Golden colored, incredibly soft solid which easily liquefies. Will catch fire and/or explode upon exposure to the atmosphere m.p. 28.40±0.01 ºC; 83.12±0.02 ºF b.p. 678.4 ºC; 1253.1 ºF density 1.873 g/cc; 116.928 pound/cubic foot	memory peg
	1860 Robert Bunsen & Gustav Kirchhoff, Germany
	cæsius = sky blue (Latin)

In the Spring of 1860 Robert Wilhelm Bunsen (1811-1899) and Gustav Robert Kirchhof (1824-1887) examined with the flame of a spectroscope a drop of Dürkheim mineral water. One would recognize the light of Sodium, Potassium, Lithium, Calcium, and Strontium. But after precipitating lime, strontia, magnesia, you see the lines of Sodium, Potassium, and Lithium, and in addition, two remarkable blue lines, very close together for which no known substance gives such rays. They wrote in 1860 that

"Spectrum analysis should become important for the discovery of hitherto unknown elements. If there should be substances that are so sparingly distributed in nature that our present means of analysis fail for their recognition and separation, then we might hope to recognize and to determine many such substances in quantities not reached by our usual means, by the simple observation of their flame spectra. We have had occasion already to convince ourselves that there are such now unknown elements. Supported by unambiguous results of the spectral-analytical method, we believe we can state right now that there is a fourth metal in the alkali group besides potassium, sodium, and lithium, and it has a simple characteristic spectrum like lithium; a metal that shows only two lines in our apparatus: a faint blue one, almost coinciding with Sr, and another blue one a little further to the violet end of the spectrum and as strong and as clearly defined as the lithium line." Note.

"I have been very fortunate with my new metal...I shall name it cesium because of its beautiful blue spectral line. Next Sunday I expect to find time to make the first determination of its atomic weight."

Extraction of Caesium compounds by Bunsen involved concentration of the Caesium and other impurities by evaporation of large volumes of mineral water. Bunsen prepared chlorides, carbonates and other salts of Caesium. He studied the properties of these salts and attempted to prepare Caesium metal but was unsuccessful.

Bunsen and Kirchhoff's samples of Caesium and Rubidium salts. University of Heidelberg (Click on photo for source).

• Mary Elvira Weeks, Discovery of the Elements, comp. rev. by Heny M. Leicester (Easton, Pa.: Journal of Chemical Education, 1968), pp. 598-603.