The recollections of Lincoln's legal colleagues (John T. Stuart, Henry Clay Whitney, Ward Hill Lamon, and William Herndon) all agree that Lincoln took blue mass pills,[25] which were commonly prescribed for hypochondriasis[lower-alpha 2] and melancholia.[26] It has been used since the 16th century to treat syphilis and by the mid-19th century was prescribed for a wide variety of ills.[26] The active ingredient of blue mass is elemental mercury – a substance now known to be a neurotoxin in its valproic state[27][lower-alpha 3] and has been known to be poisonous for centuries.[26][lower-alpha 4]