Spectral analysis
Chemical analysis by light: each element in a flame gives its own set of bright lines, and the same element as a gas absorbs the same lines.
In autumn 1859 the German physicist Gustav Kirchhoff used this to explain Fraunhofer's dark lines (1814) in the solar spectrum, and in 1860 he and the chemist Robert Bunsen published a method of chemical analysis by spectra.
The composition of matter could now be read from its light, including the Sun and the stars: caesium and rubidium were discovered this way in 1860 and 1861. The same work produced Kirchhoff's radiation law and the notion of the black body, whose spectrum forty years later led to the quantum.
Why this date
exact: Kirchhoff's communication on the Fraunhofer lines to the Berlin Academy in autumn 1859; joint paper by Kirchhoff and Bunsen, Chemische Analyse durch Spectralbeobachtungen, Annalen der Physik 1860; predecessor: Fraunhofer's dark lines 1814 (published 1817)
What it grew from
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Transparent blown glass
prerequisite for · Confidence: medium
The spectroscope is a glass prism and the glass lenses of a collimator and a telescope; Fraunhofer's dark lines, whose explanation started spectral analysis, were noticed while measuring the refraction of different kinds of glass.
Dispute: No historian has been found who asserts the spectroscope's dependence on glass as such: it is known only that the Fraunhofer lines were discovered while testing glass and that the Kirchhoff-Bunsen instrument consists of telescopes with lenses and a prism. The 1860 prism was hollow, made of glass plates and filled with carbon disulphide, so the dispersing medium was not glass; glass (flint) prisms were used by Fraunhofer and later by Steinheil.
Link sources
- Jenkins R. V. «Fraunhofer, Joseph», Dictionary of Scientific Biography (via Encyclopedia.com) Verified 20 Sep 2026, quotation assembled from fragments
In order to determine precisely the optical constants of glass, Fraunhofer in 1814 used the two bright yellow lines in flame spectra as a source of monochromatic light. ... While conducting these tests, he observed the effect of the refracting medium on light, comparing the effect of light from flames with light from the sun, and found that the solar spectrum was crossed with many fine dark lines.
- Kirchhoff G., Bunsen R. «Chemische Analyse durch Spectralbeobachtungen», Annalen der Physik und Chemie 110 (1860) (1860) Verified 20 Sep 2026
Between the objective lenses of telescopes B and C is the hollow prism F of 60° refractive angle; it is filled with carbon disulphide.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
- Jenkins R. V. «Fraunhofer, Joseph», Dictionary of Scientific Biography (via Encyclopedia.com) Verified 20 Sep 2026, quotation assembled from fragments
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Dark lines of the solar spectrum
prerequisite for · Confidence: high
Spectral analysis grew directly out of the explanation of Fraunhofer's lines: Kirchhoff showed that the dark D lines of the Sun stand exactly where the yellow lines of a sodium flame stand and arise from absorption, and his first communication of 1859 was titled precisely On the Fraunhofer lines.
Link sources
- Rosenfeld L. «Kirchhoff, Gustav Robert», Dictionary of Scientific Biography (Encyclopedia.com) (1973) Verified 20 Sep 2026
It had long been known that the dark D lines, noticed in the solar spectrum by Fraunhofer (1814), coincided with the yellow lines emitted by flames containing sodium.
- Rosenfeld L. «Kirchhoff, Gustav Robert», Dictionary of Scientific Biography (1973) Verified 20 Sep 2026
In particular, the interposition of a sodium flame of low temperature is sufficient to produce artificially the dark D lines in the spectrum of an intense light source which did not show them originally.
- «Kirchhoff, Gustav Robert», Encyclopedia of World Biography (1859) Verified 20 Sep 2026
the dark lines [Fraunhofer lines] of the solar spectrum which are not caused by the terrestrial atmosphere, arise from the presence in the glowing solar atmosphere of those substances which in a flame produce bright lines in the same position.
- Britannica, «Spectroscopy: Historical survey» (2024) Verified 20 Sep 2026
Joseph von Fraunhofer, repeating more carefully an earlier experiment by a British scientist, William Wollaston, had shown in 1814 that the spectrum of the Sun's electromagnetic radiation does not grade smoothly from one colour to the next but has many dark lines, indicating that light is missing at certain wavelengths because of absorption.
- Jenkins R. V. «Fraunhofer, Joseph», Dictionary of Scientific Biography (via Encyclopedia.com) Verified 20 Sep 2026
Somewhat later he noted that some of these lines appeared to correspond to the bright doublet of lines in many flame spectra; yet he noted further that while the pattern observed for the sun and planets appeared identical, the patterns for the sun, Sirius, and other bright stars differed from one another.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
Further reading
Not evidence: these carry no quotation and support nothing we wrote. They are books and scans to read next.
- Rosenfeld L. «Kirchhoff, Gustav Robert», Dictionary of Scientific Biography (Encyclopedia.com) (1973) Verified 20 Sep 2026
What grew out of it
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Periodic law
influenced · Confidence: medium
Spectral analysis yielded four new elements (caesium 1860, rubidium 1861, thallium 1861, indium 1863) that entered the 1869 table: the pair Rb, Cs serves Mendeleev as an example of regular weight increments among analogues, while thallium and indium he places wrongly and corrects in 1871; later spectroscopy also yielded gallium (1875), the first confirmation of the predictions. No direct statement by a historian on the influence of spectroscopy on the discovery of the law has been found.
Link sources
- Mendelejeff D., Zeitschrift für Chemie 12, 405-406, 2 (1869) Verified 20 Sep 2026
Chemically analogous elements have either similar atomic weights (Pt, Ir, Os), or weights which increase by equal increments (K, Rb, Cs).
- Giunta C., 1869 (2024) Verified 20 Sep 2026
The element which belongs between cadmium (Cd) and tin (Sn) is indium (In), and Mendeleev put indium there in the next version of his table.
- Science History Institute, «Robert Bunsen and Gustav Kirchhoff» (2023) Verified 20 Sep 2026
These discoveries inaugurated a new era in the means used to find new elements.
- Lente G. «Where Mendeleev was wrong: predicted elements that have never been found», ChemTexts 5 (2019) Verified 20 Sep 2026, quotation assembled from fragments
the introduction of the spectroscopic method in 1860 ... reduced the incidence of false element identifications
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
- Mendelejeff D., Zeitschrift für Chemie 12, 405-406, 2 (1869) Verified 20 Sep 2026
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Quantum of action
influenced · Confidence: medium
Kirchhoff's spectral work of 1859 produced the radiation law and the notion of the black body; the energy distribution of the black body over wavelength, measured at the Physikalisch-Technische Reichsanstalt (Lummer and Pringsheim, Rubens and Kurlbaum, 1899-1900), is the very spectrum for which Planck derived his formula with the quantum of action.
Dispute: The line of descent holds, but it runs not through chemical spectral analysis (the lines of the elements) but through Kirchhoff's radiation law and measurements of the spectral energy distribution of the black body. The decisive measurements by Rubens and Kurlbaum (October 1900) used the residual-ray method (multiple reflection from crystals), not a prism spectroscope; prism spectrometers were used by Lummer and Pringsheim (1899). Historians (Kragh, DSB) trace the quantum back to Kirchhoff's law of 1859, not to the spectroscope as an instrument.
Link sources
- Kragh H. «Max Planck: the reluctant revolutionary», Physics World, December 2000 (2000) Verified 20 Sep 2026
The study of black-body radiation had begun in 1859, when Robert Kirchhoff, Planck's predecessor as professor of physics in Berlin, argued that such radiation was of a fundamental nature.
- Encyclopedia of World Biography, «Gustav Robert Kirchhoff» Verified 20 Sep 2026, quotation assembled from fragments
Spectroscopy turned out to be the magic key...half a century later it ushered in the era of modern atomic physics.
- Deutsches Patent- und Markenamt, «Gustav Robert Kirchhoff» (2024) Verified 20 Sep 2026
This direction of development ultimately led to Planck's quantum hypothesis. Kirchhoff is therefore one of its pioneers.
- Kangro H. «Rubens, Heinrich», Dictionary of Scientific Biography (Encyclopedia.com) (1975) Verified 20 Sep 2026
sets out the decisive measurements that led to Planck's new radiation formula.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
- Kragh H. «Max Planck: the reluctant revolutionary», Physics World, December 2000 (2000) Verified 20 Sep 2026
Node sources
- Kirchhoff G., Bunsen R. «Chemische Analyse durch Spectralbeobachtungen», Annalen der Physik und Chemie 110 (1860) (1860) Verified 20 Sep 2026
A method of qualitative analysis can be based on these lines, whereby the field of chemical reactions is greatly widened and hitherto inaccessible problems are solved.
- Rosenfeld L. «Kirchhoff, Gustav Robert», Dictionary of Scientific Biography (Encyclopedia.com) (1973) Verified 20 Sep 2026
In the course of his preparatory work in the autumn of 1859, Kirchhoff made an unexpected observation.
- Britannica, «Gustav Kirchhoff» (2024) Verified 24 Sep 2026
They discovered two new elements, cesium (1860) and rubidium (1861).
- Britannica, «Spectroscopy: Historical survey» (2024) Verified 20 Sep 2026, quotation assembled from fragments
Joseph von Fraunhofer...had shown in 1814 that the spectrum of the Sun's electromagnetic radiation does not grade smoothly from one colour to the next but has many dark lines
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
Further reading
Not evidence: these carry no quotation and support nothing we wrote. They are books and scans to read next.