Tunis has also some celebrity for similar products, which however do not reach Europe. A recent traveller[1024] states that the rose grown there, and from which attar is obtained, is Rosa canina L., which is extremely fragrant; 30 lb. of the flowers afford about 1½ drachms, worth 15s. When at Genoa, in 1874, one of us (F.) had the opportunity of ascertaining that excellent oil of rose is occasionally imported there from Tunis.
The butyraceous oil which may be collected in distilling roses in England for rose water is of no value as a perfume.
Description—Oil of rose is a light yellow liquid, of sp. gr. 0·87 to 0·89. By a reduction of temperature, it concretes owing to the separation of light, brilliant, platy crystals of a stearoptene, the proportion of which differs with the country in which the roses have been grown, the state of the weather during which the flowers were gathered, and other circumstances less well ascertained. The oil produced in the Balkans solidifies, according to Baur, at from 11 to 16° C. In some experiments made by one of us[1025] in 1859, the fusing point of true Turkish attar was found to vary from 16 to 18°; that of a sample from India was 20° C.; of oil distilled in the south of France, 21 to 23°, of an oil produced in Paris, 29°; of oil obtained in distilling roses for rose water in London, 30 to 32° C.
From these data, it appears that a cool northern climate is not conducive to the production of a highly odorous oil; and even in Bulgaria experience shows that the oil of the mountain districts holds a larger proportion of stearoptene than that of the lowlands.
Turkish oil of rose is stated by Baur to deviate a ray of polarized light 4° to the right, when examined in a column of 100 mm. The oil from English roses which we examined exhibited no rotation.
Chemical Composition—Rose oil is a mixture of a liquid constituent containing oxygen, to which it owes its perfume, and the solid hydrocarbon or stearoptene already mentioned, which is entirely destitute of odour. The proportion which these bodies bear to each other is extremely variable. From the Turkish oil, it may be obtained to the extent of 18 per cent., and from French and English to 35, 42, 60 or even 68 per cent.
Though the stearoptene can be entirely freed from the oxygenated oil, no method is known for the complete isolation of the latter. As obtained by Gladstone[1026], it had a sp. gr. of 0·881 and a boiling point of 216° C.
With regard to the stearoptene of rose oil, the analyses of Théodore de Saussure (1820) and Blanchet (1833) long since showed its composition to accord with the formula CₙH₂ₙ. The experiments of one of us[1027] confirm this striking fact, which assigns to the stearoptene in question a very exceptional place among the hydrocarbons of volatile oils, all of which are less rich in hydrogen.
Rose stearoptene separates when attar of roses is mixed with alcohol. We have isolated it also from oil obtained from Mitcham roses, by diluting the oil with a little chloroform and precipitating with glacial acetic acid or spirit of wine, the process being several times repeated. The stearoptene was lastly maintained for some days at 100° C.; thus obtained, it is inodorous, but when heated evolves an offensive smell like that of heated wax or fat. At 32·5° it melts; at 150° vapour is evolved; at 272° C. it begins to boil, soon after which it turns brown and then blackish. Stains of the stearoptene on paper do not disappear by the heat of the water-bath and the relapse of some days.
If cautiously melted by the warmth of the sun, the stearoptene forms on cooling microscopic crystals of very peculiar shape. Most of them have the form of truncated hexahedral pyramids, not however belonging to the rhombohedric system, as the angles are evidently not equal; many of them are oddly curved, thus §. Examined under the polarizing microscope, these crystals from their refractive power make a brilliant object.