The first investigation of a Eucalyptus oil was undertaken by M. Cloez (Compt. Rend., 1870, p. 687) upon the oil of E. globulus. Trees of this species of Eucalyptus had for some years previously been successfully cultivated in many parts of Europe, although the young shoots of the trees grown at Paris did not withstand the frost, and it was due to this circumstance that Cloez was led to investigate the essential oil obtainable from the leaves. It was from this investigation that the name Eucalyptol was derived, it being given to the portion of the oil distilling at about 175° C.; and although, in the light of subsequent research his results were not strictly correct, yet the facts therein submitted are interesting and valuable, particularly for the following reasons: -

1. He mentions that he obtained the oil from both green and dry leaves, and also from material from Melbourne, and that the oil distilled under these different conditions was always similar, thus indicating the practical constancy of constituents in the oil of identical species of Eucalyptus.

2. On passing hydrochloric acid gas into the oil, he obtained a crystalline mass, the liquid portion becoming of a beautiful blue-violet colour. Although Cloez did not follow the matter up, it is now known that this colour reaction was due to the presence in the oil of the sesquiterpene (aromadendrene). This blue-violet colour is always obtained when this sesquiterpene is treated with the halogen acids.

Homeyer (Arch. Pharfn. (3) v., p. 293), soon after Clociz's research came to very different conclusions on examining 3 kilos of a Eucalyptus oil, the specific gravity of which was 0.8762 at 120 C. This oil had probably been distilled from E. dives or E. radiata, species which were, at that time, looked upon as forms of E. amygdalina. This is probably the first time that a difficulty was experienced in arranging results by different observers on Eucalyptus oils, and was due to the products having been obtained from species belonging to distinct groups of these trees.

The correct placing of Eucalyptol chemically was due to E. Jahns (Ber. 17, 2941), who isolated it fairly pure (specific gravity 0.923 at 160 C. and B.P. 176-177° C), from the compound it forms with dry hydrochloric acid gas. He also showed its analogy with cineol, and that its formula was C10Hl8O.

M. R. Voiry (Compt. Rend. 1888, p. 1419) describes a method for the preparation of pure Eucalyptol (cineol) by freezing it out of the oil. He also states that from the oil of E. globulus were obtained both acetic and formic acids, and shows that both butyric and valeric aldehydes also occur.

Although these aldehydes, which are very irritating when inhaled, are present in most crude Eucalyptus oils, yet, like the other constituents, they occur in varying amounts, being most pronounced in those cineol-pinene oils in which the corresponding esters occur in greatest quantity, such for instance as those of E. cinerea, E. goniocalyx, E. Maideni, etc.

The presence of both ethyl and amyl alcohols was determined by Bouchardat and Olivier (Bull. Soc. Chim. III. 9, p. 429) in the lower-boiling portions of Eucalyptus oil. To these may now be added methyl, iso-butyl and normal butyl alcohols which have been determined during these investigations.

Numerous articles dealing with the special study of several of the constituents of Eucalyptus oils are here appended; such are those relating to cineol, geraniol, terpineol, piperitol, eudesmol, the monohydric alcohols, the cyclic (aromatic) aldehydes, citral, citronellal, piperitone, the phenols, the free acids, the various esters, the terpenes (such as pinene, phellandrene, terpinene), the sesquiterpene, and the paraffin stearoptene.

The remaining constituents, also enumerated in the following list, usually occur in small amount, although some of them are quite of frequent occurrence, as, for instance, cymene, and the lower-boiling aldehydes. Although these constituents are not here treated separately, they will be found enumerated under the particular species in the oil of which they occur.

The list of constituents so far determined in Eucalyptus oils is as follow: -

* Constituents marked with an asterisk are the outcome of these investigations.