CHEMISTRY AND THE SPECTRUM BEFORE BUNSEN AND KIRCHHOFF' TILLMON H. PEARSON and AARON 1. IHDE University of Wisconsin, Madison, Wisconsin
PRODUCTION of colors by refraction was known to Seneca (1). Kepler (2) made use of an equilateral glass prism for the study of the subject. The Archbishop of Spalatro, Marco Antonio de Dominis (S), explained the rainbow by postulating that the light reflected from the inner surfaces of raindrops was colored by passing through different thicknesses of water. Descartes (4) improved the explanation and made it in terms of refrangibility. He also calculated the angle of the bow. It was Newton (6),however, who recognized the significance of the phenomenon. He wrote: "I procured me a Triangular glass-Prisme to try therewith the celebrated Phoenemena of Colours." After sunlight from a circular hole in the shutter had passed through his prism he observed, as others had before him, an elongated and rainbow-hued spot on the wall. His sea~chfor understanding was aided by a second prism which revealed that the various colors of the spectrum could be refracted again but without further change in color. The colors of the blue end were refracted most and those of the red end least, just as had happened when the light passed through the first prism. Newton also observed that the second prism could be used to recombine into white light the colors separated by the first prism. The ideas of Newton on light were immediately attacked. Robert Hooke, who had a color theory of his own as the result of his work on thin films, was especially vociferous in his denunciation. The reception of his first wientific p p e r c?usrtl Xewron to withdr:rw from scitmrific discussi~~n.Most of his l a t r r publications came only after prolonged persuasion on the part of his friends. I t is of interest to note that his book, "Optiks," was only published in 1704, the year after Hooke's death. The eighteenth century saw little progress in the further understanding of light. Scheele (6) in 1777 reported the effects of spectral colors on the darkening of silver chloride. It was well known among silver miners that a certain ore found as a white mineral, horn silver, turned dark upon exposure to sunlight. Scheele exposed samples of the chloride to the various colors of the spectrum, noting that the darkening was most rapid a t the violet end. His results were verified and extended in 1801 bv Johann Wilhelm Rit,t,er (7) who found the dark region beyond the violet portion'oi the spectrum still more effective in producing darkenine.
than the visible violet. The same discovery was made independently by William Hyde Wollaston (8).
It was the discovery of the infrared region of the spectrum a year earlier which had prompted Ritter to investigate the possible exist,ence of radiant energy beyond the violet. The famed astronomer, William Herschel (9) in studying the illuminating and heating power of the various colors of the spectrum observed a steady increase in heating power from the violet t o the red. He used the arrangement illustrated (Figure 1). The blackened bulb of thermometer 1 was placed in the color range under test while thermometers 2 and 3 t o the side of the spectral area served as controls. When the bulb of thermometer 1 was placed in the dark region beyond the visible red it showed an even faster temperature rise than it did when the red color fell upon it, revealing the passage of radiant heat of low refrangibility through the prism. Wollaston's paper (8) verifying the existence of ultraviolet light also reported the presence of dark lines in the spectrum of sunlight. Up to this time investigators had been working with highly impure spectra resulting from circular openings or wide slits admitting the beam of light. Wollaston used a narrow slit, not more than a twentieth of an inch in width. When he viewed the light from the slit a t the distance of 10 or 12 feet, through a flint-glass prism held near the eye, he saw the beam separated into red, yellowish . . *
'Presented before the Division of the History of Chemistry at the 118th Meeting of the American Chemical Society, Chicago, Illinois, September, 1950. 261
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green, blue, and violet. In the resulting spectrum he reported seven dark lines. Fk-e of the lines were reported as being on the boundaries of the colors observed, but the other two u-ere observed within the yellowish green and the blue regions. These lines, plus many additional ones, were independently rediscovered in 1814 by Frannhofer (10). The young lens manufacturer, in studying the refractive index of glass samples for particular colors, noticed a bright double line (of sodium) in the spectrum of the flame which he was using. This same double line was observed in alcohol and sulfur flames as well as in oil and tallow light in exactly the same position and was therefore useful in the determination of indexes. In order to find out if a similar line could be observed in the solar spectrum, he allowed sunlight to pass through a narrow slit and fall upon a flint-glass prism set in front of a telescope. Instead of a bright yellow double line he observed countless vertical liues darker than the colored parts of the spectrum in which they occurred. Eight of the most prominent lines were labeled A to H, and nearly 600 lines were observed, all but the less definite ones being mapped (Figure 2).
Pipur. 2.
Fraunhofer's Map of the Solar Spastrum
He reported on the effect of slit width and types of prisms on spectral purity. His experiments convinced him that the lines were due to the nature of sunlight rather than to diffraction caused by the slit or to optical illusions. During the remaining years of his life he continued his studies of spectra without ever realizing the significance of the lines which today bear his name. The spectra of light from several of the heavenly bodies were also observed by Fraunhofer (10, 11). The spectrum of moonlight showed the stronger lines of sunlight in the same place. The planets Venus and Mars showed fainter spectra with lines D, E, b, and F being positively identified. Study of the spectra of the stars was di5cult but revealed a lack of similarity with the solar spectrum. It was not possible to distinguish lines in the orange and yellow for Sirius but there was a strong streak in the green and two streaks in the blue which seemed unlike any of the lines in planetary spectra. Castor showed a spectrum similar to Sirius. The D line was identified in the spectrum of Pollux and Betelgeuse and possibly in that of Procyon. In 1821 Fraunhofer (12) reported experiments with diffraction gratings. His first gratings were made of wires wrapped closely around a flat frame. Later he made gratings by ruling lines on glass covered with gold foil. With his gratings he determined the wave length of the l i e labeled D. With gratings made of wires ranging between 0.04 and 0.6 mm. in thickness he
obtained values lying between 0.0005882 and 0.0005897 with a mean of 0.0005888 mrn. The accepted modern value is 0.0005896 mm. for Dl and 0.0005889mm. for Do (1s). The phenomenon of diffraction, first reported in 1665 in the posthumous treatise of the Jesuit mathematics professor a t Bologna, Francesco Grimaldi (I$), had received little attention during the century in which Newton's corpuscular theory of light had been dominant. Now, with the re-emergence of the undulatory theory as a result of the work of Young and Fresnel, diffraction took on a new significance and the diffraction grating was destined to play an important role in the study of spectra. The extension of Fraunhofer lines into the infrared was shown by John Herschel (15), son of William, in 1840. He covered a paper with gum and lampblack to make it readily absorptive of heat. After dipping in alcohol the paper was exposed to the spectrum of the sun. The alcohol, instead of evaporating evenly in the dark region beyond the visible red, as would have been the case if the spectrum were continuous, evaporated unevenly, leaving several moist patches which indicated that the dark lines apparently carried beyond the visible spectrum. Later experiments with more refined techniques soon verified this assumption. David Brewster (16) reported in 1834 on lines produced by sunlight passed through "nitrous acid gas." He stated as the object of his inquiries, '%he discovery of a general principle of chemical analysis in which simple and compound bodies might be characterized by their action on definite parts of the spectrum." After first observing that sulfur attacked the violet end of the spectrum of lamp light and iodine vapor the middle part, he turned his attention to the brown oxide of nitrogen. The spectrum was crossed by hundreds of lines or bands. Upon increasing the thickness of the gas the lines grew more distinct in the red and yellow region and broadened in the blue and violet. The same effect was noted when the thickness of the gas remained constant but the gas was heated, an experiment with attendant dangers since the tubes frequently exploded. Brewster reported the noncoincidence of the lines with the lines of the solar spectrum and the absence of the liues in Fraunhofer's map. Brewster's work was immediately extended by John Frederic Daniel1 and William Hallows Miller (17) who passed the light of a gas lamp through halogen vapors before examining the prismatic spectrum with a small telescope. In the spectrum of bromine vapor in air they observed the colors interrupted by more than 100 lines. When the concentration of bromine was increased, the blue end disappeared while the lines in the red grew stronger. They also investigated the absorption of iodine vapor and chlorine. The investigation of absorption spectra was resumed some years later by William Allen Miller (18). None of the fourteen colorless gases studied showed lines. When colorless elements were united, however, they
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might form colored gases which did show lines while colored elements like iodine might show no lines when in gaseous compounds. The characteristic bright lines in the spectra of certain metallic salts on heating were first observed by Thomas Melvill (19) in 1752. This young Scottish investigator died during the next year and his observations apparently went unnoticed. The difference in flame colors given by sodium and potassium salts appears to have been used as a means of distinguishing salts of these elements by Marggraf (20) by 1758. John Herschel (21) reported in 1822 that the flame colors of the chlorides of strontium, calcium; barium, and copper, and that of boric acid were resolved on passing through a prism, bright lines being apparent on a dark background. He believed the bright lines would be useful in detecting small quantities of a substance and touched upon the subject in his
