The German Genius
Page 33
Berzelius and other older chemists never really came to grips with organic substitution reactions, and the next moves were made mainly by French, Alsatian, and German chemists who gravitated to von Liebig’s Giessen laboratory. In this environment the structures and the properties of the aromatic compounds were gradually isolated, until the fundamental reality was grasped: organic chemistry is very largely the chemistry of “functional groups”—a term not yet invented—attached to a relatively inert hydrocarbon skeleton. Charles Frédéric Gerhardt, a Swiss brought up in Alsace, who studied under von Liebig at Giessen, was the first to understand how structure and function were related.15 For example:
Brilliant as this insight was, this picture concealed a more complex—but more fundamental—truth. What was behind this concept was something not discovered until the 1860s: valency.16 Valency, in everyday language, is “the combining power of one atom for another”—in a way, the number of “hooks” an atom has available to join it to its neighbors. By the 1850s, water was identified as H2O (hydrogen monovalent, oxygen bivalent), but the behavior of carbon was still perplexing, since methane was CH4, ethane was C2H6, ethylene was C2H4, and acetylene C2H2. Is the valency of carbon, 4, 3, 2, or 1? The answer, eventually, was found to be four, and what accounts for the difficulty nineteenth-century chemists faced is that carbon atoms form chains and rings with each other.
Once this phenomenon had been discovered, organic chemistry gave up more of its structural secrets. Here are some modern structural formulae, where “R” equals “radical,” the simplest of which is “methyl”:
The man who did more than anyone else to explain the operating principles of organic chemistry was August Kekulé. However, the circumstances of his various “discoveries” were controversial, and even after all this time they still divide historians of science.
He was born on September 7, 1829, in Darmstadt, where von Liebig had been born a generation earlier; Kekulé sounds and looks as if it is French, but in fact the family was originally Bohemian nobility.17 August studied architecture at Giessen, but fell under von Liebig’s spell and switched to chemistry. Later he argued that his architectural training (such as it was) had helped him to think in pictures—and this played a vital role when he came to identify the structure of carbon compounds.
In 1854 he visited London and there, one summer evening, he made the first of his controversial claims. He said he had an important dream. These dreams aroused suspicion because, by their very nature, they could not be corroborated, and other scientists suspected Kekulé invented them to establish his own priority in his various claims to have identified the structure of organic substances. To give some idea of the controversy aroused, Archibald Scott Couper had written his first paper on organic bonds in 1858. Kekulé, however, said he had his dream about the same phenomenon in 1854 but didn’t say so until 1890.
Organic chemistry may have had a difficult birth, but once the nature of the benzene ring was understood, the relatives of benzene—naphthalene, toluene, phenol (carbolic acid), cresols—soon became available on a vast scale, extracted from coal tar, producing a vast range of wealth-generating products: aniline dyes, trinitrotoluene, carbolic soap, creosote, naphthalene mothballs—the list is impressively long. The dyestuff industry led the way but “aromatic chemistry,” a term coined by Kekulé, proliferated over the following decades, producing endless industrial chemicals, but also powerful drugs like aspirin in 1899 and Paul Ehrlich’s pioneering antisyphilitic drug Salvarsan in 1909 (see Chapters 18 and 20).18
Benzene was at the center of this activity. Its formula, eventually understood as C6H6, was so stable that it could be transformed into many derivatives by substitution reactions without itself decomposing. Kekulé said that the structure of benzene came to him in yet another dream, this time in the winter of 1861–62, in Ghent. He said he dreamed of a snake that had seized hold of its own tail, leading Kekulé to publish his theory of the ring structure in 1865. (Arthur Koestler remarked dryly that this was “probably the most important dream in history since Joseph’s seven fat and seven lean cows.”)
As John Buckingham has observed, “The benzene structure that emerged from the 1860s is a thing of considerable beauty and intellectual satisfaction…Like the DNA structure of nearly a century later, it had to be right.”19 The ring is the key, meaning there are no reactive loose ends. Every carbon atom has two valencies that are used to bond it to its neighbors, while a third “hook” attaches it to a hydrogen atom. This leaves one over for a bond with something else. A complete understanding of benzene’s valency was not possible before the rise of quantum theory in the 1930s (see Chapter 32), but in the mid-nineteenth century chemists did begin to suspect that three-dimensional geometry might play a role in chemical reactions. This realization would help give rise to particle physics at the end of the nineteenth century, confirming Thomas Nipperdey’s point that the revolution in the natural sciences in the nineteenth century had a more far-reaching impact than the revolution brought about by Kepler, Galileo, and Newton.
This new theoretical understanding had extremely practical consequences, accounting for the heroic developments in commercial chemistry after the 1860s, which helped Germany become a world economic—and then military—power. In 1862, in a letter to von Liebig, Wöhler worried at the large array of chemists being produced by German universities and queried their fate.20 Only three years later, when Hermann Kolbe was appointed professor of chemistry at the University of Leipzig, he asked for—and was granted permission to build—a laboratory for 132 students. Von Liebig laughed at the folly of it all, yet when it opened in 1868, it was immediately swamped by the demand.21
THE AGE OF FERTILIZERS
Von Liebig, as we have noted, was a combustible character. It should, therefore, not be too much of a surprise to learn that, around 1840, he underwent a sudden scientific change of life and abandoned the theoretical aspects of organic chemistry for the much more practical interests of agriculture.22 The change was, nonetheless, provoked by his interest in carbon. In an analysis of strawberries and fruits, he had found that, in a given area of land, whether it was cultivated fields or “wild forest,” the same total quantity of carbon was produced each year in the composition of whatever plants grew in it. This was the starting point for what became a bitter argument as to whether such carbon derived from the atmosphere or from the humus in the soil. The dispute arose because von Liebig had long been interested in the source of nitrogen. He had found ammonia in the body of every plant he investigated and this persuaded him that it must come from ammonia dissolved in rainwater, which, he found, always contained certain amounts of that substance. The more he looked at plants, the more uniformities he found. Such uniformities, von Liebig thought, could not be accidental, and he concluded, controversially, that the nutrients in the soil and air were inorganic not organic.23
He put this together into what has been called “the most comprehensive picture of the problem of plant nutrition that had ever been presented.” Von Liebig’s Organic Chemistry in Its Applications to Agriculture and Physiology, London, 1840, beginning with the role of carbon in plant nutrition, refuted the then widely held view that humus—decayed plant matter—formed the main nutrient substance for plants. Von Liebig’s second argument was that the source of carbon assimilated into plants is the atmosphere. The function of plants, he argued, 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.”24
This argument for a chemical understanding of the internal processes of vegetables (further evidence to discredit the “vital force”) was, however, not what made von Liebig’s book the sensation it was. What drew attention was his view that certain nutrient materials—external sources—were essential for plant growth, for these were conclusions that impinged directly on agricultural practices. For von Liebig, the very idea of fertilizer meant adding to the soil what nutritional elements were not supplied
naturally from the atmosphere. Fertilizers, he said, should comprise not humus but bases such as lime, potash, and magnesia, plus phosphoric acid, the best source of which was pulverized animal bones.
Von Liebig’s book provoked intense interest among agriculturalists, particularly in Britain and America. At Rothamsted experimental farm in England, von Liebig’s fertilizers were tested on wheat and found to have no noticeable effect on production, whereas ammonium salts added to the soil brought about great improvements in the harvests year after year. These results blew a hole in von Liebig’s entire “mineral theory,” or so it was thought, and his ideas were dismissed. Von Liebig didn’t give way, however, though it would be another decade before he solved the problem. He had been too worried that soluble salts would be leached away by rainwater—in fact, the topsoil absorbed them. And, despite the early doubts, von Liebig’s book changed completely attitudes to scientific agriculture. Before 1840, the conventional wisdom was that both plants and animals needed organic—previously living—material in order to survive. Following von Liebig, it began to be accepted that the nutrient substances of plants were inorganic. This utterly transformed one basic belief about agriculture, namely that the production of food had fixed limits. It was now accepted, in contrast, that no such limits applied.
THE DISCOVERY OF THE CELL
At much the same time as the discovery of the benzene ring and the understanding of the nature of fertilizer, German biologists were also at work on the discovery of the cell. This, the idea that all forms of life are composed of “independent, but cooperative” units that we now call cells ranks as one of the seminal discoveries in biology.25 The first person to observe cells was Robert Hooke (1635–1703), curator of experiments at the Royal Society in London, whose Micrographia appeared in 1665. In later centuries, many others, benefiting from the ever-improving microscopes, observed “globules” or “vesicles,” of different sizes and shapes, in both animal and vegetable tissue. We know from a letter that the Dutchman Anton van Leeuwenhoek of Delft wrote to Robert Hooke in March 1682 that he had already observed a darker body inside cells that would come to be called the nucleus.26 By the end of the eighteenth century, most botanists accepted that plants were composed largely of cells, with Kaspar Friedrich Wolff (1733–94) one of the first to advocate that the fundamental subunit of all tissues—animal and vegetable—was a vesicle or globule and which, like others before him, he sometimes called a cell. However, no one had ever suggested—in print anyway—that plant cells and animal cells were homologous, and no one knew how cells divided or how new cells were formed. In 1805 Lorenz Oken (1779–1851) put forward the view that all living forms, plants as well as animals, were composed of “infusoria,” these being simple organisms like bacteria or protozoa, in other words, the simplest and most primitive forms of life then known.27
But the first man to advance thought to its modern understanding was Jan Evangelista Purkyne
Purkyne
From his earliest years, he entertained the notion that there were fundamental parallels between animal and plant cells. The 1830s saw more progress, with several experiments clarifying the structure of such animal tissues as skin and bone, these papers referring to “granules,” “Körnchen,” “Körperchen,” and “Zellen” the idea that there was “homology” between some plant cells and some animal cells, says Henry Harris, was gaining strength.29 Then there was the fact that Franz Bauer, an Austrian who was a superb botanical artist, highlighted the nucleus in his drawings. These had been made as early as 1802 but were not released until the 1830s, when he made it plain he regarded the nucleus as a regular feature of cells.30 The nucleus was actually so named by Robert Brown, custodian of the botanical collections at the British Museum (and the man who identified “Brownian motion”), but his suggestion was made the most of in Germany, the word being used as an alternative to “Kern” (kernel). The nucleolus, within the nucleus, was first observed by Rudolph Wagner in 1835, though to begin with he called it a “Fleck,” and then “the germinative spot” (“macula germinative”).31
Purkyne
In November 1832, Karl Asmund Rudolphi, professor of anatomy and physiology at the University of Berlin, died. The vacant chair was occupied the following year by a man who was to become one of the more famous nineteenth-century biologists, Johannes Müller.33 In 1835 Müller published a monograph on the comparative anatomy of the myxinidae (hagfish), in which he described the similarity between cells in the notochord (the neural channel in the spine) and plant cells. This was a crucial observation, all the more so as Theodor Schwann became Müller’s assistant. Schwann would capitalize on Müller’s insight but only after his momentous meeting with the botanist Matthias Jakob Schleiden.
Schleiden’s career had followed a familiar pattern. He first took up legal studies, obtaining a doctorate at the University of Heidelberg in 1827.34 He didn’t enjoy legal work, however, and changed professions, beginning a degree in natural science at Göttingen in 1833 and subsequently transferring to Berlin. Schleiden was invited to work in Müller’s laboratory and it was there that he met Theodor Schwann.
Though a late convert to botany, Schleiden was always very keen on the microscope and played an important role in its introduction in biological research. (He is thought to have had a hand in the establishment of the Zeiss optical works in Jena.)35 In 1838 Schleiden released “Beiträge zur Phytogenesis” in Müller’s Archiv, a journal that the Berlin professor had started and that had become one of the most respected periodicals of the time. This article, immediately translated into English and French, was the first airing of the cell theory, which, according to tradition, was conceived in a conversation between Schleiden and Schwann on the subject of phytogenesis. Schleiden was impressed by Robert Brown’s identification of the cell nucleus (1832) and used that as his starting point. The nucleus was then called the cytoblast, and according to Schleiden, “as soon as the cytoblast reaches its final size, a fine, transparent vesicle forms around it: this is the new cell.” Schleiden described this cell as “the foundation of the vegetable world.” While this paper clearly announced “the advent of plant cytol
ogy,” Schleiden did so by asserting that cells are “crystallised inside an amorphous primary substance,” which was quite wrong (italics added). Nevertheless, his botany textbook, published in 1842, the Grundzüge der wissenschaftlichen Botanik, gave over a large section to plant cytology, and in so doing transformed the teaching of botany, attracting many people to what they felt was a new science.36 Schleiden himself never fully appreciated the true significance or role of the nucleus, or cytoblast, but his fellow biologists in Müller’s laboratory more than made up for this shortcoming.
His friend and colleague Theodor Schwann was a biologist for fifty years yet devoted only five of those years (1834–39) to the subject for which he is best known. Schwann’s most famous monograph was published the very same year, 1838, in which Schleiden released his “Beiträge” article. Schwann began by outlining the structure and growth of the cells of the notochord and of cartilage. He did so, Schwann said, because their architecture “most closely resembles” that of plants and because cell formation from the “Cytoblastem” is clearly demonstrated. The second section bore a title that reflected his argument and tone: On cells as the foundation of all tissue in the animal body. Purkyne