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Justus von Liebig—Leading Teacher of Organic Chemistry

      Baron Justus von Liebig was born on May 12, 1803, in Darmstadt in southern Germany. His father owned a drug and painting materials business, and young Justus learned to perform chemical experiments in the small laboratory affiliated with the business. Because his interest in chemistry was much greater than his interest in other school subjects, young Justus was apprenticed to an apothecary. However, he left his apprenticeship after less than a year and returned home to continue his chemical experiments.
      At the age of 17 years, Liebig went to the University of Bonn (Germany) to study chemistry; he received his doctorate in 1822. Analytical chemistry was not well advanced in Germany at that time, so Liebig obtained a grant from Grand Duke Louis I of Hesse (1753–1830) to study in Paris (France). When Liebig published his work on fulminate of mercury, it came to the attention of the German naturalist and statesman Alexander von Humboldt (1769–1859), who arranged for Liebig to work with French chemist Joseph-Louis Gay-Lussac (1778–1850), the master of chemical analysis. Gay-Lussac emphasized the need to pay strict attention to the precise determinations of the composition of substances and directed Liebig to apply these techniques to organic substances.
      At the urging of von Humboldt, Louis I had Liebig appointed extraordinary professor at the University of Giessen (Germany) in 1824. Liebig's first goal was to make available to a large number of students the same opportunities that he had had with Gay-Lussac. In collaboration with a mineralogist and a mathematician, Liebig proposed that a pharmaceutical training institute be established. Although the proposal was not approved, Liebig was allowed to set up a laboratory in a military barracks. In 3 years, chemistry dominated the curriculum, and the 20 positions in the laboratory were filled.
      Liebig's first laboratory consisted of a single room. He had to buy most of the supplies and pay his assistant from his own salary. The institute was designed to enable students to progress systematically from elementary procedures to independent research under the guidance of an established investigator. The school was successful because of Liebig's analytic skills and teaching ability. As he trained others, he extended his own studies beyond what he could accomplish by himself. This approach-a team of many students with 1 scientist in charge-permitted extensive study of specific subjects. The example soon spread to other experimental sciences. Liebig eventually relinquished the teaching of this system to his assistants, while he worked with advanced students. Two of his assistants, Carl Remigius Fresenius (1818–1897) and Heinrich Will, published textbooks that outlined his methods. Many investigations on the methods for preparing inorganic substances were conducted in Liebig's laboratory.
      In 1829, Liebig completed an extensive study of the decomposition of various chemical combinations by the use of chlorine. He and Friedrich Wöhler (1800–1882), who previously had synthesized urea, studied the composition of uric acid. In his studies of urea, Liebig discovered a compound he named “hippuric acid.” He also developed methods to analyze nonnitrogenous compounds. In 1835, Liebig discovered aldehydes. He was able to analyze many more compounds in a shorter time than his competitors because he could entrust the analyses to his students. With the help of many students, his laboratory produced hundreds of analyses annually.
      Liebig was at the peak of his experimental productivity in the late 1830s. His grasp of theoretical issues was firm but also more flexible than before. Additional financial support allowed him to expand his training and research programs. In 1840, English physician-chemist Henry Bence Jones (1814–1873) spent 6 months in Liebig's laboratory studying his analytic techniques.
      During these years, Liebig had disagreements with French chemists Jean-Baptiste-André Dumas (1800–1884) and Théophile-Jules Pelouze (1807–1867), as well as Swedish chemist Jöns Jakob Berzelius (1779–1848), concerning Dumas' ether theory.
      Liebig's major contributions to organic chemistry were based on the methods that he devised or refined and the many compounds and reactions that he discovered or described. Although few of his theories were original, he was considered the leading chemist of his day.
      Turning his attention to agricultural chemistry, Liebig analyzed straw, hay, and fruits and concluded that the same land, whether cultivated or forested, produced the same total quantity of carbon in the composition of whatever plants grew on it. When he examined the alkaline earth elements in plants, he noted that those grown in various localities contained different proportions of metals. For example, pine trees in one region contained magnesium, whereas those in another region did not. He theorized that the differences were due to compounds in the soil and that the role of fertilizer is to supply ammonia and such salts as potassium silicate, calcium phosphate, and magnesium phosphate. He refuted the theory that humus was one of the main nutrients for plant growth. He believed that nitrogen was supplied by means of ammonia washed out of the atmosphere, that the role of fertilizer was to add elements that plants could not obtain from the atmosphere, and that the best source of phosphorus was pulverized animal bones. Subsequently, he revised his previous opinion and stated that farmers should supplement the natural supply of nitrogen by adding ammonium salts. He still maintained that mineral nutrients were essential for fertilizers.
      In 1845, Liebig developed a formula for making artificial fertilizers using mineral salts, but his fertilizers did not increase the crop yield. To provide answers, Liebig purchased a plot of land near Giessen and began to study the effect of his fertilizer. In 4 years, he was able to convert the plot into a fertile field. He finally realized that the delayed benefit was due to the fusion of potash with calcium carbonate in the soil. Because this compound is insoluble, plant roots cannot absorb it.
      In another study, Liebig's fertilizers did not improve the yield, but the addition of ammonium salts did. Critics claimed that Liebig's mineral theory was invalid. However, Liebig argued that he had never said that agricultural yields were dependent only on the mineral constituents in the soil or that one should not add ammonia. He argued that, in most cases, it is superfluous to add ammonia and that fertilizers cannot be evaluated by their nitrogen content. He again stated that nitrogen is replenished from the atmosphere but that the minerals come only from the soil. He emphasized that a single constituent will increase the crop yield only if the other necessary elements also are present in greater quantities. Consequently, few of the essential minerals might become the controlling factor. This generalization became known as Liebig's “Law of the Minimum.” In 1862, he published the seventh and last edition of his Chemistry and Its Applications to Agriculture and Physiology.
      Liebig became interested in the chemical aspects of physiology and maintained that animal heat was produced solely by the oxidation of nutrients to carbon dioxide and water. He claimed that animals could convert dietary starch to fat, contrary to the view of French chemists that the source of fat was plant nutrients. Liebig and his students also made contributions to the dye industry. His assistant, August W. von Hofmann (1818–1892), analyzed a light oil from the distillation of coal tar and isolated aniline. Hofmann substituted other organic radicals for the remaining hydrogen atoms in aniline and thus laid the foundation for the synthetic dye industry.
      Liebig tired of teaching and moved to Munich (southern Germany) in 1852, where a laboratory had been built for him. This laboratory was small, and he had only a few selected assistants and students. He spent most of his time writing and developed a series of evening talks on scientific topics of broad interest. He died in Munich on April 18, 1873, at the age of 69 years, the day after the death of his old friend and student Henry Bence Jones. Liebig was honored on a stamp issued by Germany in 1953 to celebrate the 150th anniversary of his birth and another issued by the German Democratic Republic in 1978.