Wolfang Pauli wrote that he had gone by train to pay a visit to Niels Bohr and at the very moment of the accident in James Frank’s laboratory his train stopped for a few minutes in Gottingen station. If, before Bohr, spectroscopy was an essentially empirical science that went little farther than to catalogue absorption and emission wavelengths, the new atomic theory offered a guide to the interpretation of the experimental results and, as often happens, theory and experiment cooperated with each other to yield the interpretation of various observed phenomena. Bohr then turned his attention to molecules, obtaining correct results for the hydrogen molecule but surprisingly not so for the helium atom which also has two electrons. Otto Stern and his collaborators investigated fundamental points concerning the kinetic theory of gases, verified spatial quantization, measured the magnetic moment of the proton, verified the de Broglie relation for waves made by helium atoms etc.