Considered broadly the variation in the amounts of oil obtainable from the leaves of any particular species of Eucalyptus depends largely on the season and on the condition of growth of the material employed. In times of drought the secretion of oil appears to diminish, or else it is used up in the struggle for existence, but after an abundance of rain the trees soon again yield the normal amount of oil.

Under ordinary conditions the larger quantity occurs in the spring and early summer, when the growth is more vigorous, falling again during the winter months, and with most of the cineol oil-producing species the product becomes correspondingly richer in that constituent as the oil decreases in amount. Not only is this the case with the oil while in the leaf, but even after extraction an increase in cineol has been observed, more particularly with oils derived from species belonging to one well-defined group. Several instances of this increase in cineol after extraction will be found recorded in this work.

The identity of the constituent responsible for this alteration is not at present clear, although the change is apparent mostly in those oils in which the formation of the insoluble deposit has been observed. This question is more fully considered in the article dealing with this deposit.

The increased yield is usually associated with an increase in the terpenes characteristic of the oil of the species, as pinene in the pinene-cineol oils, phellan-drene and pinene in the phellandrene-pinene-cineol oils; and phellandrene in the phellandrene-cineol oils.

The yield of oil from the "suckers" or adventitious shoots is, with many species, greater than from the mature lanceolate leaves, and in some cases this increase is considerable. The constituents characteristic of the oil of a particular species are, as a rule, also those of the product from the abnormal growth, a fact of considerable economic importance.

In a paper (Proc. Roy. Soc. Vic. XII, 10, and Pharm. Journ. 3, 5, 270), Mr. Bosisto deals with the problem, whether the Eucalyptus is a fever-destroying tree. In this paper he calculates the probable amount of oil in gallons existing in these trees over a large area of country at any one time. But when we consider the varying amounts of oil contained in the leaves of the several species such approximations must be merely guesses. By referring to the table of yields it will be seen that the oil obtainable from the various species ranges from 3.5 per cent, down to practically nothing; so that without a fair approximation of the extent or range of the various species is determined, it appears hopeless to attempt to solve the problem of available supply by this method. That it is practically unlimited in Australia we readily admit, and also that cineol oils conforming to the British Pharmacopoeia standard can be produced in any quantity required. This statement is also true for the phellandrene oils. It cannot but be observed what a great variety of oils are obtainable from the several Eucalyptus species, and how large a proportion of these contain a predominance of terpenes, principally pinene and phellandrene. The yield from trees giving terpene oils is often large, especially those in which phellandrene predominates, and this fact has naturally led to the exploitation of these species for mineral separation and other industrial purposes.

By referring to the table it will be seen that such species as E. polybractea, E. Morrisii, E. Smithii, E. cordata, etc, all of which yield oils rich in cineol, almost hold their own, in regard to yields, with many of the "peppermint" or phellan. drene.bearing species.

It is to be expected that the yields of oil will influence largely the commercial consideration of distillation, and for this reason we append the following table, which in almost every case records the results of our own distillations; included are a few the authenticity of which there is no doubt. The figures given represent the mean of the results obtained with the leaves of any individual species recorded in the list, and in many cases these spread over more than twenty. five years. The leaves and terminal branchlets were taken in all instances as would be done for distillation on a commercial scale, and the leaves in all cases were as fresh as it was possible to obtain them. Of course, in some instances, evaporation had taken place, as some of the material came hundreds of mites, both by land and sea, packed closely in bags. We have given the average yields in preference to that of any one distillation, because this more nearly represents the actual amount of oil that would be obtained if the species was commercially distilled.

In practically the whole of the species recorded in the table the oil was distilled from the mature leaves and not from abnormal leaves, so that in these cases the results may be considered to represent the minimum yields.

The yield per 1,000 pounds of material has also been calculated, as the statement in pounds and ounces appeals more directly to the commercial world.

E. obtusiflora and E. terminalis did not yield sufficient oil to collect.