Cineol forms a crystalline compound with phosphoric acid of specific gravity about 1.75, and this reaction is taken advantage of for its detection, as well as for its quantitative determination. It is not a satisfactory reagent, however, when only a small quantity of cineol is present in the oil, and even if containing as much as 20 or 30 per cent. it is often necessary to start the crystallisation by the addition of a minute crystal of cineol phosphate, prepared with a richer oil, and to use an ice cold bath. The combination between the cineol and the phosphoric acid is a very loose one, and even at room temperature the compound slowly changes with separation of cineol.

The reddish colouration given to Eucalyptus oils by phosphoric acid when cineol is tested by this method, is caused by the reaction between the acid and the sesquiterpene, a constituent which occurs in most Eucalyptus oils, sometimes in considerable quantity. In the oils of some species, however, it is present in very small amount, as for instance in those of the group to which E. Australians belongs, so that in the oils of those species the colouration is usually somewhat indistinct. This is also the case when the oils have been rectified by direct distillation, as in this way the greater portion of the sesquiterpene, or other high-boiling constituent, mostly remains behind in the still, so that the colouration is not shown so distinctly as with the crude oils. The selective influence of cineol for phosphoric acid does not however take place, and the appearance of the pink colouration may thus be deceptive if considered as an indication of the end reaction'.

It has been considered, and is often so stated, that the cineol and phosphoric acid enter into combination in molecular proportions; thus forming a solid compound with a definite composition. Helbing and Passmore (Pharmacological Record No. XXXV) describe this method for determining cineol, and assume the following equation: - C10Hl8O + H3PO4 = C10Hl8OH3PO4, in which the cineol represents 61.1 per cent.

The question is, however, does the phosphoric acid enter into combination with the cineol in molecular proportions, or does it combine in the form in which it is used? If the latter, then the statement that the compound contains 61.1 per cent. of cineol is not correct.

We have endeavoured to solve this problem, with the result that our investigation shows that the phosphoric acid, specific gravity about 1.75, combines with the cineol in its diluted state, so that correctly speaking no general formula can be given, as commercial phosphoric acid has not always the same concentration. From the results of this work, which was carried out with the oils of E. cinerea, E. Smithii, and E. Morrisii, it was found that the mean cineol content in the perfectly dry powdery compound was 59.47 per cent., while the phosphoric acid (H3PO4) only represented 89.27 per cent., the difference being water. The H3PO4 in the acid employed was 89.4 per cent. The theoretical mean for the cineol from the H3PO4 found was 59.56 per cent., thus being in very fair agreement.

The results showed that 59.5 was approximately the amount of cineol in 100 parts of cineol-phosphate, and not 61.1 per cent., as was previously supposed.