Professor Byron L Youtz uses a stream of electrons accelerated through mercury vapor to show that the kinetic energy of the electrons is transferred to the mercury atoms only in discrete packets of energy. Establishes the association of the quantum of energy with a line in the spectrum of mercury. Points out that the Frank-Hertz experiment was one of the earliest indications of the existence of internal energy states within the atom. Includes an epilogue by James Franck. A PSSC (Physical Science Study Committee) film.