,
circa 1846 Liebig left Paris to return to Darmstadt in April 1824. On 26 May 1824, at the age of 21 and with Humboldt's recommendation, Liebig became a professor
extraordinarius at the
University of Giessen. His methods of organic analysis enabled him to direct the analytical work of many graduate students. Liebig's students were from many of the German states, as well as Britain and the United States. They helped create an international reputation for their
Doktorvater. His laboratory became renowned as a model institution for the teaching of practical chemistry. In 1833, Liebig convinced chancellor
Justin von Linde to include the institute within the university. Weighing carbon and hydrogen directly, rather than estimating them volumetrically, significantly increased the method's accuracy of measurement. Brock suggests that the availability of a superior technical apparatus was one reason why Liebig was able to attract so many students to his laboratory. Liebig also popularized the use of a counter-current water-cooling system for distillation, still referred to as a
Liebig condenser. Although it was not widely adopted until after Liebig's death, when safety legislation finally prohibited the use of
mercury in making
mirrors, Liebig proposed a process for
silvering that eventually became the basis of modern mirror-making. In 1835, he reported that
aldehydes reduce
silver salts to metallic silver. After working with other scientists,
Carl August von Steinheil approached Liebig in 1856 to see if he could develop a silvering technique capable of producing high-quality optical mirrors for use in
reflecting telescopes. Liebig developed blemish-free mirrors by adding copper to ammoniated silver nitrate and sugar. An attempt to commercialize the process and "drive out mercury mirror-making and its injurious influence on workers' health" was unsuccessful.
Organic chemistry One of Liebig's frequent collaborators was
Friedrich Wöhler. They met in 1826 in Frankfurt, after independently reporting on the preparation of two substances,
cyanic acid and
fulminic acid, that apparently had the same composition, but very different characteristics. The
silver fulminate investigated by Liebig was explosive, whereas the
silver cyanate found by Wöhler was not. After reviewing the disputed analyses together, they agreed that both were valid. The discovery of these and other substances led
Jöns Jacob Berzelius to suggest the idea of
isomers, substances that are defined not simply by the number and kind of atoms in the molecule, but also by the arrangement of those atoms. Throughout these transformations, "a single compound" (which they named
benzoyl) "preserves its nature and composition unchanged in nearly all its associations with other bodies." The 1830s were a period of intense investigation of organic compounds by Liebig and his students, and of vigorous debate about the theoretical implications of their results. Liebig published on a wide variety of topics, personally averaging 30 papers per year between 1830 and 1840. and
Ludwig Büchner (1824–1899).
Plant nutrition By the 1840s, Liebig was attempting to apply theoretical knowledge from organic chemistry to real-world problems of food availability. His book
Die organische Chemie in ihrer Anwendung auf Agricultur und Physiologie (
Organic Chemistry in its Application to Agriculture and Physiology) (1840) promoted the idea that chemistry could revolutionize agricultural practice, increasing yields and lowering costs. It was widely translated, vociferously critiqued, and highly influential. Early studies of photosynthesis had identified carbon, hydrogen, oxygen, and nitrogen as important, but disagreed over their sources and mechanisms of action. Carbon dioxide was known to be taken in and oxygen released during photosynthesis, but researchers suggested that oxygen was obtained from carbon dioxide, rather than from water. Hydrogen was believed to come primarily from water. Researchers disagreed about whether sources of carbon and nitrogen were atmospheric or soil-based. Liebig's difficulties in reconciling theory and practice reflected that the real world of agriculture was more complex than was at first realized. By the publication of the seventh German edition of
Agricultural Chemistry he had moderated some of his views, admitting some mistakes and returning to the position that nitrogen-based fertilizers were beneficial or even necessary. In 1863 he published the book "Es ist ja die Spitze meines lebens" in which he revised his early perceptions, now appreciating soil life and in particular the biological
nitrogen fixation. Nitrogen fertilizers are now widely used throughout the world, and their production is a substantial segment of the chemical industry.
Plant and animal physiology Liebig's work on applying chemistry to plant and animal physiology was especially influential. By 1842, he had published
Chimie organique appliquée à la physiologie animale et à la pathologie, published in English as
Animal Chemistry, or, Organic Chemistry in its Applications to Physiology and Pathology, presenting a chemical theory of metabolism.
Liebig and the chemistry of food Methods of cookery Liebig drew upon his work in plant nutrition and plant and animal metabolism to develop a theory of nutrition, which had implications for cookery. In his
Researches on the Chemistry of Food (1847) Liebig argued that eating not only meat fibre, but also meat juices, which contained various inorganic chemicals, was important. These vital ingredients would be lost during conventional boiling or roasting in which cooking liquids were discarded. For optimum nutritional quality, Liebig advised that cooks should either sear the meat initially to retain fluids, or retain and use cooking liquids (as in soups or stews).
Liebig's Extract of Meat Company , Germany Building on his theories of the nutritional value of meat fluids, and seeking an inexpensive nutrition source for Europe's poor, Liebig developed a formula for producing beef extract. The details were published in 1847 so that "the benefit of it should ... be placed at the command of as large a number of persons as possible by the extension of the manufacture, and consequently a reduction in the cost". Production was not economically feasible in Europe, where meat was expensive, but in
Uruguay and
New South Wales, meat was an inexpensive byproduct of the leather industry. In 1865, Liebig partnered with Belgian engineer
George Christian Giebert, and was named scientific director of
Liebig's Extract of Meat Company, located in
Fray Bentos in Uruguay. Other companies also attempted to market meat extracts under the name "Liebig's Extract of Meat". In Britain, a competitor's right to use the name was successfully defended on the grounds that the name had fallen into general use and become a
generic term before the creation of any particular company. Liebig's company initially promoted their "meat tea" for its curative powers and nutritional value as a cheap, nutritious alternative to real meat. But such claims did not hold up to scrutiny. In 1868 the German physiologist Edward Kemmerich ran an experiment involving feeding the extract to dogs, every one of which died. After claims of its nutritional value were questioned, the company emphasized its convenience and flavor, marketing it as a comfort food. The Liebig company worked with popular cookery writers in various countries to popularize their products. German cookery writer
Henriette Davidis wrote recipes for
Improved and Economic Cookery and other cookbooks.
Katharina Prato wrote an Austro-Hungarian recipe book,
Die Praktische Verwerthung Kochrecepte (1879).
Hannah M. Young was commissioned in England to write
Practical Cookery Book for the Liebig Company. In the United States,
Maria Parloa extolled the benefits of Liebig's extract. Colorful calendars and trading cards were also marketed to popularize the product. He is considered to have made possible the invention of
Marmite, because of his discovery that yeast could be concentrated to form
yeast extract.
Infant Formula Liebig produced some of the world's first
infant formula, a
breast-milk substitute for babies who could not breast-feed. However, the product proved controversial, even though Liebig did not make any royalties off it. Liebig first came up with the idea based on the struggles of his favorite daughter, Johanna, who struggled to breastfeed her daughter, Carolina, who was born in 1864. (Johanna did not want to seek a
wet-nurse, a common but controversial practice at the time.) Carolina, according to Liebig, thrived on the formula. But other scientists were skeptical. One of them, a French doctor in Paris named Jean-Anne-Henri Depaul, decided to test his formula on four infants whose mothers could not suckle. Liebig himself prepared the first batches of formula. Depaul first gave it to a set of
twins, who were born somewhat
premature and weighed 2.24
kilograms (4.93
pounds) and 2.64 kg (5.82 lbs.). Both died within two days. Depaul tried it on a third baby, born full-term at 3.37 kg (7.43 lbs.); it soon began passing green "starvation stools" and died within three days. A fourth child, weighing 2.76 kg, also developed green stools and died within four days. At this point, Depaul stopped the experiment. At first, Depaul kept the experiment to himself. But he attended a meeting of the
French Academy of Medicine. And while didn't want to say anything at first, he felt he had to after another member of the Academy rose to speak, a
pharmacist named
Nicholas-Jean-Baptiste-Gaston Guibourt. Guibourt had grave doubts about Liebig's artificial
milk, calling it "fake milk" (in
French, "lait factice"). As historian Caroline Lieffers has written, "He [i.e., Guibourt] worried that the substance would either spoil in liquid form or lose its nutritive quality and convenience in solid form." Upon hearing Guibourt speak, Depaul felt it incumbent upon him to speak as well, and mentioned his experiments with Liebig's formula. Many
ethical questions were quickly raised. Publications in France generally supported Depaul, while German publications rallied to Liebig's defense. == Major works ==