Description—Rosemary has sessile, linear, entire, opposite leaves about an inch in length, revolute at the margin; they are of coriaceous texture, green and glabrous above, densely tomentose and white beneath. Examined under a lens, the tomentum both of the leaves and young shoots is seen to consist of white stellate hairs; in that of the shoots which is less dense, minute oil-glands are discernible. These glands are of two kinds, large and small, and probably do not yield one and the same oil. The flowers have a campanulate 2-lipped calyx, and a pale blue and white corolla, the upper lip of which is emarginate and erect, the lower 3-lobed with the central lobe concave and pendulous. The whole plant has a very agreeable smell and a strong aromatic taste. It flowers in the early spring.
Production of Essential Oil—Rosemary is cultivated on a very small scale in English herb-gardens, and though a little oil has been occasionally distilled from it, English oil of rosemary is an article practically unknown in commerce. That with which the market is supplied is produced in the south of France and on the contiguous coasts of Italy. The plant, which is plentifully found wild, is gathered in summer (not while in flower) and distilled, the operator being sometimes an itinerant herbalist who carries his copper alembic from place to place, erecting it where herbs are plentiful, and where a stream of water enables him to cool a condenser of primitive construction.
Oil of rosemary is also produced on a somewhat large scale in the island of Lesina, south of Spalato in Dalmatia, whence it is exported by way of Trieste, even to France and Italy, to the extent of 300 to 350 quintals annually.[1794]
Some of the French manufacturers of essences offer oil of rosemary at a superior price as drawn from the flowers, by which we presume is meant the flowering tops, for the separation of the actual flowers would be impracticable on a large scale. The great bulk of the oil found in commerce is however that distilled from the entire plant.
Chemical Composition—The peculiar odour of rosemary depends on the essential oil, which is the only constituent of the plant that has afforded matter for chemical research.
Lallemand (1859) by fractional distillation, resolved oil of rosemary into two liquids,—the one a mobile hydrocarbon boiling at 165° C. and turning the plane of polarization to the left; the other, boiling between 200° and 210° C., deposits when exposed to a low temperature a large quantity of camphor. Gladstone (1864) found the oil to consist almost wholly of a hydrocarbon, C₁₀H₁₆. This, according to our experiments, constitutes about ⁴/₅ of the oil; it deviates the plane of polarization to the left, whereas a fraction boiling at 200° to 210° C. deviates to the right. By warming the latter with nitric acid, we observed the odour of common camphor, and may therefore infer that a compound, C₁₀H₁₈O, is present in the oil under examination.
From Montgolfier’s investigations (1876) it would appear that the stearoptene or camphor above alluded to is a mixture of a dextrogyrate and a lævogyrate substance.
Uses—The flowering tops and dried leaves are kept by the herbalists, but are not used in regular medicine. The volatile oil is employed as an external stimulant in liniments, and also as a perfume. Rosemary is popularly supposed to promote the growth of the hair.