Alkaloid, vegetable alkali, a name given to vegetable extracts possessing the property of uniting with acids to form salts in the same manner as ammonia. The first alkaloid was discovered by Serturner in 1804 in opium; but little importance was attached to the announcement, and it was not till 1817 that the real value of morphine was demonstrated and the existence of vegetable alkalies fully shown. Since that time the list of alkaloids has rapidly increased, until at the present time (1872) they number more than 100. There are two classes, volatile liquids and permanent solids. The former contain simply carbon, hydrogen, and nitrogen, and only three of them are known, confine, nicotine, and sparteine. The solid and most numerous alkaloids contain carbon, hydrogen, nitrogen, and oxygen. The organic bases are colorless and generally crystalline. They are insoluble or slightly soluble in water, the best solvent being alcohol. Ether dissolves some of them; chloroform and the hydrocarbons are also good solvents. They generally possess powerful medicinal properties. Numerous artificial alkaloids have been formed, the most important of which is aniline. The natural base coniine has also been made artificially.

Some of the best known of the vegetable alkaloids are nicotine, quinia, morphia, strychnia, brucia, aconitina, atropia, and caffeine.