During the last years of the Nineteenth Century, the development of new techniques
and the refinement of measuring apparatuses provided an abundance
of new data, whose interpretation implied deep changes in the formulation of
physical laws and in the development of new phenomenology.
Several experimental results lead to the birth of the new physics. A brief list
of the most important experiments must contain those performed by H. Hertz
about the photoelectric effect, the measurement of the distribution in frequency
of the radiation emitted by an ideal oven (the so-called black body radiation),
the measurement of specific heats at low temperatures, which showed
violations of the Dulong–Petit law and contradicted the general applicability
of the equipartition of energy. Furthermore we have to mention the discovery
of the electron by J. J. Thomson in 1897, A. Michelson and E. Morley’s
experiments in 1887, showing that the speed of light is independent of the
reference frame, and the detection of line spectra in atomic radiation.