aug 17, 1821 - Discovered how the relative atomic mass of the middle element in a group of three is almost halfway between the lightest and heaviest ones in the group. He called this pattern “triad”. He organized many of the elements that (at the time) were discovered that way. The difference of relative atomic mass between three elements.
Added to timeline:
Date:
aug 17, 1821
Now
~ 204 years ago