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Speak the names Bunsen, Kirchoff or Crookes and every spectroscopist worth their salt will know of whom you speak. Say the name Paul-Émile (Francois) Leçoq de Boisbaudran and you will be met with a blank stare. Yet this French chemist and spectroscopist, who grew up with no formal education and worked in his father's wine business, deserves to be ranked among the elite of late 19th century spectroscopy, for he discovered the elements gallium, samarium, and dysprosium; perfected methods of separating the rare earths; and authored Spectres Lumineux (1874), one of the earliest texts dealing with spectroscopy. |
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Born in Cognac, France in 1838, Boisbaudran grew up in a supportive family whose watchwords were, according to Sir William Ramsay, "justice, kindness, and the sense of personal responsibility." Boisbaudran owed his early education to his mother, who, as the educated daughter of an army officer, taught her son classics, history, and foreign languages. By studying the course books of the École Polytechnique, he established a strong background in physics and chemistry, and with the support of his uncle, he built a home laboratory where gallium (a.k.a. Mendeleev's ekaaluminum) was discovered. One of Boisbaudran's early contributions occurred in 1869, the same year that Mendeleev published his first periodic table of the elements, when he recognized that in addition to chemical similarities, certain groups of elements exhibited physical similarities in their spectra. "The spectral lines of the alkali metals (and alkaline earths) classified by their refrangibilities (the measure of how much the light's path is bent when refracted by a prism), are placed, as the chemical properties, following the order of atomic weight." In 1874, Boisbaudran began the investigation of a zinc blende from a mine in the Argeles Valley in the Pyrenees. When he dissolved the ore and exposed it to metallic zinc, a deposit formed that, when heated and examined with a spectroscope, revealed two lines that had never been observed previously. He describes his discovery thusly: "Between three and four in the evening of August 27, 1875, I found indications of the probable existence of a new elementary body in the products of the chemical examination of a blend from Pierrefitte. The oxide, or perhaps a sub-salt, is thrown down by metallic zinc in a solution containing chlorides and sulfates. It does not appear to be the metal itself which is reduced by the zinc. The extremely small quantity of the substance at my disposal did not permit me to isolate the new body from the excess of zinc accompanying. The few drops of zinc chloride in which I concentrated the new substance gave under the action of the electric spark a spectrum composed vhiefly of a violet ray, narrow, readily visible, and situated at about 417 on the scale of wavelengths. I perceived also a very faint ray at 404." Subsequently, he isolated larger quantities of the element he named "gallium" after the Latin name for France, presenting 3.4 grams of solid gallium to the Academy of Sciences in December of 1875 and later presented a specimen of the liquid metal. What is perhaps most impressive is that it was his own law of spectra and not Mendeleev's periodic table that led him to his discovery. He was searching for the missing element between aluminum and indium and based his search on the proposition that the arrangement of lines within chemically-similar elements are repeated according to the same general arrangement. A detailed description of his discovery of gallium (Annales de Chimie (5) Vol. 10 (1877) p. 100-141) can be found on the WWW as part of the Classic Papers page of the Chem Team of Diamond Bar High School (click here). Boisbaudran's two volume treatise Spectres Lumineux (1874) described the results of extensive spectral examinations of 35 elements. The purpose of this work was to establish a proposed relationship of spectral wavelength and atomic mass in which the displacement of observed emission lines from related elements did not correspond to the magnitude of molecular forces, but rather to the mass of the elements. While not the oldest book on spectroscopy, it is certainly one of the earliest, predated by H. Schellen, "Spectrum Analysis in its Application to Terrestrial Substances, and the Physical Constitution of the Heavenly Bodies" (London, 1872). A copy of Spectres Lumineux in PDF format can be found at the Gallica website (in French - click on ouvrage for document #2 to download the PDF). Boisbaudran continued his spectroscopic investigations, concentrating on rare earth elements, and he is credited with the discovery of dysprosium, and samarium. It appears that his spectroscopic skills also supported the work of prominent chemists such as Jean-Charles Galissard de Marignac (discoverer of ytterbium and gadolinium).Boisbaudran spoke English well, but often translated his French thoughts too literally. A story is told by Sir William Ramsay of the elderly English lady who was startled by Boisbaudran's description of his soup as "devilish hot." How times have changed! He married late in life, and after 1895, his scientific contributions diminished as a result of severe anchylosis (stiffening) of the joints. He died in 1912 at the age of 74, with his impressive accomplishments rewarded with the Cross of the Legion of Honor, the 1879 Davy Medal and the Prix Lacaze. The author thanks Marvin Margoshes for his donation of a copy of Spectres Lumineux to the NIST library, and Lisa Greenhouse of the NIST Research Library for her help in discovering references. Information from the following books was used in the preparation of this article:
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