Image and Reality

Kekulé, Kopp, and the Scientific Imagination

Alan J. Rocke

• Publisher: University of Chicago Press
• Year: 2010
• ISBN:
  9780226723358
• Topic: Biological Sciences
• Subtopic: Science General
• Index: Biological Sciences

Nineteenth-century chemists were faced with a particular problem: how to depict the atoms and molecules that are beyond the direct reach of our bodily senses. In visualizing this microworld, these scientists were the first to move beyond high-level philosophical speculations regarding the unseen. In Image and Reality, Alan Rocke focuses on the community of organic chemists in Germany to provide the basis for a fuller understanding of the nature of scientific creativity.

Arguing that visual mental images regularly assisted many of these scientists in thinking through old problems and new possibilities, Rocke uses a variety of sources, including private correspondence, diagrams and illustrations, scientific papers, and public statements, to investigate their ability to not only imagine the invisibly tiny atoms and molecules upon which they operated daily, but to build detailed and empirically based pictures of how all of the atoms in complicated molecules were interconnected. These portrayals of "chemical structures, " both as mental images and as paper tools, gradually became an accepted part of science during these years and are now regarded as one of the central defining features of chemistry. In telling this fascinating story in a manner accessible to the lay reader, Rocke also suggests that imagistic thinking is often at the heart of creative thinking in all fields. Image and Reality is the first book in the Synthesis series, a series in the history of chemistry, broadly construed, edited by Angela N. H. Creager, John E. Lesch, Stuart W. Leslie, Lawrence M. Principe, Alan Rocke, E.C. Spary, and Audra J. Wolfe, in partnership with the Chemical Heritage Foundation.

ONE

Ether/Or

But a state of rest as well? O! to believe that everything in the liquid is arranged lock-step and in repose—that is an outdated concept, which became untenable when, in the March-days of the science, springtime broke out for chemical theory too (according to the calendar followed by a portion of the human world, it was in the late summer of 1850), on the broader basis of the motion of the smallest parts of the components of compounds.

HERMANN KOPP¹

Who does not remember the revolution brought about, when Wurtz discovered the compound ammonias and Williamson introduced the type HHO?!

CARL SCHORLEMMER²

Springtime for Chemistry?

In 1882, the eminent chemist and historian of chemistry Hermann Kopp published an imaginative 105-page expedition into the world of molecules, Aus der Molecular-Welt, in which he exercised his (and his reader’s) “mind’s eye” to describe anthropomorphized spectacles that are forever hidden from the bodily eye. In addition to its wry humor, the work is filled with obscure topical and literary allusions, including the epigraph just cited, and hardly any real scientists’ names can be found anywhere in the book—not even Kopp’s own name on the title page.³ In the epigraph above, Kopp’s evocation of springtime and the month of March suggests an intertwining of two metaphors: broadly, the start of the season of blissful warmth, freedom, and growth; and more specifically, the “März-Tage” that for Germans uniquely call to mind the ebullient start of the (ultimately failed) liberal democratic revolution in March 1848—German historians ever after having adopting the terms “Vormärz” and “Nachmärz” to denote the eras adjoining that watershed month. But to what beneficent revolution for chemical theory in “the late summer of 1850” was Kopp pointing?

Kopp was almost certainly referring to a paper by Alexander W. Williamson, then just twenty-six years old, read at the annual meeting of the British Association for the Advancement of Science in Edinburgh on 3 August 1850. One piece of evidence for this is Kopp’s explicit reuse of a turn of phrase found in Williamson’s concluding sentence: “In using the atomic theory, chemists have added to it of late years an unsafe, and, as I think, an unwarrantable hypothesis, namely, that the atoms are in a state of rest. This hypothesis I discard, and reason upon the broader basis of atomic motion.”⁴ In addition to matching the “late-summer” timing and the parallel phrasing of words, in an earlier explicitly historical account Kopp had already made clear to his readers how very revolutionary he regarded the work of the English chemist to have been.⁵

Though few chemists took immediate notice of it, Williamson’s 1850 paper was indeed groundbreaking in more than one respect. As the first of an important series of papers on the formation of ethers, the constitutions of molecules, and reaction dynamics, we will see in this chapter how this work would lead to dramatic changes in chemical theory. Though these changes can be expressed in conventional terms, the real key to the revolutionary character of Williamson’s contributions, I suggest, lay in his then-unfashionable eagerness to take seriously the reality of the molecular world, even though direct sensual or instrumental access to that level was not possible.

Whether consciously or only instinctively, Williamson understood that molecular reality could best be grasped and explored scientifically using vivid mental images, the very sort that Kopp would later portray so engagingly in Molecular-Welt, with the additional assistance of “paper tools” such as molecular formulas and chemical equations. Because Williamson provides a particularly striking example of a lively chemical imagination, because of his early and fundamental role in the “quiet revolution” in chemistry, and because of his powerful influence on Kekulé and on Kopp, whose work will loom large in these pages, we need to examine this interesting figure with care.

The Education of Alexander Williamson

Born in London of Scottish parents, Williamson’s childhood was comfortable, due to his parents’ substantial means.⁶ His father, also named Alexander, was an official in the East India House who moved in an elevated cultural circle that included James and John Stuart Mill. About 1840 the family relocated to the Continent, and young Alexander benefited from education in Paris, Dijon, and Wiesbaden. He learned to speak and to write nearly perfect French and German. In 1841 Williamson matriculated at the University of Heidelberg with the intent to study medicine, but the lectures of Leopold Gmelin soon decided him for chemistry. After three years of intensive study he transferred to the University of Giessen to earn his doctorate with the great master Justus Liebig, whose famous laboratory in those years had become the destination of pilgrimages by would-be chemists from around the world.

En route to his doctoral degree Williamson published three small but excellent pieces of research in inorganic chemistry, while devoting most of his attention to a new theory (of which we know no details) concerning the controverted phenomenon of electrolysis. William Brock justly writes of a “peculiar trait in his character” according to which a series of obsessive secondary interests outside of chemistry nearly always ran parallel to the research that would ultimately bring him renown. These secondary interests, Brock notes, “sometimes show amazing fertility of invention [but] could not fail to have dissipated his energies.”⁷ Williamson had an extraordinary power of imagination combined with remarkable discipline and self-criticism. In a letter to his father (probably from 1845) Williamson wrote, “I often find [a solitary] walk of as much real service to me in my progress as a whole week’s labour [in the laboratory]. The great difficulty in a research such as that I am now pursuing consists not so much in performing the experiments once fixed upon, as in inventing and choosing from those most calculated to attain the desired object.”⁸ In all of his scientific work, a prominent characteristic of his style was the meticulous and unusually clever rational design of his experiments. “If you know clearly what you want to do,” he wrote in another letter, “there is always a way of doing it.”⁹

At some point Williamson encountered difficulties that Liebig attributed to his deficient mathematical training. Williamson therefore resolved to make himself “a complete mathematician, or failing in that to turn shoemaker.”¹⁰ For this purpose, and with the aid of a letter of introduction from John Stuart Mill, Williamson moved to Paris to study with Auguste Comte. With the help of his parents’ money, and probably residing with them, he devoted himself for two years to intensive study, and then spent a third year pursuing experiments in organic chemistry. Williamson’s enthusiastic though temporary embrace of Comte’s “philosophie positive” could only have reinforced those elements of critical, empirical, and skeptical approaches inculcated by his previous education, influenced as it was by utilitarianism and the “philosophical radicalism” of his father, Jeremy Bentham, and the Mills. One may reasonably presume that Williamson also read in philosophical works of the day, including John Herschel, William Whewell, and the Scottish Common-Sense philosophy of Thomas Reid and Dugald Stewart.¹¹

Comtean positivism was distinct from the late nineteenth-century positivism of Ernst Mach or that of the Vienna Circle logical positivists, for in Comte’s work there were distinct realist impulses superadded to the dominating desire to extirpate metaphysics. Moreover, Comte actually had a healthy respect for the beneficial role of hypotheses in science—even, under certain conditions, hypothetical entities that were in principle unobservable.¹² But Williamson’s friend John Stuart Mill, for a time a positivist fellow-traveler, ended by criticizing (and severely disappointing) Comte, and Williamson soon was to follow a similar path.¹³

Interpreting Chemical Atoms

When Williamson and his contemporaries thought about the symbols that appeared in the chemical formulas they bandied about, just what did they have “in mind”? To provide even a provisional answer to this question, we need briefly to review the early history of the atomic theory in chemistry.¹⁴

In 1803 John Dalton developed a method to derive relative weights for the presumed smallest portions (chemical atoms) of each of the known elements, and molecular formulas for all the compounds that they form. For Dalton and other early chemical atomists, it was necessary first to assume molecular formulas for certain simple substances in order to calculate relative atomic weights from analytical data. For instance, Dalton posited that each molecule of water was composed of one atom of hydrogen and one of oxygen, and he represented this molecule by drawing two contiguous dissimilar circular symbols. With such a presumed molecular formula, and considering the measured proportions of the elements that form water, Dalton’s atom of oxygen had to weigh 8 relative to the weight of his hydrogen atom (conventionally considered as 1, to provide a fiducial standard).¹⁵ Once one decided upon a set of relative elemental atomic weights, any pure compound could in principle be represented atomistically. Soon after Dalton had shown the way, other early chemical atomists offered slightly differing choices for assumed formulas and atomic-molecular representations.

In 1814 the London physician-chemist William Wollaston proclaimed that “practical convenience” rather than deep theory really ought to be the only guide to the determination of relative atomic weights, and proposed what he called “equivalents” for the elements that nearly (but not quite) matched Dalton’s “atomic weights.” Wollaston’s putatively pragmatic “equivalents” proved popular in competition with the more obviously theoretically derived atomic weights that were developed by the influential Swedish chemist Jacob Berzelius between 1813 and 1826. British chemists tended to prefer Wollaston’s system, whereas German chemists, such as Liebig, Friedrich Wöhler, and Robert Bunsen, preferred Berzelius’s. French chemists mostly used a third variant, developed by Joseph-Louis Gay-Lussac and Jean-Baptiste Dumas. Yet a fourth variant was proposed in 1842, which will be discussed below.

In short, during the first half of the nineteenth century European chemists simply could not agree (e.g.) on whether the true relative atomic weight of the oxygen atom was 8 or 16, or whether carbon was 6 or 12. These varying atomic weights consequently required varying numbers of oxygen and carbon atoms in the formulas of their compounds, since those who thought (e.g.) that oxygen was 8 required twice as many oxygen atoms in their formulas as the O = 16 advocates did. All of this worked against interpreting any one of these systems as an actual portrayal of molecular reality.

How, indeed, should one interpret the symbols in a molecular formula? Dalton thought that each of the symbols in his formulas must signify an actual “atom,” in the sense of an absolutely unsplittable entity, much like an invisibly small but very real billiard ball—which is why he chose to represent his atoms by distinctive iconic circles, or spherical wooden models. Few chemists thereafter took such an unreflectively realist position. At the other extreme, some regarded chemical formulas purely conventionally, as a mere aid to memory in representing the empirical facts of chemical analysis and having no real referent in the microworld at all.

There were, of course, middle positions between ontological realism and extreme conventionalism or positivism. Berzelius devised the alphanumeric system which, in slightly modified form, chemists still use today in order to designate the presumed atomistic compositions of molecular formulas rather than to make any detailed statement about the nature of the atoms themselves. Each of Berzelius’s letters, such as the three entities in his preferred water formula, H₂O, might reasonably be taken to refer to a quantity of matter, the real micro-characteristics of which were deliberately elided. A Berzelian chemical (as opposed to physical) atom, by this more cautious interpretation, was simply a packet of elemental matter of a certain relative weight, a packet that might possibly, for all we know, have internal parts or structure, but that seemed to operate integrally across known chemical transformations. Berzelius’s approach was no less theoretical than Dalton’s, but appeared to set epistemological limits on what was knowable about the microworld.

As Klein has convincingly argued, a major reason for the popularity of the Berzelian formula system was surely its semiotic ambiguity—or flexibility. A Berzelian formula could be easily taken to occupy any of these philosophical positions, from the extreme ontological to the purely numerical. Independent of any particular philosophical interpretation, by about 1830 these formulas began to be used by workers such as Dumas and Liebig in a generative fashion, much as mathematicians use their equations on paper or biologists use physical tools such as microscopes, to understand the course of chemical reactions and to provide heuristic guidance for further investigation. This marked a major transition for the culture of organic chemistry, from what had exhibited a predominantly natural-historical character to what now became a highly experimental approach, with the preparation of new artificial substances placed in the foreground. The use of Berzelian “paper tools” was a sine qua non for this metamorphosis.¹⁶

...