Dr. Henry Jekyll was respected medical physician and scientist who theorized that every man had two sides: good and evil. In his experiments Jekyll sought to separate his good self from his evil self using a chemical, but instead of separating into two halves he instead physically transformed into his evil side, who took the name Edward Hyde. Initially Jekyll enjoyed being Hyde because Hyde was free to do all the forbidden things Jekyll could not, but Hyde soon grew stronger and began to take over: Jekyll began changing into Hyde even without the aid of the chemical, and could only remain Jekyll through regular doses of it.
Jekyll hypothesized that the chemical's effectiveness was due to an impure salt among the ingredients: failure to obtain a quantity of the salt with this particular impurity meant that the doctor could never permanently change back into his good half. Jekyll ultimately committed suicide as Hyde rather than risk remaining Hyde forever. This was all confessed in a letter to Jekyll's childhood friend and lawyer, Gabriel John Utterson.
League of Extraordinary Gentlemen
Dr. Jekyll was an English duck scientist who created the "moneybags formula," a chemical intended to make people more generous. When he tested it on himself however he accidentally created a kind of alter-ego, "Uncle Moneybags." As Moneybags, the wealthy Jekyll gave away his entire fortune and his mansion was foreclosed. Although during one of his rare moments of lucidity Jekyll created an antidote, he was unable to use it, and, after being arrested by the police for disturbing the peace with his antics, was put away in an insane asylum.