The demonstration of the adulteration of an essential oil by chemical means offers many difficulties. We devote particular attention to the physical characteristics, for experience has shown us that the olfactory organ—provided it is very expert—is often able to determine the genuineness of any aromatic substance when other tests have given only uncertain results, or can give certain results only in the hands of experts. To make this test, however, quite reliable, it is necessary to be familiar with the substances in their pure unadulterated condition. The manufacturer of perfumery, therefore, should spare neither trouble nor pecuniary sacrifices to obtain possession of absolutely genuine specimens of those essential oils, even in minute quantities, which he intends to employ. Such samples should be carefully preserved (protected from heat, evaporation, daylight, etc.) for the purpose of immediate comparison with the oils to be purchased. As above stated, the physical properties of the essential oils usually furnish the means of recognizing their purity, and these give more reliable results to the practical perfumer than the chemical tests. The most valuable points are furnished by the boiling-point, the congealing-point, and the density of the oils. The following table gives the boiling and congealing points of the most important essential oils in degrees of the centigrade thermometer, together with the density (or specific gravity); where two figures are given, they indicate the extreme limits found in genuine samples. Special characteristics of some essential oils with reference to their action at low temperatures or their melting-point are given in the column “Remarks.” Oil of turpentine, paraffin, wax, and spermaceti being frequently used for the adulteration of essential oils, have been included in the table. If accurate results are aimed at in the examination of an essential oil according to this table, the specific gravity should be determined by means of a scale sensitive to one one-thousandth gram, and the thermometer should be graduated to the tenth of a degree. TABLE SHOWING THE APPROXIMATE DENSITY, BOILING AND CONGEALING POINTS OF THE MOST IMPORTANT ESSENTIAL OILS USED IN PERFUMERY.