Uncle Tungsten: Memories of a Chemical Boyhood (2001) (24 page)

All of the tables I made, all of the tables I saw, ended with uncertainty, ended with a question mark, centered around the ‘last’ element, uranium. I was intensely curious about this, about Period 7, which started with the as-yet-unknown alkali metal, element 87, but only got as far as uranium, element 92. Why, I wondered, should it stop here, after only six elements? Could there not be more elements, beyond uranium?

Uranium itself had been placed by Mendeleev under tungsten, the heaviest of the Group VI transition elements, for it was very much like tungsten, chemically. (Tungsten formed a volatile hexafluoride, a very dense vapor, and so did uranium – this compound, UF
₆
, was used in the war to separate out the isotopes of uranium.) Uranium
seemed
like a transition metal,
seemed
like eka-tungsten – and yet, I felt somehow uncomfortable about this, and decided to do a little exploring, to examine the densities and melting points of all the transition metals. As soon as I did this I discovered an anomaly, for where the densities of the metals steadily increased through Periods 4, 5, and 6, they unexpectedly declined when one came to the elements in Period 7. Uranium was actually
less
dense than tungsten, though one would have expected it to be more so (thorium, similarly, was less dense than hafnium, not more so, as one would have expected). It was precisely the same with their melting points: these reached a maximum in Period 6, then suddenly declined.

I was excited about this; I felt I had made a discovery. Was it possible, despite all the similarities between uranium and tungsten, that uranium did
not
in fact belong in the same group, was not even a transition metal at all? Might this also be the case for the other Period 7 elements, thorium and protoactinium, and the (imaginary) elements beyond uranium? Could it be that these elements were instead the beginning of a second rare-earth series precisely analogous to the first one in Period 6? If this was the case, then eka-tungsten would not be uranium, but an as-yet-undiscovered element, which would appear only after the second rare-earth series had completed itself. In 1945, this was still unimaginable, the stuff of science fiction.

I was thrilled, soon after the war, to find that I had guessed right, when it was revealed that Glenn Seaborg and his coworkers in Berkeley had succeeded in making a number of transuranic elements – elements 93, 94, 95, and 96 – and found that these indeed were part of a second series of rare-earth elements (which, by analogy with the first rare-earth series, the lanthanides, he called the actinides).«50»

The number of elements in the second series of rare earths, Seaborg argued, by analogy with the first series, would also be fourteen, and after the fourteenth (element 103) one might expect ten transition elements, and only then the concluding elements of Period 7, ending with an inert gas at element 118. Beyond this, Seaborg suggested, a new period would start, beginning, like all the others, with an alkali metal, element 119.

It seemed that the periodic table might thus be extended to new elements far beyond uranium, elements that might not even exist in nature. Whether there was any limit to such transuranic elements was not clear: perhaps the atoms of such elements would become too big to hold together. But the principle of periodicity was fundamental, and could be extended, it seemed, indefinitely.

While Mendeleev saw the periodic table primarily as a tool for organizing and predicting the properties of the elements, he also felt it embodied a fundamental law, and he wondered on occasion about ‘the invisible world of chemical atoms.’ For the periodic table, it was clear, looked both ways: outward to the manifest properties of the elements, and inward to some as-yet-unknown atomic property which determined these.

In that first, long, rapt encounter in the Science Museum, I was convinced that the periodic table was neither arbitrary nor superficial, but a representation of truths which would never be overturned, but would, on the contrary, continually be confirmed, show new depths with new knowledge, because it was as deep and simple as nature itself. And the perception of this produced in my twelve-year-old self a sort of ecstasy, the sense (in Einstein’s words) that ‘a corner of the great veil had been lifted.’

A Pocket Spectroscope

W
e had always celebrated Guy Fawkes Night, before the war, by setting off fireworks. Bengal lights, burning brilliantly green or red, were my favorites. The green, my mother had told me, was due to an element called barium, the red to strontium. I had no idea at that point what barium and strontium were, but their names, like their colors, stayed in my mind.

When my mother saw how enthralled I was by these lights, she showed me how, if one threw a pinch of salt on the stove, the gas flame suddenly flared and turned a brilliant yellow – this was due to the presence of another element, sodium (even the Romans, she said, had used it to give their fires and flares a richer color). So, in a sense, I was introduced to ‘flame tests’ even before the war, but it was only a few years later, in Uncle Dave’s lab, that I learned they were an essential part of chemical life, an instant way of detecting certain elements, even if present in minute amounts.

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 1830
s
and 1840
s
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 1940
s
), 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 named 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 1890
s
, 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.