The alkaloids _papaverine_, _narcotine_, _narceïne_, contained in opium, are derivatives of _iso_quinoline, as also is _berberine_, found in the common barberry (_berberis vulgaris_). Papaverine, which occurs in opium to the extent of about one per cent., was first isolated by Merck in 1848. Its constitution has been established by Goldschmidt. _Narcotine_ is, next to morphine, the most abundant constituent of opium. The study of the products of its hydrolysis and oxidation—viz., _opianic acid_ and _cotarnine_, both of which substances have long been known—has indicated its probable structure. _Narceïne_ is closely allied to narcotine, and can, indeed, be obtained from it by combining the latter alkaloid with methyl iodide and treating the compound with caustic potash. The constitution of _berberine_, which is one of the few coloured vegeto-alkaloids known, has been worked out by Perkin. As yet nothing definite is known concerning the structure of the most important and largest constituent of opium—viz., _morphine_; or of its congeners _codeïne_ and _thebaine_. Grimaux, however, in 1881 converted morphine into codeïne by treatment with methyl iodide and potash; hence the two alkaloids stand in a relation somewhat similar to that in which narceïne stands to narcotine. There is very little doubt that the three alkaloids are very closely related, and that a knowledge of the constitution of one of them would immediately elucidate the structure of the others. They are probably phenanthrene derivatives. _Quinine_ and _cinchonine_, the most important of the cinchona alkaloids, are quinoline compounds, and are closely related in constitution. But the many attempts to unravel their structure have yielded no definite results up to the present. CHAPTER IX STEREO-ISOMERISM: STEREO-CHEMISTRY The first gropings in the search for light on the inner structure of molecular groupings may be said to date from Biot’s work on polarisation. In 1815 Biot, a pupil of Malus, made the remarkable discovery that a number of naturally occurring organic compounds—_e.g._, sugar, tartaric acid, oil of turpentine, camphor, etc., are _optically active_—that is, rotate the plane of polarisation in one direction or the other. The property had previously been observed in quartz, and was assumed to be connected with the crystalline character of that substance. Biot, however, pointed out that the case of oil of turpentine which is a liquid, and the cases of the other substances when in solution, showed that crystalline character had no necessary connection with the phenomenon, but that it must be dependent upon the internal or molecular arrangement of the optically active substance.