These are the substances to which many plants owe their characteristic or essential odors. On this account they are often spoken of as "essential oils," or as the "essences" of plants.

They differ from the fixed oils in that -

1. They are volatile, therefore can be distilled and do not leave a permanent grease stain.

2. They do not form soaps with alkalies.

3. They are soluble enough in water to impart to it their odor and taste.

4. They do not become rancid, but on exposure to light and air tend to oxidize and resinify.

They mix freely in any proportions with chloroform, ether, and the fixed oils, and are all soluble in absolute alcohol. Some, like oil of turpentine, require several times their own weight of official alcohol for complete solution. They are all mixtures, some of them quite complex.

Occurrence

Most of them are found in plants, and each in a definite part of the plant from which it is derived, e. g., oil of orange in the rind of the fruit; oil of cinnamon in the bark; oil of rose in the petals. From these parts they are obtained either by distillation or by means of a suitable solvent, such as benzin, which is afterward removed. Some of the delicate essential oils used in perfumery, as violet and heliotrope, are obtained by spreading the petals or flowers between wax plates, and afterward separating the absorbed oil from the wax.

A few of the volatile oils do not exist in the living plant, ancf are formed either by the action of ferments on glucosides in the presence of water, as the oil of bitter almonds, or by destructive distillation. These latter are known as empyreumatic oils.

For convenience, the volatile oils preexisting in the plant may be grouped according to their nature, and those developed in the plant part by artificial means may be grouped according to their method of production.

A.

Existing in plant as such:

1.

Terpenes, Cxhx (oils of turpentine, juniper, etc.).

2.

Terpenes + stearoptens (oils of lemon, peppermint, etc.).

B.

Not existing in plant as such, but developed from plant constituents:

3.

From enzyme action (oils of mustard and bitter almond).

4.

Empyreumatic (oil of cade, oil of tar, creosote).

Group 1 is composed of oils which are mixtures of terpenes (hemiterpenes, terpenes, sesquiterpenes, diterpenes, pinene, etc., C10H16), the simplest hydrocarbon oils of the aromatic series. Of all the volatile oils, they are the least soluble in water and the most ready to resinify and deteriorate. Examples are: the oils of copaiba, cubebs, erigeron, juniper, and turpentine. The last named consists almost wholly of dextrorotary pinene.

Group 2 includes the mixtures of terpenes which are holding in solution one or more oxygenated bodies (of variable chemic nature, as aldehydes, ketones, ethers, acids, etc.). The terpene portion is known as the eleopten, and the oxygenated portion as the stearopten. The latter is usually solid, though sometimes liquid. It can be separated from the eleopten by cold (as the menthol of peppermint oil) or by fractional distillation. It is not always readily soluble in 95 per cent. alcohol. Examples of stearoptens which are separated and used by themselves are camphor and menthol. It is to the stearopten that the characteristic odor of these oils is chiefly due, but the amount of stearopten present varies with the different oils. For example, the oils of orange or lemon contain only a small percentage of their peculiar stearopten and are nearly all eleopten, while the oils of winter-green and birch are almost entirely composed of a liquid stearopten, which chemically is methyl salicylate.

The oils of this group are for the most part more soluble in water, and, because of the stearopten, more agreeable in flavor than those of Group 1, so they are largely used in the manufacture of the medicated waters and spirits. Some of them are heavier than water, as the oil of cinnamon.

Group 3 contains those oils which do not preexist in the living plant, but result from ferment action in the presence of water. The official ones are the oil of bitter almond and the volatile oil of mustard. (For the reactions in the development of these oils see under Glucosides above.)

Group 4 contains the empyreumatic oils, those which do not preexist in the plant, but result from its destructive distillation. The official ones are: Oil of cade (oleum cadinum), from juniper wood, and oil of tar (oleum picis liquidae), from the wood of Pinus palustris and other species of pine. Both have a tarry odor, and are added to ointments for the treatment of chronic skin diseases. The syrup of tar (syrupus picis liquidae), in dose of 15 minims (1 c.c), is also used as an expectorant.

Creosote is a mixture of phenols and phenol derivatives, obtained during the distillation of wood-tar, and has some of the properties of a volatile oil. The beechwood creosote is considered best for medicinal purposes.

The volatile oils have marked pharmacologic actions, but do not belong to a single pharmacologic group. Their action will be considered in Part II.

10. The resins are all, or nearly all, mixtures of several different substances. They are an ill-defined group, forming amorphous masses which have a conchoidal shining fracture. They are insoluble in water and soluble in ether, chloroform, and the volatile oils. Many, but not all, of them are soluble in alcohol, and most of them dissolve in alkali with the formation of a non-detergent resin-soap, which is miscible with water. Their composition is still a subject of study. Some of them, and perhaps all of them, are formed by the oxidation of volatile oils, in association with which in the plant they mostly occur. Common rosin, and the resins of jalap, podophyllum, and scammony are official resins.

11. The oleoresins are the natural plant exudates which contain both volatile oil and resin. Balsam of copaiba, Canada balsam, and crude turpentine are examples, common rosin and oil of turpentine being the components of crude turpentine. (These natural oleoresins must be distinguished from the pharmaceutic oleoresins, which are artificial ethereal extracts of oily and resinous drugs, i. e., extracts made with ether.)

12. The gum resins are generally oleoresins in natural admixture with gum. They are obtained by the evaporation of the milky juices of certain plants. On rubbing a gum resin with water the gum dissolves, and with the oil and resin forms a milky emulsion. Asafetida and gamboge are examples.

13. The balsams are resinous or oleoresinous exudates which contain benzoic or cinnamic acid, or both. These latter impart a "balsamic" odor. Benzoin, storax, balsam of Tolu, and balsam of Peru are official examples. Many fragrant substances are incorrectly called "balsam," e. g., balsam of copaiba and Canada balsam, both of which are oleoresins. In some instances the resins, oleoresins, gum resins, and balsams are the only commercial representatives of their respective plants.

Keratin is obtained from horn by dissolving out the albuminous matter with artificial digestion, and macerating the residue in ammonia. It is soluble in alkalies and insoluble in acids, and is employed as a coating for pills and capsules which it is desired to have pass through the stomach without action - the so-called "enteric" pills. Theoretically, if the pills are given after meals, the coating should not dissolve in the stomach, and the medicinal agents should be set free only when the pills reach the alkaline intestinal contents. As a matter of fact, however, commercial keratin is not always proof against disintegration in the stomach, and as a coating must be considered unreliable.