Historical clue formulations

• Serotonin, e.g. — 2012-06-22
• Nitrogen compound — 2007-01-07
• Serotonin, e.g. — 2006-10-06
• Nitrogen compound — 2006-01-18
• Toluidine, e.g. — 2002-05-25
• Chemical compound — 1993-12-18