Quinoline is an organic compound that occurs in coal-tar, bone-oil, and may be prepared by numerous reactions. It appears to consist chemically of a benzene (q.v.) and pyridine ring united together by two common carbon atoms. It is a colourless liquid and possesses a powerful and penetrating odour. It has a specific gravity of 1.095, and boils about 239°. In its chemical behaviour it acts as a basic substance, uniting with acids to form compounds analogous to salts. It forms a large series of derivatives, formed by the replacement of some of the hydrogen by other elements. As this substitution can take place in either the benzene or the pyridine ring, there are a very great number of isomeric forms. It has of late years derived very much additional interest from the fact that many of the alkaloids and other compounds possessing tonic properties appear to be derivatives of quinoline or of an isomeric substance termed iso-quinoline.