Dobereiner arranged elements in increasing order of their atomic weight in well-marked groups of three-element called trides.
So, the pattern of arranging the dements in each triads, that the atomic weight of middle elements was found to be the arithmetic mean of the atomic weights of other two elements, called Dobereiner's law of trials.
Elements: Li Na K
Atomic weight: 7 23 39
So, 23 = (7+39) / 2