold, and congeals with difficulty; is soluble in 35 parts of cold and 6 parts of boiling alcohol; ether dissolves it freely. Sp. gr. ·915 to ·918. Av. prod. Sweet almonds, 46%; bitter a., 41%.
Pur. It is extensively adulterated with poppy, nut, and teel oil, and not unfrequently with refined rape or colza oil. (See above.)