Of considerable importance in the further development of the chemistry of volatile oils are the investigations of Berthelot from 1852 to 1863, which involve principally the hydrocarbons contained in these oils. He studied first of all the hydrocarbon of turpentine oil3) and its isomers and polymers obtained from its hydrochloride. By heating pinene hydrochloride with barium stearate or sodium benzoate he obtained a new hydrocarbon which he regarded as "camphene proper"1) and which is identical with the camphene of to-day. This new camphene was either dextrogyrate, laevogyrate or optically inactive according to the turpentine oil employed. Berthelot, therefore, distinguished between the following hydrocarbons:

1) This difference was first ascertained in the laboratory of Schimmel & Co. (Report of Schimmel & Co. April 1892, 30) and verified by Grimaux in 1893 (Compt. rend. 117, 1189).

2) Report of Schimmel & Co. October 1895, 46.

3) Compt. rend. 55 (1862), 496 and 544; also Liebig's Annalen, Suppl. II. (1862-63), 226.

1. Terebentene (l-pinene) from French turpentine oil, laevogyrate2) b. p. 161 °. It yields a laevogyrate monohydrochloride, also under proper conditions an inactive dihydrochloride (dipentene dihydro-chloride).

2. Terecamphene (l-camphene) from terebentene hydrochloride, laevo-gyrate, m. p. 45°, b. p. 160°. With hydrogen chloride it forms a dextrogyrate hydrochloride.

3. Australene (d-pinene) from American turpentine oil, b. p. 161°, dextrogyrate like its hydrochloride. Its behavior to hydrogen chloride is analogous to that of terebentene.

4. Austracamphene (d-camphene) from australene hydrochloride. It corresponds to terecamphene.

5. Inactive camphene (i-camphene) can be obtained by proper treatment from the hydrochloride of terebentene as well as that of australene.

6. Terebene,3) b. p. 160°.

These six hydrocarbons are isomeric and have the formula G10 H16. With these the following are polymeric:

1. A liquid hydrocarbon boiling at 250° which is probably a sesqui-terebene, C15H24.

2. Diterebene (Deville's colophene) C20H32, an inactive liquid boiling at about 300°.

3. Several polyterebenes C10„H16n) optically inactive liquids becoming more and more viscid, which boil between 360° and a dark red heat.

After a discussion of the methods of formation of the individual hydrocarbons, Berthelot continues:

1) Soubeiran and Capitaine in 1840 had applied the term camphene to all hydrocarbons C5H8 (Liebig's Annalen 34, 311).

2) The rotatory power of volatile oils was first observed by Biot in 1817 in connection with French oil of turpentine (Memoires de l'Acad. des Sc. 13), later also with oil of lemon. The turpentine oil was shown to be laevogyrate, the oil of lemon dextrogyrate. In 1843, Leeson of London found that American turpentine oil possessed a rotatory power opposite to that of the French oil. This observation was soon after verified by Pereira and Guibourt. Pereira introduced the terms laevo-gyrate and dextro-gyrate (Pharm. journ. 5 [1845], 70).

3) This substance which was considered a chemical unit by Berthelot was shown by Riban to be a mixture of a terpene, cymene and camphor. Power and Kleber in 1894 found camphene, dipentene, terpinene and cymene in terebene. (Pharm. Rundsch. 12 [1894], 16.)

In accordance with the known facts, the hydrocarbon C10H16 - e.g. tere-bentene - may be regarded as the starting point of two series:

1. Of a monatomic or camphol1) series (monohydrochlorides or chlorine esters of camphol, C10H17CI; camphene, C10H16; camphol alcohols, C10H180); justified in supposing six atoms of the turpentine oil to be arranged in similar manner as in benzene. Further, that the methyl and propyl groups in the turpentine oil occupy the same relative positions as in cymene.1)

2. Of a diatomic or terpil series (dihydrochlorides, C10H18Cl2; terpilene, C10H16; hydrate C10H20O2).

Each of these two series constitutes a larger group, which can be divided into secondary series (australene, terebentene, etc.) the parallel and isomeric members of which occur in twos; each has as type an inactive hydrocarbon, namely camphene in the first group, terpilene in the second.

A similar, but much less detailed classification was attempted by Gladstone2) in 1864, after having determined the specific gravity, the index of refraction and the optical rotation of a number of oils. By means of fractional distillation he isolated the hydrocarbons of various oils, rectified them by distillation over sodium and arranged them into three large groups:

1. Hydrocarbons of the formula C10H16 which boil between 160 - 170°;

2. Hydrocarbons of the formula C15H24 which boil between 249 - 260°;

3. Colophene, C2oH32, b. p. 315°, representing the third group.

About this time the word terpene was introduced, evidently by Kekule. In his Lehrbuch der organischen Chemie (1866) vol. 2, p. 437, the following statement occurs:

". . . . andrerseits das Terpentinol und die zahlreichen mit ihm isomeren Kohlenwasserstoffe, welche im allgemeinen als Terpene bezeichnet werden mogen."

His investigations, preceded by those of Barbier and Oppen-heim were of considerable importance, inasmuch as by revealing the relations between the terpenes and cymene, they threw new light on the molecular structure of these hydrocarbons.

Almost simultaneously Barbier3) and Oppenheim4) obtained cymene by heating the dibromide of terpin either by itself or with aniline. By the action of iodine on turpentine oil, Kekule5) obtained the same hydrocarbon. He, therefore, thought himself

1) Berthelot changed the name borneol to camphol. Liebig's Annalen 110 (1859), 368; from Compt. rend. 47 (1858), 266.

2) Journ. chem. Soc. 17 (1864), 1. A second contribution appeared eight years later. Ibidem, 25 (1872), 1.

3) Compt. rend. 74 (1872), 194. 4) Bed. Berichte 5 (1872), 94. 5) Bed. Berichte 6 (1873), 437.

This view of the constitution of the terpenes was the predominant one for a long time. It is only recently that researches have revealed facts not in harmony with this view. With this the question of the constitution of the terpenes had its origin. Important in this direction was also the synthesis of a terpene - the polymerization of isoprene to dipentene - by Bouchardat2) in 1875.