Among the naturally occurring pyridine alkaloids may be named _piperine_, found in black pepper, and _conine_, the poisonous principle of hemlock (_conium maculatum_). The latter alkaloid was prepared synthetically by Ladenburg in 1886; as first obtained it differed from the naturally occurring product, which is dextro-rotatory, in being optically inactive. Ladenburg surmised that the synthetic preparation stood to the naturally occurring compound in the same relation that racemic acid stood to tartaric acid, and that, by treatment in the manner employed by Pasteur, the racemic modification of conine might be separated into its dextro- and lævo-constituents. This was found to be the case; but the separated dextro component was now found to be distinctly more optically active than the pure, natural variety. It was, in fact, an isomeric modification—_iso_-conine. By heating this to 300° it was transformed into ordinary conine, identical in all respects with the natural alkaloid. Ladenburg has also effected the synthesis of piperine by condensing piperidine and piperinic acid. _Nicotine_, the alkaloid of tobacco, was discovered by Posselt and Reimann in 1828. Its constitution was first ascertained by Pinner, and it was synthetically obtained by Amé Pictet, in 1904, as an inactive substance, capable of being resolved by the crystallisation of its tartrates into a dextro- and lævo-modification, the latter of which was identical with that found in the tobacco leaf. _Atropine_ and _hyoscyamine_—the poisonous principles of belladonna and henbane—are isomeric alkaloids, the former of which is optically inactive, and the latter is lævo-rotatory. Atropine is, in fact, the racemic modification. The constitution of both alkaloids is known, and their synthesis is now possible. The successive steps may be thus indicated: 1. _Synthesis of glycerin_ (Faraday, Kolbe, Melsens, Boerhave, Friedel, and Silva). 2. _Glycerin to glutaric acid_ (Berthelot and De Luca, Cahours and Hofmann, Erlenmeyer, Lermantoff, and Markownikoff). 3. _Glutaric acid to suberone_ (Brown and Walker, Boussingault). 4. _Suberone to tropidine_ (Willstätter). 5. _Tropidine to tropine_ (Willstätter, Ladenburg). 6. _Synthesis of tropic acid_ (Berthelot, Fittig and Tollens, Friedel, Ladenburg, and Rügheimer). 7. _Tropine and tropic acid: atropine_ (Ladenburg). The alkaloid _cocaïne_, contained in the leaves of _erythroxylon coca_ and now employed as a local anæsthetic, was discovered by Niemann in 1860. It has been shown to be closely related to atropine in constitution, and has now been synthetically prepared in the dextro-modification.