In the history of chemical elements the discovery of a new element often directly affected the discovery of another one. Thus, the discovery of thallium was a catalyst for the discovery of indium–the last of the classic group of four elements identified by spectral analysis.
The stage was set in the German town of Freiberg; and the main characters were F. Reich, professor of physics in the Mining Academy and his assistant Th. Richter. The time was the year of 1863. Interested in some properties if thallium, discovered two year earlier, F Reich decided to obtain a sufficient amount of the metal for his experiments. Searching for natural sources of thallium, he analysed samples of zinc ores mined at Himmelsfürst. In addition to zinc the ores were known to contain Sulphur, arsenic, lead, silicon, manganese, tin, and cadmium, in a word, quite a number of chemical elements. Reich believed that thallium could be added to the list. Although time-consuming chemical experiments did not produce the desired element, he obtained a straw-yellow precipitate of an unknown composition. It was told that when C. Winkler (subsequently the discoverer of germanium) entered Reich’s laboratory the latter showed him a test-tube with the precipitate and said that it contained sulphide of a new element.
It would have been surprising if F. Reich had not used spectroscopy to prove his assumption. Of course, Reich did use it but, unfortunately, he was colour-blind and, therefore, asked his assistant Richter to perform spectral analysis. Th. Richter succeeded in the very first attempt: in the spectrum of the sample he saw an extremely bright blue line which could not be confused with either cesium blue line or any other line. In a word, the observation was quite definite. Reich and Richter came to the conclusion that the ores of Himmelsfurst contained a new chemical element. They named it “indium” after “indigo”, a bright blue dye. There is an interesting fact that does credit to F. Reich. The first reports about the discovery of indium were signed by the two scientists. Reich, however, believed that this was unjust and that the honour of the discovery belonged solely to Richter.
Soon after the two scientists had proved the existence of natural indium with the help of spectroscopy, they obtained a small amount of it. Indium compounds turn the flame of a Bunsen burner blue-violet and so bright that presence of the new element could be established without a spectroscope. Subsequently Reich and Richter studied some properties of indium, with Winkler giving them considerable help.
When metallic indium, although contaminated, was prepared, Richter submitted the samples to the Paris Academy of Sciences in 1867 and estimated their value at 800 pounds sterling which was quite a lot of money at the time.
Chemical properties of indium were described soon after its discovery but its atomic mass was at first determined incorrectly (75.6). Mendeleev saw that this atomic mass would not correctly place indium in the periodic table and suggested to increase it by about 50 per cent. Mendeleev proved to be right and indium occupied its place in the third group of the periodic table.