Historical clue formulations

• Gather on the surface, chemically — 2019-07-19
• Gather on a surface, in chemistry — 2013-04-04
• Gather in a condensed layer — 2007-07-08
• Act on like a carbon filter — 2006-03-25
• Gather in a thin layer of molecules — 1999-08-20
• Gather on the surface, chemically — 1997-06-13