3) Caspar Neumann's researches on volatile oils can be found in the second volume of his Chymia medica dogmatico-experimentalis, oder Grundliche mit Experimenten bewiesene Medizinsche Chemie. Published by Christ. Heinr. Kessel. 4 vols. Zullichau 1749-1755.
1) Geoffroy's and Rouelle's work on volatile oils was published in the Memoires de l'academie royale des sciences de Paris for the years 1730 to 1760.
Aside from the publications already mentioned, the interest shown in the study of volatile oils toward the close of the seventeenth and during the course of the eighteenth century is possibly best shown by the number of dissertations on the subject which were written at German universities under the stimulus of a number of university teachers.8)
1) In 1789, Remler of Erfurt collected and tabulated the observations relative to the yield and properties of volatile oils published up to that year.
A similar tabular compilation taking into consideration also the origin of the oils was published in the Journal de pharmacie for August 1834 by Raybaud of Paris in connection with the industrial exposition of the previous year. A German translation appeared in Buchner's Repert der Pharm. for 1835, vol. 51, p. 54. Two further treatises on this subject appeared by G. H. Zeller in 1850 and 1855 respectively in the Jahrbuch fur praktische Pharmacie und verwandte Facher. The former appeared also as a separate under the title of Studien uber atherische Oele, Landau, 1850; the latter under the title Ausbeute und Darstellung der atherischen Oele. Stuttgart, 1855.
2) The new Dispensatory: Containing the theory and practice of pharmacy, a description of medicinal simples, according to their virtues and medicinal qualities, the description, use and dose of each article etc. Intended as a correction and improvement of Quincy. London 1753.
3) The more important ones are herewith enumerated:
1670. De oleorum destillatiorum natura et usu in genere. Dissertatio ab
David Kellner. Helmstadii. 1696. De oleis destillatis. Dissertatio ab Henrico Rosenberg. Jena?. 1744. De oleis destillatis empyreumaticis. Dissertatio ab Christian Lindner.
Francofurti ad Viadrum.
1744. De sale volatili oleoso solido in oleis aethereis nonnunquam reperto. Dissertatio ab Fr. Gunther. Francofurti ad Viadrum.
1745. De oleis vegetabilium essentialibus. Dissertatio ab A. Fr. Walther. Lipsiae.
1746. De spiritu rectore in regno animali, vegetabili et fossili, atmosphaerico. Dissertatio ab Gottfried de Xhora. Leydas.
1747. De oleorum destillatorum usu multiplice principue in castris. Dis-sertatio ab Joh. Paul Ziegler. Altorfii.
1748. Dissertatio chemica inauguralis sistens Dosimasiam concretionum in nonnullis oleis aethereis observatum ab F. Hagen. Regiomontanae.
The investigations reported in these dissertations, however, rest on false premises and, therefore, produced no valuable results. Research based on the phlogistic theory and the doctrines of Boerhaave and Hoffmann concerning the constitution of volatile oils, could hardly be expected to yield results of any importance. As is well known, water was resolved into its elements as late as 1766, the elements composing the atmosphere were discovered in 1774, and the phlogiston theory was disposed of in 1785.
How crude the notions concerning the chemical nature of volatile oils were even at the time of Scheele, is shown in a dissertation1) of the year 1765 accepted by the University of Jena. From it the following propositions or conclusions are quoted:
"The essential constituents of volatile oils are of two kinds, solid and liquid. To the first class belong sulphur, phlogiston, earth and salts; to the second class air, fire and water. The presence of the first is revealed by the inflammability of the oils, for every object that burns with a flame contains much sulphur or phlogiston. The color as well as the coloration of the oil likewise argue in favor of their presence. Some oils are yellow, others green or blue; with age, the colors become darker. As is known, all coloration is due to particles of sulphur or phlogiston. Such oils have a penetrating odor, which is caused by their content of volatile saline sulphur particles. They, therefore, contain sulphur or phlogiston in sufficiently large quantities.
"In the course of time these oils are converted into a resinous mass, a change that is not conceivable without phlogiston.
"Volatile oils always burn with a smoking flame. All soot, however, consists of earth, salt, water and phlogiston. When the oils are treated, with nitric acid, a residue of earth and carbon remains.
"Some volatile oils have a higher specific gravity than water. This is due to their larger content of earthy constituents and salts."
1752. De oleis essentialibus aethereis eorumque modo operandi et usu.
Dissertatio ab Johann Friedr. Vangerow. Hallae. 1759. De oleis destillatis aethereis. Dissertatio ab Fr. W. Eiken. Helmstadii. 1765. De parti bus oleorum aethereorum constitutivis. Dissertatio ab
Johannes Christ. Schmidtius. Jenae. 1765. De parti bus oleorum aethereorum constitutivis. Dissertatio ab J. Fr.
Faselius. Jena?. 1765. De oleis vegetabilium essentialibus, eorumque partibus constitutivis.
Dissertatio ab W. B. Trommsdorff. Erfurti. 1778. De adulterationibus oleum aethereorum. Dissertatio ab K. W. Chr.
1) De partibus oleorum aethereorum constitutivis. Dissertatio inaug-uralis per Johannes Christianus Schmidtius. Jenae d. 30. Masrz 1765.
Only a superficial insight into the nature of the volatile oils was obtained through the study of their properties and their behaviour toward strong chemical reagents, a study that was taken up at the beginning of the 18. century. Indeed, many of these experiments yielded no results whatever. This is true e. g. of the repeated distillations of volatile oils with chalk1) and burned lime, such as were undertaken by the otherwise excellent chemist Wilhelm Homberg (born 1652, died 1715) about the year 1700.