Liebig, Justus von

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Liebig, Justus von

(b. Darmstadt, Germany, 12 May 1803; d. Munich, Germany, 18 April 1873)


Justus Liebig was the second of the nine children of Johann Georg and Maria Karoline Moserin Liebig. He taught chemistry at the University of Giessen from 1825 until 1851, and for the remainder of his life at the University of Munich. In 1826 he married Henriette Moldenhauer, who survived him. There were five children: Georg, Hermann, Agnes, Johanna, and Marie. In 1845 he was made a baron. Liebig achieved prominence in general analytical, organic, agricultural, and physiological chemistry. He wrote many hooks and articles popularizing chemistry, and sought to turn the results of his research to social use by marketing such products as a meat extract and artificial milk. He played a leading role in the formation of scientific laboratories and agricultural experiment stations. His scientific reputation, however, rests mainly on his role in the development of organic chemistry between 1829 and 1839.

As a child Liebig learned to perform chemical operations in the small laboratory his father maintained to supply a family drug and painting materials business. He reproduced many of the experiments described in the chemical books available in the town library. After watching an itinerant trader make the explosive silver fulminate, Liebig produced the same compound. He was much less interested in ordinary school subjects. Recognizing his preferences, his father apprenticed him to an apothecary. Within a year, however, he returned home to continue his experiments. In 1820 he went to the University of Bonn to study chemistry under Wilhelm Gottlob Kastner. He followed Kastner to Erlanger, where he received the doctorate in 1822. Liebig found Kastner unskilled in analyses and unable to provide comprehensive chemical instruction, and he continued his own investigation of fulminate of mercury. Liebig soon realized he would have to go abroad to complete his education, and he obtained from Grand Duke Louis I of Hess a grant to study in Paris, where he remained from 1822 until early 1824.

Liebig attended the lectures of Gay-Lussac, The Nard, and Dugong, where he encountered a rigorous, quantitative, experimental chemistry unlike anything he had found in Germany and learned for I he first time some of the general principles connecting his knowledge of particular compounds and processes. Continuing his own investigations of the fulminates, he submitted a memoir on the subject to the Academe des Sciences in December 1823, He argued that silver and mercury fulminate were salts of a peculiar acid which could not be separated from the metals except by combining with other metals, with which it formed a number of new salts. Among those impressed by his paper was Humboldt, who arranged for Liebig to work with Gay-Lussac. For Lie Big this opportunity was the most important of his life, for with Gay-Lussac he not only mastered methods of analysis, but learned to pursue investigations systematically.

Their collaboration soon resulted in another memoir on the fulminates, a paper making apparent that Liebig had learned from Gay-Lussac to pay stricter attention to precise determinations of the quantitative elementary composition of substances. Among their new experiments was a combustion analysis of the percentages of carbon, hydrogen, and oxygen in the fulminates. Although Lie Big had produced silver fulminate entirely from inorganic reagents, he and Gay-Lussac applied to it a modification of the method for organic substances that Gay-Lussac had helped to develop. Thus, although working in inorganic chemistry, Liebig was introduced to the general problem of organic analysis to which he afterward made major contributions.

Laboratory at Giessen. Humboldt once again gave timely aid by recommending to Louis I of Hesse that lie provide Liebig with an academic position, and in 1824 Liebig was appointed extraordinary professor at the University of Giessen. He returned to Germany determined to make opportunities similar to that afforded him by his work with Gay-Lussac available to a larger number of students. Together with a mineralogist and a mathematician, he proposed that a pharmaceutical training institute be established. Their proposal was turned down, but they were permitted to set up the institution by themselves in a recently evacuated military barracks. By 1827 chemistry dominated the instruction, and the twenty places in the laboratory were soon filled.

Liebig’s first laboratory consisted of one unventilated room, containing a large coal oven in the center and benches around the walls. Liebig had to buy most of the supplies and pay his assistant from his own modest salary. Nevertheless this laboratory saw the beginning of a whole new node of training scientists. It was the first institution deliberately designed to enable a number of students to progress systematically from elementary operations to independent research under the guidance of an established scientist. A carefully planned program of exercises led the students from one stage to the next. Liebig’s success derived in part from his own analytical skill and his magnetic qualities as a teacher, but in part also from the state of knowledge in chemistry. The science had attained to general principles which were technically demanding and which invited application to a nearly limitless number of particular cases. For the first time it offered great scope for clearly delimited research projects under the leadership of one man in command of the field. Even as he trained others, he was able to extend his own investigations far beyond what he could perform with his own hands. If other chemists wished to compete with him, they had little choice but to imitate his approach, and the example soon spread to other experimental sciences.

Students began by learning a system of qualitative analysis, that is, an orderly way to separate and identify the acids and bases present in a solid or solution of unknown composition. To develop this system Liebig drew upon a great knowledge of the chemical properties of the acids, alkalies, alkaline earths, metals, and salts in order to standardize a sequence of operations by which the presence of any of these substances in combination with any others in a given material could most effectively be determined. The procedure began with application of general reagents which separated the components of a solution into groups that could then be further separated and distinguished by more specific reactions. Liebig believed that mastery of these reactions was the first requirement for a chemist. In developing his analytical system he drew on the traditions of distinguished analysts both before and after Lavoisier. He later turned over the elementary teaching of this system to assistants, while he gave his attention to advanced students. Two of his assistants, Fresenius and Will, published textbooks outlining the methods which in modified form have been used by students ever since.

During his first five years in Giessen, Liebig carried out many investigations concerning methods for preparing known inorganic substances. When Wöhler published an analysis of silver cyanate giving the same composition that he and Gay-Lussac had found for silver fulminate, Liebig confirmed the unusual result. He drew from it the important new idea that two chemical compounds with entirely different properties could have the same elementary compositions, differing only in the manner in which the elements were joined. In 1829 he completed an extensive study of the decomposition of a variety of chemical combinations by means of chlorine. Among the compounds with which he reacted the gas was silver cyanide. He was attempting to convert the cyanic acid which Wöhler had previously investigated into a similar acid which Georges Serullas had produced from cyanogen chloride, but which seemed to contain more oxygen. Learning that Wöhler was, pursuing a similar investigation, Liebig wrote to him, and they collaborated to settle the question of the relation between the two acids. After encountering a series of difficulties, Liebig found that Serullas’s analysis of his cyanic acid was incorrect, that its percentage composition was the same as that of Wöhler’s acid. Wöhler inferred that the two compounds were isomers, according to Berzelius’ recent definition of that term, and renamed them, respectively, cynauric acid and cyanic acid. Liebig and Wöhler continued the cooperation begun in this study, became close friends, and made several important investigations in common.

Liebig also utilized chlorine in an effort to determine the composition of uric acid. The reaction produced cyanic acid, ammonia, and oxalic acid. Probably hoping to emulate Wöhler’s synthesis of urea, Liebig tried unsuccessfully to compose uric acid from oxalic acid, potassium cyanate, and ammonia. Despite the indecisive result, his interest in uric acid carried him from inorganic into organic chemistry. He thought he might find a clue to the composition of the acid in an earlier discovery by Foureroy and Vauquelin that horse urine yields benzoic acid. Liebig soon ascertained that the acid in the urine was distinct from benzoic acid but yielded the latter on distillation. He named the new compound hippuric acid.

Liebig’s study of hippuric acid confronted him with some general difficulties in the elementary analysis of organic compounds. The customary combustion analysis, he found, gave unreliable results for nitrogen, because of the very small proportion of that element in the acid. He was able to reduce the error by using a method that he and Gay-Lussac had devised earlier for burning the compound in a vacuum. To determine the hydrogen content more accurately, he measured it in a separate analysis. This method enabled him to burn a larger amount of the compound than he could use in measuring the carbon, for the necessity to handle the carbonic acid gas that the latter element produced limited the quantity of the compound that could be analyzed. Liebig applied the same procedures to other organic acids, and entered the discussions arising then over the general question of the best method for elementary analysis. The procedures worked out by Gay-Lussac, Thenard, and Berzelius between 1810 and 1820 had become the customary method, but chemists were still seeking to improve on it. In J830 Liebig wrote a critique of several recent efforts, arguing that the techniques he himself had worked out for hippuric acid best solved the crucial problems.

When Liebig turned to the analysis of alkaloids, he met further obstacles. He was attracted to these distinctive plant substances because an investigation often of them by Pelletier and Dumas had given the paradoxical result that the nitrogen in the various compounds, the supposed source of their alkaline properties, was not proportional to the quantities of acid with which each could combine. Liebig set out to improve the measurement of nitrogen, but he soon found that carbon also posed special diffculties. Because of the unusually large molecular weights of the alkaloids, errors of the order of I percent in the determination of carbon could lead to incorrect empirical formulas. The solution, he thought, would be to make it possible to analyze larger amounts of the organic compound, just as he had done earlier for hydrogen. In order to achieve this, Liebig made a crucial technical innovation. In his apparatus the combustion gases passed through a tube containing calcium chloride, which absorbed the water vapor. Then they entered a tube bent into a triangle, with five bulbs blown into it. A caustic potash solution in this tube completely absorbed the carbonic acid. He could then simply weigh the increase in the weight of the tube and dispense with the collection of gases pneumatically for non-nitrogenous compounds. He could now analyze in one operation ten times the amount of organic compound as by the older methods. In addition he made a number of small refinements, resulting in a much simplified, reliable procedure that soon became standard. He could, however, find no such satisfactory solution for the problem of nitrogen. He was able to improve the results Dumas and Pelletier had published for the nitrogen in the alkaloids, but he did this mostly by sheer persistence, repeating his analyses many times. The corrections he made partially resolved the theoretical problem with which he had begun, for he showed that within one class of the plant bases the nitrogen was strictly proportional to the quantities of sulfuric acid which would combine with them.

Although Liebig’s new method for non-nitrogenous compounds did not immediately produce results strikingly betted than had previously been attained, the simplifications he introduced eliminated much of the tedium and extraordinary skill formerly demanded. Not only could he analyze many more compounds in far shorter time than his predecessors, but he could entrust analyses to his students. Because he had many students, his laboratory began turning out hundreds of analyses annually. Liebig could now determine the elementary compositions not only of the most important organic compounds, but of all of the products of the reactions encountered in most of his investigations. His combustion apparatus became a symbol of the new era of organic chemistry that he helped to establish.

Liebig’s organic research soon involved him in a question which was growing increasingly important as a consequence of the ability to depict the numbers of atoms of each element in more and more organic compounds. How are these atoms ordered in the molecules? The discoveries of different compounds with the same elementary proportions added urgency to the problem. In 1830 Berzelius showed that tartaric acid and racemic acid have identical compositions but distinguishable properties. On the basis of this and several previously reported examples, he predicted that such situations were commonplace, and named the compounds so related isomeric bodies. Liebig and Wöhler from their experiences with fulminic acid and cyanic acid were well prepared to appreciate the importance of isomerism. Impressed with Berzelius’ investigation, they treated the results of their work on cyanic acid and cyanuric acid similarly and inferred that these compounds could be explained only “by a different arrangement of their molecules.”

Dumas had already proposed one way to approach the problem of the ordering of atoms in compounds. An investigation of ethers (esters) in 1827 and 1828 had led him and Polydore Boullay to believe that olefiant gas, which they called hydrogène bicarboné, C2H2, acted as a base and combined with water, an acid, or both to form the various ethers and alcohol. [Note: Each chemist’s original names and formulas are used here because the substitution of modern equivalents would be misleading. Dumas and most French chemists at this time adopted the atomic weight for carbon proposed by Gay-Lussac. Liebig followed Berzelius in taking a weight for carbon double Gay-Lussac’s value. Consequently Dumas’s formulas depict twice as many carbon atoms as those of Berzelius and Liebig. Berzelius’ barred symbols indicating double atoms, dots for oxygen atoms, and variant symbols for elements such as chlorine and nitrogen have not been used.]

Dumas and Boullay represented a series of ethers as a sequence of compounds containing ammonia in place of olefiant gas. Their discussion implied that the elementary particles in organic molecules form persistent subgroups exchanged as units in reactions, just as ammonia is; and they assumed in their formulas that the groupings were in accordance with the electrochemical theory of Berzelius. Their theory attracted a great deal of attention, but Liebig and Wöhler dismissed it as an ingenious idea with no empirical foundation. They considered their own finding, i.e., that the same elements are combined in different ways to form cyanic acid and cyanuric acid, to be a sufficient refutation of Dumas’s view. But Dumas for his part could see the same phenomena as additional support for his theory.

Despite his skepticism about Dumas’s attempts to define groups of atoms which function as units in organic compounds, Liebig soon began to apply similar reasoning to other substances. In 1831 he analyzed camphor and camphoric acid. He depicted the composition of camphor as 6(2C + 3H) + O, and that of camphoric acid as 5(2C + 3H) + 50, and he suggested that the unit 2C + 3H “acts as an elementary body.” He admitted that his opinion could only be regarded as a hypothesis. A year later he and Wöhler were able to provide a far more impressive illustration of their case for a unit appearing in a series of compounds.

In 1830 Pierre Robiquet and Antoine Boutron Charlard bad converted the oil of bitter almonds to benzoic acid by oxidation. They had also formed from the oil a neutral compound which decomposed to benzoic acid and another crystalline substance which formed the same acid. They named the crystalline material amygdalin, and inferred that the oil was a “benzoic radical.” During the summer of 1832, Liebig and Wöhler repeated and extended the experiments of the French pharmacists. They were able to show that the conversion of oil of bitter almonds to benzoic acid can be represented as the replacement of two atoms of hydrogen by one of oxygen. They also produced a series of related compounds containing bromine, iodine, sulfur, and cyanide. From their elementary analyses they deduced that a “benzoyl radical,” C14H10O2, persisted unchanged through all of these reactions. Although unable to isolate the radical, they presented persuasive evidence that it actually existed. When Berzelius saw their result, he praised it as the most important step yet made in plant chemistry.

Liebig had several times undertaken studies of the action of chlorine on alcohol and on ether, but so many compounds had formed that he had given up the investigations. Late in 1831 he had more success. He devised a new method to decompose the alcohol, passing the chlorine gas through it and periodically driving off the hydrochloric acid that formed by heating the solution. This process left behind a new compound which he named chloral and which he wrote C9Cl12O4. He discovered that chloral decomposed with alkalies to yield another compound containing only carbon and chlorine C2Cl5. He emphasized that these compositions satisfied not only the elementary analyses but the relationship of chloral to its decomposition products.

The hydrocarbon radical which Dumas and Boullay had defined in 1828 continued to be influential. The boldness of their generalization combined with the uncertainty of their evidence invited others to challenge their views, in part because their theory involved an explanation of one of the most important reactions in organic chemistry—the conversion of alcohol to ether by the action of sulfuric acid. The reaction was unusually complex because, depending on the conditions, a number of other products also formed. By 1827 chemists had become particularly interested in a compound known as “sulfovinic acid,” which appeared to accompany the formation of ether. The acid contained sulfur and a substance composed of carbon and hydrogen. Dumas and Boullay believed that the sulfur was in the form of sulfurous acid, and that the other substance was “oil of wine,” another product of the etheritcation reaction that they had discovered and given the formula C4H3. Henry Hennell determined, however, that the hydrocarbon contained equal portions of carbon and hydrogen, and that therefore it was probably Dumas’s and Boullay’s hydrogène bicarboné. Unlike Dumas and Boullay, Hennell considered sulfovinic acid an essential intermediary in etherification, supporting his position with several elegant and persuasive experiments. Georges Serullas also analyzed the compounds associated with etherification, and concluded that the hydrocarbon in sulfovinic acid was hydrogene bicar-bone, but combined with water to form ether. The conflict of opinions drew Liebig and Wöhler to study the acid in 1831.

The Ether Question. Liebig and Wöhler were able to show easily that Dumas’s and Boullay’s formula, C4H3, was based on incorrect elementary proportions. It was more difficult to decide whether the olefiant gas in sulfovinic acid was combined with water, in the form of ether, or directly joined with anhydrous sulfuric acid. Since they could not isolate the acid itself, they tried to deduce its composition indirectly by analyzing barium sulfovinate. Their results fitted either the interpretation that sulfovinic acid contains anhydrous sulfuric acid combined with alcohol or, as they thought more probable, hydrated sulfuric acid combined with ether. Either view, they thought, lent itself to a satisfactory explanation of the intermediary action of sulfovinic acid in the formation of ether.

Even though Serullas, Hennell, and Liebig and Wöhler had overturned certain of Dumas’s and Boullay’s conclusions, they had not undermined the general theory that alcohol, ethers, and related compounds were combinations of the same hydrocarbon base. The critics envisioned these compounds in the same way, and even strengthened the theory by making sulfovinic acid and oil of wine fit within the scheme more convincingly than its authors had done. Berzelius was not persuaded that the compounds were necessarily constituted in this way, but he acknowledged that such a representation placed so many phenomena in a simple and convenient order that the idea was worth attention. He believed, however, that the radical was not olefiant gas itself, but a multiple of it, which he named etherin.

New research soon forced further reassessments of the olefiant gas radical. In 1833 Gustav Magnus published the results of an investigation of sulfovinic acid, in which he claimed that the formula contained one molecule of water more than Liebig and Wöhler had assigned it. From arguments based on analogy and on the conditions under which sulfovinic acid was formed, Magnus inferred that two of the three atoms of water were attached to the hydrocarbon in the compound, forming alcohol with it. By reacting anhydrous sulfuric acid with absolute alcohol, he obtained a new acid containing only one molecule of water; he named it ethionic acid. In this acid, he asserted, anhydrous sulfuric acid is combined with ether in place of the alcohol in sulfovinic acid. Magnus depicted both alcohol and ether as compounds of the etherin radical, but he disputed Dumas and Boullay’s view that the radical acts as a base analogous to ammonia. Meanwhile Pelouze was studying an acid similar to sulfovinic acid, but formed by the action on alcohol of phosphoric acid instead of sulfuric acid. Phosphovinic acid was more stable, so that Pelouze was able to dry and analyze it directly. The compound yielded carbon and hydrogen in the proportions necessary to form alcohol, from which finding Pelouze concluded that it was constituted from that substance combined with phosphoric acid. He too challenged the theory that the etherin radical plays the part of a base in such compounds. Liebig examined a sample of Pelouze’s barium phosphovinate and saw that the substance absorbed water so rapidly that Pelouze’s combustion analyses had shown an excess of hydrogen and carbon over the true composition of the dry salt. Liebig therefore analyzed the crystalline salt and subtracted from the measured carbon and hydrogen the elements of the water of crystallization. The results, he concluded, fitted the view that the salt was composed of phosphoric acid and ether.

The new studies of phosphovinic, sulfovinic, and ethionic acid induced Berzelius to change his views concerning the first-order compound radicals in organic chemistry. None of these acids could contain etherin and water, he thought, because the water ought then to be removable, like the water of crystallization in the corresponding salts of ordinary bases. Furthermore, if ether consisted of etherin plus one atom of water, and alcohol of etherin and two atoms of water, then sulfovinic acid ought to be convertible to ethionic acid by driving off some of the water. Neither did that occur, however, and therefore Berzelius asserted that alcohol and ether could not be hydrates of the same radical. Discarding the etherin radical, Berzelius proposed that alcohol and ether were oxides of different radicals composed of carbon and hydrogen. Ether he considered to be C4H10+O and alcohol to be C2H6+O. The compounds were, he thought, equivalent to the oxides of inorganic chemistry.

When Berzelius expressed these views to Liebig in a letter (May 1833), Liebig wrote back enthusiastically that they were the only satisfactory explanation of all the phenomena. He had, he said, been thinking along similar lines himself. Several months later he wrote an article for a chemical dictionary in which he took up the principal evidence for Dumas and Boullay’s theory, and systematically refuted it. The first argument was that olefiant gas was supposed to combine directly with sulfuric acid to produce sulfovinic acid. Using olefiant gas specially purified of the ether and alcohol vapor which he believed usually to be mixed with it, Liebig found that the reaction no longer took place; he therefore asserted that it was not the olefiant gas but the ether ordinarily accompanying it that actually combined with sulfuric acid. The second case was a compound Zeise had recently produced by reacting alcohol with platinum chloride. Zeise had concluded that the resulting salt contained carbon and hydrogen in the proportions for olefiant gas, and no oxygen. According to Liebig, however, Zeise’s own data fit better with a formula including oxygen, joined with carbon and hydrogen to form ether rather than olefiant gas. The most difficult evidence to counter involved a compound which Dumas and Boullay had produced by reacting dry ammonia with oxalic ether. From the fact that the resulting salt, which they named ammonium oxalovinate, yielded carbonic acid and nitrogen in the ratio of 8:1; and from some indirect considerations, they inferred that it was composed of oxalic acid, ammonia, and the radical hydrogène bicarboné. On repeating the reaction, Liebig found that the product he obtained was oxamide, with a ratio of carbonic acid to nitrogen of only 2:1. This outcome seemed to him to eliminate the last foundation of the etherin theory, for that radical could not be assigned to the Composition of oxamide. Liebig believed that in eliminating Dumas and Boullay’s theory he had firmly established that of Berzelius. He had no doubt that the radical of ether, C4H10, would soon be isolated, even though he failed in his first attempt.

Although Liebig supported Berzelius’ conception of the ether radical, he could not accept the latter’s opinion that alcohol was the oxide of a different radical. Some of the reactions of sulfovinic acid and phosphovinic acid, as well as the conversions of alcohol to compound ethers, involved principally the removal of water—so he argued—and were best explained by assuming that alcohol is the hydrate of ether. By thus rejecting one aspect of Berzelius’ theory, Liebig attained much greater generality for Berzelius’ ether radical. Giving it the name ethyl, and the symbol E, Liebig presented rational formulas based on it for over twenty compounds. Among them were the following, compared here with their interpretation according to the etherin theory;

 Ethyl RadicalEtherin Radical
RadicalE = C4H10C4H8
Hydrochloric ether (ester)E+CI2C4H8+2HCl
Sulfovinic acid(EO+H2O)+2SO3C4H8+2SO3+2H2O

Controversy With Dumas. Dumas strongly defended his own theory during the following years. He maintained his position that under the circumstances in which he had carried out the reaction of ammonia with oxalic acid, he did obtain ammonium oxalovinate, and he supported his view of its constitution with a fuller analysis of its elementary composition. Searching for a situation in which he could test the consequences derived from his theory, he noted that, according to his view, alcohol contains hydrogen in two states. Four volumes of hydrogen are combined with oxygen in water, and eight volumes with carbon in the hydrocarbon radical. It ought, therefore, to be possible to distinguish these two states in the reactions of the compound ethers. He found a verification of this prediction in the formation of chloral, the compound recently isolated by Liebig. Reexamining the reactions and products Liebig had investigated, Dumas detected a small amount of hydrogen, which Liebig had missed, both in chloral and in the decomposition product Liebig had described. Dumas renamed the latter chloroform. He gave the two compounds, respectively, the formulas C8H2Cl6O2 and C2HC13. Stressing that in the reaction producing chloral from alcohol, ten volumes of hydrogen were replaced by only six of chlorine, Dumas explained the anomaly in terms of his view of the composition of alcohol, and of two substitution rules that he postulated. He then showed that similar rules could account for the conversion of alcohol to acetic acid. Later in the same year, Dumas and Eugene Peligot discovered a new compound “isomorphic” with alcohol, and a series of compounds analogous to those formed by alcohol. They interpreted these in terms of a new hypothetical radical, methylene, C4H4, corresponding to the hydrogène bicarboné radical. These investigations won Liebig’s grudging admiration, in spite of his continuing dissatisfaction with Dumas’s theoretical deductions.

In 1835 Liebig began deliberately to seek reactions which might contradict Dumas’s ether theory. One such opportunity arose when he repeated some of the experiments of Magnus which had corrected his own work with Wöhler on sulfovinic acid. From the composition and reactions of Magnus’ ethionic acid, Liebig inferred that during the formation of that compound one atom of oxygen from the sulfuric acid in the process must combine with two atoms of hydrogen from the ether to form water. Liebig considered that reaction to be proof that ether is not a hydrate of olefiant gas, for the water would not form at the expense of the elements of the ether and sulfuric acid if it were already present in the ether.

Shortly afterward Liebig discovered an important new compound which he thought revealed another fatal flaw in Dumas’s theory. Since 1831, when Dobereiner had sent him an “ether-like” fluid obtained from the oxidation of alcohol, Liebig had tried several times to identify the resultant compounds. Dobereiner maintained that the fluid contained an “oxygen ether.” In 1833 Liebig found two distinct compounds present. One of them, Dobereiner’s oxygen ether, he examined more thoroughly and renamed acetal. Early in 1835 he was able to isolate the other substance. It turned out to be an unknown and extraordinarily volatile compound, which he called aldehyde. By analyzing aldehyde and its ammonium salt, he determined its composition to be C4H8O2 and concluded that it is formed from alcohol by the loss of four hydrogen atoms. In keeping with his view of alcohol as the hydrate of ether, C4H10O + H2O, he postulated that aldehyde is C4H6O + H2O, and described it as the hydrate of C4H6O, an unknown oxide of a hypothetical hydrocarbon, C4H6. Liebig pointed out that if Dumas’s view of the composition of alcohol as C4H8 + H4O2 was accepted, then aldehyde might be considered an oxide containing the same hydrocarbon radical, or C4H8 + 2O. This interpretation required fewer new hypothetical compounds. Liebig argued, however, that it would be impossible in these terms to account for the conversion of alcohol to aldehyde. It would have to be assumed either that the hydrogen in the water of the alcohol is oxidized, or that the alcohol gives up all its water at the same time that two atoms of oxygen are added to its radical. He regarded both possibilities as absurd.

At the same time Liebig drew a refutation of another feature of Dumas’s theory from work done under his direction by Regnault. Dumas had listed the “oil of the Dutch chemists” first among the binary combinations of the hydtogene bicarboné radical. It was a substance obtained by reacting olefiant gas with chlorine, which he represented as C8H8, Cl4. Regnault produced from the oil a new ether-like substance of composition C4H6Cl2 (in Liebig’s notation). Regnault concluded that in the reaction half of the chlorine of the Dutch oil was separated, together with enough of the hydrogen to form hydrochloric acid. It followed, he asserted, that the chlorine in the Dutch oil was combined in two different ways, so that Dumas’s view of the compound as a simple combination of chlorine and olefiant gas could not be correct. Liebig triumphantly concluded that the “first limb” of Dumas’s ether theory had been proven completely false.

In the fall of 1836 Liebig appended a lengthy footnote attacking Dumas’s ether theory to a joint memoir reporting investigations he had made with Pelouze. First he countered the idea that olefiant gas is the basis of the composition of ether by rejecting supporting arguments that Dumas had derived from the reactions of analogous compounds. Then he enumerated ways in which the behavior of ether differed from that of hydrates and resembled that of oxides. Finally, he tried to prove that Dumas’s rules of substitution, applied to his own formulas for ether and alcohol, failed to give results in accord with the reactions these compounds undergo, and that Dumas had used the rules inconsistently. He showed in detail how the conversion of alcohol to acetic acid, either directly or through the intermediary of aldehyde, and the formation of chloral, led to contradictions if Dumas’s formulas and rules were applied strictly. Liebig believed that his note thoroughly destroyed the credibility of Dumas’s ether theory and his rules of substitution.

As he confronted Dumas’s theory, Liebig had presumed that he and Berzelius stood united in support of a shared ether theory, for he still regarded his view of alcohol as the hydrate of ether to be only an improvement on Berzelius’ original theory. It was therefore very disappointing to find on reading Berzelius’ Jahresbericht for 1836 that he was depicted as disputing Berzelius’ own theory of the composition of ether and alcohol. Berzelius discounted Liebig’s arguments and maintained his previous opinion that alcohol was too unlike ether to be merely a hydrate of it. In February 1837 Liebig wrote Berzelius to try to overcome his objections; he was anxious, he said, to reach an agreement, lest their division encourage the opponents of “the new ether theory.” Later in the year Liebig found a new occasion to strike a blow at this opposition and simultaneously to try to dispel Berzelius’ reservations. Recently Zeise had defended his earlier conclusion that the double salt formed by alcohol and platinum chloride was a combination of the etherin radical. Liebig asserted that even though Zeise’s new analyses appeared more accurate than the older ones, the fit between observation and theory was fortuitous; his formula was based on elementary analyses alone, unsupported by relationships between the compound and its decomposition products. Zeise was, according to Liebig, basing his interpretation mostly on a prior and misguided allegiance to the etherin theory. Reactions which could remove the oxygen and hydrogen of the water of hydration of alcohol demonstrated that that hydrogen was in a different state from the rest of the hydrogen in the compound. There were no grounds at all, however, for distinguishing any of the hydrogen atoms in ether from the others, so that ether must be a simple oxide of the hydrocarbon, rather than a hydrate. Finally Liebig tried to counter various objections that defenders of the etherin theory had raised against treating ether as an oxide.

Liebig’s adamant defense of his ether theory and his relentless opposition to that of Dumas seems hard to reconcile with the attitude that both he and Dumas professed, that all of their theories were necessarily provisional. In principle both recognized that no theory yet available was general enough to encompass all the relevant data. They believed that they must formulate broad theories even though these might prove wrong, in order to organize a burgeoning mass of analytical results. Like Dumas and Berzelius, Liebig repeatedly said that debates over divergent theories were beneficial because the defenders of each were led to discover and analyze new compounds while searching for support for their views. In this judgment they were correct, for almost every investigation conducted to advance a particular theoretical goal revealed compounds and reactions which helped expand the foundations of chemistry. The stubbornness with which Liebig sought to advance his theory and to discredit that of his rival probably owed less to the issues than to the personalities.

As Dumas’s influence grew after 1832, Liebig began to perceive him as the leader of the French school of organic chemistry, a school which he thought stood in opposition to himself and Berzelius. He believed that Dumas was using his dominant position to impose his views on the other chemists in Paris. As an influential independent voice, Liebig felt he had a duty to oppose these tendencies and to rescue French chemistry from the “false route” on which Dumas was directing it. Liebig was also resentful because he thought that the younger French chemists still acted as though no important investigations were going on outside Paris. In seeking to overthrow Dumas’s ether theory Liebig was in part trying to make the French acknowledge him and his followers as a leading force in organic chemistry. Besides these calculated reasons for treating his scientific differences with Dumas as political struggles, there were strong emotional elements moving Liebig. Since 1830 he and Dumas had opposed each other repeatedly on both theoretical and experimental questions, and during the course of their published debates each had allowed himself to interject into his arguments personal criticisms which wounded the pride of the other. Liebig was particularly prone to give free rein to the expression of heated reproaches which caused the disputes to take on a harshness out of proportion to the depth of their intellectual differences. Their scientific differences were not fundamental, and in 1837 it appeared possible that the two men might be able to reconcile them.

Since 1831 Pelouze had been Liebig’s closest friend and colleague in Paris. They corresponded frequently, exchanging scientific news and research ideas. In 1836 Pelouze spent the summer in Giessen working with Liebig. When Liebig attached his attack on Dumas’s ether theory to the joint memoir reporting these investigations, he was intentionally linking Pelouze with himself as the representative of his school in Paris, hoping that Pelouze could act as a counterbalance to Dumas’s position there. No sooner had he done this, however, than it appeared that the alliance might have been constructed at Pelouze’s expense; Pelouze had become a candidate for the Academy of Sciences, where Dumas had much influence. Liebig reflected that if he could reach a reconciliation with Dumas, Pelouze would have a better chance. Accordingly he wrote Dumas in May 1837, explaining away as best he could his motives for combating Dumas’s theory and offering to put their quarrels behind them so that they could cooperate in the future. Dumas seemed pleased at the prospect of ending the hostilities, which he acknowledged had been a burden, and accepted Liebig’s suggestion that they publish together. Liebig visited Dumas in October on his return from a trip to England; and the meeting went so well that Liebig left Paris feeling that all of the points in dispute had been settled and that he had converted Dumas to his ether theory. Dumas composed a note “Sur l’etat actuel de la chimie organique,” which he presented in both their names to the Academy. The note suggests that Dumas had not necessarily accepted Liebig’s views, for their common beliefs were stated in propositions so general as to skirt the points of contention. He proclaimed their faith that the laws of combination and reactions in organic chemistry were the same as those of inorganic chemistry, but that the role played in the latter by elements was represented in the former by radicals. Further, the characterization of these radicals had been the constant task of both men for ten years, and they regretted that they had debated their differences of opinion so heatedly. They announced their new alliance and appealed to all chemists to join their effort to classify all organic compounds according to the radicals they contained. This “manifesto,” as it was soon called, was most unfortunate. Instead of resolving Dumas’s and Liebig’s disagreements, it suppressed them. Moreover it publicly committed them to a doctrine which both were already coming to doubt. Liebig had publicly and privately questioned whether fixed radicals really exist. Instead of uniting other chemists, the document divided them. Older chemists got the impression that Dumas and Liebig claimed to have originated organic chemistry and were dismissing the contributions of their predecessors. Younger chemists saw the program as an attempt by the two to set themselves up as privileged directors of the work of everyone else. Sensing the hostile reactions, Liebig sought to dissociate himself from the note and prevented its publication in German.

In 1837 Liebig regarded the defense of his ether theory as one of his principal tasks, and by the end of that year he was confident that he had eliminated all opposition to it. Yet two years later he acknowledged that the opposing views were basically the same. He now represented the compounds involved in the earlier debates as combinations of an acetyl radical, Ac = C4H6 and claimed that this system satisfied the aims of both of the earlier theories. This sudden switch in his attitude resulted from a complex blend of scientific and personal developments.

The origins of the acetyl radical were in Liebig’s 1835 paper on aldehyde, which he had described as a combination of the hypothetical hydrocarbon C4H6. Shortly afterward Regnault produced two new compounds by reacting alcohol with bromine and iodine; he assigned to them the formulas C4H6Br and C4H6I. He thought these compounds and the Dutch oil, C4H6Cl, could best be regarded as combinations of the hypothetical radical C4H6, which he named aldehyden because he considered it the same as the radical which Liebig had discovered in aldehyde. Berzelius renamed Regnault’s aldehyden “acetyl.” From measurements of gas densities Berzelius confirmed the application of Regnault’s formulas to acetic acid and aldehyde, and by 1838 he wrote in his Jahres-bericht that there was a firm basis for the acetyl theory.

In his new system of 1839 Liebig extended the acetyl combinations to include the compounds formerly interpreted by the etherin or ethyl theories. Thus, if acetyl, C4H6, = Ac;

thenAcH2 = olefiant gas;
 AcH4 = ethyl;
 AcH4O = ether;
andAcH4O + H2O = alcohol.

Liebig added a long list of related compounds. Neither etherin nor ethyl appeared any longer as fundamental radicals, but as combinations of acetyl with hydrogen. The formulas were compatible both with his own view that ether is an organic oxide and with the view of Dumas and Boullay that the compounds of ether resemble those of ammonia. The man who had recently contrasted the emptiness of the etherin theory with the soundness of his own, now stated that from his new standpoint “both … theories have the same foundation.” He could now compromise on the issue partly because he could appreciate more fully than before that the composition of ether had been less important for itself than as a testing ground for general principles of organic composition. He had already been to some degree aware of that. He had written in 1837 that the question of the constitution of ether embraced the general question of whether organic materials which contain oxygen were oxides of a complex radical or combinations of a radical with a compound body. He believed, therefore, that in settling the problem of ether he would orient all future investigations in organic chemistry. By 1839 he had recognized that so much could not depend on one case. The existence of organic oxides was so well established in other situations that he no longer needed to defend that concept by insisting that ether was the oxide of the ethyl radical. Liebig was also more flexible because he had become far less confident that there was any satisfactory single way to envision organic composition. He was readier to accommodate his views with those of Dumas because he was coming to see that Dumas’s theoretical flexibility was more viable than the fixed principles that Berzelius maintained. Within this period the man whom Liebig had regarded since 1830 as his mentor came to appear to him as an increasingly rigid voice of the past. These shifts in Liebig’s attitude grew largely out of the problem of organic acids.

The Problem of Acidity . By 1830 the unifying conception derived from Lavoisier’s oxygen theory of acids was no longer intact. The discovery that muriatic and other acids contained no oxygen had led to the division of acids into two categories, oxacids and hydracids. When a hydracid reacted with an inorganic base it was thought not to combine directly but to decompose the base. The hydrogen formed water with its oxygen, while the metal of the base replaced the hydrogen of the acid. Oxacids were thought to be combined with a quantity of water which, unlike the water of crystallization, could not be removed by drying. These hydrated acids combined directly with bases, which displaced the combined water. According to Berzelius, a given acid, when reacted with different bases, always combined with quantities of the base containing the same amount of oxygen. If the resulting salt were neutral, the acidic oxide had the same number of atoms of oxygen as did the basic oxide. A few oxacids could be obtained both in the hydrous and anhydrous states, but in most cases the anhydrous acid was hypothetical. Its composition was inferred by subtracting the quantity of water separated during the formation of a neutral salt from the formula determined by analyzing the free hydrated acid. In this system the organic acids formed the class of oxacids of compound radicals. For the case of acetic acid these considerations led to the formulas C4H6O3) for the theoretical aqueous acid; C4H6O3 + H2O for the aqueous acid; and C4H6O3 + BaO for the barium salt.

Most of the organic acids fitted very well into this scheme, but two anomalies developed. Oxalic acid contained so little hydrogen that the subtraction of a molecule of water from its formula left the anhydrous acid with no hydrogen at all. This peculiarity caused Dulong to propose in 1816 that oxalic acid was a hydracid, H2 + C4O4. The second difficulty concerned citric acid. From the analysis of its lead salt Berzelius had given the anhydrous acid the formula C4H4O4. During the summer of 1832 Jules Gay-Lussac, who was studying chemistry in Liebig’s laboratory, performed an analysis of copper citrate, the results of which seemed to fit the formula for the hydrate of the acid rather than the anhydrous acid. Consequently Liebig asked Berzelius to reexamine his earlier analyses. A few months later Berzelius reported back that his new results were quite puzzling. The proportions of acid and base in the lead salt varied according to the acidity or alkalinity of the solution in which it formed, a phenomenon which he ascribed vaguely to the propensity of the acid to form acidic and basic salts. The sodium salt formed only a single combination of fixed proportions, but the amount of removable water of crystallization was equivalent to 2 1/3 atoms. The dry salt could be represented as NaC4H4O4+H2O, but when heated further it lost one-third of an atom more than the formula indicated it contained. Berzelius could not give a satisfactory explanation for these results. It occurred to Liebig that the contradictions might be resolved if citric acid were considered to be C6H6O6 or C3H3O3. He suggested for the sodium salt the formula

2(C6H6O6 + 3NaO + 11H2O (or 12H2O),

which he thought accounted fairly closely for the amount of water lost in drying the salt. Berzelius objected, however, that the error would still be too large, and that it was contrary to experience for a neutral salt to have more atoms of base than of acid. Both men agreed that the problem could not be solved until organic chemistry had progressed further.

During the same years the number of organic acids under investigation began to grow rapidly. Some were isolated from natural materials; others, known as “pyrogenic” acids, were produced by heating the natural acids. In 1832 Liebig interpreted the conversion of meconic acid to “metameconic acid” as the removal of one atom of carbonic acid from each atom of meconic acid, leaving one-half atom of meta-meconic acid. Soon afterward Pelouze learned to produce simple controlled decompositions systematically, by distilling various acids at moderate, constant temperatures. In this way he found a sublimate of lactic acid which contained two equivalents of water less than the ordinary lactic acid. He converted tannic acid to gallic acid, interpreting the reaction as the loss of one equivalent each of carbonic acid and water. He treated similarly the conversion of malic acid to fumaric acid, and tartaric acid to pyrotartaric acid. Liebig followed these investigations with close interest and judged Pelouze’s new approach to be one of the most important discoveries in organic chemistry.

Following the prevalent opinion of the time, Pelouze assumed that the pyrogenic acids were oxacids. Liebig, however, already considered the division between oxacids and hydracids to be unnatural. He and Wöhler had thought for a while of interpreting benzoic acid as a hydracid in 1832. A year later he speculated briefly that tartaric acid and malic acid might be hydracids. Early in 1836 he still seemed uncertain of his position on acids, but by the end of that year he was trying to persuade Berzelius that several organic acids were hydracids. He also asserted that there are organic acids that neutralize one, two, and three atoms of base. This break with Berzelius’ general conception of salts was inspired by Thomas Graham’s recent discovery that there are three types of phosphoric acid containing respectively three atoms, two atoms, and one atom of water replaceable by bases. Liebig believed that a number of organic acids are analogous to phosphoric acid. According to Berzelius, potassium cyanurate is C3N3H3O3 + KO. Liebig contended, however, that the so-called neutral potassium salt is C6N6H2O4 + 2KO, whereas the so-called acid salt is C6N6H4O5 + KO. He discovered a new silver salt which he depicted as C6N6O3 + 3AgO. Thus in the second case one atom of base replaced one atom of water; in the first, two atoms replaced two of water; and in the third, three atoms replaced three of water. With meconic acid he obtained a silver salt containing two atoms of silver oxide and another containing three of silver oxide with no water. The same principles could explain the anomalies in the composition of citric acid.

A new analysis of silver citrate gave Liebig slightly more silver and less water than Berzelius had found, enabling Liebig to substitute for Berzelius’ formula,C4H4O4 + AgO, one that required three atoms of silver. Liebig wrote it C12H10O11 + 3AgO. Tripling the formula eliminated the fractional atoms Berzelius had encountered and favored the interpretation that citric acid, too, can neutralize three atoms of base. Liebig also argued that tartar emetic has four atoms of replaceable water. From Liebig’s first letter, Berzelius thought that the replacement of water in these reactions represented the ordinary removal of water from acids and salts by heating. In a second letter Liebig emphasized that he had dried silver citrate and the salts of cyanuric acid at room temperature. His results, therefore, must directly involve the constitution of the acids themselves.

Liebig especially wanted to clarify that point because it was one of the reasons that he did not think his results could be interpreted by an adaptation of the oxacid theory to the idea of multiple bases. He was now convinced that alkali and metallic bases did not simply replace water already present in the acid, but that the oxygen of the bases combined with hydrogen to form the water. His other reason was that silver oxide, a weak base, could displace all the water, whereas potash, a strong base, could displace only part of it. This was anomalous under the old theory, but predictable in terms of hydracids. Silver is easily reducible and would therefore more readily give up its oxygen to the hydrogen. Consequently cyanuric acid was not 3Cy2O + 3Ag, but C6N6O6. He thought that potassium sulfate might similarly be regarded as K + SO4 rather than KO + SO3 Sulfates would then be exactly analogous to chlorides, and all compounds would fit into a simple, harmonious system.

Liebig returned to these ideas nine months later, as he set out on the reformulation of organic chemistry that he and Dumas had decided upon during their meeting. On 16 November 1837 he sent Dumas a letter proposing that they begin their collaboration by extending to the rest of the organic acids the consequences of his hypothesis that all acids are hydracids. Dumas hesitated and cautioned Liebig to prudent about proposing theories. Although annoyed by this reaction, Liebig wrote again, explaining his ideas more fully, and succeeded in overcoming Dumas’s reservations. In December Dumas presented a short note, “Sur la constitution de quelques acides,” to the Academy summarizing the views on organic acids upon which they agreed. The contents came mostly from Liebig’s two letters. The note summarized the arguments for considering citric acid, meconic acid, and cyanuric acid as tribasic, and stressed that by tripling the formula for citric acid the authors had resolved the dilemma that Berzelius’ research had posed. They asserted that the phenomena could be envisioned in a simpler, more direct manner, if these acids were regarded as hydracids. Using tartaric acid as an example, they showed that in the old system complicated double formulas were necessary to represent salts such as cream of tartar. But if tartaric acid were a hydracid, C8H4O12, H8 then all of its compounds would appear as ordinary substitutions of different amounts of metal for equivalent amounts of the hydrogen. Thus:

The anhydrous acid assumed in the old theory would not exist. Relying entirely on the advantage of simplicity in representation as justification, and presenting only a few of the examples Liebig had worked out, the note did not reveal the full scope of his argument.

Even as he exchanged letters with Dumas concerning how they would proceed in their new venture, Liebig was becoming increasingly anxious about Berzelius’ reaction to his acid theories. He wrote Berzelius repeatedly, expanding and developing his earlier arguments, and pleading for considerate, forbearing criticism. Berzelius tactfully referred to the analyses underlying Liebig’s ideas as “highly interesting discoveries,” but could not accept the explanation that the acids were hydracids. By February 1838 Liebig’s hope that he could win Berzelius over was waning, and the rapprochement with Dumas had proved to be only temporary. When Pelouze read Dumas and Liebig’s note on acids in the Comples rendas de l’ Academie des sciences he saw that Dumas had referred to the fact that many citrates in addition to those investigated by Berzelius lose one-third of an atom of water. Pelouze was convinced that Dumas had appropriated without acknowledgment his own research on citrates, work which he had mentioned to Dumas in the fall of 1837. Pelouze therefore wrote Liebig expressing his displeasure. At first Liebig did not consider the problem serious, but during the exchange of letters and the events that followed he once again became suspicious of Dumas’s motives. Moreover, Liebig had moved ahead with his research on the organic acids while, as far as he could see, Dumas had done no significant work on the problem. He persuaded himself, therefore, that Dumas was only seeking to share in the credit. by mid-February, Liebig was nearly ready to publish his own results: he wrote Dumas that he had decided to give up their collaboration and that he would continue independently.

Liebig’s massive article on organic acids, “Über die Constitution der organischen Sauren,” appeared in April 1838. The first part contained his analyses of the acids and their salts and the remainder treated three related theoretical questions. Liebig gave detailed arguments for considering that some organic acids neutralize more than one base and that all acids are hydrogen acids. In addition he tried to account for the molecular rearrangements involved in the conversion of some of the organic acids to their related pyrogenic acids. In part he was extending the approach of Pelouze, but he was attempting also to explain in terms of changes in composition the decrease in the number of atoms of base which the resulting acids could neutralize.

The interpretive portion began with a discussion of phosphoric acid based on Graham’s work. After reviewing the three series of phosphoric acid salts, Liebig proposed various ways in which the isomeric changes in the acid might be envisioned to account for the differences in the number of bases with which the three forms could combine. He also pointed out that the property of the acid of combining with more than one base revealed the uncertainty underlying the customary conception of neutrality. He suggested that the property that most clearly distinguished acids like phosphoric from the majority of acids was the ability to form true double salts such as sodium-potassium phosphate; for this capacity was the same as that of combining with more than one atom of a single base.

Liebig next applied the principles he had drawn from the example of phosphoric acid to the organic acids which embodied similar combining properties. From this viewpoint he depicted the constitution of cyanuric, meconic, citric, tartaric, mucic, malic, gallic, and aspartic acids, their salts, and their pyrogenic products. The case of meconic acid will illustrate his approach:

C14H2O11 + 3H2O dried acid

C14H2O11 + 3AgO silver salt

Liebig showed that the pyrogenic acids related to meconic, citric, tartaric, and gallic acids usually neutralized only one or two atoms of base. For example, heat and concentrated acid transformed meconic acid into comenic acid, which could combine with two bases. The constitution of comenic acid he considered to be

C12H4O8 + 2H2O crystalline acid

In the formation of comenic acid two atoms of carbonic acid were removed. That did not account, however, for the loss of one-third of the saturation capacity, a change caused, according to Liebig, by the incorporation of one atom of the previously separable water into the acid radical itself, where it could no longer be exchanged for another base. The further conversion of comenic acid to pyromeconic acid involved the separation of another atom of carbonic acid and the transfer of another atom of water to the radical. The resulting constitution, C10H8O5 + H2O, accorded with the capacity of the acid to neutralize only one atom of base. Liebig described in a similar manner the conversions of other acids to their pyrogenic products, although some of the changes were more complex, and in some cases unsolved difficulties prevented him from accounting fully for the reactions.

In the last section of his article, Liebig noted that he had built the preceding interpretations on current conceptions of acids and bases. For certain of the phenomena, however, he could find no explanation in that system. Besides the previously mentioned paradox that silver oxide displaced more water than did potash, there was another question. Why, if the saturation capacity of an acid depends on water it contains, do those acids which lose part of their saturation capacity in the modifications that displace some of the water not regain both when they are placed again in water? Such situations suggested that it might be profitable to consider generalizing the view that some acids are hydracids. He ascribed the strangeness of resulting formulas such as SO4 + K mostly to habit, and argued that the hydrogen theory was as valid a way as the conventional theory to envision acids containing oxygen. The close similarity of various reactions of bases with the halogen acids and with oxygen acids such as sulfuric made it unlikely that water was formed in the first type and merely separated in the second. The hydrogen theory led to some deeper insights into the nature of compounds, which the oxygen theory could not provide. A comparison of the acids formed by the various oxidation states of chlorine showed that their saturation capacities depended only on the amount of hydrogen present, not on the composition of the radicals. Liebig thought he could apply the same principle to numerous organic acids; various complicated combinations of elements could be added to their radicals without changing their saturation capacity. After completing a few more general arguments, Liebig sketched only briefly with a single example what the constitution of organic acids would be like in the framework of the hydrogen theory.

C12H4O10 + H4 = meconic acid

C10H6O6 + H2 = pyromeconic (comenic) acid

Liebig carefully presented his theory as a hypothesis which might not necessarily express the true constitution of the compounds. He maintained only that it had guided him in his investigations, that it brought certain relations into a simple and general form, closing gaps that the prevailing theory could not close, and that he was deeply persuaded that the theory would lead chemists to important discoveries.

The paper on organic acids was one of Liebig’s finest achievements, reflecting the best of the attributes that had marked his previous work. He based his position on precise analyses of numerous compounds. Some concerned substances he had discovered, but many were refinements of analyses done by others. He had not originated the theories he defended, but had greatly extended approaches drawn from Davy, Graham, Pelouze, and others. Through his extensive knowledge of compounds and reactions, he was able to amass impressive evidence for his inferences. He displayed a realistic sense of the value and limits of theoretical conceptions. He utilized flexibly such currently accepted foundations of reasoning as the radical theory. He was able to weld these elements into a comprehensive, unifying whole. He took a major step in one of the most important revisions in general chemical theory since the acceptance of Lavoisier’s system of chemistry: a revision completed a few years later in the more universal statement of the hydrogen theory of acids by Liebig’s former student, Gerhardt.

For Liebig his investigation of the organic acids only continued to be a source of personal turmoil. In part this was inevitable, for the new system he was building entailed the disruption of that which had guided Berzelius for many years and which had been the foundation for their scientific partnership. Recognizing the seriousness of the breach between them, both men made strenuous efforts to repair it. Liebig tried again and again to overcome Berzelius’ disapproval of his new ideas. Diplomatically he portrayed them as generalizations of interpretations Berzelius himself had given in specific cases, and he appealed to Berzelius to join him in exploring the consequences of his theory. Berzelius studied Liebig’s views with care, but he could not subscribe to them. Within the framework in which he viewed organic composition, Liebig’s formulas were incomprehensible. Both men acknowledged that no single theory was exclusively true, which was a step toward reconciling their intellectual differences, but at the same time non-rational factors were making harmony difficult. Liebig had recently published the Anleitung zur orgnaischer Körper, in which he had included several criticisms of details of Berzelius’ methods. Berzelius was hurt by what seemed to him an abrupt dismissal of his earlier contributions, and began to feel that Liebig was trying to enhance his own reputation at the expense of his colleagues. Liebig expressed regrets at his thoughtlessness, and the two men appeared reconciled; but they were estranged again by Pelouze’s quarrel with Dumas.

In January 1838 Dumas had assured Pelouze that his claims concerning the citrates would be given full credit in an article that he and Liebig would publish as soon as Berzelius had confirmed Liebig’s analysis of silver citrate. By March, Pelouze felt that he could wait no longer, and he wrote Berzelius to find out if Berzelius actually intended to do the analyses in question. If not, Pelouze said, he planned to present his own case at the Academy of Sciences. In his letter Pelouze described Dumas’s conduct in very harsh terms. Berzelius, who had long distrusted Dumas, wrote a letter to the Academy in which he not only supported Pelouze’s assertion that the explanation given in the joint note of Dumas and Liebig of the loss of water in citrates was the same as that which Pelouze had written him, but attacked Dumas’s theory of substitutions and the joint theory of organic acids as well. After Pelouze read this letter at the Academy, he and Dumas engaged in an acrimonious debate over what had actually happened. Liebig was soon drawn into the fracas. Pelouze and Dumas both sought to enlist Liebig’s support, each writing to persuade him that honor and self-interest required Liebig to join him in defense against the accusations of the other. For a time Liebig sided with Dumas and unsuccessfully attempted to persuade Pelouze to drop the affair. Berzelius’ open attack on his theory of organic acids greatly angered Liebig; and, in a letter to Dumas, he was led in turn to criticize Berzelius’ acid theory. He authorized Dumas to read the scientific portions of the letter at the Academy.

As the dispute continued, however, Liebig’s old suspicions about Dumas reasserted themselves. On the basis of a partial knowledge of what was happening in Paris, reflected especially in an inaccurate report in the daily press, Liebig decided that Dumas had violated his confidence and was manipulating him for personal advantage. By June 1838 he had made a “total break” with Dumas and thought of him as an “implacable” enemy. With Berzelius, however, he tried hard to repair the damage the events of the preceding weeks had done to their relationship. Patiently the two men renewed their efforts to reconcile their views about organic acids. Yet they were inexorably drifting apart, and Liebig was beginning to see Berzelius as a spent force in chemistry. Despite the personal enmity between Liebig and Dumas, they were coming to share a general conviction that organic chemistry was so much more complex than inorganic that new rules were required for it, and that whatever promised to simplify and order some of the phenomena deserved consideration. Like it or not, during the next two years Liebig was drawn toward an intellectual alliance with his most formidable directed rival against his most revered friend.

During the years that Liebig was preoccupied with the ether theory and with organic acids, he also carried out two important investigations with Wöhler. In October 1836 Wöhler wrote that he had discovered a way to transform amygdalin to oil of bitter almonds and hydrocyanic acid, by distilling it with manganese and sulfuric acid, and he invited Liebig to join in pursuing the topic. Two days later he made a more remarkable discovery. It had occurred to him that perhaps the transformation of amygdalin could be effected by the albumin in the almonds, in a manner similar to the action of yeast on sugar. He was able to confirm his expectation, for either crushed almonds or an aqueous emulsion derived from them produced the reaction. Wöhler suspected that the decomposition was an example of what Berzelius had recently defined as catalysis. Liebig and Wöhler then divided up the detailed examination of the properties and composition of amygdalin. They precipitated from the emulsion of almonds a substance which when redissolved retained its action. They named the active substance “emulsin.” Its effectiveness in very small quantities confirmed that it acted like yeast. Liebig wrote up the memoir “Über die Bildung der Bittermandelöls,” which they published in 1837. but the critical ideas and experiments came from Wöhler. Their characterization of emulsin, following that of diastase by Payen and Persoz, and of pepsin by Theodor Schwann, helped to generalize the conception of specific fermenting actions, which became a central theme in organic and physiological chemistry during the next decade.

Wöhler also provided the initial impulse for their other major joint investigation. In June 1837 he wrote Liebig that he had found a way to identify the constituents of uric acid by decomposing it in water with lead peroxide as the oxidizing agent. The products formed were urea, carbonic acid, and a colorless crystalline substance. Invited by Wöhler to take up the investigation with him, Liebig immediately determined the elementary composition of the new substance and identified it with allantoin, a compound long known to be present in the allantoic fluid of calves. The composition of uric acid was C5N4H4O3 and that of allantoin C4N4H6O3. Both men then started to examine other means of oxidizing uric acid. When Liebig attended the meeting of the British Association for the Advancement of Science later in the summer, he presented a preliminary report on the new work. He depicted the reaction which formed allantoin as:

C10N4H4O6+ Pb2O4
1 atom uric acid2 atoms peroxide of lead

On the basis of the reaction and the conventional assumption that a compound containing four elements must be formed by a union of binary compounds, Liebig interpreted uric acid as a compound of urea with a peculiar unknown acid, the latter containing the radical of oxalic acid combined with cyanogen. He represented this formulation as 4(CO + Cy) + urea. Wöhler came to Giessen at Christmas so that he and Liebig could complete the project together; but they found such an astonishing number of new compounds that they were unable to finish. Liebig continued the experiments through the spring, and the results were finally ready for publication in June 1838.

The paper, “Über die Natur der Harnsäure,1” described the conversion of uric acid into allantoin and then the products of its decomposition by nitric acid. In dilute nitric acid and ammonia, uric acid produced ammonium oxalurate, C6N4H8O8. In more concentrated cold nitric acid it yielded crystals of a water-soluble substance which they named alloxan and gave the formula C8N4H8O10. If an excess of ammonia were present the reaction produced instead the ammonium salt of purpuric acid, a remarkable compound identified earlier by William Prout. Liebig and Wöhler renamed it murexid and considered its composition to be C12N10H12O8. Under other circumstances uric acid produced a substance they named alloxantin, the composition of which, C8N4H10O10, was closely related to that of alloxan. Alloxan could readily be converted to alloxantin by reducing agents, and back again with oxidizing agents. The diverse products of the direct decomposition of uric acid gave rise in turn to numerous other products. Among those that Liebig and Wöhler discovered and named were thionuric acid, uramil, uramilic acid, dialuric acid, alloxic acid, mesoxalic acid, and mycomelinic acid.

The complexity of the reactions made it difficult for Liebig and Wöhler to draw unequivocal conclusions concerning the rational constitution of the compounds involved. They elaborated essentially the same interpretation of the relation between uric acid and allantoin that Liebig had summarized the previous summer. They considered, as an admittedly provisional and “prejudiced” view, that urea preexisted in uric acid. Assuming this to be so, they subtracted the formula for urea from that for uric acid and derived a hypothetical body, C8N4O4, which they named uril. They then proposed interpretations of reactions such as the conversion of uric acid to alloxan based on the supposed separation of uric acid into urea and uril. In general they recognized that the diversity of decomposition reactions made it impossible to deduce from the products that the same ones preexisted in the original compounds. Often two or more explanations of a reaction seemed equally conformable to the result. The most general conclusion they reached was that uric acid demonstrated that organic compounds undergo “innumerable metamorphoses’ a characteristic setting them quite apart from inorganic compounds.

Further research on the decomposition products of uric acid occupied Liebig’s laboratory during 1839, although he began to spend more of his time writing textbooks. He was now supreme in his field, and at the age of thirty-six at the peak of his experimental productivity. His grasp of theoretical issues was firmer yet more flexible than ever before, and he headed the largest scientific laboratory and school in the world. By August of that year additions to his laboratory and improved financial support enabled him to expand his training and research programs. Yet within a year Liebig had nearly given up those experimental areas in which he excelled and was devoting his energy to applications of chemistry in agriculture and physiology. To some extent his switch was a response to new opportunities; but it reflected also a weariness with the work he had been doing, and especially with the disputes which pervaded organic chemistry. In 1839 and 1840 the debates over Dumas’s substitution theory reached a climax, as Dumas responded to new objections by Berzelius. Liebig supported the basic conception of substitution, and dissociated himself from Berzelius’ alternative explanations of the reactions Dumas had used to support his theories. Early in 1840, however, Pelouze joined the opposition to Dumas and urged Liebig also to join in refuting the substitution theory. As he saw himself in danger of being pulled into further arguments dominated by personalities more than by issues, Liebig began to feel an aversion toward the activities of chemistry. He therefore turned to new subjects, in part to escape the impasse he had reached in his old field.

If Liebig’s significance were to be judged in terms of his lasting tangible contributions to organic chemistry, the emphasis would have to be on the methods he devised or refined, and the many compounds and reactions he discovered or described. Few of his theories were highly original, and none of them definitive. Yet a summation Of Liebig’s achievements in terms of discoveries and general theories is bound to underrate his stature, for that is measurable more by the way in which he participated in a crucial stage in the development of chemistry. The manner in which he did organic chemistry was as important as the tabulation of his results. Through the reliability of his analyses, the thoroughness of his examination of all the products and reactions related to any particular problem, the quality of his reasoning, and the soundness of his judgment about most theoretical issues, Liebig helped his contemporaries and successors to see more clearly how the science ought to be pursued. Similarly, his new way of training chemists was as important as the many excellent chemists he himself taught. He helped to set standards in another way in his role as the critical editor of the Annalen der Chemie und Pharmacie, a journal he turned into the preeminent publication in chemistry. Liebig did as much as any one person to bring about the era of large-scale research, in which the ability to organize men became as critical as the ability to conceive and carry out experiments. Between 1830 and 1840 he was at the very center of the rapidly growing field of chemistry and he gave it an impetus that was felt long after he had ceased actively to participate in it.

Agricultural Chemistry. Liebig moved into his new areas of interest at his usual pace. Within four months after he began systematically studying the relation of organic chemistry to agriculture and physiology, he had produced the first version of one of the most important books in the history of scientific agriculture. In a letter to Berzelius in April 1840, he wrote that he had derived his general arguments from several surprising discoveries. First, from analyses of straw, hay, and fruits, he reached the “very remarkable result” that a given area of land, whether cultivated field or forest, produces each year the same total quantity of carbon in the composition of whatever plants grow on it. That inference became the starting point for his argument that the carbon must derive from the atmosphere rather than from humus in the soil. The problem of the source of nitrogen had “long occupied” him, he said, but after he found ammonia in the sap of every plant he investigated he became persuaded that the nitrogen must come from ammonia dissolved in rainwater, and he found that all of it contained determinate amounts of ammonia. Next Liebig examined the alkalies and alkaline earths in plants. Because they were always present, he presumed that they were essential, and set out to explain why they nevertheless varied according to the soil in which the plants grew. He surmised that the total equivalent quantity of base must be constant for a given species of plant, but that one type of base might substitute for another. He verified this prediction by reference to analyses that other chemists had made of the ashes of evergreens. Pine trees grown in one locality contained magnesia, while those grown in another place did not, but the total oxygen content (and thus by current acid-base theory the saturation capacity) of the potash, lime, and magnesia was in both cases the same. Fir trees grown in different regions also yielded different proportions of the specific alkalies, but the same total, a total which differed from that of the spruce ashes. Liebig thought these uniformities could not be accidental, and he explained them by postulating that the bases served to neutralize the organic and inorganic acids which were characteristic, essential constituents of each type of plant. Liebig drew a last key observation from the sad experience of a farmer near Göttingen who had grown absinthe in his fields to use as a source of potash; he found afterward that he could not raise ordinary crops in the field because the potash of the soil was exhausted. From all these considerations Liebig concluded that the true purpose of a fertilizer is to supply ammonia and such salts as potassium silicate, calcium phosphate, and magnesium phosphate to plants. He wrote Berzelius that a large number of practical applications for farmers could be developed from his results, and he would describe them in a forthcoming book.

Liebig’s letter implied that he had derived his views on agricultural chemistry chiefly from investigations he had carried out himself. No doubt he felt he had, and the observations and calculations he made may well have been decisive in shaping his opinions. Yet in the short time he had devoted to the subject he could not have mastered the whole field of investigation, and the full treatment he published shortly afterward shows that he drew most of his arguments from the work of others and from strongly deductive reasoning. He had ably and impressively reviewed an area of activity that was new to him but that had been pursued with varying intensity for several decades. He joined the scattered conclusions of his predecessors into the most comprehensive picture of the problems of plant nutrition that had ever been presented, using his own findings at certain crucial points to decide between alternative theories.

Liebig’s Die organische Chemie in ihre Anwendung auf Agricultur und Physiologie (Brunswick, 1840; English trans., Organic Chemistry in Its Applications to Agriculture and Physiology [London, 1840]) begins with a discussion of the role of carbon in plant nutrition. He refuted the widely held theory that humus, the product of the decay of plant matter, formed the main nutrient substance for plant growth and supported the view that the source of the carbon assimilated into plant substances is the atmosphere. The stability of the carbonic acid content of the air, despite the continual exhalation of that compound by animals, required that something else must be continually removing it from the atmosphere. Even though the percentage of carbonic acid in the air was very small, the atmosphere contained an ample amount to supply all of the plant material on the surface of the earth. The most important plant function, Liebig asserted, was to separate the carbon and oxygen of carbonic acid, releasing the oxygen and assimilating the carbon into compounds such as sugar, starch, and gum, In a later section he admitted uncertainty as to whether carbon was separated and joined with the elements of water to produce these compounds, or whether the plant in fact separated the oxygen from the water and combined its hydrogen with carbonic acid. He favored the latter alternative, because it was evident that in the formation of waxes, oils, and resins, the hydrogen in excess of the proportions of water must come from the decomposition of water.

Liebig denied the view that the consumption of oxygen and the exhalation of carbonic acid by plants at night constituted a “true respiration.” That transformation was, he thought, a purely chemical decomposition. He believed that the starch, sugar, and gum formed by the primary nutritive process were transported throughout the plant and converted by numerous further metamorphoses into the special constituents of such parts as the flowers and fruit. Liebig’s general picture of plant physiology was not original; he had obtained most of it from the earlier investigations of Priestley, Senebier, Ingen-Housz, and Saussure. Through his knowledge of recent developments in organic chemistry, however, Liebig was able to give many more examples of chemical processes of the kind that might occur in plants. From the elementary compositions of various plant compounds he could calculate the amounts of carbonic acid and water consumed, and of oxygen released, in the formation of a given quantity of each compound. Although he had not investigated directly the chemical processes in plants, he could depict impressively the types of metamorphoses which he supposed took place by likening them to the metamorphoses he had produced in the laboratory. The creation in the laboratory of such important organic compounds as formic acid, oxalic acid, and urea enabled him to predict that vegetative processes would be explained through chemistry. One of his main purposes was to persuade botanists and physiologists that they must pay more attention to chemistry if they were to make further progress.

This glimpse of a chemical understanding of the internal processes of vegetables was, however, not what gave Liebig’s book its most forceful impact. Whatever explanations he gave of the necessity for certain nutrient materials in terms of their role in plant growth, his views on the external sources of the substances he deemed essential were what aroused immediate interest, for these conclusions directly impinged on agricultural practices. The source of hydrogen was no problem, since all agreed that it was water. Nitrogen was a more pressing question. To support his claim that plants are supplied with nitrogen by means of ammonia washed out of the atmosphere, Liebig had to dispose of the fact that the ammonia in the air is so small in amount as to be undetectable, whereas nitrogen gas constitutes about 80 percent of the atmosphere. His confirmation that ammonia exists in rainwater provided him with an important supporting argument, but his conviction was based on his general chemical experience. Atmospheric nitrogen is one of the least reactive of substances, whereas ammonia enters readily into many organic compounds, a large number of which Liebig had investigated. Therefore he considered it most probable that ammonia is also the medium through which nitrogen is introduced into the constitution of plants. His reasoning was typical of the approach which Liebig and other organic chemists brought to the discussion of physiological questions; that is, they expected to predict the behavior of the substances within living organisms from their knowledge of chemical properties of elements and compounds in laboratory reactions. The separation of carbon from oxygen by plants was the one great and mysterious exception to this rule: “a force and capacity for assimilation which we cannot match with an ordinary chemical action, even the most powerful.”

Turning to the inorganic constituents of plants, Liebig elaborated his view that plants need alkalies and alkaline earths to neutralize their essential acids. Analyses of ash contents, he contended, form the basis for determining the requirements of different plants for bases. Because these contents differ from one plant to another, some plants can flourish in soils that will scarcely support others. After finishing his discussion of plant nutrients, Liebig drew the appropriate lessons for agriculture. Cultivation of the same crop year after year gradually diminishes the fertility of the soil as the inorganic constituents essential to the growth of that crop are depleted. Rotation of crops can extend the resources of a given field because one plant can utilize those particular bases that the other did not require, but eventually the soil will become totally infertile unless these elements are returned to it. Allowing land to lie fallow slowly restores its fertility because the weathering of minerals gradually restores the acids and bases. A rational agriculture, however, would sustain fertility more effectively by supplying artificially the necessary elements. Liebig defined fertilizer as the means for adding to the soil the nutritional requirements of crops not supplied naturally from the atmosphere. The composition of a good fertilizer therefore varied according to the ash content of each specific crop. Fertilizers should ordinarily be composed of bases such as lime, potash, and magnesia, but since all plants contain phosphoric acid that compound should be included also. Liebig considered the best source of phosphoric acid to be pulverized animal bones. The bones should be dissolved in sulfuric acid and the phosphoric acid would thus be freed to combine with the bases in the soil. The best available sources of the other inorganic salts were plant ashes and human and animal excrement. But Liebig looked forward to the day when chemical industries would be able to produce salts to make up specific fertilizers for each crop.

Liebig was then rather ambivalent about the need for nitrogen in fertilizers. The ammonia derived from rainwater was sufficient, he argued, for natural plant growth, but plants could convert added ammonia into greater yields of nitrogenous organic compounds, and therefore increase the supply of the important nitrogenous constituents of human and animal diets. Gypsum, calcium chloride, and sulfuric acid were effective in increasing growth because they transferred ammonia from ammonium carbonate to less volatile salts so that the ammonia remained in the soil until absorbed by plants.

Liebig’s book excited an unexpectedly intense interest among practical agriculturalists, especially in England, where his former student, Lyon Playfair, acted as translator and propagandist. Playfair arranged for large-scale tests of Liebig’s ideas on several farms. In America some of his ideas also aroused a lively response. Liebig thus became more deeply involved in the application of his general ideas to actual farming practices. The changing titles that he gave his book symbolized his shifting conception of the scope of the topic with which he was dealing. The work appeared first in French (April 1840) as volume one of a Traite de chimie organique, with the explanation that Liebig was publishing it separately because the material lay outside of chemistry proper. The German edition, four months later, reflected that situation directly in its title. In later editions the adjective “organic” disappeared.

Although he had taken up the subject as an application of organic chemistry, which he regarded as the chemistry of materials of biological origin, the problem had proved to have different boundaries; the analysis of inorganic compounds had in fact become dominant. The main additions Liebig made in the second edition were efforts to specify more concretely the types of fertilizers that would be suitable for particular crops in given localities. Thus he discussed the mineral constituents characterizing the major crops in England as a guide to the selection of inorganic fertilizers. Anticipating the substitution of artificial for natural fertilizers, he suggested a method for converting the ammoniacal liquid left over from industrial production of coal gas into a purified ammonium sulfate. Stressing that knowledge of the composition of soils was the basis for the whole system of rational agriculture, he appended a long list of soil analyses carried out earlier by Kurt Sprengel. In the third edition (1843), Liebig revised his previous opinion that agriculturists should supplement the natural supply of nitrogen by adding or fixing ammonium salts. The addition of nitrogenous fertilizers alone, he said, could not augment the fertility of a field, for its productivity increased or decreased in direct proportion to the mineral nutrients provided in its fertilizer. In keeping with his greater emphasis on the mineral requirements of plants, he added analyses performed in his laboratory of the ash contents of various crops.

In 1845 Liebig took another step toward the practical implementation of his theories; he devised instructions for making artificial fertilizers formulated entirely of mineral salts that were combined in proportions corresponding to the ash contents of various crops. Muspratt and Co. of Liverpool put them into production. The venture proved a severe setback for Liebig’s views, for nowhere did the fertilizers cause increased yields sufficient to cover costs. Surprised and dismayed, Liebig purchased a plot of sterile grazing land outside Giessen and began to investigate why his fertilizer was ineffective. Between 1845 and 1849 he managed to convert the plot into a fertile field, but he remained puzzled by the slowness of the action of the fertilizer. Not until over a decade later did he identify the cause of the difficulty. In order to prevent the potash from being washed out of the soil, he had fused it with calcium carbonate into a highly insoluble combination, and its insolubility had prevented the plant roots from absorbing the alkali. By this time he realized that he need not have worried about soluble salts being removed by rainwater, because the topsoil itself absorbed and held them. Meanwhile the error had threatened to discredit the general principles he had espoused.

At Rothamsted, in England, Lawes and Gilbert tested one of Liebig’s fertilizers on wheat and found no noticeable increase in production, whereas ammonium salts added annually brought about significant improvements year after year in the harvests. Lawes and Gilbert regarded these results as a refutation of Liebig’s entire “mineral theory,” and other agriculturists came to regard it as a hastily conceived mistake. The success with guano, a substance rich in ammonia, seemed further to contradict Liebig’s views. Liebig defended himself ably, however, in a series of polemic tracts published during the 1850’s. Lawes and Gilbert’s experiments were not decisive, he argued, because they had conducted their investigation under conditions which precluded a fair test of his fertilizer. From their description of the fields they had used, Liebig inferred that the soil already contained such ample quantities of the essential minerals that the provision of more thorough fertilizers could not significantly affect its productivity. Furthermore, he asserted that the English experimenters had misrepresented him, for he had not claimed that agricultural yields are dependent solely on the addition of mineral constituents to the soil, nor that it is never useful to add ammonia. He had only stated that in most cases it is superfluous to supplement the natural supply of ammonia and that fertilizers cannot be evaluated by their nitrogen contents. The current fashion for fertilizing with nitrogenous salts, he believed, would simply cause the mineral stores of the soil to be more rapidly exploited by quicker plant growth; if not supplemented by proportionate quantities of mineral fertilizers that practice would only deplete the soil sooner. Farmers, he contended, must think beyond the objective of obtaining the largest possible crop in a given year. In order to protect their capital, they must preserve the productivity of their land for future years by restoring to it all that their harvests removed. Nitrogen is replenished from the atmosphere but minerals come from the soil alone, and therefore it was the minerals with which they must be chiefly concerned.

While defending his theories, Liebig also clarified them. The discrepant effects of fertilizers in different situations led him to perceive that the addition of any one constituent to a field has value only in relation to the availability of the other required constituents of a given crop. If one is lacking, even though all others are in excess, plants will still not thrive. The addition of a single constituent will increase the crop only if a particular soil can deliver the other necessary constituents in greater quantities as well. Depending on the circumstances, therefore, any of the essential minerals might become the controlling factor. This generalization became known as Liebig’s “Law of the Minimum.”

In 1862 Liebig completed an expanded seventh edition of his Chemistry in Its Applications to Agriculture and Physiology; the sixth edition had appeared in 1847. The new book was the most comprehensive statement of his views, backed with the most extensive analytical and field data, and it was also his last major scientific endeavor. His theories remained controversial and vulnerable. Some of the issues dividing him from his critics could not be settled at the time, for the question of the source of nitrogen depended on phenomena they did not yet understand. Liebig’s role in agricultural chemistry was far different from his earlier role in organic chemistry. In both areas he displayed an acute perception of central issues and was a powerful advocate for his own opinions; but in organic chemistry he had also been the most distinguished experimentalist of his time. He could support his theoretical positions with analyses he knew to be more reliable than those of most of the chemists who disagreed. In agricultural chemistry he did carry out important analyses of plant composition and tested fertilizers on a small scale, but he was aware that for the large-scale investigations which would decide the key questions he was dependent on other people. He could act only as catalyst and critic, providing the chemical principles and the theory which he claimed would lead agriculturists to achieve significant investigations. When these agriculturists reached conclusions which appeared to contradict his views, he could not improve on their experiments, but only complain that they had failed to follow his reasoning. He could not direct the emergence of a new science from the inside, as he had done in organic chemistry, but only draw on the influence he had attained through his earlier leadership to induce others to follow the program he had laid out.

In agricultural chemistry, Liebig also indulged in bolder speculative conclusions than he had done in his previous investigations. In organic chemistry his deep knowledge of the properties and reactions of compounds had helped to control his theoretical bent. He acquired his knowledge of agriculture more hastily and largely at secondhand, and his reasoning from chemical principles was less checked by anticipations of complicating physiological or physiographical factors. From the rebuffs of his initial views, he learned to be a good deal more cautious. Yet he justly claimed that even though he had made mistakes, even though there were in the first edition of his book the most extraordinary inconsistencies and the greatest disorder, its appearance had completely changed the nature of the problem of scientific agriculture. Before 1840 it was generally believed that both plant and animal life were dependent on the circulation of an organic, previously living material. Now, whatever opinion individuals held on specific points, they agreed that the nutrient substances of plants were inorganic. That change had transformed the objectives of agriculture, for under the older conception the potential production of foodstuffs would seem to have a fixed limit, whereas in the new view an unbounded increase in organic life appeared possible.

Liebig’s students and followers provided much of the means for the rigorous scientific study of agriculture that he envisioned, as they began to set up experiment stations in Europe and the United States. In that way they gradually removed the questions involved from the control of the scientifically unsophisticated farmers who had caused Liebig so much trouble. His abrupt change of subjects in 1840 thus marked a crucial step in the emergence of modern scientific agriculture.

Physiology . After finishing his first treatise on agricultural chemistry in 1840, Liebig entered with equal intensity into the study of animal chemistry. He brought to this topic the same confidence that his knowledge of the chemical properties of organic compounds would enable him to infer the transformations occurring within living organisms; the application of quantitative chemical methods would solve the problems that physiologists had failed to solve. He had already become interested in nutrition in 1838 when G. J. Mulder found that the elementary compositions of plant albumin, animal fibrin, casein, and albumin were identical. During the next three years Liebig’s students verified and extended Mulder’s results. From that basis Liebig concluded that animals receive the chief constituents of their blood already formed in their nutrients. He developed the idea that the nitrogenous plant substances were assimilated in the blood and organized tissues of animals, while starch, sugar, and other nonnitrogenous compounds were consumed in respiration. During 1841 and 1842 Liebig expressed his physiological ideas in three articles, then elaborated them more extensively in his book Die Thierchemie oder die organische Chemie in ihrer Anwendung auf Physiologie und Pathologie (Brunswick, 1842; English trans., Animal Chemistry or Organic Chemistry in Its Applications to Physiology and Pathology [London, 1842]). He maintained that animal heat is produced solely by the oxidation to carbon dioxide and water of the carbon and hydrogen of the nutrient compounds. The idea dated from Lavoisier’s experiments on combustion and respiration, but during the intervening years some physiologists had proposed other views, and the most thorough experimental investigations undertaken had cast some doubt on it. Liebig had no new evidence to prove Lavoisier correct, but from his understanding of chemical reactions it seemed to him that there was no other possibility. Liebig broadened the combustion theory by perceiving that the respiratory oxidations were net reactions, encompassing the cumulative results of all of the transformations that nutrients may undergo while in the body. He also examined the mutual proportionalities between the internal nutritive metamorphoses within an animal, its respiratory exchanges, intake of food, excretions, and production of heat or work.

Beyond establishing such general principles, Liebig attempted to deduce the actual chemical transformations which the three classes of nutrients undergo within the body; and in order to depict these more concretely, he constructed hypothetical chemical equations similar in form to those he and his colleagues had been using to interpret the reactions of organic compounds in their laboratories.

Liebig’s Animal Chemistry aroused sharply divergent reactions. Some accepted it uncritically as a revelation of the true inner workings of animals and humans. Others were so antagonized by its speculative excesses that they refused to credit the important insights it contained. Even those who reacted against it, however, began to view the chemical phenomena of life differently than they had before, for Liebig had provided one of the first comprehensive pictures of the overall meaning of the ceaseless chemical exchanges which form an integral part of the vital processes. A few men adopted it as a guide for further investigation, and Bischoff and Voit based the field of energy metabolism on Liebig’s ideas. As with his agricultural chemistry, Liebig’s physiological writings provided an impetus which outlasted the refutation of some of his specific theories.

By transferring his investigations into agriculture and animal chemistry, Liebig had hoped to avoid further disputes with his old rival Dumas, but to his dismay Dumas’s interests were moving in the same direction. In August 1841 Dumas published a lecture which expressed nutritional ideas similar in general outline to those Liebig had already presented in his agricultural chemistry, and also to those on animal chemistry which he had begun to teach in Giessen. Liebig now accused Dumas of plagiarism, a charge that could only exacerbate the earlier bitterness between them. Over the following years Liebig engaged Dumas and his associates Boussingault and Payen in a debate over the most prominent difference between their respective physiological views, Liebig had asserted that animals can convert dietary starch into fats, whereas the French chemists insisted that the source of all animal constituents, including fats, is in plant nutrients. The French chemists carried out extensive feeding experiments on bees, geese, hogs, and cattle, which were expected to demonstrate the sufficiency of dietary fat, but which instead proved Liebig to be correct. The outcome enhanced the prestige of Liebig’s physiological views.

Industrial Chemistry . Liebig did not directly involve himself in industrial chemistry as he did in agricultural chemistry, but his indirect influence was nearly as great. Many of his students founded or worked in the new large-scale chemical factories, and a number of the analytical methods he developed for research purposes found important industrial applications. Most decisive, however, was his effect on the dye industry. In 1843 Ernest Sell, a former student who had recently set up a plant to distill coal tar, sent Liebig a sample of a light oil which he had produced. Liebig asked one of his assistants, August Wilhelm Hofmann, to analyze the oil. Hofmann separated two organic bases, one of which, afterward named aniline, reacted with concentrated nitric acid to give a deep blue fluid that became yellow and then deep scarlet when heated. Liebig, who had already been interested in colored organic compounds as potential sources of dyes, predicted that aniline would have important effects on the dye industry. At first, Hofmann was more interested in using the new substance to help solve a theoretical question previously raised by Liebig: organic bases contain a hypothetical compound, amid (NH2), combined with an organic radical, so that the radical substituted for one of the hydrogens of ammonia. Hofmann, who had found by the usual elementary analyses that aniline has the formula C12H7N, interpreted its composition in the light of Liebig’s view, as

Hofmann then began to try to substitute other organic radicals for the remaining hydrogen atoms. He followed this general approach over the next two decades, and it eventually yielded many of the compounds which formed the basis for the synthetic dye industry that burgeoned after 1860. Thus Liebig provided the institutional, analytical, and conceptual framework within which Hofmann began the study that helped give birth to the first industry based entirely on the application of the results of systematic scientific research.

In 1839 Liebig advanced the theory that fermentation is caused by the decomposition of a nitrogenous material, the reaction of which evokes a similar decomposition in another compound such as sugar. Although formulated mainly because of his dissatisfaction with Berzelius’ definition of catalysis, which in Liebig’s view provided no explanation for the phenomena it designated, the theory placed Liebig in the middle of the debate over whether fermentation is a vital or a purely chemical process. Ultimately his position drew him into a famous controversy with Pasteur, who defended with great resourcefulness the belief that living microorganisms are essential for fermentation.

In 1852 Liebig left Giessen for Munich. His old laboratory had continued to attract students from all over Europe, and increasingly from the United States, but he himself had grown weary of the unremitting burdens of his teaching. In Munich he had an institute built to his specifications, with better research facilities than in Giessen and a large lecture hall. He no longer carried on a formal program of instruction, but allowed only a few selected assistants and students to work in his laboratory. He himself continued to do some experiments, mostly following up certain aspects of his earlier research; but he devoted most of his time to writing, especially in support of his agricultural ideas. In the livelier milieu of Munich he also became more involved in society than he had been. He instituted a popular series of evening talks on scientific topics of broad interest. Although he remained quite active time until his illnesses, it was for Liebig a more quiet time than those twenty-eight hectic years when he was one of the most formidable figures of a formidable scientific age.


The articles by Liebig and his contemporaries on which this text is based are mostly found in Poggendorff’s Annalen der Physik and Chemie, Liebig’s Annalen der Pharmacie,and the Annales de Chimie et de Physique. Indispensable sources are Berzelius and Liebig Ihre Briefe von 1831-1845, Justus Carrière, ed. (Munich, 1893;repre. Wiesbaden, 1967); and Aus Justus Leibig’s and Friedrich Wöhlers’s Briefwechsel in der Jahren 1829-1873, A.W. Hofmann, ed. (Brunswick, 1888). The correspondence of Berzelius and Pelouze in Jac.Berzelius Berzelius Brev, H. G. Soderbaum, ed. (Uppsala,1941), supp.2, contains useful references to Liebig.

Convenient descriptions of the analytical methods used in Liebig’s laboratory are Justus Liebig, Instructions for the Chemical Analysis of Organic Bodies, William Gregory, tr. (Glasgow, 1839), and Henry Will, Outlines of the Course of Qualitative Analysis Followed in the Giessen Laboratory(London, 1841).

Carlo Paoloni,Justus von Liebig. Eine Bibliographic sämtlicher Veroffentlichungen (Heuidelberg: Carl Winter, 1968), contains a full bibilography of Liebig’s articles and books and published collections of letters, and a list of secondary literature concerning Liebig.

The most important collections of MSS of Liebig are in the Bibliothek der Liebig-Museum-Gesellschaft and the Universitatsarchiv of the Justus Liebig Universitat in Giessen, which contain especially documents concerning Liebig’s laboratory; and the Bayerische Staatsbibliothek in Munich, which has an extensive collection of correspondence.

The discussions in the present article based on unpublished letters between Liebig, Pelouze, and Dumas are derived from the following sources: the letters from Liebig to Pelouze and to Dumas are preserved in the Dumas dossier at the Archives of the Academic des Sciences in Paris; letters to Liebig from Pelouze and from Dumas are in the Bayerische Staatsbibliothek. I am grateful to these two institutions for making available to me microfilms of these documents.

Liebig’s laboratory in Giessen is preserved as a museum in the state in which it existed after the additions of 1839.

The most extensive biography of Liebig is Jakob Volhard, Justus Von Liebig (Leipzig, 1909). A.W. Hofmann, The Life-Work of Liebig, Faraday lecture for 1875 (London, 1876), gives a laudatory but valuable discussion of Liebig’s personality and contributions to organic chemistry. Many of the chemical investigations discussed above are summarized in J.R. Partington, A History of Chemistry,IV (London, 1964). For Liebig’s agricultural chemistry, see F.R. Moulton, ed.,Liebig and After Liebig:a Century of Progress in Agricultural Chemistry, Publications of the American Association for the Advancement of Science, no. 16 (Washington, 1942); and Margaret W. Rossiter, Justus Liebig and the Americans: a Study in the Transit of Science, 1840-1880 (unpub. diss., Yale Univ., 1971).

For Liebig’s animal chemistry, see F. L. Holmes, “Introduction,”, to Justus Liebig, Animal Chemistry(New York, 1964); and Claude Bernard and Animal Chemistry (Cambridge, Mass., 1974); and Timothy O. Lipman, “Vitalism and Reductionism in Liebig’s Physiological Thought” in Isis,58 (1967), 167-185.

Liebig’s role in the controversies over fermentation is treated in Joseph S. Fruton, Molecules and Life(New York, 1972). Liebig’s laboratory at Giessen is discussed most recently in J.B. Morrell, “The Chemist Breeders: the Research Schools of Liebig and Thomas Thomson”, in Ambix,19 (1972), 1-47. For Liebig’s influence on the dye industry, see John J. Beer, The Emergence of the German Dye Industry(Urbana, Ill.,1959).

F. L. Holmes

Liebig, Justus von

views updated Jun 08 2018


(b. Darmstadt, Grand Duchy of Hesse-Darmstadt, 12 May 1803; d. Munich, Germany, 18 April 1873),

chemistry. For the original article on Liebig see DSB, vol. 8.

Liebig’s life encompassed innovation in teaching, important contributions to organic chemistry and, above all, the significant application of chemistry to agriculture, physiology, medicine, nutrition, and industry, as well as to the popularization of chemistry. He has attracted considerable attention since Frederic L. Holmes’s fine article was published in 1973. Historical interest has been concentrated on the publication of critical editions of Liebig’s extensive correspondence with other chemists and pharmacists, his publishers, and the chancellor of the University of Giessen; the development of a deeper understanding of his role in organic chemistry, especially through his improvement of analytical techniques and the use of chemical formulas; the enrichment of knowledge concerning Liebig’s methods for training chemists and the influence of the Giessen school on the international development of laboratories and research schools; the investigation of the worldwide impact of the publication of his writings on agricultural and physiological chemistry; and the role of his Chemische Briefe in the popularization of chemistry. While Jakob Volhard’s two-volume German biography of Liebig (1909) remains the essential introduction, there is now a substantial English and German biography by William H. Brock (1997) that examines the controversial nature of Liebig’s opinions and character, portraying him as a gatekeeper who helped to transform public and governmental awareness of chemistry and its essential role in a modern society.

Liebig became a world celebrity during his lifetime and was one of the most significant nineteenth-century scientists in shaping an international vision of science. For Liebig, science was a body of knowledge that ignored and transcended national boundaries. What could be said of German or British agriculture applied equally to that in Italy, America, or Japan. He wanted to universalize the localized nature of knowledge and practice and did this by forging educational tools, the exchange of information and research, and the popularization of chemical knowledge. One vital aspect of his determination to communicate the central significance of chemistry was his use of Roman instead of Gothic (Fraktur) type in his monthly periodical Annalen der Chemie and in the German editions of his books, knowing that this typography made it easier for foreign readers (for whom Fraktur was as unintelligible as Greek or Cyrillic print). Translation of his work was another important means of communication, and he did everything possible to encourage it. For example, Die organische Chemie in ihrer Anwendung auf Agricultur und Physiologie (1840) appeared in at least nineteen editions in about nine different languages. Such internationalism reached its long-term fulfillment in the twentieth century when Liebigs Annalen der Chemie was amalgamated into the European Journal of Organic Chemistry in 1998.

Liebig’s fame was not so much that he made a startling new chemical discovery. It was largely due to his demonstration with Friedrich Wöhler that it was possible to use the paper tools of Berzelian chemical symbols to make sense of analytical results by inspecting and juggling with the compositions of reactants and products. As Ursula Klein (2003) has shown, the 1830s produced novel ways of individuating, identifying, and classifying organic compounds, chief of which was the exploitation of Jacob Berzelius’s chemical formulas and their manipulation on paper in an attempt to understand the composition of the dazzling parade of new derivatives that were totally

unknown in nature. Liebig, Wöhler, and Jean-Baptiste Dumas excelled at this practice and were considerably helped by the sophisticated method of organic analysis with the so-called Kaliapparat that Liebig developed in 1830 when attempting to understand the composition of plant alkaloids. Replication of these gravimetric experiments by Melvyn C. Usselman and others (2005) has shown historians how accurate Liebig’s results were for carbon, hydrogen, and oxygen content (though nitrogen content remained an acute problem). This development of a rapid and accurate method of gravimetric organic analysis using the Kaliapparat acted as a trigger for the explosion of organic (as opposed to inorganic) chemistry. These two techniques, paper chemistry and accurate compositional analysis, forged a new ontology of carbon chemistry, as opposed to the tradition of vegetable and animal chemistry, and enabled chemists to classify and interpret analyses in terms of common groups or radicals and, later, in terms of “chemical types.”

The “Giessen Model.” . However, Liebig’s contribution to the perfection of inorganic analysis and its dissemination must not be underestimated. Building upon the long historical tradition of tests for mineralogical composition, at Giessen he taught systematic methods of inorganic analysis, though he left it to pupils and assistants such as Carl Fresenius and Heinrich Will to publish these methods in the 1830s. These systematic group separation methods (wet qualitative and quantitative analysis) were taught to every student of practical chemistry into the 1950s.

The fame and celebrity status that Liebig sought as a young man came about through teaching these systematic methods of inorganic and organic analysis. Beginning with a majority of pharmacy students, he successfully attracted an international body of chemistry students to Giessen where, from 1835 until he left for Munich in 1852, he engaged in line-production research investigating the chemistry of living systems of plants and animals. Whether Giessen was the model for future research schools has been the subject of great historical interest since the publication of Jack Morrell’s heuristic model in 1972. Joseph S. Fruton (1990) has contrasted Liebig’s style of research leadership with other chemists and biochemists and has also provided a valuable list of the majority of the active researchers who studied with him at Giessen or Munich. Alan J. Rocke (2003) suggests that Liebig was able to establish something new and unique in chemical education by exploiting isomerism, the use of formulas to understand composition, and the Kaliapparat to ensure accurate quantitative compositions. This may explain why and how Liebig’s “Giessen model” spread rapidly far and wide. Liebig himself bombastically advertised the Giessen method in his polemic against the Prussian government in 1840. His letters to his publisher Friedrich Vieweg demonstrate his control of the monthly Annalen der Pharmacie and the ambitious nature of his own book publication program, as well as his ambition to publish German translations of important English cultural works by Charles Darwin and John Stuart Mill. Other letters to Justin von Linde, the Catholic chancellor of the University of Giessen, are revelatory in two respects. On the one hand, they demonstrate Liebig’s inexhaustible dedication to chemistry, and on the other, his determination to promote his university as a leading European institution of scientific learning. Finally, his correspondence with Georg von Cotta, the aristocratic owner of the Augsberger Allgemeinen Zeiting, reveals Liebig’s ambitions as a popular writer and his determination to promote chemistry as the central science for economic prosperity through his Chemische Briefe from 1844 onward.


Carlo Paoloni, Justus von Liebig. Eine Bibliographie sämtlicher Veröffentlichungen (Heidelberg, Germany: Carl Winter, 1968), the standard bibliography cited by Holmes, is not entirely reliable. A good guide to the literature up to 1996 is found in William H. Brock, Justus von Liebig: The Chemical Gatekeeper (Cambridge, U.K. and New York: Cambridge University Press, 1997; paperback, 2002); German translation Justus von Liebig: Eine Biographie des großen Wissenschaftlers und Europäers (Braunschweig and Wiesbaden, Germany: Vieweg, 1999). The following bibliography lists primary sources published since the 1973 entry, but, with some exceptions, only secondary works published since 1996.


Animal Chemistry (New York, 1842). Edited by Frederic L. Holmes. New York: Johnson Reprint, 1964. Holmes’s introduction, pp. vii–cxvi. For a German translation of Holmes’s essay, see Büttner and Lewicki (2001), cited below, pp. 1–107.

Die Organische Chemie in ihrer Anwendung auf Physiologie und Pathologie (Braunschweig, 1842). Facsimile edition with appendix containing reprints of essays on the history of physiological chemistry, edited by Wilhelm Lewicki. Pinneberg, Germany: AgriMedia Verlag Alfred Strothe, 1992.

Die Chemie in ihrer Anwendung auf Agricultur und Physiologie, 9th posthumous edition by Philipp Zöller (1876), facsimile in 2 vols. Holm, Germany: Agrimedia, 1995. Issued with a supplementary volume, edited by Wilhelm Lewicki, containing reprints of essays on the history of agricultural chemistry.

Aus Justus Liebig’s und Friedrich Wöhler’s Briefwechsel in den Jahren 1829–1873. 2 vols. Edited by August Wilhelm Hofmann. Braunschweig, Germany: Friedrich Vieweg, 1888. Reprint edited by Wilhelm Lewicki. Göttingen, Germany: Jürgen Cromm, 1982. Hofmann’s edition is an uncritical and massively expurgated extracts of the whole correspondence. A complete critical edition of the surviving 1,700 letters is in production at the University of Regensburg, edited by Christoph Meinel and Thomas Steinhauser.

Berzelius und Liebig: Ihre Briefe von 1831–1845. Edited by Justus Carrière. Munich, Germany: J.F. Lehmann, 1893; 2nd ed., 1898. Reprint edited by Till Reschke. Göttingen, Germany: Jürgen Cromm, 1978.

Justus von Liebig “Hochwohlgeborner Freyherr”: Die Briefe an Georg von Cotta und die anonymen Beiträge zur Augsburger Allgemeinen Zeitung. Edited by Andreas Kleinert. Mannheim, Germany: Bionomica-Verlag, 1979.

Liebigs Experimentalvorlesung. Vorlesungsbuch und Kekulés Mitschrift. Edited by Otto Paul Krätz and Claus Priesner. Weinheim, Germany: Verlag Chemie, 1983. Facsimiles and transcriptions of Liebig’s Giessen lectures on organic chemistry, together with Kekulé’s student notes of 1848 and valuable commentaries.

Justus von Liebig und August Wilhelm Hofmann in ihren Briefen (1841–1873). Edited by William Hodson Brock. Weinheim, Germany: Verlag Chemie, 1984. With English abstracts; note supplement by Heuser and Zott below (1988).

Justus von Liebig. Briefe an Vieweg. Edited by Margarete Schneider and Wolfgang Schneider. Braunschweig and Wiesbaden, Germany: Vieweg & Sohn, 1986. Liebig’s letters to his publisher, 1823–1872.

The Letters from Gerrit Jan Mulder to Justus Liebig (1838–1846). Edited by Harry A. M. Snelders. Amsterdam: Rodopi, 1986.

Justus Liebig und Julius Eugen Schlossberger in ihren Briefen von

1844–1860. Edited by Fritz Heße and Emil Heuser, eds. Mannheim, Germany: Bionomica-Verlag, 1988.

Justus von Liebig und August Wilhelm Hofmann in ihren Briefen, Nachträge 1845–1869; Justus von Liebig und Emil Erlenmeyer in ihren Briefen von 1861–1872. Edited by Emil Heuser and Regine Zott. Mannheim, Germany: Bionomica-Verlag, 1988. The Hofmann-Liebig letters supplement the edition by Brock.

Justus von Liebig und der Pharmazeut Friedrich Julius Otto in ihren Briefen von 1838–1840 und 1856–1867. Edited by Emil Heuser. Mannheim, Germany: Bionomica-Verlag, 1989.

Die Nachlässe von Martius, Liebig und den Brüdern Schlagintweit in der Bayerischen Staatsbibliothek. Edited by Anne Büchler and Rolf Schumacher. Wiesbaden, Germany: Otto Harrassowitz, 1990. Lists Liebig’s archives in Munich.

Universität und Ministerium im Vormärz: Justus Liebigs Briefwechsel mit Justin von Linde. Edited by Eva-Marie Felschow and Emil Heuser. Giessen, Germany: Verlag der Ferber’schen Universitäts-Buchhandlung Giessen, 1992.

Die streitbaren Gelehrten: Justus Liebig und die preußischen Universitäten. Edited by Regine Zott and Emil Heuser. Berlin: ERS-Verlag, 1992. Documents the controversy arising from Liebig’s polemic concerning the state of science teaching in Prussia.

Justus von Leibig and Hermann Kolbe in ihren Briefen, 1846–1873. Edited by Alan J. Rocke and Emil Heuser. Mannheim, Germany: Bionomica Verlag, 1994.

Kleine Schriften. Edited by Hans-Werner Schütt. Hildesheim, Germany, Zürich, and New York: Olms-Weidmann, 2000. A reprint of Liebig’s papers from Annalen der Chemie und Pharmacie.

Justus Liebig in Grossbritannien: Justus Liebigs Briefe aus Grossbritannien an seine Frau Henriette. Edited by Günther Klaus Judel. Giessen: Liebig-Gesellschaft, 2003.


Berichte der Justus Liebig-Gesellschaft zu Giessen, vols. 1 (1990) to date. Giessen, Germany: Justus Liebig-Gesellschaft.

Billig, Christine. Pharmazie und Pharmaziestudium an der Universität Giessen. Stuttgart, Germany: Wissenschafttliche Verlagsgesellschaft, 1994. Liebig’s pharmaceutical teaching.

Brock, William H. “Breeding Chemists in Giessen.” Ambix 50 (2003): 25–70.

Büttner, Johannes, and Wilhelm Lewicki, eds. Stoffwechsel im tierischen Organismus: Historische Studien zu Liebigs, “ThierChemie.” Seesen, Germany: HisChymia Buchverlag, 2001.

Finlay, Mark R. “Justus von Liebig and the Internationalization of Science.” Berichte der Justus Liebig-Gesellschaft 4 (1998): 57–76.

Fruton, Joseph S. Contrasts in Scientific Style: Research Groups in the Chemical and Biochemical Sciences. Philadelphia: American Philosophical Society, 1990. Pages 277–307 contain comprehensive lists of Liebig’s students. Reprint in Büttner & Lewicki (2001), pp. 373–412.

Heilenz, Siegfried. Eine Führung durch das Liebig-Museum in Giessen. Giessen: Verlag Liebig-Gesellschaft, 1994. An illustrated guide to Liebig’s laboratory.

Holmes, Frederic L. “The Complementarity of Teaching and Research in Liebig’s Laboratory.” Osiris 5 (1989): 121–164.

———. “Justus Liebig and the Construction of Organic Chemistry.” In Chemical Sciences in the Modern World, edited by Seymour H. Mauskopf, 119–134. Philadelphia: University of Pennsylvania Press, 1993. Hormuth, Stefan, ed. Justus Liebig: Seine Zeit und Unsere Zeit; Der streitbare Gelehrte; Die “Chemischen Briefe.” 3 vols. Giessen: Justus Liebig-Universität, 2003. Three scholarly illustrated exhibition catalogs celebrating the 200th anniversary of Liebig’s birth.

Jaschke, Brigitte. Ideen und Naturwissenschaf:. Wechselwirkungen zwischen Chemie und Philosophie am Beispiel des Justus von Liebig und Moriz Carrière. Stuttgart, Germany: Chemisches Institut der Universität Stuttgart, 1996. Carrière was Liebig’s son-in-law.

Kirschke, Martin. Liebigs Lehrer Karl W. G. Kastner (1783–1857): Eine Professorenkarriere in Zeiten naturwissenschaftlichen Umbruchs. Berlin-Diepholz: GNT Verlag, 2001.

Klein, Ursula. Experiments, Models, Paper Tools: Cultures of Organic Chemistry in the Nineteenth Century. Stanford, CA: Stanford University Press, 2003. For Liebig’s exploitation of chemical formulas

———. “Contexts and Limits of Lavoisier’s Analytical Plant Chemistry: Plant Materials and Their Classification.” Ambix 52 (2005): 107–158. Discusses the assimilation of vegetable chemistry into organic chemistry in the 1830s.

Meimberg, Paul, ed. Justus v. Liebig 1803–1873, special issue of Gießener Universitätsblätter, Jahrgang 6 (April 1973). Contains valuable essays and list of archives in Liebig Museum.

Munday, Pat. “‘Politics by Other Means’: Liebig and Mill.” British Journal for the History of Science 31 (1998): 403–418. On Liebig’s promotion of a translation of J. S. Mill’s Logic.

Rocke, Alan J. “Origins and Spread of the ‘Giessen Model’ in University Science.” Ambix 50 (2003): 90–115.

Schwedt, Georg. Liebig und seiner Schüler: Die neue Schule der Chemie. Berlin: Springer, 2002. A popular illustrated study.

Strube, Wilhelm. Justus Liebig: Eine Biographie. Beucha, Germany: Sax-Verlag, 1998. A short biography.

Usselman, Melvyn C. “Liebig’s Alkaloid Analyses: The Uncertain Route from Elemental Content to Molecular Formulae.” Ambix 50 (2003): 71–89.

Usselman, Melvyn C., Alan J. Rocke, Christine Reinhart, et al. “Restaging Liebig: A Study in the Replication of Experiments.” Annals of Science 62 (2005): 1–55. A study of the Kaliapparat.

Werner, Petra, and Frederick L. Holmes. “Liebig and the Plant Physiologists.” Journal of the History of Biology 35 (2002): 421–441.

W. H. Brock

Liebig, Justus von

views updated May 14 2018

Liebig, Justus von


Justus von Liebig, one of the founders of modern chemistry, was born on May 12, 1803, in Darmstadt, Hesse, Germany. His father was a manufacturer of drugs and paints. As an adolescent Liebig performed many experiments using materials from his father's business while neglecting other studies. He was apprenticed to an apothecary at age fifteen; however, his real interest was chemistry. He enrolled at the University of Bonn in 1820 to attend the lectures of Wilhelm Kastner. When Kastner left for the University of Erlangen, Liebig followed him there and received his doctoral degree in 1822 after only two years of study.

Liebig soon realized that his knowledge of chemistry was deficient and using the patronage of the grand duke of Hesse was able to study in Paris from 1822 to 1824. Paris was the leading center for the study of chemistry at this time, and here Liebig was able to attend the lectures of such famous chemists as Joseph Gay-Lussac, Louis Thénard, and Pierre Dulong. Parisian chemistry stressed a rigorous and quantitative approach that was lacking in European chemistry. In Paris Liebig had the opportunity to work in the laboratory of Gay-Lussac, where he acquired skills in the elemental analysis of inorganic and organic compounds, as well as in the systematic methodology of chemical research.

In 1825 Liebig returned to Germany and was offered the professorship of chemistry at the University of Geissen. He stayed in Geissen until 1851, at which time he was called to the chair of chemistry at the University of Munich where he remained until his death on April 18, 1873.

In Geissen Liebig drawing from his studies in Paris established the model for chemical education that was soon copied by other German educators. His students learned by working in the laboratory with their mentor, starting with simple procedures, working their way through more complex exercises, and finally graduating to their own independent research.

One of Liebig's many achievements at Geissen was the development of more efficient combustion techniques for the elemental analysis of organic compounds. The impetus for this was his inability to get a good result in his analysis of a compound he had isolated from urine (which he had named hippuric acid) using the conventional methods that were available to him. These methods, which were tedious and time-consuming, included the use of very small amounts of a given sample, which amplified the experimental error. In 1830 Liebig devised a technique that allowed the use of larger samples and he was able to quantify the amounts of carbon and hydrogen in organic compounds. The trapping of the gaseous combustion products water vapor and carbon dioxide on pre-weighed absorbents was part of his technique. This procedure greatly reduced error and was simple enough so that Liebig's students were able to analyze all types of organic compounds almost routinely, which greatly enhanced the existing knowledge of the variety of organic compounds in nature.

Liebig pioneered methods for the analysis of nitrogen, sulfur, and halogens in organic compounds, in addition to his contributions to the analysis of carbon and hydrogen in these compounds. Liebig was the founding editor of one of the first chemical journals, Annalen der chemie in 1832.

see also Gay-Lussac Joseph-Louis; Organic Chemistry; Proteins.

Martin D. Saltzman


Brock, W. H. (1997). Justus von Liebig: The Chemical Gatekeeper. New York: Cambridge University Press.

Liebig, Justus von

views updated May 18 2018

Liebig, Justus von (1803–1873) German chemist; pioneered physiological chemistry; published Tieren Chemie in 1842; mistakenly concluded that protein provided the energy for muscle work (see also Fick, adolf; Wislicenus, johannes).