Döbereiner's triads

In the history of the periodic table, Döbereiner's triads were an early attempt to sort the elements into some logical order by their physical properties. In 1829, the German chemist Johann Wolfgang Döbereiner published a report of his previous observations that there were groups of three elements (hence "triads") which had similar physical properties. He also noted that some quantifiable properties of elements (e.g. atomic weight and density) in a triad followed a trend whereby the value of the middle element in the triad would be exactly or near exactly predicted by taking the arithmetic mean of values for that property of the other two elements.