The alchemists had known since the 1500s that salts could impart various colors to a flame. In the 1700s, Andreas Sigismund Marggraf (1709-1782) demonstrated that sodium salts could be differentiated from potassium salts by the former’s yellow color and the latter’s purple hue. For other salts, however, some ambiguity might exist. For example, lithium, calcium, and strontium would impart a red color, although, careful observation could note subtle differences — viz., lithium was carmine; strontium was scarlet, calcium was brick red. The invention of the spectroscope in 1818 by Joseph Fraunhofer (1787-1826) allowed the observation of different light wavelengths (colors); the exact measurement of these wavelengths showed each element had its own spectral pattern. It then became easy to differentiate the elements, and even to recognize and identify elemental mixtures. (Fraunhofer is best known for his “Fraunhofer lines,” the dark lines in the spectrum of the sun). In 1854 David Alter (1807-1881), an American physician, physicist, and inventor, had the foresight to predict that spectroscopic analysis would even someday be used to identify the elements in stars.
Just before the discovery of the periodic table, four new elements were discovered by the new science of spectroscopy. In 1859 Robert Wilhelm Bunsen (1811-1899) and Gustav Robert Kirchhoff (1824-1887) in Heidelberg, Germany, invented the flame spectroscope, an instrument that allowed the identification of elements by their emission spectra. In fact, the Bunsen burner was invented purposely to produce a colorless flame so that the emission colors of the analyzed salt would not be masked. (The claim has been made that Michael Faraday (1791-1867), who designed the initial version of the burner, should actually be credited with the invention of the “Bunsen burner”.) This instrument consisted of three parts: (1) a flame and collimator, which would narrow the light from the incandescent sample into a beam; (2) a prism that could be rotated to admit and pass on various wavelengths of light; and (3) a telescope to observe the color being emitted by the glowing sample. This optical instrument allowed for very precise measurement of wavelengths of light that were being emitted by samples heated to incandescence in the flame. With this invention Bunsen and Kirchoff together founded the science of spectroscopic analysis. In the main hall of the Chemistry Building of Heidelberg University, an exhibit shows the spectral analyzer that the two scientists designed (see figure). On the outside wall of the historic university, a plaque reminds us of the extreme importance of this technique in identification of elements, even on celestrial objects! (see figure). It reads (translated): “In this building Kirchhoff in 1859 turned spectral analysis which he founded with Bunsen, on the sun and stars, thereby opening up the chemistry of the universe.”
Bunsen and Kirchhoff, in 1860, studied a concentrate of mineral water (from Dürkheim, a spa 60 kilometers to the west) using their spectroscope. The usual lines of sodium, potassium, lithium, calcium, and strontium could be observed. However, two new blue lines were also observed, which they attributed to a new element. They named it cesium from the color. A few months later, they observed new, deep red spectral lines of a new element they named rubidium. Salts of these two elements were later isolated in small quantities from concentrations of huge quantities of the spa water, and eventually in larger quantities from minerals from Bohemia (1862) and Italy (1882).
The third element discovered spectroscopically was thallium. Sir William Crooks (1832-1919), who shared with Bunsen, Kirchhoff, and Lecoq de Boisbaudran (1838-1912) the honor of establishing the science of spectral analysis, was studying some waste from a sulfuric acid plant in the Harz Mountains of central Germany. Analyzing the residues for selenium and tellurium, he observed a new intense green line, which he attributed to a new element he named thallium for “green branch.” To isolate a sample of thallium, the French chemist Claude-Auguste Lamy (1820-1878) in 1873 extracted the sulfuric acid prepared from burned pyrites (sulfide ores) from Belgium. He studied thallium compounds extensively and discovered they were extremely poisonous.
The fourth element discovered spectroscopically was indium. Ferdinand Reich (1799-1882) and his assistant Hieronymus Theodor Richter (1824-1898) at the Freiberg School of Mines discovered this element in sphalerite (zinc sulfilde) ores from the Himmelfürst Mine in Germany (the same mine that furnished the ore in which germanium was later discovered). Reich wanted to conduct a spectroscopic analysis of the ore, but he was colorblind and preferred to entrust the examination to his assistant Richter. Upon placing a platinum loop of zinc blende ore in a flame, Richter observed an intense indigo color which was obvious even without a spectroscope. Reich and Richter were able to easily prepare the elemental metal by reduction of the oxide with charcoal. They found that this metal was admixtured with zinc prepared from the mine and could be isolated more easily from the zinc itself than from the ore.
From Agricola’s time until the 1800s, chemistry and mineralogy were united disciplines. In fact, many of the chemists of these times were also mineralogists who were making their chemical discoveries through the analysis of ores. Today, several excellent mineralogical collections of historical value may be viewed at famous museums, such as the Musée de Minéralogie at the École des Mines in Paris and the Werner Museum at Technische Universität Bergakademie Freiberg, Germany, where several elements were discovered.
With the new spectroscopic method, an invaluable tool was now available to find a wide array of new elements, and was particularly important in the analysis of the chemically similar elements of the rare earth group. This story will be told in the next chapter, which describes the discovery of the periodic table.