One had only to put a speck of the element or one of its compounds on a loop of platinum wire and put this in the colorless flame of a Bunsen burner to see the colorations produced. I explored a whole range of flame colors. There was the azure blue flame produced by copper chloride. And there was the light blue – the ‘poisonous’ light blue, as I regarded it – produced by lead and arsenic and selenium. There were lots of green flames: an emerald green with most other copper compounds; a yellowish green with barium compounds, some boron compounds too – borane, boron hydride, was highly inflammable and burned with an eerie green flame of its own. Then there were the red ones: the carmine flame of lithium compounds, the scarlet of strontium, the yellowish brick red of calcium. (I read later that radium also colored flames red, but this, of course, I was never to see. I imagined it as a red of the most refulgent brilliance, a sort of ultimate, fatal red. The chemist who first saw it, so I imagined, went blind soon after, the radioactive, retina-destroying red of radium being the last thing he ever saw.)
These flame tests were very sensitive – much more so than many chemical reactions, the ‘wet’ tests one also did to analyze substances – and they reinforced a sense of elements as fundamental, as retaining their unique properties however they were combined. Sodium, one might feel, was ‘lost’ when it combined with chlorine to form salt – but the telltale presence of sodium yellow in a flame test served to remind one that it was still there.
Auntie Len had given me James Jeans’s book The Stars in Their Courses for my tenth birthday, and I had been intoxicated by the imaginary journey Jeans described into the heart of the sun, and his casual mention that the sun contained platinum and silver and lead, most of the elements we have on earth.
When I mentioned this to Uncle Abe, he decided it was time for me to learn about spectroscopy. He gave me an 1873 book, The Spectroscope, by J. Norman Lockyer, and lent me a small spectroscope of his own. Lockyer’s book had charming illustrations showing not just various spectroscopes and spectra, but bearded, frock-coated Victorian scientists examining candle flames with the new apparatus, and it gave me a very personal sense of the history of spectroscopy, from Newton’s first experiments to Lockyer’s own pioneering observations of the spectra of the sun and stars.
Spectroscopy indeed had started in the heavens, with Newton’s decomposition of sunlight with a prism in 1666, showing that it was composed of rays ‘differently refrangible.’ Newton obtained the sun’s spectrum as a continuous luminous band of color going from red to violet, like a rainbow. A hundred and fifty years later, Joseph Fraunhofer, a young German optician, using a much finer prism and a narrow slit, was able to see that the entire length of Newton’s spectrum was interrupted by odd dark lines, ‘an infinite number of vertical lines of different thicknesses’ (he was able, finally, to count more than five hundred).
One needed a brilliant light to get a spectrum, but it did not have to be sunlight. It could be the light of a candle, or limelight, or the colored flames of the alkali or alkaline earth metals. By the 1830s and 1840s these, too, were being examined, and an entirely different sort of spectrum was now seen. Whereas sunlight produced a luminous band with every spectral color in it, the light of vaporized sodium produced only a single yellow line, a very narrow line of great brilliance, set upon a background of inky blackness. It was similar with the flame spectra of lithium and strontium, except these had a multitude of bright lines, mostly in the red part of the spectrum.
What was the origin of the dark lines Fraunhofer saw in 1814? Had they any relation to the bright spectral lines of flamed elements? These questions presented themselves to many minds at the time, but remained unanswered until 1859, when Gustav Kirchhoff, a young German physicist, joined forces with Robert Bunsen. Bunsen was a distinguished chemist by this time, and a prolific inventor – he had invented photometers, calorimeters, the carbon-zinc cell (still used, with negligible change, in the batteries I pulled to pieces in the 1940s), and, of course, the Bunsen burner, which he had perfected to investigate color phenomena more closely. They were an ideal pair, Bunsen a superb experimentalist – practical, technically brilliant, inventive – and Kirchhoff with a theorizing power, a mathematical facility, that Bunsen perhaps lacked.
In 1859, Kirchhoff performed a simple and beautifully designed experiment, which showed that the bright-line and dark-line spectra – the emission and the absorption spectra – were one and the same, the corresponding opposites of the same phenomenon: the capacity of elements to emit light of characteristic wavelength when vaporized, or to absorb light of exactly the same wavelength if they were illuminated. Thus the characteristic line of sodium could be seen either as a brilliant yellow line in its emission spectrum, or as a dark line in exactly the same position in its absorption spectrum.
Directing his spectroscope to the sun, Kirchhoff realized that one of the countless dark Fraunhofer lines in the solar spectrum was in exactly the same position as the bright yellow line of sodium – and that the sun, therefore, must contain sodium. The general feeling, in the first half of the nineteenth century, had been that we would never know anything about the stars beyond what could be gained by simple observation – that their composition and chemistry, in particular, would remain perpetually unknown, and so Kirchhoff’s discovery was greeted with astonishment.«51»
Kirchhoff and others (and especially Lockyer himself) went on to identify a score of other terrestrial elements in the sun, and now the Fraunhofer mystery – the hundreds of black lines in the solar spectrum – could be understood as the absorption spectra of these elements in the outermost layers of the sun, as they were transilluminated from within. On the other hand, a solar eclipse, it was predicted, with the central brilliance of the sun obscured and only its brilliant corona visible, would produce instead dazzling emission spectra corresponding to the dark lines.
Now, with Uncle Abe’s help – he had a small observatory on the roof of his house, and kept one of his telescopes hitched up to a spectroscope – I saw this for myself. The whole visible universe – planets, stars, distant galaxies – presented itself for spectroscopic analysis, and I got a vertiginous, almost ecstatic satisfaction from seeing familiar terrestrial elements out in space, seeing what I had known only intellectually before, that the elements were not just terrestrial but cosmic, were indeed the building blocks of the universe.
At this point, Bunsen and Kirchhoff turned their attention away from the heavens, to see if they could find any new or undiscovered elements on the earth using their new technique. Bunsen had already observed the great power of the spectroscope to resolve complex mixtures – to provide, in effect, an optical analysis of chemical compounds. If lithium, for example, was present in small amounts along with sodium, there was no way, with conventional chemical analysis, to detect it. Nor were flame colors of help here, because the brilliant yellow flame of sodium tended to flood out other flame colors. But with a spectroscope, the characteristic spectrum of lithium could be seen immediately, even if it was mixed with ten thousand times its weight of sodium.
This enabled Bunsen to show that certain mineral waters rich in sodium and potassium also contained lithium (this had been completely unsuspected, the only sources hitherto having been certain rare minerals). Could they contain other alkali metals too? When Bunsen concentrated his mineral water, rendering down 600 quintals (about 44 tons) to a few liters, he saw, amid the lines of many other elements, two remarkable blue lines, close together, which had never been seen before. This, he felt, must be the signature of a new element. ‘I shall name it cesium because of its beautiful blue spectral line,’ he wrote, announcing its discovery in November 1860.
Three months later, Bunsen and Kirchhoff discovered another new alkali metal; they called this rubidium, from ‘the magnificent dark red color of its rays.’
Within a few decades of Bunsen and Kirchhoff’s discoveries twenty more elements were discovered with the aid of spectroscopy – indium and thallium (which were also nam
ed for their brilliantly colored spectral lines), gallium, scandium, and germanium (the three elements Mendeleev had predicted), all the remaining rare-earth elements, and, in the 1890s, the inert gases.
But perhaps the most romantic story of all, certainly the one that most appealed to me as a boy, had to do with the discovery of helium. It was Lockyer himself who, during a solar eclipse in 1868, was able to see a brilliant yellow line in the sun’s corona, a line near the yellow sodium lines, but clearly distinct from them. He surmised that this new line must belong to an element unknown on earth, and named it helium (he gave it the metallic suffix of -ium because he assumed it was a metal). This finding aroused great wonder and excitement, and it was even speculated by some that every star might have its own special elements. It was only twenty-five years later that certain terrestrial (uranium) minerals were found to contain a strange, light gas, readily released, and when this was submitted to spectroscopy it proved to be the selfsame helium.
The wonder of spectral analysis, analysis at a distance, had literary resonances as well. I had read Our Mutual Friend (written in 1864, just four years after Bunsen and Kirchhoff had launched spectroscopy), and here Dickens imagined a ‘moral spectroscopy’ whereby the inhabitants of remote galaxies and stars might analyze the light from the Earth to gauge its good and evil, the moral spectrum of its inhabitants.
‘I have little doubt,’ Lockyer wrote at the end of his book, ‘that, as time rolls on…the spectroscope [will] become…the pocket companion of everyone amongst us.’ A small spectroscope became my own constant companion, my instant analyzer of the world, whipped out on all sorts of occasions: to look at the new fluorescent lights that were beginning to appear in London Tube stations, to look at solutions and flames in my lab, or at coal fires and gas flames in the house.
I also explored the absorption spectra of compounds of all sorts, from simple inorganic solutions to blood, leaves, urine, and wine. I was fascinated to find out how characteristic the spectrum of blood was even when dried and how small a quantity was needed to analyze in this fashion – one could identify a faint bloodstain more than fifty years old and distinguish it from a rust stain. The forensic possibilities of this intrigued me; I wondered if Sherlock Holmes, along with his chemical explorations, had used a spectroscope too. (I was especially fond of the Sherlock Holmes stories, and even more of the Professor Challenger ones which Conan Doyle had written later – I identified with Challenger; I could not identify with Holmes. In The Poison Belt, spectroscopy plays a crucial role, for it is a change in the Fraunhofer lines of the sun’s spectrum that alerts Challenger to the presence of an approaching poison cloud.)
But it was the bright lines, the brilliant colors, the emission spectra I always came back to. I remember going to Piccadilly Circus and Leicester Square with my pocket spectroscope, and looking at the new sodium lights that were being used for street lighting, at the scarlet neon advertisements, and at the other gas-discharge tubes – yellow, blue, green, according to the gas used – which now turned the West End into a glory of colored lights after the long blackout of the war. Each gas, each substance, had its own unique spectrum, its own signature.
Bunsen and Kirchhoff had felt that the position of the spectral lines was not only a unique signature of each element, but a manifestation of its ultimate nature. They seemed to be ‘a property of a similar unchangeable and fundamental nature as the atomic weight,’ indeed a manifestation – as yet hieroglyphic and indecipherable – of their very constitution.
The complexity of spectra (that of iron, for example, contained several hundred lines) in itself suggested that atoms could hardly be the small, dense masses which Dalton had imagined, distinguished by their atomic weights and little else.
One chemist, W.K. Clifford, writing in 1870, expressed this complexity in terms of a musical metaphor:
…a grand piano must be a very simple mechanism compared with an atom of iron. For in the spectrum of iron there is an almost innumerable wealth of separate bright lines, each one of which corresponds to a sharp definite period of vibration of the iron atom. Instead of the hundred-odd sound vibrations which a grand piano can emit, the single iron atom appears to emit thousands of definite light vibrations.
There were a variety of such musical images and metaphors at the time, all concerned with the ratios, the harmonics, which seemed to lurk in the spectra, and the possibility of expressing them in a formula. The nature of these ‘harmonics’ remained unclear until 1885, when Balmer was able to find a formula relating the position of the four lines in the visible spectrum of hydrogen, a formula that enabled him to predict correctly the existence and position of further lines in the ultraviolet and infrared. Balmer, too, thought in musical terms, and wondered whether it might be ‘possible to interpret the vibrations of the individual spectral lines as overtones of, so to say, one specific keynote.’ That Balmer was on to something of fundamental importance, and not some numerological mumbo jumbo, was immediately recognized, but the implications of his formula were wholly enigmatic – as enigmatic as Kirchhoff’s discovery that the emission and absorption lines of elements were the same.
CHAPTER EIGHTEEN
Cold Fire
My many uncles and aunts and cousins served as a sort of archive or reference library, and I would be referred to different ones for specific problems: most often to Auntie Len, my botanical aunt, who had played such a lifesaving role in the grim days of Braefield, or Uncle Dave, my chemical and mineralogical uncle, but there was also Uncle Abe, my physics uncle, who had started me on spectroscopy. Uncle Abe was consulted rather rarely at first, because he was one of the senior uncles, six years Uncle Dave’s senior and fifteen years my mother’s. He was regarded as the most brilliant of his father’s eighteen children. He was intellectually formidable, although his knowledge had come through a sort of osmosis, not formal training. Like Dave, he had grown up with a taste for physical science, and like Dave, he had gone geologizing to South Africa as a young man.
The great discoveries of X-rays, radioactivity, the electron, and quantum theory had all occurred in his formative years and were to remain central interests for the rest of his life; he had a passion for astronomy and for number theory as well. But he was also perfectly capable of turning his mind to practical and commercial ends too. He played a part in developing Marmite, the widely used vitamin-rich yeast extract developed early in the century (my mother adored this; I hated it), and, in the Second World War, when normal soap was difficult to get, he helped develop an effective fat-free soap.
Though Abe and Dave were alike in some ways (both had the broad Landau face, with wide-set eyes, and the unmistakable, resonant Landau voice – characteristics still marked in the great-great-grandchildren of my grandfather), they were very different in others. Dave was tall and strong, with a military posture (he had served in the Great War and in the Boer War before that), always carefully dressed. He would wear a wing collar and highly polished shoes even when he worked at his lab bench. Abe was smaller, somewhat gnarled and bent (in the years that I knew him), brown and grizzled, like an old shikari, with a hoarse voice and chronic cough; he cared little what he wore, and usually had on a sort of rumpled lab smock.
The two were associated formally as codirectors of Tungstalite, though Abe left the business end to Dave and spent all his time in research. It was he who developed a safe and effective way of ‘pearling’ lightbulbs with hydrofluoric acid in the early 1920s – he had designed the machines to do this in the Hoxton factory. He also worked on the use of ‘getters’ in vacuum tubes – highly reactive, oxygen-hungry metals like cesium and barium which could remove the last traces of air from a tube – and, earlier, he had patented the use of Hertzite, his synthetic crystal, for crystal radios.
He had developed and patented a luminous paint, and this was used in military gunsights in the First World War (it may have been decisive, he told me, in the Battle of Jutland). His paints were also used to illuminate the dials of Ingerso
ll watches and clocks. He had, like Uncle Dave, big, capable hands, but where Uncle Dave’s were seamed with tungsten, Uncle Abe’s were covered with radium burns and malignant warts from his long, careless handling of radioactive materials.
Both Uncle Dave and Uncle Abe were intensely interested in light and lighting, as was their father; but with Dave it was ‘hot’ light, and with Abe ‘cold’ light. Uncle Dave had drawn me into the history of incandescence, of the rare earths and metallic filaments which glowed and incandesced brilliantly when heated. He had inducted me into the energetics of chemical reactions – how heat was absorbed or emitted during the course of these; heat that sometimes became visible as fire and flame.
Through Uncle Abe, I was drawn into the history of ‘cold’ light – luminescence – which started perhaps before there was any language to record things, with observations of fireflies and glowworms and phosphorescent seas; of will-o’ – the-wisps, those strange, wandering, faint globes of light that would, in legend, lure travelers to their doom. And of Saint Elmo’s fire, the eerie luminous discharges that could stream in stormy weather from a ship’s masts, giving its sailors a feeling of bewitchment. There were the auroras, the Northern and Southern Lights, with their curtains of color shimmering high in the sky. A sense of the uncanny, the mysterious, seemed to inhere in these phenomena of cold light – as opposed to the comforting familiarity of fire and warm light.
There was even an element, phosphorus, which glowed spontaneously. Phosphorus attracted me strangely, dangerously, because of its luminosity – I would sometimes slip down to my lab at night to experiment with it. As soon as I had my fume cupboard set up, I put a piece of white phosphorus in water and boiled it, dimming the lights so that I could see the steam coming out of the flask, glowing a soft greenish blue. Another, rather beautiful experiment was boiling phosphorus with caustic potash in a retort – I was remarkably nonchalant about boiling up such virulent substances – and this produced phosphoretted hydrogen (the old term), or phosphine. As the bubbles of phosphine escaped, they took fire spontaneously, forming beautiful rings of white smoke.