Cesium was the first element to be discovered using spectroscopic means by Robert W. Bunsen and Gustav R. Kirchhoff in 1860, the year after they invented the spectroscope. Cesium, from the Latin caesius, meaning heavenly blue, was named after the color of the most prominent line in its spectrum (= 455.5 nm). It can be identified qualitatively in a flame test from the pale violet light given off by the electronic transitions in the excited metal atoms. Natural cesium consists of a single stable isotope, Cs–133. It occurs chiefly as a hydrated aluminosilicate mineral known as pollucite, 2Cs2O2Al2O39SiO2H2O, mined in the Bernic Lake region of Manitoba.