An important place in Wallach's researches is occupied by the successful efforts to unravel the isomeric complications so prevalent in the terpene group. Thus special investigations dealt with the study of optical isomerism, molecular refraction, cleavage of cycles as well as their formation, hydration and other physical and chemical problems.
After Wallach had removed the principal difficulties in the investigation of volatile oils, other chemists also developed a successful activity. A. v. Baeyer's valuable investigations into the constitution of the terpenes and related compounds appeared in the Berichte der deutschen chemischen Gesellschaft since 1893.
Whereas Wallach and v. Baeyer investigated primarily the cyclic compounds, Semmler paid special attention to chain compounds. He showed that the alcohols geraniol and linalool and the aldehydes citral and citronellal, which occur frequently in volatile oils, are chain compounds; also that they, like the more or less closely related cyclic compounds, can be converted into cymene. Later, Semmler's activity extended to practically all fields of volatile oil and terpene chemistry. Of the terpenes he examined sabinene, camphene, phellandrene, camphenilone. Among other problems, Semmler cleared up the constitution of buchu camphor, of santalol and of myrtenol. To him we are also indebted for our knowledge of the constituents of the oils of asarum, of East Indian sandalwood, of pilea oil, ayapana oil, etc. Practically all of his results have been published in the Be-richte der deutschen chemischen Gesellschaft.1)
In this connection the work of Tiemann and his colaborers, which began with the nineties of the past century, should be mentioned. This included the isolation of irone, the principal constituent of orris oil; later this led to the synthesis of ionone, which possesses the aroma of the violet. In this connection he published numerous articles on the chemistry of citral.
1) Berl. Berichte 23 (1890) to 41 (1908).
In addition to these, numerous other chemists have investigated the composition of volatile oils and the constitution of the terpenes and their oxygenated derivatives. In so far as their results pertain to the composition of volatile oils, these are included in the special part of this treatise.
The constitutional problems involved in the study of the terpenes and their derivatives are among the most difficult of organic chemistry because of the readiness with which rearrangements within the molecule take place under the influence of ordinary physical and chemical reagents. A typical example of some of the difficulties involved may be had in the ordinary camphor. Of this substance investigators had an almost unlimited supply at their disposal. Moreover, many chemists have been incessantly investigating this compound for several decades, during which period no less than thirty structural formulas received consideration. And yet it was but recently that Bredt found in his formula a generally satisfactory expression,1) a formula that finally led to the synthesis of camphor by Komppas.2)
This brief history of volatile oils would not be complete without some reference to the literature on the subject since the beginning of the past century. So long as the volatile oils were principally prepared in the laboratories of apothecary shops, the description of the oils and of the methods of their preparation was found in pharmaceutical reference works and pharmacopceial commentaries. Likewise the results of scientific and technical studies were published in pharmaceutical rather than in purely chemical journals. With the early forties, the preparation of volatile oils was taken out of the pharmaceutical laboratories and a separate literature was created. Yet Zeller's publications, already referred to, appeared first in the Jahrbuch fur praktische Pharmacie before they were published as separates.also a very meagre description of the physical properties of the oils and their behavior toward reagents. Maier's1) later treatise also takes into consideration the scientific investigations. The methods of preparation and the subject of distillation are described in detail by Mierzinski.2) A similar work was written by Askinson.3) Die Toiletten-Chemie of Hirzel,4) which passed through four editions, also "The art of perfumery" by Piesse,5) which was translated into several languages, may here be mentioned.
Zeller's8) publications contain a compilation of the yield,
1) Bredt, Versammlung D. Naturf. u. Arzte, Braunschweig 1907. Liebig's Annalen 314 (1901), 388.
2) Komppa and Hirn, Berl. Berichte 36 (1903), 4332.
3) G. H. Zeller, Stud/en uber die atherischen Ole. 1. Heft. Des chemi-schen Teils erster Abschnitt. Landau 1850. - //. Heft. Die physischen und chemischen Eigenschaften der officinellen atherischen Ole. Stuttgart 1855. - ///. Heft. Die Ausbeute und Darstelhmg der atherischen Ole aus officinellen Pflanzen. Stuttgart 1855.
The results of the earlier papers by Wallach are contained in the excellent work of Bornemann,6) whereas the Odorographia of Sawer7) emphasizes the botanical side of the subject.
Indispensable to scientific work on the chemistry of the terpenes was the monograph of F. Heusler8) which comprises the otherwise scattered literature up to the close of the last century. Translated into English by F. ). Pond9) and supplemented, it was published in the U. S. in 1892. Restricted to the sesquiterpenes is the smaller monograph by O. Schreiner.10)