Starlight Detectives

by Alan Hirshfeld

  When a second laboratory explosion in 1869 threatened Bunsen’s remaining eyesight, a crowd gathered on the plaza outside his home until his physician announced from the balcony that the patient was all right. That evening Bunsen’s students returned in a torchlight procession and serenaded their beloved professor with a rendition of Gaudeamus Igitur.

  At its peak, the Heidelberg laboratory held upwards of seventy students and visiting chemists from around the world. Among them were such chemical giants as Dmitri Mendeleev and Julius Lothar Meyer, cocreators of the periodic table of the elements. Of the estimated hundred Americans who passed through the laboratory, thirty-three obtained their doctoral degree under Bunsen, and forty-six became chemistry professors themselves.

  Bunsen had neither the time nor the inclination for a family life. Asked by one faculty wife why he hadn’t married, he replied, “Heaven forbid, when I should return at night, I should find an unwashed child on each step.” (The staircase in Bunsen’s home rose twenty-five steps to the second floor.) Housekeeping was a task reserved for the laboratory. At home, Bunsen tossed unopened mail into a vacant room, to be sifted through every few weeks by a subordinate. Arrayed around the baseboard of another room was his accumulation of well-worn boots. So rarely did he host social gatherings that a group of faculty wives commandeered his house and invited him to his own party.

  Modesty was Bunsen’s hallmark. Only on mandatory occasions did he don his waistcoat full of medals; these he hid from public view by buttoning up his overcoat, even during summers, and avoiding well-traveled streets. “The only value such things had for me,” he offered late in life, “was that they pleased my mother, and she is now dead.” (Celebratory gatherings were not completely wasted on Bunsen: he gathered up used champagne corks to bring back to the laboratory.) In his lectures, he never took credit for his own discoveries, declaring instead, It has been found . . . The prominent English physicist John Tyndall summed up Bunsen as “the man who came nearest my ideal of a University teacher. He was every inch a gentleman, and without a trace of affectation or pedantry.”

  At the same time, Bunsen had a knack for the bon mot. When a student tried to explain his repeated absences by insisting that he was hidden by a pillar, Bunsen replied, “Ah, so many sit behind the pillar.” On a trip to England in 1862, he was mistaken by a woman for the deceased Baron von Bunsen, a noted Prussian scholar and diplomat. Asked whether he had completed his great work, God in History, the chemist Bunsen lamented, “Alas, no, my untimely death prevented me from accomplishing my design.”

  Once at Heidelberg, Bunsen used his carbon-zinc battery to electrically decompose chemical compounds into their constituent elements, producing ultrapure samples of chromium, manganese, magnesium, aluminum, sodium, barium, calcium, and lithium. Despite appearances, Bunsen was no production-line automaton, content to churn out reference data and chemical specimens; his thinking was interdisciplinary, and he reached freely into physics, geology, and mathematics. (His math teacher was the eminent Carl Friedrich Gauss.) “A chemist who is not a physicist is nothing,” Bunsen asserted. Pursuing his physics muse, Bunsen joined Henry Roscoe in the darkened attic of the chemistry building in what would become a decade-long study of the photochemical action of sunlight. The interaction of light and matter intrigued Bunsen, who wondered whether elements might be uniquely identified by the color of light they emit when heated to incandescence. It was for these explorations that he developed his eponymous burner, with its hot, yet virtually colorless, gas flame.

  From the start, Bunsen leveraged his renown to promote the growth of the physical sciences at Heidelberg. Through the 1850s, fully 97 percent of the university’s state funding went to chemistry. In addition to being director of the chemistry laboratory, Bunsen was dean of the philosophy faculty (precursor to the modern-day college of arts and sciences), a position through which he exerted considerable power to shape Heidelberg’s scholarly future. Mid-nineteenth-century Germany was fragmented into a confederation of kingdoms, grand duchies, and electorates—Baden, Prussia, Bavaria, Hesse-Darmstadt, plus a host of others—each vying for cultural supremacy, if not outright power. In this rivalry, the reputation of universities served as a proxy for more typical economic or military measures of success. Prestige was conferred upon a state by the eminence of its resident scholars—especially scientists. Substantial salaries, cultural amenities, and research support were proffered to convince elite scientists to join the “team.” Personnel shuffled from Heidelberg to Munich, Munich to Berlin, Berlin to Potsdam, as the respective state treasuries sweetened their enticements. No academic was more politically adept than Bunsen at securing government funds toward this end. At times, he led hiring forays into new areas of research; when instead he sensed the Baden government trending toward established fields, he promoted candidates for these positions.

  In 1854, when a faculty slot opened up, Bunsen pressed the Baden government for the appointment of a novel sort of researcher: equal parts physicist and mathematician, a scientist unconstrained by the experimental laboratory or by disciplinary boundaries. In this effort, he was opposed by his colleagues, who advocated for an industrial or a biological physicist. When the haggling was over, Bunsen’s candidate prevailed: Gustav Kirchhoff, a thirty-year-old physicist at the University of Breslau in Prussia, whose mathematical skill and cognitive potency had impressed him when their paths had crossed two years earlier. Together, the pair would form one of history’s great scientific partnerships—and would alter the course of cosmic exploration as profoundly as the advent of the telescope.

  Gustav Kirchhoff was destined for an extraordinary life—at least so he had thought when not wracked with self-doubt over his height and his mathematical aptitude. He had written dutifully to his parents and to his brother Otto, revealing cracks in a facade otherwise burnished by academic excellence. “I am currently quite annoyed at my slight stature,” Kirchhoff once confided to Otto when he was eighteen, “and would enjoy the university more if my size and age were in better agreement.” This adolescent lack of confidence persisted throughout Kirchhoff’s adulthood, according to physicist Emil Warburg, who knew him for many years.

  Burdensome as they were, Kirchhoff’s neurotic demons never managed to seize up his creativity. While an undergraduate at the University of Königsberg in 1845, Kirchhoff had developed the now-famous set of algebraic rules that enable the calculation of voltages and currents in multi-loop electric circuits. So impressive was this achievement that the university awarded him a doctoral degree a year later at the age of twenty-two.

  Following a two-year stint as an unpaid lecturer and scientist-gadabout in Berlin, Kirchhoff found himself, in 1850, teaching introductory physics at the University of Breslau (today’s Wrocław, in Poland). Compared to the scientific bustle and urban energy of Berlin with its half-million inhabitants, his new locale was a letdown. Kirchhoff labored in anonymity on his electrical research, finding few like-minded colleagues among Breslau’s faculty. But a job is a job, he had reluctantly come to admit, and bread has to be put on one’s table.

  Kirchhoff had been hired as an experimental physicist, a far remove from the abstract, mathematical physics that fired his curiosity and filled his brief résumé. At the time, Germany’s academic centers valued laboratories, equipment, and measurement; the exploratory potential of paper-and-pencil theoretical physics had yet to be established. To Kirchhoff, mathematics was the language of nature, its symbols and operators the means by which the physicist inquired of nature’s design. Through mathematical modeling and analysis, the theorist develops testable predictions of the behavior of, say, matter or light. If verified by experiment, the proposed theory is broadened or extended to more complex situations. Thus, science is a cyclical process of experiment driven by theory and theory driven by experiment, leading to discovery of the laws that govern the physical universe.

  But experimental, not theoretical, research was what Breslau was paying Kirchhoff to undertake. Disheart
ened, he wrote hopefully to his father in Königsberg, “It will do me some good to move into experimental work, just as a plant grows stronger when it’s moved into new soil.” Despite his limited prospects, Kirchhoff’s tepid optimism would prove prophetic. The new soil in which he would flourish was soon to be tilled, not in Breslau, but in Heidelberg by Robert Bunsen.

  Kirchhoff would undoubtedly have noticed the middle-aged, bearish man who materialized in his introductory physics class in 1851. To count the renowned Herr Bunsen among his listeners—not only that afternoon, but as it happened, for the rest of the semester—was a distinct honor for a young professor barely a year into his post. It was also one of few bright spots in what had otherwise been a depressing term at Breslau, a circumstance compounded by a conflict with his senior colleague in physics. “My stay in Breslau has recently become more pleasant,” Kirchhoff briefed his mother in May 1851. “At the beginning of the semester arrived the new chemistry professor, Bunsen, previously in Marburg. I find his manner very appealing and we have had wonderful times together. He is a man of extraordinary kindness.”

  Kirchhoff and Bunsen stood at opposite poles, both physically and temperamentally. The sight of the pair strolling the streets of Breslau—and later Heidelberg—drew attention, as though one were witnessing the encounter of two different species of man. The mountainous Bunsen stood nearly a head taller than Kirchhoff, even without his trademark stovepipe hat. Where Bunsen was voluble and imposing, Kirchhoff was soft-spoken and wiry as a gymnast (which he was in his youth). Bunsen radiated confidence, while Kirchhoff held close his conflicted self-assurance. But in the arena of scientific debate, Kirchhoff could dispatch any of his peers with authority, yet without offense, a talent that delighted his gregarious companion.

  Each day, after Kirchhoff’s lecture, the two men gathered other professors and headed over to the beer hall for an evening meal and quaff. Kirchhoff and Bunsen were inseparable: they took daily constitutionals to discuss developments in science, sat next to each other at the theater, and traveled together during school holidays. It was during his walks with Kirchhoff, Bunsen reported, that his best ideas came to him.

  Kirchhoff was distressed when Bunsen left Breslau in August 1852 to head the chemistry institute at Heidelberg, some five hundred miles away. The two corresponded frequently until 1854, when Bunsen led with stunning news: he had persuaded the Baden government to appoint Kirchhoff to Heidelberg’s recently vacated senior faculty position in physics. Kirchhoff would become director of his own physical institute, subordinate to no one. (Aware of Kirchhoff’s insecurities, Bunsen advised him to stifle his habitual self-effacement when dealing with the ministry.)

  Gustav Kirchhoff, Robert Bunsen, and Henry Roscoe, photographed in Manchester, England, in 1862.

  Before extolling nominee Kirchhoff’s virtues, the ever-shrewd Bunsen informed the Baden government, with evident regret, that the “first notables of science” in Berlin, Göttingen, Königsberg, and Vienna would never decamp to Heidelberg, given its deficient experimental facilities. However, a junior faculty member—namely, Kirchhoff—might be induced to accept a job offer despite the shortcomings. Kirchhoff’s résumé was sold on his dual strength in physical theory and experiment, a combination that opened the door to fruitful, cross-disciplinary research collaborations with Bunsen. In his request to the ministry, Bunsen quoted a remark by Wilhelm Weber, a prominent physicist, that “two scientists who by working together multiply their achievements . . . lends a special radiance to the university where it is achieved.” In fact, Weber’s hyperbole underestimated the synergistic power of the collaboration.

  In every way, the Heidelberg position was a step up for Gustav Kirchhoff: in title, from assistant professor to full professor; in salary, from 1,050 florins to 1,600 florins, plus 400 florins for housing; in prestige, from a regional university to an internationally acclaimed academic center; in focus, from experimental physics to theoretical physics—or any combination of the two he desired. In October 1854, with anxiety tempering his joy, Kirchhoff set off for Heidelberg and his much-anticipated reunion with Bunsen. (Three years later, Bunsen further elevated Heidelberg’s scientific stature by hiring Hermann von Helmholtz, who performed seminal studies on the conservation of energy, fluid mechanics, electromagnetism, and thermodynamics.)

  Kirchhoff and Bunsen took up their social routine where they had left off, their daily outings now relocated to the so-called Philosopher’s Walk on the southern slopes of the Heiligenberg, across the Neckar River from the university. In 1857, when Kirchhoff married Clara Richelot, daughter of his former math professor, Bunsen found himself a welcome adjunct to the new couple. The three of them joined a dramatic reading group in which they played roles from Goethe, Schiller, and Shakespeare. To the Kirchhoffs’ children, Bunsen was “Onkel Hofrat”—from his honorific as a government councilor—who arrived every Christmas Eve bearing an armload of gifts. Clara tried to wrest Bunsen from his near-total immersion in his work, with marginal success. If anything, she was undermined by her own husband, whose research partnership with Bunsen evolved into an all-consuming passion for both men.

  During the late 1850s, driven in part by his photochemical investigations with Henry Roscoe, Bunsen sought to identify chemical elements by their distinctive color when ignited in his gas burner or an electric arc. The results proved equivocal. Whether viewed directly or through filters, the various colors of combustion were typically too similar to differentiate between substances. Kirchhoff observed the experiment and at once recognized its shortcomings: The luminous emission of any incandescent element is an admixture of many colors, or wavelengths, all of which flood the eye simultaneously. What Bunsen needed was a spectroscope to sort out the constituent colors by their wavelength before they enter the eye. Instead of perceiving a riotous blend of colors, observers would see an ordered spectrum: various hues situated in different positions along a viewing plane, akin to reassembling a group of people alongside each other according to height.

  Bunsen’s goal—now shared by Kirchhoff—was to demonstrate that the spectrum of a given element is unique and can therefore be used to identify an element remotely by its light alone. Yet for Kirchhoff, there was an additional challenge: to explain the physical basis of the emission and absorption of light from matter. Here, in 1859, at the crossroads of chemistry and physics and of theory and experiment, the duo embarked on their first scientific collaboration.

  Chapter 14

  WHAT’S MY LINE?

  The world is moved along, not only by the mighty shoves of its heroes, but also by the aggregate of the tiny pushes of each honest worker.

  —Helen Keller, Optimism: An Essay, 1903

  RAINBOWS ARE SEEMINGLY MAGICAL APPARITIONS: fleeting, insubstantial, their twin footings at once anchored to the horizon yet responsive to one’s movements. Poised between stormy gloom and sunbeam daylight, rainbows infuse the sky with ghostly colors, arranged with a geometric perfection that charms the eye and stirs the soul. No wonder people have puzzled over their origin and infused them with religious and mythological significance. Through the ages, proto-scientific observers have postulated a host of physical causes, some far afield, others quite close to the mark. A rainbow is nature’s most flamboyant expression of the broad phenomenon of color, an aspect of light that astronomers gradually harnessed to great advantage.

  The multicolored character of light has been studied with man-made contrivances since at least the thirteenth century. Trying to imitate the action of a raindrop, Leonardo da Vinci placed a water-filled glass globe in his sunlit window and noted the array of colors cast onto the floor. In an appendix to his 1637 Dioptrique, Rene Descartes sketched the formation of an artificial rainbow by a triangular glass prism, a scheme more clearly depicted in Robert Boyle’s treatise Colours, from 1664. Two years later, inspired by Boyle’s work, Isaac Newton directed a sunbeam from a hole in the window shade into his darkened room and through a prism. On the opposite wall was projected a mult
icolored band, formed from overlapping images of the round aperture in the window shade. In his report to the Royal Society in London, Newton stated that, “Light it self is a Heterogeneous mixture of differently refrangible Rays.” Within this amorphous spectrum, as he called it, Newton identified precisely seven primary colors (violet, indigo, blue, green, yellow, orange, and red), a number without physical significance, but chosen to accord with the seven notes of the major scale in music. Again taking a lead from Boyle, he refocused the dispersed colors to restore the original white beam of sunlight.

  In a darkened room, a prism decomposes a beam of sunlight into its constituent colors; a second prism repeats the process for a narrow section of the original spectrum.

  During the mid-eighteenth century, Scottish scientist Thomas Melvill observed that a flame, when sprinkled with substances such as sea salt, saltpeter, or potash, yields a spectrum that is not continuous like the Sun’s, rather a sequential array of discrete, colored bands. Half a century later, English physician William Hyde Wollaston closed his medical practice to become a scientist, funding his diverse research from the invention of a method that rendered platinum malleable. Wollaston developed an optical rig to gauge the refractive power of translucent materials, such as glass, amber, melted spermaceti, even the lens of an ox-eye. To illuminate his samples, he directed sunlight through a narrow slit, before dispersing it with a prism. The spectrum projected by a prism is a composite of contiguous or overlapping colored images of the luminous source, whether a candle flame, a glowing lightbulb filament, or—in both Newton’s and Wollaston’s setups—a sunlit aperture. A mere twentieth of an inch wide, Wollaston’s slit-shaped aperture produced a sharper spectrum than Newton’s original round aperture, whose projected colors bled into one another. (Newton mentioned his use of slit apertures in the 1704 treatise Opticks.)