A sesquiterpene occurs in most crude Eucalyptus oils, perhaps in all of them, although it is present in but small amount in many of those belonging to the "Peppermint" group. It does not appear to be associated with one terpene more than another, because it has been found in quantity in some of the pinene oils (those occupying the earlier portion of the series), in which phellandrene is absent; and also in oils in which pinene is only present in small quantity, with phellandrene the principal terpene. It is also found in the oils belonging to the cineol-pinene group.

The sesquiterpene was first detected in quantity in the oil of E. Dawsoni, where it occurs with phellandrene; it was also present in quantity in those of the following species: - E. eximia (with pinene); E. nova-anglica (with pinene); E. trachyphloia (with pinene); E. affinis (with pinene and cineol); E. maculata (with pinene and cineol); E. acmenioides (with pinene and phellandrene); E. crebra (with pinene, phellandrene, and cineol); E. viminalis (with pinene, phellandrene, and cineol); E. hcemastoma (with phellandrene), and as a well-defined constituent in the oils of many other species. The oil of E. nova-anglica contains the sesquiterpene in great quantity, and over 50 per cent. of the crude oil distilled above 2550 C. It appears to be a constant constituent in those oils in which it is found, and in that of E. hcernastoma distilled from material sent from Gosford, N.S.W., and from Barber's Creek, N.S.W., localities about 140 miles apart, practically the same amount of the sesquiterpene was present. The oil from the Gosford sample gave 50 per cent. boiling between 255° and 2800 C, and that from Barber's Creek, 55 per cent. distilling between 245° and 2830 C. The oil of E. hcernastoma was the one employed originally for the separation of the sesquiterpene. In all the above oils the sesquiterpene was accompanied by a sesquiterpene alcohol in larger or smaller amount.

There appears to be only the one sesquiterpene in Eucalyptus oils, because the high-boiling portions of those of several species were added together, and the product obtained from this, by fractional distillation, finally over sodium, was practically identical with that obtained from the oil of E. hcernastoma in a similar manner.

Although the sesquiterpene has been prepared as free as possible from associated constituents by fractional distillation, finally over sodium, yet it cannot, as thus obtained, be considered pure. All efforts, so far, to form crystallised chemical compounds with it have not been successful; the constants cannot, therefore, be given with any very great degree of accuracy. Attempts Were made to form the crystalline dihydrochloride, the nitrosochloride, and the nitrosite, but these were not satisfactory; nor did it appear possible to obtain a solid sesquiterpene alcohol from it by treatment with glacial acetic acid and sulphuric acid, so that its derivatives have yet to be prepared.

When treated with bromine, very energetic action takes place with the evolution of hydrobromic acid; the bromide is of a very dark colour, and not obtained in a crystallised condition.

A characteristic test for this sesquiterpene is the very fine colour reactions it gives with bromine, and with the halogen acids.

The bromine test is best carried out in the following manner: -

One or two drops of the sesquiterpene are dissolved in 2 or 3 c.c. of glacial acetic acid, and the vapour of bromine allowed to pass down the tube until it reaches the liquid; a crimson colour is immediately formed, which rapidly passes downward and throughout the whole of the liquid; if agitated the whole becomes crimson, soon changing to violet, and in a short time to a deep indigo-blue colour, which, under favourable conditions, remains for some days. This test is exceedingly delicate, but the bromine should not be added in excess. The reaction appears to be due to the formation of hydrobromic acid immediately the bromine meets the sesquiterpene, and identical colour reactions were obtained when one or two drops of hydrobromic acid were added directly to the acetic acid solution, not mixing the two liquids. On heating the blue liquids, they became reddish-brown, but usually recovered the indigo-blue colour on standing.

When one or two drops of concentrated hydrochloric acid were added to the acetic acid solution, prepared as for the previous tests, a crimson colour was obtained at once; this soon changed to violet, and, on standing some hours, to an indigo-blue colour. Here again the colours were more brilliant if the liquids were not mixed by agitation.

When one or two drops of concentrated sulphuric acid were passed to the bottom of an acetic acid solution prepared as above, a bright crimson colour was at once formed at the junction of the liquids; on mixing these, the crimson colour first formed, changed to a purplish-brown rather than to the violet colour formed with the halogen acids. On boiling, the solution first became violet, then deep crimson.

When a few drops of phosphoric acid (specific gravity 175) were added to the acetic acid solution prepared as above, a rose-madder colour formed at the junction of the liquids; on mixing the acids after the colour had formed, the liquid changed in a few minutes to crimson, and then slowly to violet. It is thus evident that the pink colour given to Eucalyptus oils when treated with phosphoric acid is due to the influence of the sesquiterpene.

In order to prepare the sesquiterpene in as pure a condition as possible, 300 c.c. of the crude oil of E. nova-anglica were directly distilled under reduced pressure, and the fractionation repeated until finally 50 c.c. were obtained boiling at 124-1250 C, at 10 millimetres pressure.

It was not easy to entirely separate the lævo-rotatory sesquiterpene alcohol from the dextro-rotatory sesquiterpene, and it was necessary to finally distil over sodium before an oil with a constant rotation was obtained. The sesquiterpene, when thus prepared, had a slight yellowish tinge, was quite mobile, had a not unpleasant odour, nor was it readily soluble in alcohol.

The constants for the sesquiterpene were determined as follows: - Boiling point, under ordinary atmospheric pressure 260-2650 C, and under 10 millimetre's pressure 124-1250 C.; specific gravity at 150 C. = 0.9222; rotation aD 4- 470; and refractive index at 200 = 1.4964.

Molecular refraction calculated for CI5H24 with one double bond = 64.45; found 64.89. It thus appears that aromadendrene contains one double linkage in the molecule.

An analysis gave the following results: -

0.1366 gram. gave 0.4388 gram. CO2 and 0.1502 H2O. Carbon 87.6 per cent. and hydrogen, 12.2 per cent. Cl5H24 requires 88.3 per cent. C, and 11.77 per cent. H,

In a paper read by one of us before the Roy. Soc. N.S.W., Nov., 1901, the name Aromadendrene was suggested for this sesquiterpene.

A sample of the sesquiterpene was similarly prepared from the oil of E. Baileyana. The constants were in agreement with those recorded above, with the exception that the sesquiterpene was lævo-rotatory. It boiled at 123-1250 C. at 10 millimetres; had specific gravity at 150 C. = 0.9240; optical rotation aD - 3.7°; and refractive index at 200 = 1.4964.

The characteristic colour reactions will detect the presence of this sesquiterpene in many crude Eucalyptus oils. It can also be shown by this method to occur in that of Angophora lanceolata.