Olea Volatilia. [Olea Destillata]. Volatile Oils.

Related entry: Oils

Olea Aetherea, Aetherolea; Ethereal Oils, Essential Oils, Distilled Oils; Huiles Volatiles, Huilea Essentielles, Huilea Distillees, Essences, Fr.; Flüchtige Oele, Aetherische Oele, G.; Essenze, It.; Esencias, Sp.

These are sometimes called distilled oils, from the mode in which they are usually procured; sometimes essential oils, from the circumstance that they possess, in a concentrated state, the organoleptic properties of taste and odor of the plants from which they are derived. The word essence is used in same countries to designate the volatile oils themselves, but in America this word has unfortunately been largely applied to the alcoholic solutions of the volatile oils, usually called the spirits, and the word soluble essence has been used to describe hydro-alcoholic solutions of volatile oils which possess the property of mixing clear with water.

The term Aetherolea is used in some countries to distinguish the volatile oils in general from those which are produced by destructive distillation, like oil of amber, oil of cade, oil of tar, etc., which are given the collective name pyrolea (pyroligneous oils).

They exist in all odoriferous vegetable tissues, sometimes pervading the plant, sometimes confined to a single part; in some instances contained in distinct cells, and partially retained after desiccation, in others formed upon the surface, as in many flowers, and exhaled as soon as formed. Occasionally two or more are found in different parts of the same plant. Thus, the orange tree produces one oil in its leaves, another in its flowers, and a third in the rind of its fruit. In a few instances, when existing" in distinct cells, they may be obtained 'by pressure, as from the rind of the lemon and orange; but they are generally procured by steam distillation. (See page 738.) Some volatile oils, as those of bitter almond and mustard, are formed in the presence of moisture out of substances of a different nature pre-existing in the plant, and may be subsequently distilled.

The volatile oils are usually colorless when freshly distilled, or at most yellowish, but some are brown, red, green, or blue. There is reason, however, to believe that in all instances the color depends on foreign matter dissolved in the oils. Septimus Piesse succeeded, by the fractional distillation of certain volatile oils, in separating a blue liquid, which, by repeated rectification, he has obtained quite pure. In this state it has the sp. gr. 0.910, and a fixed boiling point of 302.3° C. (576° F.), and yields a dense blue vapor having peculiar optical properties. He named this principle azulene, and believed that upon it depends the blueness of volatile oils wherever existing. The yellowness of the oils he ascribed to the resin resulting from their oxidation, the green and brown colors to a mixture of azulene and resin in various proportions. The formula of azulene he gave as C₁₆H₂₆O. (Chem. News, Nov. 21, 1863, p. 245.) Gladstone named this blue coloring constituent caerulein, and stated that it contains nitrogen and is colored green by acids and alkalies. The volatile oils differ from the fixed oils in not yielding glycerol when treated with alkalies.

The volatile oils have a strong odor, resembling that of the plants from which they were procured, though generally less agreeable. Their taste is hot and pungent, and, when they are diluted, is often gratefully aromatic. The greater number are lighter than water, though some are heavier; their sp. gr. varies from 0.847 to 1.17. They vaporize at ordinary temperatures, diffusing their peculiar odor, and are completely volatilized by heat. When distilled alone, they nearly always undergo partial decomposition. Heated in the open air, they take fire and burn with a bright flame attended with much smoke. Almost all of them are optically active, and advantage may sometimes be taken of this property to detect adulterations of one of these oils with another. The refractive index is another physical constant that is being studied in connection with volatile oils, but the accumulation of data is not yet sufficiently large to give the factor much value as yet. Exposed at ordinary temperatures, some of them absorb oxygen, assume a deeper color, become thicker and less odorous, and are ultimately converted into resin. This change takes place most rapidly under the influence of light. Before the alteration is complete, the remaining portion of oil may be recovered by distillation.

The volatile oils are sometimes hydrocarbons, although with these are frequently associated alcohol- or ketone-like bodies called camphors, and products of oxidation known under the general name of resins, and undoubtedly formed from the hydrocarbons. Phenols, aldehydes, esters, ethers, and sulphides are also represented. The hydrocarbons are generally known as terpenes, from oil of turpentine, which is taken as a type. Olea aetherea sine terpeno is the name proposed by Schweissinger for concentrated volatile oils made so by the removal of the non-fragrant hydrocarbon, and representing from two to thirty volumes of the ordinary essential oils. Thus, one volume of the concentrated oil represents two volumes of the oils of anise, cassia, fennel, gingergrass, mentha crispa, mentha piperita, cloves, sassafras, and star anise; two and one-half volumes of the oils of bergamot, caraway, and lavender; four volumes of cumin and rosemary; five volumes of thyme; six volumes of coriander; eight volumes of calamus; ten volumes of absinthe; twenty volumes of juniper; thirty volumes of angelica, lemon and orange. It is asserted that these concentrated oils are more permanent, more soluble in alcohol and in water, have a finer odor, and are of constant composition, thus enabling the specific gravity and boiling point to be used as tests of purity. They should be kept in the dark. (Ph. Centralh., 1888, No. 25.) Under the name of "terpeneless volatile oils," similar products can now be found in the market; they are undoubtedly superior to the ordinary volatile oils both in odor and strength.

Wallach, to whom much of our knowledge on volatile oils is due, divides the hydrocarbons into classes, as follows:

1.—True terpenes, of the formula C₁₀H₁₆, of which there are two main groups: (a) the terpane group, uniting with two molecules of haloid acid or four atoms of bromine; this group includes limonene, dipentene, sylvestrene, terpinolene, terpinene, thujene, and phellandrene, and its members boil between 175° and 185° C. (347° and 365° F.); (b) the camphane group, uniting with one molecule of haloid acid or two atoms of bromine; this group includes pinene, bornylene, camphene, and fenchene, and its members boil between 151° and 161° C. (303.8° and 321.8° F.).

2.—Hemiterpenes, of the formula C₅H₈, such as isoprene.

3.—Polyterpenes, such as cedrene, cubebene, etc., of the formula C₁₅H₂₄ (sesquiterpenes); colophene, of the formula C₂₀H₃₂; and caoutchouc, of the formula (C₁₀H₁₆)x.

Hydrocarbons other than those of the terpene class or derivable from them, occur very sparingly in the natural oils. Thus there is of the paraffin series of saturated hydrocarbons, heptane, C₇H₁₆, occurring in the oil from the Pinus sabiniana, or California digger pine, and solid hydrocarbons of the same series in oil of rose, and probably in oils of wintergreen and sweet birch. Of the benzene series there is a single representative in cymene, C₁₀H₁₄, found in the oils of the Thymus and Monarda species.

In addition to these naturally occurring hydrocarbons, there is a class of artificially prepared hydrocarbons known as hydroterpenes, such as dihydrodipentene from dipentene, menthene and carvomenthene from menthol and carvone.

The terpenes in general are practically insoluble in water, but soluble in alcohol, ether, chloroform, benzene, petroleum benzin, and the fixed and volatile oils.

Many of the essential oils, however, owe their essential character and their value to other constituents than hydrocarbons. We shall briefly notice the most important of these, either describing them or referring to other places in the text where they are already described, and shall then give a classification of the commonly known essential oils based upon the character of the most important compounds present in them.

I. Alcohols.

Geraniol, Linalool, Citronellol, Terpineol, Menthol, Borneol, Nerol, Sabinol, Santalol.

II. Aldehydes.

Citral, Citronellal, Salicyl Aldehyde, Cumic Aldehyde, Benzaldehyde, Cinnamic Aldehyde, Anisic Aldehyde, Piperonal, Santalal.

III. Ketones.

Methyl-amyl-ketone, Ionone, Methyl-nonyl-acetone, Methyl-heptenone, Carvone, Pulegone, Menthone, Camphor, Fenchone, Thujone, Irone.

IV. Esters.

Methyl Salicylate, Methyl Anthranilate, Benzyl Acetate, Linalyl Acetate, Geranyl Acetate, Bornyl Acetate, Menthyl Acetate, Bornyl Valerate, Geranyl Tiglate.

V. Phenols and Phenol Ethers.

Chavicol, Methyl-chavicol, Anethol, Thymol, Carvacrol, Eugenol, Methyl-eugenol, Iso-eugenol, Chavibetol, Safrol, Iso-safrol, Thymohydroquinone dimethyl ether, Asarol, Apiol, Diosphenol.

VI. Neutral Bodies, Oxides, Etc.

Cineol (Eucalyptol).

VII. Sulphides and Sulphur Compounds.

Diallyl Disulphide, Allyl Isothiocyanate, Dimethyl Sulphide, Butyl Isothiocyanate, Vinyl Sulphide, Acrinyl Isothiocyanate, Phenyl Ethyl Isothiocyanate.

The classification of the volatile oils in any rational and practical manner is a difficult task.

Any system that is devised is either exceedingly complex and cumbersome or, if conciseness is attempted, there are too many inconsistencies and discrepancies. For instance, one of the favored methods of classification is to divide them into four groups, i.e., the terpenes, oxygenated oils, nitrogenated oils, and sulphurated oils. If this be critically studied it is seen that oil of lemon, which is usually given as a typical example of the terpene class, owes its real identity and value to citral, an aldehyde present in only a very small proportion (less than 5 per cent.), and that oil of bitter almond, which is given as the example of the nitrogenated class, owes its classification in this respect only to the incidental presence of hydrocyanic acid, which is frequently removed from the oil, especially when it is used for flavoring purposes. This leaves the sulphurated class, which contains only one important member and the oxygenated class, which is the repository of over 90 per cent. of oils of diversified character.

It is therefore more suitable to follow the foregoing outline adding such explanatory matter and examples as will serve to make it intelligible and valuable for either study or reference purposes.

NOTE.—Although the alcohols only were mentioned in the first group given above it will be found convenient to refer to the more important esters of these alcohols as they are taken up for discussion.

I. Alcohols and Their Esters.—This is an interesting group of open chain, unsaturated alcohols, which, with their esters and aldehydes, play a very important part in the composition of many essential oils. It is the geraniol, linalool and citronellol group.

1.—Geraniol, C₁₀H₁₇OH, or

CH3 \	C=CH.CH2.CH2.C	/ CH3
CH3 /		\\ CH.CH2OH

is a colorless liquid of a pleasant rose geranium odor, boiling at 229° to 230° C. (444.2°-446° F.), under ordinary pressure, and at 129° C. (264.2° F.) under a pressure of 16 mm. Its specific gravity is about 0.881 and its refractive index 1.478. It is optically inactive. By careful oxidation with chromic trioxide it yields its aldehyde, citral, and by heating it in an autoclave with water to 200° C. (392° F.), it is partly converted into its isomer, linalool. It is a monatomic, primary, open chain alcohol, and is the principal constituent of true geranium (pelargonium) oil, as well as of the so-called "Turkish geranium oil," or oil of palmarosa. It is a constituent of rose oil, of citronella oil and in very small proportions of a number of other oils. Of the esters of geraniol the most important is the geranyl acetate. This is a fragrant oil of specific gravity 0.917, boiling at from 128° to 129° C. (262.4°-264.2° F.), under a pressure of 16 mm. It occurs naturally in several essential oils, and can be made artificially by the action of acetic anhydride upon geraniol. Commercial geraniol is also known as rhodinol.

2.—Linalool, C₁₀H₁₇OH, or

CH3 \		/ CH3
	C=CH.CH2.CH2.C	—CH = CH2,
CH3 /		\ OH

is known in both of the optically active forms as well as in the inactive. It is a liquid having a pleasant odor, and boils under normal pressure at about 198° C. (388.4° F.), and at from 86° to 87° C. (186.8°-188.6° F.) at 14 mm. pressure. Its specific gravity ranges from 0.868 to 0.877, and its refractive index is 1.463. It is a monatomic, tertiary, open chain alcohol, and is an essential constituent of oil of linaloe, and is found in the free state or as an acetic ester in oils of bergamot, coriander, lavender, neroli, etc. The most important of its esters is linalyl acetate, which occurs in many essential oils, as in oil of bergamot. It is an odoriferous liquid of the specific gravity 0.912, and boiling at from 105° to 108° C. (221°-226.4° F.) at 11 mm. pressure. It is so prepared artificially, and the artificial ester is an important commercial article.

3.—Citronellol, C₁₀H₁₉OH, or

CH2 \\	C=CH2.CH2.CH2.CH	/ CH3
CH3 /		\ CH2.CH2OH,

is found in oil of rose and in Spanish geranium oil. It is an odoriferous oil, boiling at from 117° to 118° C. (242.6 °-244.4° F.), under a pressure of 17 mm.; specific gravity, 0.8565 at 17.5° C. (63.5° F.); refractive index, 1.457. It is an open chain alcohol closely related to geraniol and is found in oil of rose, oil of geranium, etc. When obtained from oil of rose it is laevogyrate, while when obtained by reducing the aldehyde it is dextrogyrate; in most geranium oils both varieties exist.

A second group comprises the mono-cyclic alcohols, terpineol, which is unsaturated, and menthol, a saturated compound.

4.—Terpineol, C₁₀H₁₇OH,

CH3 \	CH—COH.CH2.CH2.CH2.CH.C—CH3,
CH3 /

occurs in liquid and solid modifications, inactive, as well as in the two optically active, varieties. Besides occurring naturally it is made artificially by the dehydration of terpin hydrate, and is largely used in the perfume industry because of its lilac odor. It has a sp. gr. of 0.940 at 15° C. (59.2° F.). Its most important ester is the acetate, which occurs in cajuput and cardamom oils.

5.—Menthol, menthyl alcohol, C₁₀H₁₉OH,

CH3 \	CH—CH.CH2.CHOH.CH2.CH2CH—CH3,
CH3 /

is now official. Two of its esters are recognized as occurring with it in peppermint oil, menthyl acetate and menthyl iso-valerate. Of these the former is the more important. It is an oil having a penetrating odor, boiling at 224° C. (435.2° F.).

To a third group of compounds known as dicyclic, belongs:

6.—Borneol, C₁₀H₁₇OH, or CH.CH₂.CH₂.CH₂CH₃.C.CH₃.CH₂.CHOH.C—CH₃.—This compound occurs naturally in both optically active modifications. It also occurs optically inactive. It forms crystalline masses, which, when quite pure, melt at 203° C. (397.4° F.). Borneol can be prepared artificially by reducing its ketone (camphor) with metallic sodium. In this case, however, there results a mixture of borneol and iso-borneol. Borneol forms a number of esters, of which bornyl acetate is the most important. This compound melts at 29° C. (84.2° F.), and has a specific gravity of 0.991 at 15° C. (59° F.). It is the characteristic constituent of the pine needle oils.

Nerol is an alcohol discovered in oil of neroli. It is a stereo-isomer of geraniol. Santalol, so called, is found as an alcoholic constituent of oil of sandalwood. It has been given the formula C₁₅H₂₅OH, but is probably a mixture of several isomeric or closely related alcohols. Sabinol, C₁₀H₁₅OH, is a constituent of oil of savin.

Fenchyl alcohol, thujyl alcohol, and pulegol, all having the same empiric formula as geraniol, C₁₀H₁₇OH, do not occur naturally, but are produced from the respective ketones.

II. Aldehydes.—Of the alcohols just enumerated, only the first group are primary alcohols, and will yield aldehydes. Thus, from geraniol, there is obtained by oxidation, citral, the corresponding aldehyde, and from its isomer, linalool, the same.

1.—Citral, C₉H₁₅COH, or (CH₃)₂C.CH.CH₂.CH₂.C(CH₂)CH.COH—Although at one time called geranial, this compound is now universally known as citral, which indicates its importance in the oils of the citrus family. It is obtained most abundantly from lemon grass oil by the aid of the bisulphite process. It is an oily liquid, boiling at from 228° to 230° C. (442.4°-446° F.), under ordinary pressure, and at 110° C. (230° F.), under 12 mm. Its specific gravity is about 0.89 and its refractive index 1.49. It is optically inactive. Under the influence of alkalies, citral condenses with acetone with the elimination of water to form pseudo-ionone, C₁₃H₂₀O, which is converted into its isomer ionone by means of acids. Reduction with sodium produces the alcohol geraniol.

2—Citronellal, C₉H₁₇OH, or C(CH₃)₂CH.CH₂CH₂CH(CH₃)CH₂COH, the aldehyde of citronellol, occurs in several essential oils, as in citronella oil. It is an oily liquid of characteristic odor; specific gravity 0.8768, refractive index 1.4481. It is dextrogyrate.

Of the class of aromatic aldehydes, there are found in essential oils, salicyl aldehyde, cumic aldehyde, benzaldehyde, and cinnamic aldehyde.

3.—Salicyl Aldehyde, C₆H₄(OH)COH.—This body is an oily liquid of aromatic odor, boiling at 196° C. (384.8° F.), and having a specific gravity of 1.172. It occurs in the oils of the several varieties of Spiraea. It can also be formed by the oxidation of its alcohol saligenin, or by the action of chloroform and potassium hydroxide on phenol.

4.—Cumic Aldehyde, C₆H₄(C₃H₇)COH.— This is p-isopropyl-benzaldehyde, and is found in several essential oils, as in cumin oil. It is an aromatic oil, having a specific gravity of 0.973, and boiling at 235° C. (455° F.).

5.—Benzaldehyde, C₆H₅.COH.—This is now official.

6—Cinnamic Aldehyde, C₆H₅.CH:CH.COH. —This was official in the U. S. P. VIII.

Anisic Aldehyde, C₆H₄OCH₃)COH (oxy-methyl-benzaldehyde), is a synthetic product used in artificial hawthorn perfume.

Piperonal or Heliotropin, C₆H₃O₂CH₂COH, is a synthetic product used in heliotrope perfume.

Santalal, C₁₄H₂₃COH, is found in sandalwood oil.

III. Ketones.—This class of chemical compounds is largely represented in essential oils. Of aliphatic saturated ketones there is only one fairly important representative.

1.—Methyl-nonyl-ketone, CH₃.CO.C₉H₁₉, which is found in oil of rue, is a liquid of strong odor, boiling at 225° C. (437° F.), and specific gravity 0.8295 at 17° C. (62.6° F.). It solidifies when cooled, and melts at 13° C. 155.4° F.).

Of aliphatic unsaturated ketones there is also one occurring naturally.

2.—Methyl-heptenone, CH₃.CO.C₆H₁₁, is found in lemon grass oil and other oils, and results from the oxidation of citral or the distillation of cineolic anhydride. It has a specific gravity of 0.853 at 20° C. (68° F.) and boils at 174° C. (345.2° F.).

A methyl-amyl-ketone, C₅H₁₁COCH₃, boiling at 151° C. (303.8° F.) and having a specific gravity of 0.837 at 50° C. (122° F.), is found as a minor constituent in oil of cloves.

The most commonly occurring ketones, however, belong to the cyclic compounds:

3.—Carvone, C₁₀H₁₄O,

CH3C	// CH—CH2 \	CH—C	/ CH3
	\ CO—CH2 /		\ CH2

This compound, found in oils of caraway fennel and dill, is a colorless oil boiling at 225° C. (437° F.) and having a sp. gr. of 0.960 at 18° C. (64.4° F.). It is optically active, occurring in both forms. Its most interesting addition compound is that with hydrogen sulphide; this is obtained in crystals, decomposing with alcoholic potassium hydroxide and liberating pure carvone. When heated with glacial phosphoric acid, it is changed into the isomeric carvacrol, a phenol compound.

4.—Pulegone, C₁₀H₁₆O,

CH3CH	/ CH2—CO \	C=C	/ CH3
	\ CH2—CH2 /		\ CH3

This characteristic ketone of pennyroyal oil is a liquid boiling at 221° C. (429.8° F.), and having a specific gravity of 0.9323 at 20° C. (68° F.). An isomeric body, iso-pulegone, has been obtained artificially.

5.—Menthone, C₁₀H₁₈O,

CHCH	/ CH2—CH2 \	CHCH(CH3)2.
	\ CH2—CO /

—This occurs along with the corresponding secondary alcohol menthol, in oil of peppermint. It is an oily liquid boiling at 206° C. (402.8° F.), with a specific gravity of 0.897 at 15° C. (59° F.). By the action of oxidizing agents it can be converted into thymol, C₁₀H₁₄O.

To the class of dicyclic ketones belong the following isomeric bodies:

6.—Camphor, C₁₀H₁₆O,

/	CH2
C8H14	|
\	CO

— This most important compound of the ketone class is official.

7.—Fenchone, C₁₀H₁₆O,

CH2—	CH2——	CCH3
|	/	|
|	C(CH3)2	|
|	|	|
CH2—	CH———	CO.—

This ketone occurs as dextro-fenchone in oil of fennel, and as laevo-fenchone in oil of thuja. When purified, it forms an oil of camphoraceous odor, boiling at 193° C. (379.4° F.), and having a specific gravity of 0.9465 at 19° C. (66.2° F.).

8.—Thujone, C₁₀H₁₆O,

CH3—CH	/ CO—CH2 \	C—CH	/ CH3
	\ CH—CH2 /		\ CH3.—

This ketone is found in oils of thuja, tansy, wormwood and sage, and is identical with the bodies formerly known as tanacetone and salvone. It is an optically active liquid, boiling at from 200° to 203° C. (392°-397.4° F.) and having a specific gravity of 0.9126 at 20° C. (68° F.). A more complex ketone yet to be noted is:

9.—Irone, C₁₃H₂₀O,

(CH3)2C	/ CH(CH:CH.CO.CH3)CH.CH3	\	CH2
	\ CH:CH—	/

The characteristic constituent of orris oil, to which also the violet odor is due, is irone. It is an oil almost insoluble in water, but readily soluble in alcohol, boiling at 144° C. (291.2° F.) at 16 mm., and having a specific gravity of 0.939 at 20° C. (68° F.). In the attempt to effect the synthesis of irone, Tiemann and Krüger obtained an isomeric body, ionone, from citral, which is now used as the artificial violet odor.

IV. Acids and Their Esters.—While esters of several of the fatty acids, both saturated and unsaturated, are found in the essential oils, the compounds are not so distinctive or characteristic, that they require mention here. Some of the esters have been already described under the appropriate alcohols, see geraniol, linalool, etc. Of esters of aromatic acids there are several of importance, however, comprising the most valuable constituents of the oil in which they occur.

1.—Methyl Salicylate is the chief constituent of the oils of wintergreen and sweet birch.

2.—Methyl Anthranilate (o-amidobenxoate), C₆H₄(NH₂)COOCH₃—This is the odoriferous constituent of neroli oil. It is an oil solidifying at low temperatures in crystals melting at 24.5° C. (76.1° F.), and boiling at 127° C. (260.6° F.) at 11 mm. Its specific gravity is 1.163 at 26° C. (78.8° F.). It is strongly fluorescent and has a powerful neroli odor.

V. Phenols and Phenol Ethers.—This is an important class, largely represented in the volatile oils. Of monatomic phenols there are:

I.—Chavicol (para allyl-phenol), C₆H₄(OH)C₃H₅.—This occurs in the oil of Chavica Betle, and is a colorless liquid having a strong odor and a specific gravity of 1.035 at 20° C. (68° F.), boiling at 237° C. (458.6° F.). Still more abundantly formed in nature is its methyl ether, which is:

2—Methyl-chavicol, C₆H₄(OCH₃)C₃H₅. — This occurs in the oils of anise, star anise, sweet-basil, bay, etc. It is a colorless liquid of an anise-like odor, boiling at from 215° to 216° C. (419°-420.8° F.), and having a specific gravity of 0.979 at 12° C. (53.6° F.).

3.—Anethol, C₆H₄(OCH₃)C₃H₅, is the methyl ether of p-propenyl-phenol, and is therefore isomeric with methyl-chavicol. It can be formed from the latter by heating it with alcoholic potassium hydroxide. It is the most abundant constituent of the oils of anise, star anise and fennel. It forms white scales having a specific gravity of 0.986 at 25° C. (77° F.), melting at 21° C. (69.8° F.), and boiling at 232° C. (449.6° F.).

4.—Thymol (methyl-propyl-phenol), C₈H₃.CH₃.(OH)C₃H₇ 1:3:4.—This is one of the best known of the phenols occurring in volatile oils. As it is official its description will be found under Thymol.

5.—Carvacrol (methyl-propyl-phenol), C₆H₆.CH₃(OH)C₃H₇ 1:2:4—This important phenol, occurring in oils of marjoram and savory, is isomeric with thymol. It is a thick oil, solidifying at 0° C. (32° F.) and boiling at 236° C. (456.8° F.). It can be prepared artificially by heating carvone, an isomeric ketone, with phosphoric acid or by heating camphor with iodine.

Of diatomic phenols and their ethers, there are the following:

6.—Eugenol, C₆H₃.OH.OCH₃.C₃H₅ 4:3:1, is the methyl ether of allyl-dioxybenzene. It occurs in the oils of cloves, allspice, bay, Ceylon cinnamon and cinnamon leaves.

7.—Methyl-eugenol, C₆H₃.(OCH₃)2.C₃H₅, is found together with eugenol in oil of bay. It has a specific gravity of 1.041 at 11° C. (51.8° F.) and boils at 248° C. (478.4° F.).

8.—Iso-eugenol is the methyl ether of propenyl-dioxybenzene, and is produced from eugenol when the latter is heated with alcoholic potassium hydroxide. It is used in the manufacture of artificial vanillin.

9.—Chavibetol, C₉H₈(OCH₃)OH, is another isomer of eugenol, and occurs with chavicol in oil of betel leaves.

10.—Safrol,

C6H3	O \	CH2.C3H5,
	O /

is the methylene ether of allyl-dioxybenzene. It is found quite largely in the oils of sassafras, camphor, etc. It was official in the U. S. P. VIII.

II.—Iso-safrol is the methylene ether of propenyl-dioxybenzene, and is produced from safrol by heating the latter with alcoholic potassium hydroxide. It is used in the manufacture of piperonal (artificial heliotropin).

12.—Thymohydroquinone dimethyl-ether, C₆H₂.(OCH₃)₂.CH₃.C₃H₇, constitutes the bulk of the oil of arnica root. It is a liquid distilling at from 248° to 250° C. (478.4 °-482 ° .F.).

Of triatomic phenols and their ethers, the representative is:

13.—Asarol, C₆H₂(C₃H₅)(OCH₃)₃.—This is the trimethyl ether of propenyl-trioxybenzene. It forms the solid constituent of the oil of Asarum europaeum, and occurs in prisms melting at 61° C. (141.8° F.), and boiling at from 295° to 296° C. (563°-564.8° F.). It has also been made synthetically.

Of tetratomic phenols and their ethers, there is also a representative, as follows:

14—Apiol, C₁₂H₁₄O₄,C₉H₆(OCH₃)2O₂.CH₂, is the dimethyl-methylene ether of allyl-tetroxy-benzene. It occurs in two isomers—the apiol from oil of parsley and dill apiol from some varieties of oil of dill. Of these, the former is a crystalline solid melting at 30° C. (86° F.) and boiling at 294° C. (561.2° F.) and the latter is an oily liquid boiling at 288° C. (550.4° F.).

VI. Neutral Bodies, Oxides, etc.—One important compound of frequent occurrence in volatile oils has not been classified under any of the preceding groups, for the reason that it is relatively inert in a chemical way and is considered as an oxide of neither distinct acid or basic character.

1.—Cineol (Eucalyptol), C₁₀H₁₈O.—This is found in nature in large amounts in eucalyptus oil (hence the name "eucalyptol" sometimes given to it), in cajuput oil (hence the name of "cajuputol"), in lavender, wormseed, and other oils.

VII. Sulphides and Sulphur Compounds.

1.—Di-allyl-disulphide, C₆H₁₀S₂, is a constituent of oil of garlic and occurs in many oils belonging to the Cruciferae. It is a liquid of very unpleasant odor, boiling at 140° C. (284° F.)

2.—Allyl-isothiocyanate, C₃H₅.NCS, is the principal constituent of the volatile oil of mustard. It is, however, a decomposition product of the glucoside potassium myronate, which breaks up under the influence of the ferment myrosin. The allyl thiocyanate can also be obtained artificially by distilling allyl iodide or bromide with alcoholic potassium thiocyanate. This artificial product is largely sold in place of the natural oil. It is a liquid having a very unpleasant odor, boiling at 151° C. (303.8° F.), and having a specific gravity of 1.017 at 10° C. (50° F.).

Vinyl sulphide, (C₂H₃)₂S, is found in a variety of garlic.

Butyl isothiocyanate, CH(CH₃)C₂H₅NCS, is found in oil of horseradish, acrynyl isothiocyanate, C₇H₇ONCS, is found in white mustard and phenylethyl isothiocyanate, C₄H₄(C₆H₅)NCS, is found in oil of mignonette.

Classification.

Classification of the more commonly occurring volatile oils.—In considering the question of classifying the volatile oils, two methods of arrangement naturally suggest themselves, viz.: a classification according to the botanical natural orders to which they belong, and a chemical classification based on the most important chemical constituents of the oils themselves. While the first of these is the more readily made, it suffers from the disadvantage of being cumbrous and less readily understood except by the botanist. On the other hand, the second plan shows at a glance the sources of the valuable odoriferous and medicinally and technically important constituents for which the volatile oils are largely used. That it has not been generally adopted is no doubt due to the fact that many of the oils contain several different constituents of value, and it is therefore difficult to make an assignment of some of them to individual chemical groups. Nevertheless, it is believed that the chemical classification adopted here will be found to have practical value for reference.

Group I.—Oils containing mainly Terpenes and Sesquiterpenes.

• Cedarwood; cedrene and cedrol (an oxygenated constituent).
• Copaiba Balsam; caryophyllene.
• Cubeb; cadinene and cubeb camphor (an oxygenated constituent).
• Dog Fennel; phellandrene.
• Fleabane; d-limonene.
• Galbanum; pinene and cadinene.
• Ginger; sesquiterpene and phellandrene.
• Hemp; cannibene and other terpenes.
• Hops; humulene and tetrahydro-cymene.
• Myristica; terpenes and myristicol (an oxygenated constituent).
• Orange; limonene with small amount of citral and citronellal (both oxygenated constituents).
• Turpentine; pinene and sylvestrene.

Group II.—Oils containing mainly Alcohols and their esters.

a. Aliphatic saturated alcohols:

• Heracleum; octyl alcohol and octyl ester.

b. Aliphatic unsaturated alcohols:

• Bergamot; linalool and linalyl acetate.
• Coriander; linalool and d-pinene.
• Geranium; geraniol and geranyl esters.
• Lavender; linalyl acetate and geraniol.
• Lime; linalyl acetate and limonene.
• Linaloes; linalool and geraniol.
• Petit Grain; linalool and linalyl acetate.
• Rose; geraniol and citronellol with esters.

c. Monocyclic and dicyclic alcohols.

• Angostura; galipol, galipene, and cadinene.
• Fir Cones; bornyl acetate and terpenes.
• Golden Rod; borneol and bornyl esters.
• Juniper Berries; juniper camphor and cadinene.
• Ledum; ledum camphor.
• Lovage; terpineol.
• Mace; myristicol.
• Patchouli; patchouli camphor and cadinene.
• Peppermint; menthol and menthone.
• Pine Needles; boryl acetate and terpenes.
• Rosemary; borneol and bornyl acetate.
• Sandalwood; santalol and esters.
• Savin; sabinol and sabinyl acetate.
• Valerian; borneol and bornyl esters.

d. Aromatic Alcohols and Esters:

• Jasmine; benzyl acetate and benzyl alcohol.

Group III.—Oils containing Aldehydes as characteristic constituents.

a. Aliphatic unsaturated aldehydes:

• Citron peel; citral and limonene.
• Citronella; citronellal and geraniol.
• Lemon; Citral and citronellal (this oil might also properly be classed in Group I, as the terpenes constitute over 90 per cent. of the oils).
• Lemon grass; citral, citronellal, and methyl-heptenone.
• Verbena; citral.

b. Aromatic aldehydes:

• Bitter Almond; benzaldehyde.
• Cassia; cinnamic aldehyde.
• Cinnamon; cinnamic aldehyde and eugenol.
• Cumin; cumic aldehyde.
• Meadow Sweet; salicyl aldehyde.

Group IV.—Oils containing Ketones as characteristic constituents.

a. Aliphatic saturated aldehydes:

• Rue; methyl-nonyl-ketone.

b. Aliphatic unsaturated aldehydes:

• Lemon grass; methyl-heptenone.

c. Monocyclic and dicyclic ketones:

• Artemisia; thujone.
• Camphor; camphor.
• Caraway; carvone and limonene.
• Dill; carvone and limonene.
• Orris; irone.
• Pennyroyal; pulegone.
• Peppermint; menthone and menthol.
• Sage; thujone with borneol and cineol.
• Spearmint; carvone.
• Tansy; thujone with borneol.
• Thuja; thujone and fenchone.
• Wormwood; thujone and thujyl alcohol.

Group V.—Oils containing Esters.

a. Aliphatic Acids:

• Angelica; methyl-ethyl-acetic esters.
• Calamus; heptylic and palmitic acid and esters.
• Cardamom; acetic esters and cineol.
• German Chamomile; caproic acid esters.
• Roman Chamomile; butyric, angelic and tiglic esters.

b. Aromatic acid esters:

• Neroli; o-amido-benzoic-methyl ester.
• Sweet Birch; methyl salicylate.
• Gaultheria; methyl salicylate.

c. Undetermined acids:

• Celery; sedanolide and lactone.
• Elecampane; alantic acid and lactone.

Group VI.—Oils containing Phenols and Phenol Ethers.

a. Monatomic phenols and their ethers:

• Ajowan; thymol and cymene.
• Anise; anethol and methyl-chavicol.
• Betel; chavicol and methoxy-chavicol.
• Fennel; anethol and fenchone.
• Marjoram; carvacrol and linalool.
• Savory; carvacrol, pinene, and cymene.
• Star Anise; anethol, methyl-chavicol and safrol.
• Sweet Basil; Methyl-chavicol and d-linalool.

b. Diatomic phenols and their ethers:

• Arnica Root; thymohydroquinone dimethyl-ether.
• Bay; eugenol, methyl-eugenol and methyl-chavicol.
• Camphor; safrol, eugenol and camphor.
• Cascarilla Bark; eugenol and terpenes.
• Cinnamon Leaf; eugenol and cinnamic aldehyde.
• Cloves; eugenol and sesquiterpene.
• Pimenta; eugenol and sesquiterpene.
• Sassafras; safrol and camphor.

c. Triatomic phenols and their ethers:

• Asarum; asarol and methyl-eugenol.
• Matico; asarol.

d. Tetratomic phenols and their ethers:

• Parsley; apiol.

Group VII.—Oils containing Neutral Bodies.

• Cajuput; cineol, terpineol and terpenyl acetate.
• Eucalyptus; cineol, pinene, and aldehydes.
• Laurel Leaves; cineol and pinene.
• Myrtle; cineol, d-pinene and dipentene.
• Wormseed; cineol and dipentene.

Group VIII.—Oils containing Sulphur.

• Asafoetida; sulphides and pinene.
• Garlic; diallyl-disulphide and allylpropyl sulphide.
• Mustard; allyl-thiocyanate, allyl cyanide and carbon disulphide.
• Onion; allyl-propyl sulphide.

Properties.—Volatile oils are slightly soluble in water. Agitated with this fluid they render it milky, but the major portion separates upon standing, leaving the water impregnated with their odor and taste. This impregnation is more complete when water is distilled with the oils, or from the plants containing them. Trituration with insoluble powders, as talc, magnesium carbonate, kieselguhr, etc., renders them much more soluble, probably in consequence of their minute division. The intervention of sugar also greatly increases their solubility, and affords a convenient method of preparing them for internal use. The hydrocarbon oils are scarcely soluble in diluted alcohol, and, according to De Saussure, the solubility of volatile oils generally in this liquid is proportionate to the oxygen which they contain. The volatile oils dissolve sulphur and phosphorus with the aid of heat, and deposit them on cooling. By long boiling with sulphur they form brown, unctuous, fetid substances, formerly called balsams of sulphur. They absorb chlorine, which converts them into resinous oxidation products and then combines with these. Iodine produces a similar effect. They are decomposed by the strong mineral acids, and unite with some organic acids. When treated with a caustic alkali, some of them are saponified or otherwise decomposed. Several of the metallic oxides, and various salts which easily part with oxygen, convert them into resinous oxidation products. The volatile oils dissolve many of the proximate principles of plant and animal tissues, such as the fixed oils and fats, resins, camphor, and many of the alkaloids when in the free state. Exposed to air and light, many of them absorb oxygen and become what are termed ozonized oils which possess oxidizing properties.

Adulterations.—The volatile oils are often sophisticated. Among the greater adulterations are fixed oils, resinous substances, chloroform, hydrocarbon oils, and alcohol, but the most dangerous are those made by mixing the pure oil with the cheaper volatile oils and terpenes and fractions from other oils like limonene. The presence of the fixed oils may be known by the permanent greasy stain "which they leave on paper, while that occasioned by a pure volatile oil disappears entirely when exposed to heat. They may also in general be detected by their comparative insolubility in alcohol. Both the fixed oils and resins are left behind when the adulterated oil is distilled with wafer. If alcohol be present, the oil will become milky when agitated with water in a graduated tube, and after the separation of the liquids the water will occupy more space and the oil less than before. The following method of detecting alcohol was proposed by Beral. Put twelve drops of the suspected oil in a perfectly dry watch-glass, and add a piece of potassium about as large as the head of a pin. If the potassium remains for twelve or fifteen minutes in the midst of the liquid, there is either no alcohol present, or less than 4 per cent. If it disappears in five minutes, the oil contains more than 4 per cent. of alcohol; if in less than a minute, 25 per cent. or more. Borsarelli employs calcium chloride for the same purpose. This he introduces in small pieces, well dried and perfectly free from powder, into a small cylindrical tube, closed at one end, and about two-thirds filled with the oil to be examined, and heats the tube to 100° C. (212° F.), occasionally shaking it. If there is considerable proportion of alcohol, the chloride will be entirely dissolved, forming a solution which sinks to the bottom of the tube; if only a very small quantity, the pieces will lose their form, and collect at the bottom in a white adhering mass; if none at all, they will remain unchanged. (J. P. C., xxvi, 429.) J. J. Bernoulli -proposes as a test dry potassium acetate, which remains unaffected in a pure oil, but will be dissolved if alcohol be present, and form a distinct liquid. (See A. J. P., xxv, 82.) Distillation, catching the first portion, and testing for alcohol by the iodoform reaction, will detect very small additions of alcohol. The most dangerous adulterant of volatile oils is a liquid sold under some "fancy name," found in the markets of London and other large cities, and recommended for "reducing" essential oils; one specimen examined by John Barclay (P. J., 1896, 463) had a delicate odor, and could be mixed with oils of lemon and bergamot without being detected by odor or taste. It was believed to be a laevo-pinene. There are good reasons for believing that similar liquids are used to an enormous extent.

Sometimes volatile oils of little value are mixed with the more costly. The taste and odor afford in this case the best means of detecting the fraud. The specific gravity of the oils may also serve as a test of purity. When two oils, of which one is lighter and the other heavier than water, are mixed, they may be separated by long agitation with this fluid, and will take a place corresponding to their respective specific gravities, but it sometimes happens that an unadulterated oil may thus be separated into two portions. The difference of apparent effect produced by iodine with the several oils has been proposed as a test, and bromine was employed for the same purpose by John M. Maisch, who used both these tests preferably in the state of ethereal solution, which, as it is liable to spontaneous change by keeping, should be prepared when wanted for use. According to Liebig, when iodine is made to act on a volatile oil, a portion of it combines with the hydrogen of the oil, forming hydriodic acid, while another portion takes the place of the lost hydrogen. Oil of turpentine may be detected by remaining in part undissolved when the suspected oil is treated with three or four times its volume of alcohol of the sp. gr. 0.84; or, according to Mero, by causing the suspected oil, when agitated with an equal measure of poppy oil, to remain transparent, instead of becoming milky, as it would do if pure. The latter test will not apply to the oil of rosemary. (J. P. C., 3e ser., vii, 303.) G. S. Heppe suggests a very delicate test of oil of turpentine and most other non-oxygenated oils, when used to adulterate one of the oils containing oxygen. A piece of copper nitroprusside, of the size of a pin's head, is put into a little of the suspected oil in a test-tube, and heated until the liquid begins, to boil. The boiling must be continued only a few seconds. If the oil be pure and oxygenated, the copper nitroprusside will become black, brown, or gray; if oil of turpentine or other non-oxygenated oil be present, the deposit will be green or bluish-green, and the supernatant liquid colorless or yellowish.

The different relations of the volatile oils to polarized light may, to a certain extent, be made available for the detection of adulterations; especially where the action of the adulterating oil is in an opposite direction to that of the oil adulterated. Thus, the oils of juniper, lavender, and rosemary, rotate the plane of polarization to the left, while American oil of turpentine rotates it to the right; and if this should be added to one of the other oils it might in some degree neutralize their action, and thus offer one means for its detection. Unfortunately, the French oil of turpentine, from the juice of the Pinus maritima, acts strongly in the opposite direction. But the very strength of its left-rotatory power might lead to its detection by the abnormal increase of this power which it would impart to the oils in question. Synthetic or artificial volatile oils are now largely manufactured. They vary greatly at times in their resemblance to the natural products. They will be considered under their respective titles elsewhere, a number having received official recognition.

Volatile oils may be preserved without change in small, well-stoppered amber-colored bottles, entirely filled with the oil, and secluded from the light.

They may also be preserved by mixing with them about 5 per cent. of a bland fixed oil which will remain undissolved when the volatile oil is added to alcohol and for which allowance must be made, of course, in the volume of the oil used in formulas, etc. (LaWall, Proc. A. Ph. A., 1910, 1121.)

Manufacture.—Most of the volatile oils may be prepared by the general formula of the U. S.. P. 1870: "Put the substance from which the Oil is to be extracted into a retort or other vessel suited for distillation, and add enough water to cover it, then distil by a regulated heat into a large refrigeratory. Separate the Distilled Oil from the water which comes over with it."

A large proportion of the volatile oils of European commerce is produced in Grasse, a town of France, twenty-five miles west of Nice. For an elaborate article on this industry and methods of preparation, see A. Pharm., xxii, 473, abstracted in the P. J., vol. xv, 468. Schimmel & Co., Miltitz, Germany, are large producers of volatile oils.

Under the general observations on the Aquae, or Waters, will be found remarks upon the use of steam in preparing the Distilled Waters, which are to a considerable extent applicable also to the volatile oils.

The substances from which the volatile oils are extracted may be employed in either the recent or the dried state. Certain flowers, however, such as orange flowers and roses, must be used fresh, or preserved with salt or by means of glycerin, as they afford little or no oil after desiccation. Most of the aromatic herbs, also, as peppermint, spearmint, pennyroyal, and marjoram, are usually distilled while fresh, although it is thought by some that when moderately dried they yield a larger and more grateful product. Dried substances, before being submitted to distillation, require to be macerated in water until they are thoroughly penetrated by this fluid, and to facilitate the action of the water it is necessary that, when of a hard or tough consistence, they should be properly comminuted by slicing, shaving, rasping, bruising or other similar mechanical operation.

The water which is introduced into the still with the substance answers the double purpose of preventing the decomposition of the vegetable matter by regulating the temperature, and of facilitating the volatilization of the oil, which, although in most instances readily rising with the vapor of boiling water, requires, when distilled alone, a considerably higher temperature, and is at the same time liable to be partially decomposed. Some oils, however, will not ascend readily with steam at 100° C. (212° F.), and in the distillation of these it is customary to use water saturated with common salt, which does not boil under 110° C. (230° F.). Recourse may also be had to a bath of strong solution of calcium chloride, or to an oil bath. Other oils, again, may be volatilized with water at a temperature below the boiling point, and, as heat exercises an injurious influence over the oils, it is desirable that the distillation should be effected at as low a temperature as possible. To prevent injury from heat, it has been recommended to suspend the substance containing the oil in a wire basket, or to place it upon a perforated shelf, in the upper part of the still, so that it may be penetrated by the steam, without being in direct contact with the water. Another mode of effecting the same object is to distill it in vacuo. Duncan stated that the most elegant volatile oils he had even seen were prepared in this manner by Barry, the inventor of the process. The employment of steam heat also prevents injury, and the best volatile oils are now prepared by manufacturers in this way. Steam can be very conveniently applied to this purpose by causing it to pass through a coil of tube, of an inch or three-quarters of an inch bore, placed in the bottom of a common still. The end at which the steam is admitted enters the still at the upper part, and the other end, at which the steam and condensed water escape, passes out laterally below, being furnished with a stop-cock, by which the pressure of the steam may be regulated and the water drawn off when necessary. In some instances it is desirable to conduct the steam immediately into the still near the bottom, by which the contents are kept in a state of brisk ebullition. This method is used in the preparation of the oil of bitter almond. The same method is applicable to the preparation of distilled waters.

The quantity of water added is not a matter of indifference. An excess above what is necessary acts injuriously by holding the oil in solution when the mixed vapors are condensed, and if the proportion be very large, it is possible that no oil whatever may be obtained separate. On the contrary, if the quantity be too small, the whole of the oil will not be distilled, and there will be danger of the substance in the still adhering to the sides of the vessel and thus become burnt. Enough water should always be added to cover the solid material and prevent the latter accident. Dried plants require more water than the fresh and succulent. The whole amount of material in the still. should not exceed three-fourths of its capacity, as otherwise there would be danger of the liquid boiling over. The form of the still has an influence over the quantity of water distilled, which depends more upon the extent of surface than upon the amount of liquid submitted to evaporation. By employing a high and rather narrow vessel one may obviate the disadvantage of an excessive quantity of water. Sometimes the proportion of oil in the substance employed is so small that it is wholly dissolved in the water distilled, even though the proportion of the liquid in the still is not greater than is absolutely essential. In this case it is necessary to redistill the same water several times from fresh portions of the plant, until the quantity of oil exceeds the solvent power of the water. This process is called cohobation.

The more volatile of the oils pass with facility along with the steam into the neck of the common still, but some which are less volatile are apt to condense in the head and thus return into the still. For the distillation of the latter, a copper still should be employed having a very high head. As, after the distillation of any one oil, it is necessary that the apparatus should be thoroughly cleansed before being used for the preparation of another, it is better that the condensing tubes should be straight, rather than spiral as in the ordinary still. It should be recollected, moreover, that certain oils, such as those of anise and fennel, are rendered solid by a comparatively slight reduction of temperature, and that in the distillation of these the water employed for refrigeration should not be below 5.5° C. (42° F.).

The mixed vapors are condensed into a milky liquid, which is collected in a receiver, and, after standing for some time, separates into the oil and a clear solution of it, the former floating on the surface, or sinking to the bottom, according as it is lighter or heavier than water. The distillation should be continued so long as the liquid coming over has a milky appearance.

The last step in the process is to separate the oil from the water. For this purpose the Florentine receiver may be used. For illustrations of this, and other forms of receivers, for collecting and separating immiscible liquids, see Remington's Practice of Pharmacy, 6th edition, p. 250. This is a conical glass vessel, broad at the bottom and narrow towards the top, and very near its base furnished with a tubulure or opening, to which is adapted, by means of a pierced cork, a bent tube so shaped as to rise perpendicularly to seven-eighths of the height of the receiver, then to pass off from it at right angles, and near the end to bend downward. The condensed liquid being admitted through the opening at the top of the receiver, the oil separates, and, rising to the top, occupies the upper narrow part of the vessel, while the water remains at the bottom, and enters the tube affixed to the receiver. When the surface of the liquid attains in the receiver a higher level than the top of the tube, the water will necessarily begin to flow out through the latter, and may be received in bottles. The oil thus accumulates as long as the process continues, but it is evident that the plan is applicable only to the oils lighter than water. For the heavier oils, cylindrical vessels may be employed, to be renewed as fast as they are filled; but, as all the water cannot be removed by these plans, it is necessary to resort to some other method of effecting a complete separation. An instrument called a separator is usually employed for this purpose. It consists of a glass funnel, or a globular vessel, furnished with a stopper, and prolonged at the bottom into a very narrow tube. The lower opening being closed, the mixed liquids are introduced and allowed to stand until they separate. The orifice at the bottom is then opened, and, the stopper at the top being a little loosened, so as to admit the air, the heavier liquid slowly flows out, and may be separated to the last drop from the lighter, which floats above it. If the oil is heavier than the water, it passes out of the separator; if lighter, it remains within. According to George Leuchs, all oils obtained by distillation with water, even when perfectly clear, contain some water.

The water saturated with oil should be preserved for use in future distillations, as it can dissolve no more of the oil. One or more volatile acids are frequently found in the distilled water, as acetic, butyric, or valerianic, and Wunder has detected all three of these acids in the water distilled from chamomile flowers. (J. Pr. Chem., lxiv, 499.) For an illustration of a cheap apparatus for distilling volatile oils by steam under slight pressure, see Proc. A. Ph. A., 1894, 680.

According to Overbeck, all the volatile oils may be decolorized by distilling them from an equal weight of poppy seed oil and a saturated solution of common salt. (A. Pharm., lxxxiv, 149.)

When first procured, some volatile oils have a disagreeable somewhat empyreumatic odor, from which they may be freed by allowing them to stand for some days in vessels loosely covered with paper. They should then be filtered and introduced into small opaque bottles, which should be well stoppered so as to exclude the air. When altered by exposure to air, they may sometimes be restored to their original appearance and quality by agitation with a little recently heated animal charcoal, and the same method may be employed for freeing them from adhering water.

They may be administered upon a lump of sugar; or triturated with at least ten times their weight of sugar, forming oleosaccharates, which are then dissolved in water; or made into emulsions with water, sugar, and gum arabic. In making emulsions with volatile oils, it has been recommended first to dissolve them in one of the fixed oils, the oil of almond for example, and then to emulsionize the oleaginous solution with syrup and gum arabic. For 100 parts of water, 15 of the almond oil in which the volatile oil is to be dissolved, 10 of powdered gum arabic, and 25 of syrup may be taken. The volatile oils are often kept dissolved in alcohol under the name of essences.

Enfleurage.—This term is applied by the French to the impregnation of fixed oil and fatty matters with the odors of certain sweet scented plants, such as jasmine, tuberose, and mignonette, the oils of which are so delicate and fugitive that they cannot well be separated by distillation. The process consists in exposing the fatty matter, placed in layers, in suitable frames, to the exhalations from the flowers, which are absorbed, and give their characteristic odor to the fat. Another plan is to expose alternate layers of the flowers, and of cotton impregnated with bland fixed oil, to the sun, and afterwards to express the oil from the cotton. (A. J. P., xxix, 551.) The French sometimes give to the spirituous solutions made by extracting the odors from fats and oils with alcohol the name of Essences.

The following suggestions on preparing emulsions of the volatile oils may be useful. Oil of turpentine and other volatile oils, to be emulsionized in quantity, are most successfully treated by rubbing them with one-half their weight of powdered gum arabic, and, when intimately mixed, adding at once, with rapid trituration, water in the proportion of one and a half times the weight of the gum used. Such an emulsion is more permanent when a little fixed oil is used.

Uses.—While substances differing so widely in their source and in their chemical composition must manifestly exercise dissimilar effects upon the body, but there are certain properties common to so many of the volatile oils that a word concerning the therapeutic effects of this group of principles may be in place. Practically all of the official volatile oils are more or less irritant; some of them only feebly so and some very actively so. By virtue of this action when taken into the intestinal tract they excite peristalsis and are therefore used for the expulsion of flatus in colic as well as to enhance the effect of cathartic drugs. It is probably also to this local irritant effect that their uses as expectorants, diaphoretics, and diuretics is to be attributed. A large number of them are powerful germicides and nearly all of them more or less antiseptic. The antibacterial property of the volatile oils has been investigated especially by Martindale (Perfume and Essential Oil Record, 1910, i, p. 266) by Gilmore (P. J., 1910, p. 844) and by Peck (J. A. M. A., 1899, xxxii, p. 6). According to Martindale the oil of origanum is twenty-five times as actively germicidal as phenol and the oil of cloves about 9 times; while Peck finds that the oil of cloves prevents the multiplication of bacteria when present in about 1 part to 1100. The following volatile oils are powerfully germicidal: origanum, thyme, cinnamon, cassia, cloves, bay, and sassafras. Of considerable disinfectant power but weaker than the preceding are: peppermint, eucalyptus, cajuput, bitter almond, sandal wood, spearmint and lavender. The following are feeble in their anti-bacterial properties: anise, bergamot, juniper, cade, and turpentine. Concerning most of the other oils figures are lacking. While the cost of the volatile oils precludes their use for many disinfectant purposes yet for others they are very valuable agents, especially are they employed for their antibacterial properties in the mouth and nose.

Some of the essential oils possess marked local anesthetic powers, among which may be mentioned especially cloves, peppermint, thyme, and cajuput. For this influence they are used in toothache and pruritus.

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The Dispensatory of the United States of America, 1918, was edited by Joseph P. Remington, Horatio C. Wood and others.