Chemists now became especially interested in the action of hydrogen chloride on various terpenes and in the resulting hydrochlorides, some of which were solid, others liquid; also in the study of terpin hydrate and its decomposition products. The study of the literature pertaining to these subjects is rendered difficult by the error of regarding mixtures of several substances as chemical individuals and describing them as such;3) further by the fact that almost every author, irrespective of the work of others, coined a nomenclature of his own.4)

Crystalline pinene monohydrochloride had been discovered by Kindt,5) an apothecary, in 1803. He regarded it as artificial camphor, a view shared by Trommsdorff.6) The true composition of this compound was ascertained by Dumas in 1833.

1) Liebig's Annalen 22 (1837), 1.

2) Liebig's Annalen 52 (1844), 1. A more complete insight into the mechanism of the reaction by which mustard oil is produced was supplied by the later investigations of Will and Koerner in 1863. Liebig's Annalen 125 (1863), 257.

3) This confusion continued until Wallach cleared up the situation.

4) The historical development of this chapter of the chemistry of the terpenes is described in Terpene und Terpenderivate, einBeitragzurGeschichte der atherischen Oele by E. Kremer's (Pharm. Rundschau 9 (1891), 55, 110, 159, 217, 237; and 10 (1892), 10, 31, 60; also Proc. Wise. Acad. Sc. Arts and Letters, 8, pp. 312-262).

5) Trommsdorffs Journ. d. Pharm. 11, II. 132.

6) Ibidem, p. 135.

Crystalline dipentene dihydrochloride was discovered by Thenard in 1807. It is the salzsaures Citronenol, muriate of lemon oil, of Blanchet and Sell, the artificial lemon camphor of Dumas. These and similar substances were investigated by Soubeiran and Capitaine (turpentine oil), Deville (turpentine oil and elemi oil), Schweizer (carvene) and Berthelot (turpentine oil).1) The formation of terpin hydrate and the action of acids on this substance was studied principally by Wiggers, List, Deville and Berthelot.2)

A paper published about this time (1841) by Gerhardt and Cahours3) is of special interest in so far as it contains a definition of a volatile oil which in a general way holds good to-day. It also makes known new methods of investigation. About oils in general the authors state:

"There are, indeed, but very few oils which can be crystallized; most oils are liquid and consist of a mixture of two and even three peculiar substances, which rarely are obtained by themselves when distilled at different temperatures."

The separation of the individual substances is effected by first allowing any solid constituent to crystallize out, then the lower boiling hydrocarbon is isolated by distillation at a temperature 20 - below the boiling point of the crude oil.4) Inasmuch, however, as the hydrocarbon cannot be completely freed from oxygenated constituents in this manner it is treated with fused alkali. The oxygenated constituents also are subjected to like

1) Of later investigators of this subject Oppenheim (1864), Hell and Ritter (1884), Bouchardat and Lafont (1886), and finally Wallach (1884-1887) may be mentioned.

2) The same subject was later investigated by Oppenheim (1864), Flawitz-ky (1879), Tilden (1878-79), Bouchardat and Voiry (1887). Here also Wallach's exact investigations revealed the fact that different acids, as well as the same acid in different degrees of concentration, produce different results.

3) Liebig's Annalen 38 (1841), 67.

4) Fractional distillation, however, had been previously employed in the examination of volatile oils. As early as 1838 Walter had subjected peppermint oil to interrupted distillation, gebrochene Destination (Gmelin, Hand-buch d. Chem. [4], vol. 7 a, p. 404). In 1840 Volckel (Liebig's Annalen 35, 306) speaks of "fractional distillation." Already Blanchet and Sell in 1833 had applied fractionation with water vapor as a means of separation and had found that the first fraction of lemon oil boiled at 167°, the last fraction at 173°. j treatment with fused alkali, and cumin oil is thus made to yield cuminic acid, oil of valerian valeric acid.

Strong reagents were also employed by Rochleder, Persoz, Laurent and Gerhardt in order to obtain an insight into the nature of volatile oils. They oxidized either the entire oil or fractions thereof with chromic acid or nitric acid. Their investigations included the oils of valerian, sage, anise, staranise, fennel, cumin, cinnamon, tansy and estragon. The conclusions drawn from these oxidation experiments were in part correct, in part wrong. Thus e. g. Gerhardt pointed out the identity of dragonic acid, obtained from estragon oil, with anisic acid, and claimed that estragon oil and anise oil were absolutely identical. This conclusion was wrong, for the anethol of anise oil is paramethoxy-propenylbenzene, whereas the formation of anisic acid from estragon oil is due to the presence of paramethoxyallylbenzene.1)

Moreover, this method rendered it impossible to decide whether a substance obtained after the oxidation preexisted in the oil or not. Thus camphor was found in several oxidized oils and was regarded as an original constituent although, as was the case in the oils of valerian and sage, it had resulted from borneol. Persoz, however, seems to have had doubts as to the reliability of these conclusions, for he leaves it undecided whether the camphor obtained from oil of tansy was contained in the oil or not. As a matter of fact, tansy oil contains camphor as an original constituent,2) this being less readily attacked by the oxidizing agents than the other constituents of the oil.