Cocoa-nut oil, quantity of,
| Im- ported. | Retained for consumption. | Ex- ported. | |
|---|---|---|---|
| Year. | Cwts. | Cwts. | Cwts. |
| 1835. | 19,838 | 14,015 | 2,238 |
| 1836. | 26,058 | 26,062 | 3,158 |
| 1837. | 41,218 | 28,836 |
Olive oil, quantity of,
| Im- ported. | Retained for consumption. | Ex- ported. | |
|---|---|---|---|
| Year. | Cwts. | Cwts. | Cwts. |
| 1835. | 606,166 | 554,196 | 283,734 |
| 1836. | 2,682,016 | 1,844,622 | 150,561 |
| 1837. | 1,720,397 | 1,499,122 |
Duties on olive oil, not of Naples and Sicily, 4d.; of Naples and Sicily, 8d.; and, if in ships of these countries, 10d. per gallon.
Train oil, spermaceti, and blubber, quantity of,
| Im- ported. | Retained for consumption. | Ex- ported. | |
|---|---|---|---|
| Year. | Cwts. | Cwts. | Cwts. |
| 1835. | 24,197 | 16,114 | 8,035 |
| 1836. | 19,489 | 18,722 | 1,365 |
| 1837. | 21,823 | 21,286 |
Duties on oil taken by British ships, 1s.; by foreign fishers, £26 18s. per tun.
OILS, VOLATILE OR ESSENTIAL; Manufacture of. The volatile oils occur in every part of odoriferous plants, whose aroma they diffuse by their exhalation; but in different organs of different species. Certain plants, such as thyme and the scented labiatæ, in general contain volatile oil in all their parts; but others contain it only in the blossoms, the seeds, the leaves, the root, or the bark. It sometimes happens that different parts of the same plant contain different oils; the orange, for example, furnishes three different oils, one of which resides in the flowers, another in the leaves, and a third in the skin or epidermis of the fruit. The quantity of oil varies not only with the species, but also in the same plant, with the soil, and especially the climate; thus in hot countries it is generated most profusely. In several plants, the volatile oil is contained in peculiar orders of vessels, which confine it so closely that it does not escape in the drying, nor is dissipated by keeping the plants for many years. In other species, and particularly in flowers, it is formed continually upon their surface, and flies off at the moment of its formation.
Volatile oils are usually obtained by distillation. For this purpose the plant is introduced into a still, water is poured upon it, and heat being applied, the oil is volatilized by the aid of the watery vapour, at the temperature of 212°, though when alone it would probably not distil over unless the heat were 100° more. This curious fact was first explained in my New Researches upon Heat, published in the Philosophical Transactions for 1818. Most of the essential oils employed in medicine and perfumery are extracted by distillation from dried plants; only a few, such as those of the rose and orange flower, are obtained, from fresh or succulent salted plants. When the mingled vapours of the oil and water are condensed into the liquid state, by the refrigerator of the still, the oil separates, and either floats on the surface or sinks to the bottom of the water. Some oils of a less volatile nature require a higher heat than 212° to raise them in vapour, and must be dislodged by adding common salt to the water, whereby the heat being augmented by 15°, they readily come over. If in such distillations too much water be added, no oil will be obtained, because it is partially soluble in water; and thus merely an aromatic water is produced. If on the other hand too little water be used, the plant may happen to adhere to the bottom of the still, get partially charred, and thus impart an empyreumatic odour to the product. But as the quality of water distilled depends less upon the quantity employed, than upon that of the surface exposed to the heat, it is obvious that by giving a suitable form to the still, we may get rid of every inconvenience. Hence the narrower and taller the alembic is, within certain limits, the greater will be the proportion of oil relative to that of the aromatic water, from like proportions of aqueous and vegetable matter employed. Some place the plants in baskets, and suspend these immediately over the bottom of the still under the water, or above its surface in the steam. But the best mode in my opinion is to stuff an upright cylinder full of the plants, and to drive down through them, steam of any desired force; its tension and temperature being further regulated by the size of the outlet orifice leading to the condenser. The cylinder should be made of strong copper tinned inside, and encased in the worst conducting species of wood, such as soft deal or sycamore.