What relation about elements did dobereiner want to establish ?

What is dobereiner's triad?

"Learning is establishment of relation. The human brain works to establish the relation." The statement of:

The element M in the Dobereiner traid: S,M,Te is

What are the limitations of Dobereiner's law to triads ?

If the sum of atomic mass of 3 elements forming Dobereiner traid is 69, then atomic mass of second element