The volatile or essential oils are compounds of carbon and hydrogen, and differ from the fixed oils, which are compounds of carbon, hydrogen and oxygen, the molecular arrangements of which are similar to the metallic salts and compound ethers - possessing the radicals of certain acids united with hydro-carbons. The essential oils act as antiseptics without coagulation, and are generally absorbers of oxygen, and such of them as the oils of peppermint, cloves, caraway, cajeput, mustard and turpentine are useful obtunders of hypersensitive dentine; and some are also very efficient germicides and antiseptics. The oil of thyme and oil of cinnamon are generally considered to head the list of the essential oils in antiseptic power. From experiments noticed by Dr. Harlan, the essential oils are not miscible with water; hence when they are used as a root-dressing they are not contaminated or dissipated by the saliva or the mixed fluids of the mouth. They are capable of depositing vaporizable camphors that are powerful disinfectants, and are soluble at a temperature below body heat. They are good menstrua for iodine compounds, and oily dressings, when not too tightly confined in a pulp-chamber, are not vitiated like substances which are soluble in water. They may be allowed to remain for longer periods in the roots of teeth, when it is not possible to fill such roots at once. When an oily dressing (such as oil of pepperment or oil of cassia) is applied in a root canal, after the removal of a pulp by extirpation or its devitalization by arsenic, it acts as an anodyne and also as an antiseptic, preventing the formation of mephitic gases. The oily dressings may be pumped through roots and fistulous tracts with impunity, and will not clog delicate canals. The solutions of the essential oils are all made as follows (Dr. Black): "An excess of the oil is mixed with water by violent shaking continued for some minutes. It is then placed in the incubating oven, where the temperature is 990 F., for twelve hours. It is then again violently shaken and returned to the oven for another twelve hours, twenty-four hours in all. It is then carefully filtered, and if not perfectly clear and free from all appearance of oil, it is refiltered until it is clear. The filtrate is then used as the solution of the oil. The same plan is used in making saturated solutions of the crystalline substances."

The prominent essential oils are those of cajeput, cassia, cinnamon, cloves, eugenol,eucalyptol, fennel, mustard, caraway, peppermint, sassafras, turpentine, thyme, pennyroyal, valerian, winter-green. In dental practice the oil of cassia diluted with a bland oil like that of wintergreen is a valuable dressing for putrid root canals and abscesses. Dr. Black recommends the following mixture:

Carbolic acid (melted crystals), 1 part; oil of cassia, 2 parts; oil of wintergreen, 3 parts. Mix the oils and add the melted crystals of carbolic acid. This is known as the 1, 2, 3 mixture.

Dr. Black also says: "After thorough cleaning with the watery form of the antiseptic we need something that will be more enduring in its effects, and according to conditions, should choose between the oils and the powders. If it is an abscess with which Essential Oils 1026 we have to deal, an oil which is in itself an antiseptic, or an oil holding the antiseptic drug in solution in effective proportions, may be introduced into the cavity and so agitated as best to bring it in contact with all of its parts. This will, on account of its sparing solubility, remain in position very much longer than the watery forms, and the essential oils are very much more diffusible than the fixed oils, or, indeed, any other of the simple forms of the antiseptics with which I am yet acquainted. At the same time a sufficient amount for very extended work is contained in small compass. These are of especial value in such positions as the roots of teeth. In this position any but the most irritating of the antiseptic essential oils may be used to good advantage, and may be relied upon for many days together. In the choice of the essential oils it is by no means necessary that the most powerful of them be selected. It should be remembered in making the choice that the more powerful antiseptics are the more irritating as a rule. It is, therefore, often best to choose antiseptics of very moderate range, especially where it is only required to preserve a condition of asepsis. For instance, when a very foul root canal is opened, such an antiseptic as the oil of cassia is indicated. After appropriate cleaning, and especially in case cleaning is to be deferred for fear of forcing poisonous material through the apical foramen, it may be used in full strength; in which form it may be relied upon as a disinfectant as well. But afterward, when it is only a matter of holding an aseptic condition during the healing of the parts, an antiseptic of shorter range, that is not so liable to injure the tissues, is to be preferred. For this purpose the eucalypti extract in substance is a very useful agent. Its range of actual inhibition is very short, but its injurious effects on the tissues are also very slight, so that the healing process may go on in its presence without hindrance. Furthermore, this oil has a very extended range of restraining influence beyond its range of actual inhibition that is undoubtedly of much use. This oil may be exchanged for terpinol where a little more stimulating effect is regarded as beneficial. The oil of cloves and the oil of cinnamon seem to occupy a middle ground, and may be made useful in a large class of cases if the others are in any wise distasteful to the dentist or his patient. The oil of mustard, though a good antiseptic of short range, has an irritant action that limits its use. Yet in cases where it is desirable to arouse the tissues from a state of inactivity this action, in a high degree, is combined with the antiseptic property. It is also one of the most diffusible of the antiseptic oils thus far studied.

"Any of these oils may be used in the emulsion, if, for any reason, it is not thought well to use the oil in substance. This form is especially recommended for surfaces of suppurating wounds and the washing of abscesses. In this way small quantities of the oil may be widely diffused and left in a multitude of minute globules, to gradually dissolve where it is most wanted, forming a kind of connecting link between the true solutions and the oils in substance. For this purpose a little of the oil selected may be diffused through water by severe shaking, or better, by repeatedly filling and violently emptying the syringe. The milky emulsion thus formed may be used in the same manner as the solution.

"I have said that all antiseptics are poisons. I wish to emphasize this statement. They are antiseptic by virtue of their power of restraining life forces; and their use as antiseptics is permitted only by shades of difference in the action of certain poisons toward the different forms of life, by which they affect the fungi more prominently than the animal tissues."