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Home , Citronella (genus), Eucalyptus

784 NATURE [MAY 28, 1927

The Essential Oils of the . By Prof. JoHN READ, University of St. Andrews. rJ1HE timbers and the resinous exudations, or possible only through the combined efforts of the _j_ 'gums,' of the eucalypts have deservedly botanist and the organic chemist. Investigations of attracted much attention ; but chemically, if this nature, which were prosecuted with unflagging not economically, the greatest interest of this zeal through a period of more than thirty years by leading of centres around the R. T. Baker and H. G. Smith, of the eucalyptus oils. These ' essential ' oils are pro• Technological Museum, have demonstrated the duced abundantly in the minute -glands of remarkable constancy of composition of the leaf-oil the eucalypts, and sometimes they may also be derived from any particular of , and distilled from the and timber. Essential have rendered possible a chemico-botanical classifi• oils, as the name indicates, possess fragrant odours ; cation of the various species, through the elucidation they are more mobile and more volatile than the of certain remarkable relationships between chemi• ' fixed ' oils, with which they must not be cal and botanical characteristics in the genus. confused; further, unlike the fixed oils, they are It is usually taken for granted that ' eucalyptus unassimilable, being in no way related to the fats. oil' consists mainly, or wholly, of the familiar The early settlers in Australia were quick to eucalyptole, or cineole, the smell of which is so notice the value of the eucalypts as sources of familiar during epidemics of colds and influenza. essential oils. Dr. John White, surgeon-general In point of fact, however, cineole is merely one out to the first settlement, attracted by the strong of about forty chemical components which have odour of a common species growing been discovered in the oils of this genus since 1870. around Port Jackson, was led to make the first According to the interesting evolutionary theory distillation of a in 1788. In his of Baker and Smith, the original eucalypts were "Journal of a Voyage to " (1790, evolved in north- from the still p. 227) it is recorded that "the name Peppermint older genus , and such species still has been given to this plant by Mr. White on predominate in this region; the in this group, account of the very great resemblance between the of which the well-known Bloodwood (E. corymbosa) drawn from its leaves and that obtained is an example, possess a ' feather ' venation and from the Peppermint ( piperita) which grows are very poor in oil, of which the main component in . This oil was found by Mr. White to is the turpentine hydrocarbon, pinene. At the be much more efficacious in removing all cholicky other end of the evolutionary scale, the most complaints than that of the English Peppermint, recently evolved species occur mainly in the south• which he attributes to its being less pungent and eastern portion of the continent; the leaves have more aromatic." The species of eucalypt which a' butterfly-wing' venation and are thickly studded furnished this oil is now known as Eucalyptus with oil-glands, so that the yield of oil may exceed piper,ita, and it is common in the Sydney district four per cent. of the weight of the leaves and twigs. and the Blue Mountain Ranges of New South Wales. The Broad-leaved Peppermint (E. dives), a widely It was sixty-six years later that the first eucalyptus distributed member of this group, furnishes an oil oil factory was established in Australia by Bosisto, consisting largely of phellandrene, in association while the first chemical investigation was made by with an interesting peppermint ketone, called the French chemist, Cloez, in 1870, upon an oil piperitone, which promises to assume considerable yielded by specimens of E. globulus grown in France. importance as a commercial source of synthetic Such were the modest beginnings of the utilisa• and thymol. Certain oils in this group tion and scientific examination of eucalyptus oils. are used also in the flotation process for the Every Australian knows that a eucalyptus leaf, separation of metallic sulphides from their ores. when crushed, often emits an agreeable odour; Cineole is the chief component of the oils from many know that these odours may vary consider• an intermediate group of eucalypts ; it occurs as a ably, from tree to tree, throughout a stretch of rule in association with pinene, in such species as bush; and some are able to effect a rough classi• E. globulus, E. Smithii, E. Australiana, and many fication of these , based upon such observations. others. Oils of this are used largely in Few, however, are able to proceed beyond this point, pharmacy, and they are sometimes so rich in and it is remarkable that so little exact knowledge cineole that the crude ' first-hour oil' readily should exist, in the popular mind, of a genus which deposits a solid glacial mass of this substance when is rightly held in such esteem as an emblem of placed in a freezing chamber. Australia. In remote parts of ' bush• Chemically, therefore, it is possible to discern whackers' born and bred among the eucalypts have three main groups of eucalyptus oils; but in been known to assert that there are five kinds of addition there are many exceptional species the ' gums ' ; but Australian men of science who have leaf-oils of which contain such valuable components devoted their lives to a study of this wonderful as geraniol (E. Macarthuri), citronella! (the Citron• genus have distinguished some two hundred and scented Gum, E. citriodora) and citral (the Lemon• fifty species, and the tale is even yet incomplete. scented Ironbark, E. Staigeriana). As a rule, So interwoven are the relationships, so refined the each chemical constituent is found to increase distinctions, that in some instances discrimination through a range of species until it reaches a maxi• between closely related species has been rendered mum value in the final member. In exceptional No. 3004, VoL. 119]

© 1927 Nature Publishing Group MAy 28, 1927] NATURE 785 cases, such as that of E. Macarlhuri, the end species The harvesting of eucalyptus leaves from the alone appears to have survived. In spite of the forms is simpler than from the trees, but intricate relationships in the genus, ability to dis• the subsequent operations are the same for material criminate between the main types is not particu• from either source. In order to liberate the oil, larly difficult to acquire. As a practical aid in the leaves are brought into contact with steam, such studies, an interesting record of the character which ruptures the oil-glands and causes a slow of the leaf-venation and the disposition of the vaporisation of their contents. The primitive oil-glands may be obtained by making direct foym of bush-still consists of the cubical iron tank sun-prints of the leaves on sensitised . The so familiar to Australians ; this contains the identification of indigenous species in any particular tightly packed leaves resting upon a grating, area is helped by the fact that the chemical fastidi• below which is boiled by means of a ousness of the eucalypt is accompanied by an fire underneath the tank. As the steam forces its equally marked susceptibility to environment, so way up through the mass of leaves it becomes that changes in such factors as geological formation, charged with the vaporised oil, and the mixed rainfall, and altitude are reflected in the character vapours are condensed during their passage through of such species. an exit tube cooled by immersion in a creek, or in In harvesting eucalyptus leaves for distillation some more refined manner. The resulting mixture the trees may either be lopped or felled, and of water and oil runs down the tube and is collected although the latter method may appear extrava• in a receiving vessel, which is so constructed as to gant, yet experienced distillers often favour it. allow the relatively small layer of oil to be drawn The phamix-like eucalypt conforms to the general away from the water upon which it floats. Other motto of the Australian flora, which is ' Resurgam ! ' types of plant possess an independent boiler It combines amazing vitality with unusual rapidity which supplies steam under pressure to a series of of growth ; and so, after the lapse of a few years, digesters, these latter being sometimes sunk into the decapitated stump may have surpassed the the ground to facilitate the handling of the fresh ideal of Dean Swift by producing not merely and the spent leaves. two, but three, or even four, sturdy trunks where The prices realised by eucalyptus oils range over only one grew before. To that bizarre list of alleged a wide scale. The value of the oil depends upon Australian paradoxes which circulates so freely its chemical composition, which, although sensibly outside Australia, to the mingled amusement and constant for any particular species, varies enor• annoyance of good Australians, may thus be added mously from one species to another. The Blue the less familiar but more truthful statement that Mallee, for example, yields a cineole oil having a lopped or felled eucalyptus tree, rising on the a market value of somewhat more than a shilling stepping-stone of its dead self, may in a few years a pound to the distiller, while the citronella! oil of develop more foliage than it originally possessed. the Citron-scented Gum of Queensland brought in The mallee scrub in the Wyalong district of New as much as six shillings per pound during the War. South Wales is treated in a still more drastic The reputation of the cineole oils has suffered manner. The mallee, a type of eucalypt embracing in the past, owing to the multiplicity of species many species, is a dwarfed form, having a number yielding such oils and to the confusion which has of small stems instead of the usual single trunk. existed between these species in the field. A In the western part of New South Wales, as also in vernacular name, such as Messmate, may easily South Australia and other regions, mallee eucalypts be interpreted in half a dozen different ways, cover vast areas, and the essential oil of the Blue according to the tastes of the individuals concerned. Mallee (E. polybractea), the dominant species in It is therefore a satisfaction to find reputable oil the Wyalong district, is worked extensively for distillers adopting the systematic Latinised names ; cineole. A second species, E. oleosa, or Water and although the use of scientific nomenclature MaBee, secretes water in its , a fact which in the Australian bush may occasion some degree was fully appreciated by the aboriginal inhabitants of surprise, yet-as a ' bushwhacker ' once re• of Australia. Other important mallees are E. marked in different words-there is nothing odorata and E. cneorifolia, from which the bulk inherently difficult in the pronunciation of ' Eu• of the South Australian eucalyptus oil is extracted ; calyptus Macarthuri,' and the name is decidedly the last-named species occurs only on Kangaroo more euphonious than the synonymic Camden Island. In dealing with the Blue Mallee, the oil Woolly Butt or Paddy's River Box. Fortunately, distiller flattens and partly uproots the mallee neither E. Luehmanniana nor E. macrorhynca is scrub by driving a heavy roller over it; and after worked for oil ! the hardly used vegetation has dried in the sun As his readiness to use these strange names he completes its apparent destruction by burning shows, the oil distiller is eminently adaptable ; it off. The bare waste which repels the eye at he. has been known to write his letters with home• this juncture seems to be devoid of any germ made ink by the light of a lamp burning of life. In a short time, however, the irrepres• the oil of E. JWacarthuri. He is, indeed, a virile sible eucalypt reappears ; a pleasing dull blue and picturesque Australian type, full of the lore mantle of E. polybractea covers the landscape; of the bush. Let us hope that some day an and after the interval of a year the oil distiller is Australian master will arise to do by him as gladdened by the sight of a luxuriant growth of Thomas Hardy has done by the tranter and the mallee rising to the height of his waist. reddleman of Wessex. No. 3004, VoL. 119]

© 1927 Nature Publishing Group