As suggested by one of the commentators to my last question, I am going through Bohr's Nobel prize lecture in order to understand how quantum mechanics was developed. It describes about Planck's observation on radiations,( I hope while answering or commenting on this , someone would also explain how it was concluded from this observations that energy can change only in discrete amounts and also the derivation of Planck's formula) and then goes on to Einstein's idea about the restricting condition for vibrational energies of atoms and the specific heats of crystal . Can anyone describe what exactly what Einstein's idea was about the specific heats of crystal, how that accentuated the concept of discrete energy variation and how it led to further development of QM?