The Franck–Hertz experiment confirmed Bohr's quantized model of the atom by demonstrating that atoms could indeed only absorb (and be excited by) specific amounts of energy (quanta).
http://en.wikipedia.org/wiki/Franck%E2%80%93Hertz_experiment
I saw only one specific energy level of gas molecule used in the experiment, which is the first excited state of the molecule. Why does my book say this experiment results shows that an atom only have specific energy in different energy levels, and the stable energy levels of atoms are discrete? I think this only proves 'at least one energy level in an atom is discrete.'