"Mahn ... concludes, after an elaborate investigation, that Gr. khymeia was probably the original, being first applied to pharmaceutical chemistry, which was chiefly concerned with juices or infusions of plants; that the pursuits of the Alexandrian alchemists were a subsequent development of chemical study, and that the notoriety of these may have caused the name of the art to be popularly associated with the ancient name of Egypt." [OED]The al- is the Arabic definite article, "the." The art and the name adopted by the Arabs from Alexandrians and thence returned to Europe via Spain. Alchemy was the "chemistry" of the Middle Ages and early modern times; since c.1600 applied distinctively to the pursuit of the transmutation of baser metals into gold, which, along with the search for the universal solvent and the panacea, were the chief occupations of early chemistry.
"Use ... Sperma Cete ana with redd Wyne when ye wax old." [Sir George Ripley, "The Compound of Alchemy," 1471]
"Pythagoras was the first who called himself philosophos, instead of sophos, 'wise man,' since this latter term was suggestive of immodesty." [Klein]Modern form with -r appears early 14c., from an Anglo-Fr. or O.Fr. variant of philosophe, with an agent-noun ending. Philosophy also was used of alchemy in Middle Ages, hence Philosophers' stone (late 14c., translating M.L. lapis philosophorum, c.1130), a reputed solid substance supposed by alchemists to change baser metals into gold or silver; also identified with the elixir and thus given the attribute of prolonging life indefinitely and curing wounds and disease. (Fr. pierre philosophale, Ger. der Stein der Weisen).