Electrons bouncing off mercury atoms proved energy comes in packets

Fire electrons through mercury vapor and slowly raise the voltage, and the current doesn't climb smoothly — it collapses sharply at fixed intervals, over and over. The 1914 result showed atoms can only absorb energy in specific fixed amounts, not any amount at all, giving quantum theory one of its first direct experimental proofs.

— James Franck and Gustav Hertz, The Franck–Hertz Experiment — Verhandlungen der Deutschen Physikalischen Gesellschaft, 1914; Nobel Prize in Physics, 1925