Cesium, a rare alkaline-earth metal, was fated to become the first chemical element whose presence on Earth was established by spectroscopy, although its fate could have been different.
Back in 1846 the mineralogist A. Breithaupt, studying minerals and ores from the island of Elba, noted a coloured variety of quartzite, which he named pollux (or pollycite). The sample of pollux then fell into the hands of the German chemist K. Plattner from Freiberg, a professor of metallurgy in the Mining Academy. Plattner had a minute amount of pollux sufficient only for one analytical experiment. Having separated the constituents of the mineral and finding nothing new, Plattner, to his surprise, noted that the sum total of the constituents was only 92.75 per cent.
The reason for this remained unclear since Plattner had no pollux left. The scientist, however, established the following: pollux had the highest alkali content among all known silicates. It is now clear that cesium was safely masked by the much large amounts of sodium and potassium and Plattner was not able to extract it.
In 1860 R. Bunsen and G. Kirchhoff studied the chemical composition of various mineral spring waters by spectroscopy. After the separation of calcium, strontium, magnesium, and lithium from a sample of Dürkheim mineral water, a drop of the evaporated solution was studied spectroscopically. The scientists observed two pronounced blue lines close to each other. One of them almost coincided with the strontium line. Bunsen and Kirchhoff asserted that since no substance was known to have such spectral lines it had to be an unknown substance, belonging to the group of alkali metals. They proposed to name it “cesium” (symbol Cs) from the Latin caesius: in ancient times this word was used to describe the blueness of the upper part of the firmament. The beautiful blue vapour of cesium helped to prove the presence of a few millionths of a milligram of this substance in a mixture with sodium, lithium, and strontium.
On April 11, 1860, R. Bunsen wrote to G. Roskoe (his collaborator in a study in photochemistry) about his investigation of the new alkali metal. On May 10 he reported the discovery of cesium to the Berlin Academy of Sciences. Six months later Bunsen already had 50 g of almost pure cesium chloroplatinate. To obtain such an amount of the product, it was required to process nearly 300 tons of mineral water; about one kilogram of lithium chloride was isolated as a side product. These figures show how small was the cesium content in mineral spring waters.
Four years later the Italian Analyst F. Pizani set to study pollux, earlier investigated by Plattner. Pizani had a stroke of luck; he discovered cesium in the mineral and demonstrated that the German scientist had erroneously taken cesium sulphate for a mixture of sodium and potassium sulphates. Pure cesium, however, was separated only in 1882 by the German chemist K. Satterberg via electrolysis of a mixture of cyanides CsCN and Ba(CN)2.
In Russia Beketov prepared cesium almost at the same time and independently of Satterberg by reducing cesium aluminate (CsAlO2) with magnesium in a hydrogen flow.