The essential Oil of Eucalyptus used in medicine is obtained by aqueous distillation of the fresh leaves. It is a colorless or straw-colored fluid when properly prepared, with a characteristic odour and taste, soluble in its own weight of alcohol. The most important constituent is Eucalyptol, present in E. globulus up to 70 per cent of its volume. It consists chiefly of a terpene and a cymene. Eucalyptus Oil contains also, after exposure to the air, a crystallizable resin, derived from Eucalyptol.
The British Pharmacopoeia requires Eucalyptus Oil to contain not less than 55 per cent, by volume, of Eucalyptol, to have a specific gravity 0.910 to 0.930 and optical rotation -10 degrees to 10 degrees.

The leaves are leathery in texture, hang obliquely or vertically, and are studded with glands containing a fragrant volatile oil. The flowers in bud are covered with a cup-like membrane (whence the name of the genus, derived from the Greek eucalyptos well-covered), which is thrown off as a lid when the flower expands. The fruit is surrounded by a woody, cup shaped receptacle and contains numerous minute seeds.
Eucalyptus trees are quick growers and many species reach a great height.

There are a great number of species of Eucalyptus trees yielding essential oils. About twenty-five species are at the present time being utilized for their oil.

E. globulus, the best-known variety ( named by the French botanist De Labillardière)It was Baron Ferdinand von Müller, the German botanist and explorer (from 1857 to 1873 Director of the Botanical Gardens in Melbourne), who made the qualities of this Eucalyptus known all over the world, and so led to its introduction into Europe, North and South Africa, California and the non-tropical districts of South America.

• Eucalyptus

He was the first to suggest that the perfume of the leaves resembling that of Cajaput oil, might be of use as a disinfectant in fever districts, a suggestion which has been justified by the results of the careful examination to which the Eucalyptus has been subjected since its employment in medicine. Some seeds, having been sent to France in 1857, were planted in Algiers and thrived exceedingly well. Trottoir, the botanical superintendent, found that the value of the fragrant antiseptic exhalations of the leaves in fever or marshy districts was far exceeded by the amazingly powerful drying action of the roots on the soil. Five years after planting the Eucalyptus, one of the most marshy and unhealthy districts of Algiers was converted into one of the healthiest and driest. As a result, the rapidly growing Eucalyptus trees are now largely cultivated in many temperate regions with the view of preventing malarial fevers. A noteworthy instance of this is the monastery of St. Paolo à la tre Fontana, situated in one of the most fever-stricken districts of the Roman Campagna. Since about 1870, when the tree was planted in its cloisters, it has become habitable throughout the year. To the remarkable drainage afforded by its roots is also ascribed the gradual disappearance of mosquitoes in the neighbourhood of plantations of this tree, as at Lake Fezara in Algeria.

In Sicily, also, it is being extensively planted to combat malaria, on account of its property of absorbing large quantities of water from the soil. Recent investigations have shown that Sicilian Eucalyptus oil obtained from leaves during the flowering period can compete favourably with the Australian oil in regard to its industrial and therapeutic applications. Oil has also been distilled in Spain from the leaves of E. globulus, grown there.

In India, considerable plantations of E. globulus were made in 1863 in the Nilgiris at Ootacamund, but though a certain amount of oil is distilled there locally, under simple conditions, little attempt has hitherto been made to develop the industry on a commercial scale, Australia remaining the source of supply.

A great increase in Eucalyptus cultivation has recently taken place in Brazil as a result of a decree published in 1919 awarding premiums and free grants of land to planters of Eucalyptus and other trees of recognized value for essence cultivation.