- Collection of papers of Niels Janniksen Bjerrum (1879-1958), a Danish chemist. Bjerrum introduced the concept of three forms of molecular energy, translational, vibrational and rotational which was important in understanding vibrational spectroscopy.[2] He is also noted for the theory behind the Bjerrum length, and the Bjerrum plot. Bjerrum also performed some of the first research on the measurement of soil acidity. He also contributed to the Bohr Model of the atom. Bjerrum worked with Nernst in Berlin and contributed to chemical physics in four papers (1911-1914). The subject of the papers is the kinetic and quantum theories through absorption measurements in the infrared to elucidate the constitution and the optical and thermal properties of matter. He advanced the studies of specific heat that had been made for solids by Albert Einstein, Walther Nernst, and Lindemann. He showed the connection of specific heats and the spectrum as required by the quantum theory. Using the quantum hypothesis, the infrared absorption spectra of water vapor were shown to link to the line broadening caused by molecular rotational frequencies that vary discontinuously and to radiating atoms that do not rotate. This phenomenon was mentioned by Niels Bohr as contributing greatly to his 1913 model of the atom because it provided agreement with specific heat investigations which suggested that the rotational energy of atoms must be very small.[