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 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 1890s, the inert gases.
51
Auguste Comte had written, in his 1835
On the subject of the stars, all investigations which are not ultimately reducible to simple visual observations are… necessarily denied to us. While we can conceive of the possibility of determining their shapes, their sizes, and their motions, we shall never be able by any means to study their chemical composition or mineralogical content.