A better knowledge was attained by careful observation. Thus the crystalline deposits formed in some oils upon standing, also the congealing of certain oils at lower temperatures, which had been observed by Valerius Cordus in 1539, by Joh. Kunkel in 1685, by ). H. Link in 1717, by Friedr. Hoffmann in 1701, by Caspar Neumann in 1719 and by others, were studied. The crystalline parts were regarded as a volatile salt, later as a camphor peculiar to each oil, at times also as benzoic acid.2) Fr. Hoffmann explained the congealing of oils of rose, anise and fennel by assuming the formation of a curdled modification of the oil. Caspar Neumann in 1719 and CI. J. Geoffroy in 1726 regarded the crystals formed upon standing as camphor.3) The formation of such crystals was observed by Caspar Neumann4) in the oils of thyme, cardamom and marjoram; in peppermint oil by H. D. Gaubius5) of Leyden in 1770; in oil of mace by J. G. Wiegleb6) in 1774; in the oils of lavender, rosemary, sage and marjoram by Arezula7) in 1785. They regarded these separations as varieties of camphor, only J. G. Wiegleb thought them to be peculiar combustible salts.1)
1) Memoires de l'acad6mie royale des sciences de Paris. 1700, p. 298 and 1701, p. 129, also Chem. u. botan. Abhandlungen der Acad. d. Wissensch. zu Paris. Translated by Steinwehr. Vol. 3, pp. 155 - 157.
2) Hagen, Dissertatio chemica inauguralis sistens dosimasiam, concre-tionum in nonnullis oleis aethereis observatarum. Regiomontana; 1748.
P. J. Macquer's Dictionnaire de Chymie. Paris 1766. Deutsche Uber-setzung von J. G. Leonhardi. Leipzig 1781. Vol. 4, p. 465, footnote 9.
3) Memoires de I'acade'mie royale des sciences de Paris. 1726. p. 95. 4) De salibus alcalino fixis camphora. Berolini 1727. p. 105.
5) Adversariorum varii argumenti liber unus. Leidae 1771. Sectio 7, p. 99-112.
6) Vogel's Lehrsatze der Chemie. Mit Anmerkungen herausgegeben von J. G. Wiegleb. Weimar 1775. § 342.
7) Resultato de las experiencas hachas sobre alcanfor de Murcia con licencia. En Segovia 1789.
The action of strong acids had been observed by Glauber2) as early as 1663. The effect of strong nitric acid on a number of distilled oils was studied by Olaus Borrichius3) in 1671, by J. P. Tournefort4) in 1698, by Hasse5) in 1783; that of sulphuric acid by Jon. Kunkel6) in 1700 and by W. Homberg7) in 1701. A more detailed study of the action of strong acids on volatile oils was made by Friedrich Hoffmann8) and by CI.- J. Geoffroy9) in 1726 and by Rouelle10) in 1747. Upon distillation of oils with strong hydrochloric acid, especially if the acid was generated in an almost anhydrous condition in the experiment, it was supposed that compounds of the oil with the acid were obtained. Such a supposed compound was known to Homberg11) as early as 1709. The preparation, however, of such a compound of definite chemical composition was first accomplished by Kindt12) an apothecary in Eutin, in 1803 by the action of gaseous hydrogen chloride on turpentine oil.
The solubility and color of distilled oils also received attention during the eighteenth century. Thus Macquer in 1745 published his investigations on the solubility of distilled oils in alcohol, which were the most extensive on this subject.13) The color of oils and the changes in color were studied by Wilh. Homberg14) in
1) See p. 74, footnote 6.
2) R. Glauberii Prosperitas Germaniae. Amstelod. 1656.
3) Acta medica et philosophica Haffnienses. 1671. p. 133.
4) Historia regiae scientiarum academiae. Autore du Hamel. Parisii 1701. p. 495.
5) Crell's Neueste Entdeckungen in der Chemie 9 (1783), 38. - Crell's Chem. Annalen 1 (1785), 417.
6) Laboratorium chymicum. Hamburg 1716. p. 347.
7) Chem. botan. Abhandlungen der konigl. Acad. d. Wissensch. in Paris. Translated by Steinwehr. 1, 720.
8) Observatorium physico-chimicarum selectiorum. 1712. Liber III, p. 123. 9) Memoires de I'acade'mie roya/e des sciences de Paris. 1726, 95.
10) Ibidem. 1747, 45.
11) Chem. botan. Abhandl. d. konigl. Acad. d. Wissensch. in Paris. Translated by Steinwehr. 3, 155-167.
12) Trommsdorffs Journ. d. Pharmacie 11, II. (1803), 132.
13) Memoires de I'acadGmie royale des sciences de Paris. 1745, 4.
14) Chem. botan. Abhandl. d. konigl. Acad. d. Wissensch. in Paris. Translated by Steinwehr. 3, 155-167.
1707 and by Jacob ). Bindheim1) of Moscow in 1788. The latter arrived at the conclusion that the color depends on a larger or lesser amount of resin carried over in the process of distillation, hence the darker colored oils are apt to contain considerable resin. Bindheim also determined the yield of oils, especially of those of chamomile and peppermint, by distilling larger amounts of these herbs.
In 1793 and 1794 Margueron studied the action of frost on volatile oils and observed the formation of crystals and congealing in connection with a number of the more common oils.2)
As has already been pointed out, the phlogistic theory afforded no satisfactory basis for the study of organic substances and consequently of volatile oils. With the discovery of oxygen by Scheele and Priestley during the years 17713) to 1774 and the ingenious interpretation of this and other discoveries by Lavoisier with the aid of the balance, a reaction against the phlogistic theory set in which resulted in the inauguration of the present chemical period. The study of the chemical constitution of substances was placed on a rational scientific basis. Inorganic chemistry, having to deal with the simpler substances, profited first by the new theories of the opponents of the phlogistic school. Organic chemisty, and with it the study of the volatile oils, were benefited somewhat later.
Though of little consequence, the experience of the Dutch chemists Deimann, Troostwyck, Bond and Lauwerenburg4) should here be mentioned. They [passed the vapors of volatile oils through red hot iron tubes and examined the resulting gases. At the same time they made the bold attempt to synthesize oils by the action of gaseous hydrogen chloride on olefiant gas.