The term radical was already in use when radical theory was developed. Louis-Bernard Guyton de Morveau introduced the phrase "radical" in 1785 and the phrase was employed by Antoine Lavoisier in 1789 in his Traité Élémentaire de Chimie. A radical was identified as the root base of certain acids (The Latin word "radix" meaning "root"). The combination of a radical with oxygen would result in an acid. For example the radical of acetic acid was called "acetic" and that of muriatic acid (hydrochloric acid) was called "muriatic". Joseph Louis Gay-Lussac found evidence for the cyanide radical in 1815 in his work on hydrogen cyanide and a number of cyanide salts he discovered. He also isolated cyanogen ((CN)) not realizing that cyanogen is the cyanide dimer NC-CN. Jean-Baptiste Dumas proposed the ethylene radical from investigations into diethyl ether and ethanol. In his Etherin theory  he observed that ether consisted of two equivalents of ethylene and one equivalent of water and that ethylene and ethanol could interconvert in chemical reactions. Ethylene was also the base fragment for a number of other compounds such as ethyl acetate. This Etherin theory was eventually abandoned by Dumas in favor of radical theory. As a radical it should react with an oxide to form the hydrate but it was found that ethylene is resistant to an oxide like calcium oxide. Henri Victor Regnault in 1834 reacted ethylene dichloride (CHCH.Cl) with KOH forming vinyl chloride, water, and KCl. In etherin theory it should not be possible to break up the ethylene fragment in this way.
Radical theory replaced electrochemical dualism which stated that all molecules were to be considered as salts composed of basic and acidic oxides.