158. Uncle Tungsten - XI. Spectroscopy

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Uncle Tungsten

Chapter 17 - A Pocket Spectroscope

If you wanted to invent a family to tell the story of the development of science and the Mephistophic (I think I prefer this to Mephistophelian) world order, you couldn't do any better than the Sacks/Landau extended family.  


p213 Auntie Len had given me James Jean’s book The Stars in Their Courses for my tenth birthday, and I had been intoxicated by the imaginary journey Jeans describes 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.

p214 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... [the book] 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... 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.

p215 What was the origin of the dark lines Fraunhofer saw in 1814?...

In 1859, [Gustav] 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 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. 

Kirchoff did some other interesting things so it's worth following that link above.. 

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.
...
p216 Now, with Uncle Abe’s help -- he had a small observatory on the roof of his house [of course he did], and kept one of his telescopes hitched up to a spectroscope -- I saw this [during a solar eclipse you could see the emission spectra instead of the usual absorption spectra] for myself... [Perhaps he had an aunt who could cause solar eclipses.] 
At this point, [Robert] 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. [Not sure I see the distinction between “new” and “undiscovered” here.] 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.
...
p217 [Bunsen discovers cesium in November 1860. Bunsen and Kirchhoff discover rubidium three months later.] Within a few decades of Bunsen and Kirchhoff’s discoveries twenty more elements were discovered with the aid of spectroscopy -- indium and thallium... gallium, scandium, and germanium (the three elements Mendeleev had predicted), all the remaining rare-earth elements, and, in the 1890s, the inert gases.

...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... 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. 

This is a product of alpha decay in the uranium releasing a pair of protons with two neutrons in tow. All this alpha particle has to do is pick up a couple electrons and it becomes an atom of helium. 
...
p219 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 vibrations 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 [Johann Jacob] 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 overtures 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.


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