The frequencies of the lines in the X-ray spectra of the elements are related to the atomic numbers of the elements. If the square roots of the frequencies of corresponding lines of a set of elements are plotted against the atomic numbers, a straight line is obtained. The law was discovered by Henry Moseley (1887–1915) in 1913–14 and provided an experimental basis for ideas about the structure of the atom, particularly the idea that a chemical element is characterized by its atomic number.