in late 1852 bunsen became the successor of leopold gmelin at the university of heidelberg. there he used electrolysis to produce pure metals, such as chromium, magnesium, aluminum, manganese, sodium, barium, calcium and lithium. a long collaboration with henry enfield roscoe began in 1852, in which they studied the photochemical formation of hydrogen chloride (hcl) from hydrogen and chlorine. from this work, the reciprocity law of bunsen and roscoe originated. he discontinued his work with roscoe in 1859 and joined gustav kirchhoff to study emission spectra of heated elements, a research area called spectrum analysis. for this work, bunsen and his laboratory assistant, peter desaga, had perfected a special gas burner by 1855, which was influenced by earlier models. the newer design of bunsen and desaga, which provided a very hot and clean flame, is now called simply the "bunsen burner", a common laboratory equipment.