The flowers, the fruits and their rinds, or even the wood of some plants form the receptacles of essential oils; if they are liquid they are called essential oils _par excellence_; if they are firm they are called camphors. Besides, there are intermediate states between them: oil of rose is always viscid and solidifies even at temperatures considerably above the freezing-point of water (see under Oil of Rose). The bodies which are generally called essential oils are usually mixtures of a hydrocarbon with an oxygenated body, or an unchanged oil with another which has become altered by the influence of the oxygen of the air—a condition to which we shall recur later on. With reference to their elementary composition, essential oils may be divided into two groups: 1. Non-oxygenated essential oils. 2. Oxygenated essential oils. The non-oxgenated essential oils consist only of two elements—carbon and hydrogen; the other group, as the name indicates, contains a third element in chemical combination, and consist of carbon, hydrogen, and oxygen. Most of the essential oils of the first group have the same chemical composition: C_{10}H_{16} (10 atoms of carbon combined with 16 atoms of hydrogen). Despite the like chemical composition, all the essential oils display different physical qualities; they vary in density, in refractive power, in boiling-point (often by many degrees), and, a matter of the greatest importance for our purposes, in their odor. We may state at once that but few essential oils can be said to have a pleasant odor; that of most of them is even disagreeable and narcotic to the olfactory nerves; it is only after the oil has been extremely diluted that the odor begins to become pleasant and to resemble that of the plant from which the oil was derived. According to their physical qualities, essential oils may be described as fluids of a specific narcotic odor, colorless but very refractive, and easily inflammable. Only a few essential oils can be produced in such a state of purity as to appear perfectly colorless; usually they are more or less dark yellow in color, and some even possess a characteristic tint; thus oil of acacia is reddish-brown, oils of rose and absinth are green, oil of chamomile is blue. But a simple experiment will show that the color is not inseparably connected with the oil, for certain tinted oils can be obtained perfectly colorless by being distilled with another, less volatile oil which retains the coloring matter. The boiling-point of essential oils is in general very high —between 160° and 288° of the centigrade thermometer (C.), or 320° to 550° F. The fact that we smell the essential oils in aromatic plants so distinctly despite their high boiling-point is an evidence of their exceedingly strong influence on the olfactory nerves.