ALKALOID  , in

chemistry, a term originally applied to any organic base, i.e. a nitrogenous substance which forms salts with acids; now, however, it is usual to restrict the term to bases of vegetable origin and characterized by remarkable toxicological effects . Such bases occur almost exclusively in the dicotyledons, generally in combination with malic, citric, tartaric or similar plant-acids . They may be extracted by exhausting the plant-tissues with a dilute acid, and precipitating the bases with potash, soda, lime or magnesia . The separation of the mixed bases so obtained is effected by repeated fractional crystallization, or by taking advantage of certain properties of the constituents . A chemical classification of alkaloids is difficult on account of their complex constitution . I . A . Wyschnegradsky, and after-wards W . Konigs, expressed the opinion that the alkaloids were derivatives of pyridine or quinoline . This view has been fairly well supported by later discoveries; but, in addition to pyridine and quinoline nuclei, alkaloids derived from isoquinoline are known . The purely dhemical literature on the alkaloids is especially voluminous; and from the assiduity with which the constitutions of these substances have been and are still being attacked, we may conclude that their synthesis is but a question of time . Piperine, conine, atropine, belladonine, cocaine, hyoscyamine and nicotine have been already synthesized; the constitution of several others requires confirmation, while there remain many important alkaloids—quinine, morphine, strychnine, &c.—whose constitution remains unknown .

The following classification is

simple and convenient; the list of alkaloids makes no pretence at being exhaustive . (r) Pyridine group . Piperine; conine; trigonelline; arecaidine; guvacine; pilocarpine; cytisine; nicotine; sparteine . (2) Tropine group . Alkaloids characterized by containing the tropine (q.v.) nucleus . Atropine; cocaine; hygrine; ecgonine; pelletierine . (3) Quinoline group . The alkaloids of the quina-barks: quinine, &c.; the strychnos bases: strychnine, brucine; and the veratrum alkaloids: veratrine, cevadine, &c . (4) Isoquinoline group . ,The opium alkaloids: morphine, codeine, thebaine, papaverine, narcotine, narceine, &c.; and the complicated substances hydrastine and berberine . In addition to the above series there are a considerable number of compounds derived from purin which are by some writers classed with the alkaloids . These are treated in the article PURIN .

There are also reasons for including such compounds as muscarine, choline, neurine and betaine in this group . The greater number of these substances are of considerable medicinal value; this aspect is treated generally in the article

PHARMACOLOGY . Reference should also be made to the articles on the individual alkaloids for further details as to their medicinal and chemical properties . The chemistry of the alkaloids is treated in detail by Arne Pictet in his La Constitution chimique des alcaloides vegelaux (Paris, 1897); enlarged and translated by H . C . Biddle with the title The Vegetable Alkaloids (New York, 1904); and by J . W . Bruhl, E . Hjelt, and O . Aschan: Die Pflanzen-Alkaloide (1900) . A pamphlet, Die Alkaloidchemie in den Jahren 1900-1904, by Julius Schmidt, may also be consulted .