BOHR, NIELS (1885— ), Danish physicist, was born at Copenhagen, the son of Christian Bohr, professor of physiology at Copenhagen university. He studied at Copenhagen until 1911, in which year he took his doctor's degree. In the same year he proceeded to Cambridge, where he worked in the Cavendish laboratory under Sir J. J. Thomson, and in 1912 passed on to Sir Ernest Rutherford's laboratory at Manchester, where he came into intimate contact with those conceptions of atomic structure which he was later to turn to so brilliant account. After a year's sojourn in Copenhagen he returned to Manchester in 1914. In 1916 he was appointed professor of theoretical physics in the University of Copenhagen. Largely by his instrumentality an institute for theoretical physics was established at the university in 1920. Niels Bohr was appointed as its head. This institute has become one of the great intellectual centres of Europe and is visited by students from all over the world, including many men of mature performance. Much brilliant work on the subject of atomic structure, experimental as well as theoretical, has been carried out there.
Although Niels Bohr started his scientific life by an experimental investigation, which dealt with the determination of surface ten sion by means of oscillating jets of fluid, it is on his theoretical work in the field of atomic structure that his fame rests. In 1911 Rutherford had put forward the theory that the atom consists of an impenetrable nucleus, very small compared to the size of the atom itself, surrounded by electrons occupying the space which is, in ordinary parlance, the volume of the atom (see ATOM ; NUCLEUS) . Planck had shown how the assumption of a quantum, or unit, of radiant energy was necessary to explain the distribution of radiant energy in the spectrum, and the quantum theory had already come to occupy a very prominent place in theoretical physics. In 1913 Bohr, by a few bold yet simple assumptions, showed how the quantum theory could be applied to the problem of the structure of the atom. In three papers published in the Philosophical Magazine (1913 and 1915) he obtained a series of important results, of which the most striking were a theoretical derivation of Balmer's formulae for the hydrogen series and an explanation of the line spectrum of ionized helium, which afterwards led to a remarkable quantitative confirmation of the theory. A fact which went far to convince the most sceptical that there was something in this theory was that Bohr obtained, on purely theoretical grounds, a value for the Rydberg constant R, which agreed very closely with that obtained experimentally. Two of the basic assumptions made in these early papers remain as fundamental for all modern theories of the atomic structure, namely, that the atom can exist in a series of stationary states each characterized by a certain value of the energy, and that when the atom passes from one stationary state to another a radiation is emitted, the frequency being given by a quantum condition. The exact ways in which the stationary states are to be fixed has been somewhat indefinite since the early days of the theory.
The great success of the Bohr theory, in explaining the simplest optical and X-ray spectra, inspired a large number of workers, and, in spite of the World War, there developed a new school of theoretical physics, in which the mechanism of the motions of electrons in an atom of the Bohr pattern was mathematically investigated, with the object of explaining details of spectral systems. Bohr himself made a great advance when he showed, by means of his so-called correspondence principle (see ATOM) how the quantum theory could be made to work in co-operation with the older theory of radiation, so that the useful results of both theories could be used to supplement one another. In 1921 and 1922 Bohr showed how his atomic theory could be applied to consideration of the physical and chemical properties of the elements considered in order. This work led directly to the dis covery of the new element hafnium by Coster and Hevesy. More recently, at the Institute of Analytical Physics at Copen hagen, Bohr has inspired a number of experimental and theoretical investigations, which have led to valuable results, all on the subject of atomic structure. In 1922 Bohr was awarded the Nobel prize for physics, in 1921 the Hughes medal of the Royal Society, and in 1926 he was elected a foreign member of the Royal Society.
(E. N. DA C. A.) BOHTLINGK, OTTO VON (1815-1904), German Sanskrit scholar, was born on May 3o (June 11, N.$), 1815, at St. Peters burg (Leningrad). In 1868 he settled at Jena, and died at Leipzig on April I, 1904. Bohtlingk's works are of great value in Indian and comparative philology. His first great work was an edition of Panini's Acht Bucher grammatischer Regeln (Bonn, which was in reality a criticism of Franz Bopp's philo logical methods. This book Bohtlingk again took up 47 years later, when he republished it with a complete translation under the title Paninis Grammatik mit t]bersetzung (1887). The earlier edition was followed by V opadevas Grammatik (1847) ; Ober die Sprache der Jakuten (1851); Indische Spriiche (2nd ed., to which an index was published by Blau, 1893) ; and other works. But his magnum opus is his great Sanskrit dictionary, Sanskrit Worterbuch (1853-75; new ed., 1879-8q), which, with the assist ance of his two friends, Rudolf Roth (1821-95) and Albrecht Weber (b. 1825), was completed in 23 years.