The artificial essence of violets known as _ionone_, prepared by Tiemann in 1893, and now made commercially, is similar but not identical in structure with the true perfume—_irone_. What is known as _artificial musk_ is a trinitro-butyl toluol. _Artificial orange-flower oil_ is a methyl ester of anthranilic acid. In Vol. I. a short account has been given of the early history of the large and important group of vegetable products known as the _alkaloids_. Many of these have long been valued on account of their powerful physiological action. As they all contain nitrogen and are generally basic, they were regarded by Berzelius, and subsequently by Liebig and Hofmann, as akin to ammonia in constitution, and were classed as amines. The first experimental evidence of their nature was obtained by Gerhardt, who found that, when strychnine and certain of the alkaloids belonging to the quinine group are treated with potash, an oil was obtained which he termed _quinoline_, and which was recognised by Hofmann as identical with a substance obtained in 1834 from coal-tar by Runge, and at that time known as _leucol_. By other modes of treatment certain alkaloids—_e.g._, nicotine and conine—are found to yield pyridine, a basic substance found by Anderson, in 1846, in the fœtid liquor obtained by distilling bones, and since found in coal-tar. Others of them—_e.g._, papaverine, narcotine, etc.—yield _isoquinoline_, an oil also discovered in coal-tar, by Hoogewerff, and Van Dorp, in 1885. These three substances—quinoline, _iso_quinoline, and pyridine—constitute so many nuclei in the constitution of a large number of alkaloids. Pyridine resembles benzene in being a cyclic compound, consisting of five carbon atoms and one nitrogen atom. Quinoline stands to pyridine in much the same relation that naphthalene stands to benzene. It can be obtained synthetically, as first shown by Koenigs and Skraup, and subsequently by Doebner and Von Miller, from benzene derivatives. _Iso_quinoline, isomeric with quinoline, differs from that substance in the position of the nitrogen atom. It, too, has been synthetically prepared from benzene derivatives in a number of ways.