Editing Justus von Liebig (section)

==Research and development==
[[File:Justus von Liebig by Trautschold.jpg|thumb|right|upright|Justus von Liebig, by [[Wilhelm Trautschold]], ''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]].<ref name=Brock/>{{rp|35}} Liebig's appointment was part of an attempt to modernize the University of Giessen and attract more students. He received a small stipend, without laboratory funding or access to facilities.<ref name=Brock/>{{rp|38–41}}

The presence of existing faculty complicated his situation: Professor Wilhelm Zimmermann (1780–1825) taught general chemistry as part of the philosophy faculty, leaving medical chemistry and pharmacy to Professor Philipp Vogt in the medical faculty. Vogt was happy to support a reorganization in which pharmacy was taught by Liebig and became the responsibility of the faculty of arts, rather than the faculty of medicine. Zimmermann found himself competing unsuccessfully with Liebig for students and their lecture fees.  He refused to allow Liebig to use existing space and equipment and finally committed [[suicide]] on 19 July 1825.  The deaths of Zimmermann and Professor Blumhof, who taught technology and mining, opened the way for Liebig to apply for a full professorship. Liebig was appointed to the ''[[ordentlicher Professor|Ordentlicher]]'' chair in chemistry on 7 December 1825, receiving a considerably increased salary and a laboratory allowance.<ref name=Brock/>{{rp|38–41}}

Liebig married Henriette "Jettchen" Moldenhauer (1807–1881), the daughter of a state official, in May 1826. They had five children: Georg (1827–1903), Agnes (1828–1862), Hermann (1831–1894), Johanna (1836–1925), and Marie (1845–1920). Although Liebig was Lutheran and Jettchen Catholic, their differences in religion appear to have been resolved amicably by bringing their sons up in the Lutheran faith and their daughters as Catholics.<ref name=Brock/>{{rp|44}}

===Transforming chemistry education===
[[File:Justus von Liebigs Labor, 1840.jpg|thumb|right|Liebig's laboratory at Giessen, by Wilhelm Trautschold]]
[[File:Liebig Company Trading Card Ad 01.12.006 front.tif| thumb| right|Liebig's laboratory, Chimistes Celebres, [[Liebig's Extract of Meat Company]] Trading Card, 1929]]

Liebig and several associates proposed to create an institute for pharmacy and manufacturing within the university.<ref name=Brock/>{{rp|42}} The Senate, however, uncompromisingly rejected their idea, stating that training "apothecaries, soapmakers, beer-brewers, dyers and vinegar-distillers" was not the university's task.<ref name=Brock/>{{rp|43}} As of 17 December 1825, they ruled that any such institution would have to be a private venture.  This decision worked to Liebig's advantage.  As an independent venture, he could ignore university rules and accept matriculated and unmatriculated students.<ref name=Brock/>{{rp|42–43}} Liebig's institute was widely advertised in pharmaceutical journals and opened in 1826.<ref name=Brock/>{{rp|44–45}} Its classes in practical chemistry and laboratory procedures for chemical analysis were taught in addition to Liebig's formal courses at the university.

From 1825 to 1835, the laboratory was housed in the guardroom of a disused barracks on the edge of town. The main laboratory space was about {{convert|38|m2|abbr=on}} in size and included a small lecture room, a storage closet, and a main room with ovens and work tables.  An open colonnade outside could be used for dangerous reactions. Liebig could work there with eight or nine students at a time.  He lived in a cramped apartment with his wife and children on the floor above.<ref name=Brock/>{{rp|47}}

Liebig was one of the first chemists to organize a laboratory in its present form, engaging with students in empirical research on a large scale through a combination of research and teaching.<ref name=JLU>{{cite web|last1=Felschow|first1=Eva-Marie|title=Justus Liebig (our Eponym)|url=http://www.uni-giessen.de/cms/about/jlu/justusliebig|publisher=Justus Liebig University|access-date=5 November 2014}}</ref> 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.<ref name=Brock/>{{rp|47}} It was also significant for its emphasis on applying discoveries in fundamental research to the development of specific chemical processes and products.<ref name=Peppas>{{cite journal|last1=Peppas|first1=Nicholas A.|title=The First Century of Chemical Engineering|journal=Chemical Heritage Magazine|year=2008|volume=26|issue=3|pages=26–29|url=https://www.sciencehistory.org/distillations/article/first-century-chemical-engineering|access-date=20 March 2018}}</ref>

In 1833, Liebig convinced chancellor [[Justin von Linde]] to include the institute within the university.<ref name=Brock/>{{rp|47}} In 1839, he obtained government funds to build a lecture theatre and two separate laboratories designed by architect [[Paul Hofmann (architect)|Paul Hofmann]].  The new chemistry laboratory featured innovative glass-fronted [[fume cupboards]] and venting chimneys.<ref name=Brock/>{{rp|58}} By 1852, when he left Giessen for Munich, more than 700 students of chemistry and pharmacy had studied with Liebig.<ref name=Brock/>{{rp|57}}

===Instrumentation===
[[File:Liebig Manuel 1838 RGNb10348372.03.planche II.tif|thumb|right|Drawing of apparatus from Liebig's ''Manuel pour l'analyse des substances organiques'', 1848, ''Kaliapparat'' in lower right]]
[[File:Fünfkugelapparat.jpg|thumb|right|Modern reproduction of the ''Kaliapparat'' apparatus]]
[[File:Liebig condensers-two 1.jpg|thumb|right|Modern Liebig condenser (left) and West condenser (right)]]

A significant challenge facing 19th-century [[Organic chemistry|organic chemists]] was the lack of instruments and methods of analysis to support accurate, replicable analyses of organic materials.  Many chemists worked on the problem of organic analysis, including French [[Joseph Louis Gay-Lussac]] and Swedish [[Jöns Jacob Berzelius]], before Liebig developed his version of an apparatus for determining the carbon, hydrogen, and oxygen content of organic substances in 1830.  It involved an array of five glass bulbs, called a [[Kaliapparat]], to trap the oxidation product of the carbon in the sample following its combustion. Before reaching the Kaliapparat, the combustion gases were conducted through a tube containing hygroscopic [[calcium chloride]], which absorbed and retained the oxidation product of the hydrogen of the sample, namely water vapor.  Next, in the Kaliapparat, [[carbon dioxide]] was absorbed in a [[potassium hydroxide]] solution in the three lower bulbs and used to measure the weight of carbon in the sample.  For any substance consisting only of carbon, hydrogen, and oxygen, the percentage of oxygen was found by subtracting the carbon and hydrogen percentages from 100%; the remainder must be the percentage of oxygen. A charcoal furnace (a sheet steel tray in which the combustion tube was laid) was used for the combustion.<ref>{{Citation | last=Liebig |first=J. | title = Ueber einen neuen Apparat zur Analyse organischer Körper, und über die Zusammensetzung einiger organischen Substanzen | journal = Annalen der Physik | year = 1831 | volume = 21 |issue=1 | pages = 1–47 | doi=10.1002/andp.18310970102 |bibcode = 1831AnP....97....1L | url = https://zenodo.org/record/1423542 }}</ref> Weighing carbon and hydrogen directly, rather than estimating them volumetrically, significantly increased the method's accuracy of measurement.<ref name=Brock/>{{rp|48–51}}  Liebig's assistant, Carl Ettling, perfected glass-blowing techniques for producing the Kaliapparat and demonstrated them to visitors.<ref name=Brock/>{{rp|50}} Liebig's kaliapparat simplified the method of quantitative organic analysis and rendered it routine.<ref name=Jackson>{{cite journal|last1=Jackson|first1=Catherine M.|title=Synthetical Experiments and Alkaloid Analogues: Liebig, Hofmann, and the Origins of Organic Synthesis|journal=Historical Studies in the Natural Sciences|date=September 2014|volume=44|issue=4|pages=319–363|doi=10.1525/hsns.2014.44.4.319|jstor=10.1525/hsns.2014.44.4.319}}</ref> 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.<ref name=Brock/>{{rp|50}} His method of [[combustion analysis]] was used pharmaceutically, and certainly made possible many contributions to organic, agricultural and biological chemistry.<ref name=Brock/>{{rp|76–77}}<ref name=Rochelle>{{cite web|last1=Forrester|first1=Rochelle|title=Organic chemistry in the nineteenth century|url=http://www.rochelleforrester.ac.nz/organic-chemistry.html|access-date=6 November 2014}}</ref>

Liebig also popularized the use of a counter-current water-cooling system for distillation, still referred to as a [[Liebig condenser]].<ref name=Brock/>{{rp|84}}  Liebig himself attributed the vapor condensation device to German pharmacist [[Johann Friedrich August Gottling]], who had made improvements in 1794 to a design discovered independently by German chemist [[Christian Ehrenfried Weigel]] in 1771, by French scientist [[P. J. Poisonnier]] in 1779, and by [[Finnish people|Finnish]] chemist [[Johan Gadolin]] in 1791.<ref name=Jensen>{{citation|author1-link=William B. Jensen |last=Jensen |first=William B. |title=The Origin of the Liebig Condenser |journal=[[J. Chem. Educ.]] |year=2006 |volume=2006 |issue=83 |page=23 |doi=10.1021/ed083p23 |bibcode = 2006JChEd..83...23J }}</ref>

Although it was not widely adopted until after Liebig's death, when safety legislation finally prohibited the use of [[Mercury (element)|mercury]] in making [[mirror]]s, 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.<ref name=Brock/>{{rp|136–139}} Liebig's mirrors struggled commercially due to poor glass, which produced an off-color, greenish-yellow reflection. Rudely, Liebig commented that Frenchwomen especially hated his mirrors because Frenchwomen already looked yellow and sickly, and the mirrors just reminded them how ugly they were.<ref>{{Cite web |last=Gebelein |first=Helmut |title=Justus Liebig: Life and Work |url=https://bibliotekanauki.pl/articles/896195.pdf}}</ref>

=== Organic chemistry ===
[[File:PSM V74 D619 Liebig laboratory at giessen.png|thumb|right|Liebig laboratory, Giessen]]
[[File:Gießen, Liebig-Museum, the pharmaceutical laboratory.JPG|thumb|right|Liebig-Museum, the pharmaceutical laboratory, Giessen]]
One of Liebig's frequent collaborators was [[Friedrich Wöhler]]. They met in 1826 in Frankfurt, after independently reporting on the preparation of two substances, [[Isocyanic acid|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.<ref name=Brock/>{{rp|72}}<ref name=LWControversy>{{cite journal|last1=Esteban|first1=Soledad|title=Liebig–Wöhler Controversy and the Concept of Isomerism|journal=Journal of Chemical Education|date=September 2008|volume=85|issue=9|pages=1201|doi=10.1021/ed085p1201|bibcode=2008JChEd..85.1201E}}<!--|access-date=6 November 2014--></ref><ref name=LWCHF/>

In 1832, Liebig and Friedrich Wöhler published an investigation of the oil of bitter almonds. They transformed pure oil into several halogenated compounds, which were further transformed in other reactions.<ref>{{cite journal | last1 = Wöhler | last2 = Liebig | year = 1832 | title = Untersuchungen über das Radikal der Benzoesäure |trans-title=Investigations of the radical of benzoic acid | url = https://books.google.com/books?id=z-VAAAAAYAAJ&pg=249| journal = Annalen der Pharmacie | volume = 3 | issue = 3| pages = 249–282 | doi=10.1002/jlac.18320030302| hdl = 2027/hvd.hxdg3f| hdl-access = free }}</ref> Throughout these transformations, "a single compound" (which they named [[benzoyl]]) "preserves its nature and composition unchanged in nearly all its associations with other bodies."<ref name=Brock/>{{rp|79}}  Their experiments proved that a group of carbon, hydrogen, and oxygen atoms could behave like an element, take the place of an element, and can be exchanged for elements in [[chemical compound]]s. This laid the foundation for the doctrine of [[Radical theory|compound radicals]], which can be seen as an early step in the development of structural chemistry.<ref name=LWCHF>{{cite web | url = https://www.sciencehistory.org/historical-profile/justus-von-liebig-and-friedrich-w%C3%B6hler | title = Justus von Liebig and Friedrich Wöhler | publisher = Science History Institute | date = June 2016 | access-date = 21 March 2018 | archive-date = 20 June 2018 | archive-url = https://web.archive.org/web/20180620232155/https://www.sciencehistory.org/historical-profile/justus-von-liebig-and-friedrich-w%C3%B6hler | url-status = dead }}</ref>

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.<ref name=Brock/>{{rp|76}} Liebig not only isolated individual substances, but also studied their interrelationships and the ways in which they degraded and metamorphosed into other substances, looking for clues to the understanding of both chemical composition and physiological function.  Other significant contributions by Liebig during this time include his examination of
the nitrogen content of bases;<ref name=Brock/>{{rp|77}}
the study of chlorination and the isolation of [[chloral]] (1832);<ref name=Brock/>{{rp|83}}
the identification of the [[ethyl radical]] (1834);<ref name=Brock/>{{rp|82}}
the oxidation of alcohol and formation of [[aldehyde]] (1835);<ref name=Brock/>{{rp|84}}
the polybasic theory of organic acids (1838);<ref name=Brock/>{{rp|86–87}}
and the degradation of [[urea]] (1837).<ref name=Brock/>{{rp|88–89}}

Writing about the analysis of urine, a complex organic product, he made a declaration that reveals both the changes that were occurring in chemistry over a short time and the impact of his own work.<ref name=Brock/>{{rp|89}}  At a time when many chemists such as [[Jöns Jakob Berzelius]] still insisted on a hard and fast separation between the organic and inorganic, Liebig asserted:

{{Blockquote|text="The production of all organic substances no longer belongs just to living organisms.  It must be seen as not only probable, but as certain, that we shall be able to produce them in our laboratories.  Sugar, salicin, and morphine will be artificially produced.  Of course, we do not yet know how to do this, because we do not yet know the precursors from which these compounds arise, but we shall come to know them." |source=[Liebig and Woehler (1838)]}}

Liebig's arguments against any chemical distinction between living (physiological) and dead chemical processes proved a great inspiration to several of his students and others who were interested in [[materialism]]. Though Liebig distanced himself from the direct political implications of materialism, he tacitly supported the work of [[Carl Vogt]] (1817–1895), [[Jacob Moleschott]] (1822–1893),<ref>{{Cite thesis |last=Meneghello |first=Laura |title=Jacob Moleschott and the conception of science in the 19th century |date=2010 |degree=PhD |url=https://studenttheses.uu.nl/bitstream/handle/20.500.12932/4982/MasterThesisLauraMeneghello.pdf?sequence=1&isAllowed=y}}</ref> and [[Ludwig Büchner]] (1824–1899).<ref>{{Cite journal |last=Munday |first=Pat |date=1998 |title=Politics by Other Means: Justus von Liebig and the German Translation of John Stuart Mill's "Logic" |url=https://www.jstor.org/stable/4027874 |journal=The British Journal for the History of Science |volume=31 |issue=4 |pages=403–418 |doi=10.1017/S0007087498003379 |jstor=4027874 |issn=0007-0874}}</ref>

===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.<ref name=Brock/>

Liebig's book discussed chemical transformations within living systems, both plant and animal, outlining a theoretical approach to agricultural chemistry. The book's first part focused on plant nutrition; the second was on chemical mechanisms of putrefaction and decay.<ref name=Brock/>{{rp|148}} Liebig's awareness of both synthesis and degradation led him to become an early advocate of [[Conservation (ethic)|conservation]], promoting ideas such as the recycling of [[sewage]].<ref name=Brock/>{{rp|250–270}}

Liebig argued against prevalent theories about role of [[humus]] in plant nutrition, which held that decayed plant matter was the primary source of carbon for plant nutrition. Fertilizers were believed to act by breaking down humus, making it easier for plants to absorb. Associated with such ideas was the belief that some sort of "vital force" distinguished reactions involving organic as opposed to inorganic materials.<ref name=Hill>{{cite book|author-last1=Hill|author-first1=Jane F.|chapter=Translator's Introduction|author-last2=de Saussure|author-first2=Theodore|title=Chemical research on plant growth|date=2012|publisher=Springer|location=New York|isbn=978-1-4614-4136-6|chapter-url=https://books.google.com/books?id=cVVJAAAAQBAJ&pg=PR15|access-date=7 November 2014}}</ref>

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.<ref name=Hill/>{{rp|xv–xxi}} [[Nicolas-Théodore de Saussure]]'s experiments, reported in ''Recherches Chimiques sur la Végétation'' (1804), suggested that carbon was obtained from atmospheric rather than soil-based sources. It also indicated that water was a likely source of hydrogen. He also studied the absorption of minerals by plants, and observed that mineral concentrations in plants tended to reflect their presence in the soil in which the plants were grown. However, the implications of De Saussure's results for theories of plant nutrition were neither clearly discussed nor easily understood.<ref name=Hill/>{{rp|xxii–xxvii}}

Liebig reaffirmed the importance of De Saussure's findings, and used them to critique humus theories, while regretting the limitations of De Saussure's experimental techniques. Using more precise methods of measurement as a basis for estimation, he pointed out contradictions such as the inability of existing soil humus to provide enough carbon to support the plants growing in it.<ref name=Hill/>{{rp|xxix}} By the late 1830s, researchers such as [[Karl Sprengel]] were using Liebig's methods of combustion analysis to assess manures, concluding that their value could be attributed to their constituent minerals.<ref name=Brock/>{{rp|106}} Liebig synthesized ideas about the mineral theory of plant nutrition and added his own conviction that inorganic materials could provide nutrients as effectively as organic sources.<ref name=Brock/>{{rp|148}}

In his theory of mineral nutrients, Liebig identified the chemical elements of nitrogen (N), phosphorus (P), and potassium (K) as essential to plant growth. He reported that plants acquire carbon (C) and hydrogen (H) from the atmosphere and from water (H<sub>2</sub>O). In addition to emphasizing the importance of minerals in the soil, he argued that plants feed on nitrogen compounds derived from the air. This assertion was a source of contention for many years, and turned out to be true for legumes, but not for other plants.<ref name=Brock/>{{rp|181}}

[[File:Minimum-Tonne.svg|thumb|right|Liebig's barrel]]
Liebig also popularized Carl Sprengel's "theorem of minimum" (known as the [[Liebig's Law of the Minimum|law of the minimum]]), stating that plant growth is not determined by the total resources available, but by the scarcest available resource.  A plant's development is limited by the one essential mineral that is in the relatively shortest supply.  This concept of limitation can be visualized as "Liebig's barrel", a metaphorical barrel in which each stave represents a different element. A nutrient stave that is shorter than the others will cause the liquid contained in the barrel to spill out at that level. This is a qualitative version of the principles used for determining the application of fertilizer in modern agriculture.

''Organic Chemistry'' was not intended as a guide to practical agriculture. Liebig's lack of experience in practical applications, and differences between editions of the book, fueled considerable criticism. Nonetheless, Liebig's writings had a profound impact on agriculture, spurring experiment and theoretical debate in Germany, England, and France.<ref name=Brock/>{{rp|165}}

One of his most recognized accomplishments is the development of [[nitrogen]]-based [[fertilizer]]. In the first two editions of his book (1840, 1842), Liebig reported that the atmosphere contained insufficient nitrogen, and argued that nitrogen-based fertilizer was needed to grow the healthiest possible crops.<ref name=Brock/>{{rp|120}} Liebig believed that nitrogen could be supplied in the form of [[ammonia]], and recognized the possibility of substituting chemical fertilizers for natural ones (animal dung, etc.)

He later became convinced that nitrogen was sufficiently supplied by precipitation of ammonia from the atmosphere, and argued vehemently against the use of nitrogen-based fertilizers for many years. An early commercial attempt to produce his own fertilizers was unsuccessful, due to lack of nitrogen in the mixtures.<ref name=Brock/>{{rp|121–124}} When tested in a farmer's field, Liebig's manure was found to have no appreciable effect.<ref>Matter-of-Fact (July 1847) [https://babel.hathitrust.org/cgi/pt?id=hvd.32044048680383&view=1up&seq=214 Liebig's System of Manuring], [[The Cultivator]] series 2, volume 4, page 208 via [[HathiTrust]]</ref>

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.<ref name=Brock/>{{rp|179}}He was instrumental in the use of [[guano]] for nitrogen.<ref>Charles Mann (2011) [https://archive.org/details/1493uncoveringne00mann 1493: Uncovering the New World Columbus Created], p. 214, Knopf</ref> 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 N fixation.[http://www.vanmansvelt.nl/home/historic-and-actual-awareness-of-soil-fertility-in-agriculture-russia-western-europe-usa-draft-of-a-survey/] 
Nitrogen fertilizers are now widely used throughout the world, and their production is a substantial segment of the chemical industry.<ref name=Travis2013>{{cite journal|last1=Travis|first1=Anthony S.|title=Dirty Business|journal=Chemical Heritage Magazine|date=Spring 2013|volume=31|issue=1|page=7|url=https://www.sciencehistory.org/distillations/magazine/dirty-business|access-date=20 March 2018}}</ref>

===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.<ref name=Brock/>{{rp|185}} The experimental techniques used by Liebig and others often involved controlling and measuring diet, and monitoring and analyzing the products of animal metabolism, as indicators of internal metabolic processes.  Liebig saw similarities between plant and animal metabolism, and suggested that nitrogenous animal matter was similar to, and derived from, plant matter.  He categorized foodstuffs into two groups, nitrogenous materials which he believed were used to build animal tissue, and non-nitrogenous materials which he believed were involved in separate processes of respiration and generation of heat.<ref name=Brock/>{{rp|184}}

French researchers such as [[Jean-Baptiste Dumas]] and [[Jean-Baptiste Boussingault]] believed that animals assimilated sugars, proteins, and fats from plant materials and that animals could not synthesize complex molecules. Liebig's work suggested a common ability of plants and animals to synthesize complex molecules from simpler ones. His experiments on fat metabolism convinced him that animals must be able to synthesize fats from sugars and starches.<ref name=Brock/>{{rp|187}} Other researchers built upon his work, confirming the abilities of animals to synthesize sugar and build fat.<ref name=Brock/>{{rp|189–190}}

Liebig also studied respiration, at one point measuring the "ingesta and excreta" of 855 soldiers, a bodyguard of the Grand Duke of Hessen-Darmstadt, for an entire month.<ref name=Brock/>{{rp|191}}  He outlined an extremely speculative model of equations in which he attempted to explain how protein degradation might balance within a healthy body and result in pathological imbalances in cases of illness or inappropriate nutrition.<ref name=Brock/>{{rp|191–193}} This proposed model was justifiably criticized. Berzelius stingingly stated that "this facile kind of physiological chemistry is created at the writing table".<ref name=Brock/>{{rp|194}} Some of the ideas that Liebig had enthusiastically incorporated were not supported by further research.  The third and last edition of ''Animal Chemistry'' (1846) was substantially revised and did not include the equations.<ref name=Brock/>{{rp|195–197}}

The third area discussed in ''Animal Chemistry'' was fermentation and putrefaction. Liebig proposed chemical explanations for processes such as [[Decomposition|eremacausis]] (organic decomposition), describing the rearrangement of atoms as a result of unstable "affinities" reacting to external causes such as air or already decaying substances.<ref name=Brock/>{{rp|205}}  Liebig identified the blood as the site of the body's "chemical factory", where he believed processes of synthesis and degradation took place. He presented a view of disease in terms of chemical process, in which healthy blood could be attacked by external contagia; secreting organs sought to transform and excrete such substances; and failure to do so could lead to their elimination through the skin, lungs, and other organs, potentially spreading contagion. Again, although the world was much more complicated than his theory, and many of his individual ideas were later proved wrong, Liebig managed to synthesize existing knowledge in a way that had implications for doctors, sanitarians, and social reformers. The English medical journal ''[[The Lancet]]'' reviewed Liebig's work and translated his chemical lectures as part of its mission to establish a new era of medicine.<ref name=Brock/>{{rp|207}} Liebig's ideas stimulated medical research, led to the development of better techniques for testing experimental models of metabolism, and pointed to chemistry as fundamental to the understanding of health and disease.<ref name=Brock/>{{rp|214}}

In 1850, Liebig investigated [[spontaneous human combustion]], dismissing the simplistic explanations based on [[ethanol]] due to alcoholism.<ref name=Ford2012>{{cite journal|last1=Ford|first1=Brian J.|title=Solving the Mystery of Spontaneous Human Combustion|journal=The Microscope|year=2012|volume=60|issue=2|pages=63–72|url=http://www.mcri.org/CMSuploads/Brian%20J.%20Ford%20-%20Solving%20the%20Mystery%20of%20Spontaneous%20Human%20Combustion-85047.pdf|access-date=4 November 2014|url-status=dead|archive-url=https://web.archive.org/web/20130510231220/http://www.mcri.org/CMSuploads/Brian%20J.%20Ford%20-%20Solving%20the%20Mystery%20of%20Spontaneous%20Human%20Combustion-85047.pdf|archive-date=10 May 2013}}</ref>

===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).<ref name=Brock/>{{rp|217–218}}

Liebig was acclaimed in ''The Lancet'' for revealing "the true principles of cookery", and physicians promoted "rational diets" based on his ideas. Well-known British cookery writer [[Eliza Acton]] responded to Liebig by modifying the cookery techniques in the third edition of her ''Modern Cookery for Private Families'', and subtitling the edition accordingly.<ref name=Brock/>{{rp|218–219}}  Liebig's idea that "[[searing]] meat seals in the juices", though still widely believed, is not true.<ref name=McGee2004>{{cite book |chapter=The Searing Question | last=McGee |first=Harold | title=On Food and Cooking | publisher=Scribner | year=2004 | isbn=0-684-80001-2|page=161| edition=Revised }}</ref>

====Liebig's Extract of Meat Company====
[[File:Liebig Company Memorial Trading Card 01.12.001 front.tif|thumb|right|upright|Memorial tradecard commemorating Justus Liebig, from [[Liebig's Extract of Meat Company]] ]]
[[File:Liebig-Muenchen.jpg|thumb|right|upright|Justus Liebig statue, [[Munich]], Germany]]

{{Main|Liebig's Extract of Meat Company}}

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".<ref name=Quinologist>{{cite book|editor-last1=Mattison|editor-first1=Richard V.|title=The Quinologist|year=1883|publisher=s.n.|location=Philadelphia|pages=55–58|volume=VI|number=1 |url=https://books.google.com/books?id=_NADAAAAYAAJ&pg=PA55|access-date=4 November 2014}}</ref>

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]],<ref>{{cite web|url=https://negocios.elpais.com.uy/noticias/multinacional-liebig-fray-bentos.html |title=Liebig's in Fray Bentos|date=2 July 2018|work=[[El País (Uruguay)|El País]]|language=es}}</ref> and was named scientific director of [[Liebig's Extract of Meat Company]], located in [[Fray Bentos]] in Uruguay.<ref name=Cansler/><ref name=NickSkye>{{cite web|last1=Skye|first1=Nick|title=The Liebig chromolithographs, origins of bouillon, Marmite, Oxo and Campbell's soups|url=http://nickyskye.blogspot.com/2012/06/liebig-chromolithographs-part-1-origins.html|website=nickyskye meanderings|access-date=11 November 2014|date=25 June 2012}}</ref>

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.<ref name=Quinologist/> The judge asserted that "Purchasers must use their eyes", and considered the presentation of the products to be sufficiently different to enable the discriminating consumer to determine which of the products bore Liebig's signature and was supported by Liebig himself.<ref name=Quinologist2>{{cite book|editor-last1=Mattison|editor-first1=Richard V.|title=The Quinologist|year=1883|publisher=s.n.|location=Philadelphia|pages=184–186|volume=VI|number=1 |url=https://books.google.com/books?id=_NADAAAAYAAJ&pg=PA184|access-date=4 November 2014}}</ref>

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 [https://www.jstor.org/stable/44442675 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.<ref name=Cansler>{{cite journal|last1=Cansler|first1=Clay|title=Where's the Beef?|journal=Chemical Heritage Magazine|date=Fall 2013|volume=31|issue=3|url=https://www.sciencehistory.org/distillations/article/where%E2%80%99s-beef|access-date=20 March 2018|archive-date=20 November 2018|archive-url=https://web.archive.org/web/20181120095538/https://www.sciencehistory.org/distillations/article/where%E2%80%99s-beef|url-status=dead}}</ref>

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.<ref name="Brock" />{{rp|234–237}}

The company also worked with British chemist [[Henry Enfield Roscoe]] to develop a related product, which it registered some years after Liebig's death, under the "[[Oxo (food)|Oxo]]" trademark. Oxo was trademarked worldwide in 1899 and in the United Kingdom in 1900. Originally a liquid, Oxo was released in cubed solid form in 1911.<ref name="Brock"/>{{rp|230}}

====Marmite====
Liebig studied other foods, as well. He promoted the use of baking powder to make lighter bread, studied the chemistry of coffee-making and [[oatmeal]].<ref name="Brock"/><ref>{{Cite book|url=https://books.google.com/books?id=p4o9AQAAIAAJ|title=Scientific American, "Oatmeal"|date=1878|publisher=Munn & Company|pages=25|language=en}}</ref>{{rp|238–248}} He is considered to have made possible the invention of [[Marmite]], because of his discovery that yeast could be concentrated to form [[yeast extract]].<ref name=Boulton>{{cite book|editor-last1=Boulton|editor-first1=Chris|title=Encyclopedia of brewing|date=2012|publisher=Wiley|location=Weinheim|isbn=978-1-4051-6744-4|page=394|url=https://books.google.com/books?id=uWXcajHd3W0C&pg=PA394|access-date=4 November 2014}}</ref>

'''Infant Formula'''

Liebig produced some of the world's first [[infant formula]], a [[Breast milk|breast-milk]] substitute for babies who could not breast-feed.<ref>{{Cite journal |url=https://academic.oup.com/jhmas/article-abstract/79/1/1/7223089?redirectedFrom=fulltext |access-date=4 March 2024 |title="They Perished in the Cause of Science": Justus von Liebig's Food for Infants |journal=Journal of the History of Medicine and Allied Sciences |doi=10.1093/jhmas/jrad035 |volume=79 |date=12 July 2023 |issue=1 |first=Caroline |last=Lieffers |pages=1–22|pmid=37435903 }}</ref> 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.<ref>{{Cite web |title=Family tree of Johanna von Liebig |url=https://gw.geneanet.org/cvpolier?lang=en&n=von+liebig&p=johanna |access-date=4 March 2024 |website=Geneanet |language=en}}</ref> (Johanna did not want to seek a [[Wet nurse|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<!--Q61943621-->, 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 [[twin]]s, who were born somewhat [[Preterm birth|premature]] and weighed 2.24 [[kilogram]]s (4.93 [[Pound (mass)|pounds]]) and 2.64&nbsp;kg (5.82&nbsp;lbs.). Both died within two days. Depaul tried it on a third baby, born full-term at 3.37&nbsp;kg (7.43&nbsp;lbs.); it soon began passing green "[https://www.atlchildrens.com/diarrhea-breastfed-infants starvation stools]" and died within three days. A fourth child, weighing 2.76&nbsp;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 [[Académie Nationale de Médecine|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 [[Nicolas-Jean-Baptiste-Gaston Guibourt|Nicholas-Jean-Baptiste-Gaston Guibourt]]. Guibourt had grave doubts about Liebig's artificial [[milk]], calling it "fake milk" (in [[French language|French]], "lait factice"). As historian [https://www.kingsu.ca/about-us/staff-directory/contact_id/5790 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 [[Ethics|ethical]] questions were quickly raised. Publications in France generally supported Depaul, while German publications rallied to Liebig's defense.