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In this installment on the history of atom theory, physics professor (and my dad) Dean Zollman discusses how a chemist and a physicist teamed up to figure out why dark lines appeared in spectra.

By Dean Zollman

Dean ZollmanLast month, we saw one of the mysteries in the early days of studying light emitted by matter. When scientists looked at the spectrum of the sun, they saw dark lines in the rainbow of colors. The lines were named after Joseph von Fraunhofer (1787 – 1826) because he investigated them thoroughly. The mystery came when Fraunhofer compared the wavelengths of the dark lines with those of bright lines found in light emitted by certain salts when they were burned. Why did some of the wavelengths of the dark lines in the solar spectrum match the bright lines emitted by matter when it was heated in the laboratory?

Gustav Robert Kirchhoff and Robert Bunsen

Gustav Robert Kirchhoff (left) and Robert Bunsen (Courtesy Edgar Fahs Smith Memorial Collection, Department of Special Collections, University of Pennsylvania Library, public domain, via Wikimedia Commons)

Sorting out this mystery took a physicist and a chemist who today are known primarily for other contributions in advancing science during the 19th century. Gustav Kirchhoff (1824 – 1887) was the physicist. In addition to his work in spectroscopy, he developed rules for analyzing electrical circuits and an equation for the heat released in a chemical reaction. Robert Bunsen (1811 – 1899), the chemist, is most famous for the Bunsen burner, the device used by all of us who took a chemistry course in high school or college. Its most important property is that when properly adjusted it has a colorless flame.

Bunsen met Kirchhoff while he was on a travel grant and visited Breslau (now Wroclaw, Poland), where Kirchhoff was a professor. One account that I read said that Bunsen’s most important discovery while traveling was Kirchhoff. Later, Bunsen became a professor of chemistry at the University of Heidelberg and arranged for Kirchhoff to become a professor of physics in Heidelberg. There, they collaborated on measuring and understanding the light coming from matter.

Hold the Salt

Prior to Bunsen and Kirchhoff’s studies, an important development by William Swan (1818 – 1894) set the stage nicely. We saw in the last post that two closely spaced bright yellow lines were present in almost all spectra that scientists looked at. Swan did a series of carefully controlled experiments to identify those lines. He used a Bunsen burner with its colorless flame and put the substances on a platinum wire because platinum did not emit light.

In his experiments, he would dip a wire in distilled water and look at the light emission. Then, he would try minute quantities of a salt. He found that even the smallest amount of sodium chloride (ordinary table salt) would produce the yellow lines. Thus, he could conclude that the yellow light that seemed to come from everything was actually coming from sodium chloride. He was able to narrow the emission to the sodium in the salt. This work was confirmed by Volkert van der Willigen (1822 – 1878).

The conclusion was that previous investigators had not been careful in making their substances pure. They always had some contamination of salt. So, they always saw that yellow light and frequently got somewhat confusing results because of the impurities. (If you ever did flames tests in high school chemistry, you probably had the same problem. I certainly did.)

At that time the reigning expert on creating pure substances was Robert Bunsen. He and Kirchhoff teamed up to investigate the spectra of various elements. They devised a spectroscope which is shown below. This drawing appeared in an article published in Annalen der Physik und der Chemie in 1860. The item label F in the middle is the prism that broke the light into it constituent colors.


Kirchhoff-Bunsen spectroscope (from Annalen der Physik und der Chemie, Poggendorff, Vol. 110, 1860, public domain image via Wikimedia Commons)

These studies showed that each metal emitted a unique spectrum when it was heated in a flame. Now a spectrum could be used to identify the components of a substance. Further, one could discover new materials by finding lines in spectra which were not identified with any known substance. Bunsen and Kirchhoff discovered two previously unidentified elements – cesium and rubidium. Each of these elements was named for some of the light that it emitted. Cesium is derived caesius, the Latin word for sky blue. The name rubidium is related to the Latin word for dark red.

In the Annalen der Physik und der Chemie paper, they stated “Spectrum analysis, which, as we hope we have shown, offers a wonderfully simple means for discovering the smallest traces of certain elements in terrestrial substances, also opens to chemical research a hitherto completely closed region extending far beyond the limits of the Earth and even of the solar system.” They were certainly correct. Today spectral analysis is an important part of astronomy research as well as terrestrial bound investigations. (English translations from The Chem Team.)

About Those Black Lines

Kirchhoff then began a series of experiments to try to understand the dark lines in the solar spectrum. His basic process was to pass the solar spectrum through a flame that came from a burning element. He directed light from the sun through a sodium flame and noticed some interesting results.

The real breakthrough came when he passed the light through a lithium flame. The normal solar light had no dark lines corresponding the emission lines of lithium. Yet, after sunlight had passed through the lithium, it had dark lines that corresponded to the emission spectrum of lithium.

Even then the process of getting to the explanation took a while. Eventually Kirchhoff came to the conclusion that the lithium was absorbing some of the solar spectrum and reemitting in directions different from the one that the sunlight was travelling. This result led to the conclusion that sunlight was passing through gases before it reached the Earth. The sunlight started as a full spectrum with no dark lines when it left the surface of the sun. As it passes through the atmosphere around the sun, the elements in that atmosphere absorb some of the light. That absorption leads to the dark lines which Fraunhofer had studied earlier in the 19th century.

With these studies Bunsen and Kirchhoff connected the dark lines and the bright lines. What was doing the absorption (atoms, molecules, something else) was not clearly understood. In fact, I have avoided using the word atom in this post because it was generally not part of the explanations given by Kirchhoff and Bunsen.

And then there is the question of the mechanism of emission and absorption. What happens during either emission or absorption of light by an element? We have another half a century before we will get to an answer of that question. In the meantime, we will look at progress toward understanding that atoms were involved.


What Are Things Made of? Depends on When You Ask.

Ancient Greeks Were the First to Hypothesize Atoms

The Poetry of Atoms

Atom Theory in Ancient India

Religion, Science Clashed over Atoms

Medieval Arabic Scholarship Might Have Preserved Scientific Knowledge

Rediscovering a Roman Poet – and Atom Theory – Centuries Later

Reconciling Atom Theory with Religion

Did Atom Theory Play a Role in Galileo’s Trouble with the Inquisition?

Did Gifted Scientist’s Belief in Atoms Led to His Obscurity?

Does Atom Theory Apply to the Earthly and the Divine?

A Duchess Inspired by Atoms

Separating Atoms from Atheism

Isaac Newton: 300 Years Ahead of His Time

Issac Newton and the Philosopher’s Stone

When Chemistry and Physics Split

Redefining Elements

Mme Lavoisier: Partner in Science, Partner in Life

With Atoms, Proportionality and Simplicity Rule

Despite Evidence of Atoms, 19th Century Skeptics Didn’t Budge

Mission of the First International Scientific Conference: Clear up Confusion

Rivalry over the First Periodic Table

The Puzzle of Dark Lines amid Rainbow Colors

Dean Zollman is university distinguished professor of physics at Kansas State University where he has been a faculty member for more than 40 years. During his career he has received four major awards — the American Association of Physics Teachers’ Oersted Medal (2014), the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and AAPT’s Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.