"Quantum physics, which in contrast to the earlier theory, is characterized by the appearance of the elementary action quantum h and the designation of states in material systems by whole numbers, dates as theory only from the beginning of the twentieth century... However, some of its experimental roots extend far back into the nineteenth century. Of course, the measurements of the intensity of heat radiation which brought the change are a product of the last decade of that century. However, the photoelectric effect, and the wavelengths of the line and band spectra and also the dependence of the specific heats of certain substances on temperature had been known decades earlier. The older physics had hoped to arrive at an explanation of these findings; otherwise it is difficult to understand why Philipp von Jolly (1809-1884) told the inquiring young Planck that physics was essentially worked out and the pursuit of this science accordingly could hardly be very profitable. What appeared from time to time concerning line spectra could no longer stand up under rigid criticism when the discussion was based on the older ideas. On the other hand, quantum physics handled these problems more or less easily and in addition elucidated much of the newly acquired experimental observations."