In 1835 the philosopher Auguste Comte offered an example of knowledge that would forever lie beyond human reach. We might learn the shapes, distances and motions of the stars, he wrote, but their chemical composition was a question no method could ever answer.
Twenty four years later the method was found, and within a decade it had been used to identify an element that existed nowhere in any laboratory on Earth. Helium was discovered ninety three million miles away, and it took another twenty seven years for anyone to find some.
The Dark Lines
Newton had shown in the Opticks that white light passing through a prism separates into a band of colours. What he did not have was an instrument good enough to reveal that the band is not continuous.
The spectroscope, once it existed, turned out to be one of the few instruments that genuinely changed what humans could see. In 1802 William Wollaston noticed a few dark gaps in the solar spectrum. Joseph von Fraunhofer, a Bavarian optician of extraordinary skill, examined the phenomenon properly from 1814 with instruments of his own manufacture and mapped 574 dark lines, labelling the most prominent with letters that are still in use. The pair of closely spaced lines in the yellow he called D.
Fraunhofer was a manufacturer of optical glass, and his interest was practical: the lines gave him fixed reference wavelengths for measuring the refractive properties of his glass precisely. He measured them, mapped them, and did not know what they were.
Our edition of Newton’s Opticks contains the experiments where the analysis of light by prism was first set out systematically, a century and a half before anyone could read a chemical composition out of the result.
Kirchhoff and Bunsen
The explanation came from Heidelberg in 1859, from a collaboration between the physicist Gustav Kirchhoff and the chemist Robert Bunsen.
Bunsen had developed a gas burner producing an almost colourless flame, which meant that any colour appearing in it came from the substance being burned. Different salts gave different colours, and Kirchhoff suggested passing the light through a prism rather than judging by eye.
The results were unambiguous. Each element, heated until it glows, emits light at a set of specific wavelengths unique to that element. Sodium gives a bright yellow doublet, lithium a red line, potassium lines in the violet. The pattern identifies the substance in the way a fingerprint identifies a person.
Kirchhoff then made the connection that mattered. He passed sunlight through a sodium flame and found that the bright sodium lines and the dark solar D lines fall at precisely the same wavelength, and that the dark lines became darker. A cool gas absorbs exactly the wavelengths that the same gas emits when hot.
The dark lines in the solar spectrum are therefore the signatures of elements in the Sun’s cooler outer layers, absorbing light from the brighter interior. The chemical composition of the Sun can be read off a photographic plate.
The Eclipse of 1868
The Sun’s outer atmosphere is far too faint to observe against the glare of the disc. During a total eclipse it becomes visible, and the prominences, enormous arcs of glowing gas rising from the limb, can be examined directly.
On 18 August 1868 a total eclipse crossed India. Among the observers was the French astronomer Jules Janssen, positioned at Guntur, who trained a spectroscope on the prominences.
He saw bright emission lines, confirming that the prominences are hot gas. Among them was a yellow line very close to the sodium D pair but not coincident with either, at a wavelength of 587.6 nanometres. It was later designated D3.
Janssen also realised something with immediate practical consequences. The prominence lines were bright enough that with a narrow enough slit they might be observable in ordinary daylight, without waiting for an eclipse. He tested the idea the following morning and it worked.
In London, Norman Lockyer had reached the same conclusion independently and was building an instrument for the purpose. On 20 October 1868 he observed the prominences in daylight and recorded the same unexplained yellow line.
The two men’s letters reached the Académie des Sciences in Paris within days of each other. Rather than argue about priority, the Académie struck a medal bearing both their profiles.
An Element Nobody Had
Lockyer, working with the chemist Edward Frankland, tried to match D3 to a known substance. They tested hydrogen under a range of pressures and temperatures, which was the leading candidate, and could not reproduce it.
Lockyer proposed that the line belonged to an element not yet known on Earth, and named it after the Greek word for the Sun. The suffix he chose, applied to what he assumed was a metal, is why helium is the only noble gas whose name ends in ium.
The claim was not well received. Announcing a new element on the basis of a single line, in an object nobody could sample, struck most chemists as unwarranted, and various alternative explanations circulated for the better part of three decades. The periodic table, which Mendeleev published the following year, had no space for it.
Found at Last
The confirmation came in 1895, and it was accidental.
William Ramsay had recently isolated argon with Lord Rayleigh and was hunting for other unreactive gases. He read a report that treating the uranium mineral cleveite with acid released a gas assumed to be nitrogen. He repeated the procedure, collected the gas, purified it, and examined its spectrum.
It showed a bright yellow line. Ramsay could not measure the wavelength precisely enough with his own equipment and sent samples to Lockyer and to William Crookes, who confirmed it: D3, at 587.6 nanometres, in a sealed tube in a London laboratory.
Helium is produced inside uranium ores by radioactive decay and is trapped in the mineral. Rutherford and Thomas Royds established the mechanism in 1908 by collecting the gas emitted from a radium source and showing spectroscopically that alpha particles are helium nuclei.
Terrestrial helium turned out to be commercially significant almost immediately. In 1903 a gas well drilled at Dexter, Kansas, produced a stream that would not burn. Analysis showed nearly two percent helium, and the American plains proved to hold the world’s largest accessible supply.
What Followed
Helium is the second most abundant element in the universe, making up about a quarter of ordinary matter by mass, nearly all of it produced in the first few minutes after the Big Bang. It is rare on Earth only because it is light enough to escape the atmosphere.
Its physical properties opened an entire field. Heike Kamerlingh Onnes liquefied it in 1908 at 4.2 kelvin, the last permanent gas to be liquefied, and the extremely low temperatures this made available led him directly to the discovery of superconductivity three years later. Below 2.17 kelvin, liquid helium becomes a superfluid that flows without viscosity and climbs the walls of its container.
The spectroscopic method that found it went on to become the central tool of astrophysics. Reading the composition of stars from their spectra led to the stellar classification scheme, to the discovery that stars are overwhelmingly hydrogen and helium, and to the whole theory of stellar structure and energy generation. Redshifted spectral lines gave the expansion of the universe. The chemistry of interstellar clouds, of exoplanet atmospheres, and of galaxies at the edge of the observable universe is all read the same way.
Why the Lines Are There
Spectroscopy worked for half a century before anyone knew what produced the lines. Kirchhoff established the rules empirically, and astronomers used them with complete confidence while the underlying mechanism remained a blank.
The explanation arrived with Bohr’s atomic model in 1913. Electrons occupy discrete energy levels, and light is emitted or absorbed only when an electron moves between them, at a frequency fixed by the energy difference and Planck’s constant. Every element has a different arrangement of levels and therefore a different set of lines.
The dark bands Fraunhofer catalogued in 1814 turned out to be direct evidence of quantum mechanics, sitting in plain view a century before the theory existed.
Comte’s Mistake
The interesting thing about Comte’s claim is that it was not obviously foolish. He reasoned that chemistry requires manipulating a substance, and the stars cannot be manipulated. What he could not anticipate was that light itself would turn out to carry a complete chemical record of its source.
Thirty three years after he wrote it, an astronomer in India pointed a prism at the edge of an eclipsed Sun and found an element that no chemist had ever handled. It is the most compact rebuttal in the history of science of the claim that something is unknowable in principle.
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