A historical account of Talbot's contributions to spectroscopy
Talbot is one of the early researchers into the field of spectral analysis. Between 1826 and 1836 he made several significant contributions and deserves recognition as one of the founders of modern spectrochemical analysis.
William Henry Fox Talbot is generally regarded as one of the founders of modern photography. Of two biographical encyclopedias of science consulted, only one contains an entry but makes no mention of any contributions to spectroscopy (1). The internet sites are scarcely better (2,3). However, some older texts on spectroscopy do note his experiments with flame spectra (4–6) and the latter two considered him the founder of spectrochemisty, a title traditionally reserved for Kirchhoff and Bunsen. One such claim might well be dismissed, but two would seem to demand further investigation.
English classical scholar, mathematician, scientist, and inventor William Henry Fox Talbot was born on February 11, 1800, in Melbury, Dorset, England. He was the son of William Davenport Talbot, an officer in the Dragoons, and Lady Elizabeth Fox Stangeways, daughter of the second Earl of Ilchester. His father died when he was but five months old. However, to his good fortune, his stepfather — his mother remarried in 1804 — treated him with love (3).
Talbot entered Trinity College in Cambridge, England, in 1817 where he won prizes in Greek verse. He graduated with classical honors in 1921 and was 12th in his class in mathematics (3).
In 1826 Talbot published a paper entitled "Some Experiments on Coloured Flames." He described using an alcohol burner and simple spectroscope to observe the flames produced by salts of sodium, potassium, and strontium. Discussing the red line observed with the flame of niter or of a chlorate of potash, Talbot wrote
The red ray appears to possess a definite refrangibility and to be characteristic of the salts of potash, as the yellow ray is to the salts of soda. . . . If this should be admitted I would further suggest that whenever the prism shows a homogeneous ray of any colour to exist in a flame, this ray indicates the formation or the presence of a definite chemical compound.
In 1834, in a paper entitled "Facts Relating to Optical Science," Talbot noted that the flame spectra of strontium and lithium salts were visually indistinguishable. He wrote
The strontia flame exhibits a great number of red rays well separated from each other by dark intervals. . . . The lithia exhibits one single red ray. Hence I hesitate not to say that optical analysis can distinguish the minutest portions of these two substances from each other with as much certainty, if not more, than any other known method.
It is for this statement that Twyman (6) considers Talbot to be the discoverer of spectrochemical analysis.
In 1835 Talbot wrote on the nature of the continuous spectrum, which he correctly attributed to the heating of matter:
In short, we see that the mere presence of the lime in a heated state is the cause of the light . . . the emission of intense light by a particle of lime in this experiment without the loss of any portion of its own substance, arises from the cause above referred to — namely that the heat throws the molecules of lime into a state of such rapid vibration that they become capable of influencing the surrounding aetherial medium and producing in it the undulations of light.
Later in the same paper he comments on the dark lines in the spectrum produced by iodine vapor: "I have found by careful observation that they are not equidistant, but that they become gradually more crowded towards the blue end of the spectrum. This . . . seems a consequence of some simple general law."
Still later he notes, :
. . . I have advanced the hypothesis that the vibrations of light and those of material molecules are capable of mutually influencing each other. It remains to be seen whether the same hypothesis does not afford a clue to the explanation of this apparently complex phenomenon of absorption. . . . Let us suppose that iodine vapour is so constituted that its molecules are disposed to vibrate with a rapidity not altogether dissimilar to that of light. Now, if the different coloured rays differ also (as is probable) in rapidity of vibration, some of them will vibrate in accordance and others in discordance, with the vibrations of the iodine gas, and these accordances will succeed each other in regular order from the red end of the spectrum to the violet end; each discordance marked by a dark line or deficiency in the spectrum, because the corresponding ray is not able to vibrate through the medium but is arrested by it and absorbed.
The quotations above are all from Frank Twyman (6), managing director of Hilger Ltd. for many years, who was a significant player in the development of spectroscopic instrumentation in the early decades of the 20th century. Inasmuch as this author has no access to the originals, the papers quoted by Twyman are as follows: Brewster J. Sci. 5, 77 (1826); Phil. Mag. 3(4), 112–114, (1834); Phil. Mag. 3(7), 113 (1834).
Of course, Talbot's experiments were not performed in a vacuum. In 1802, British physicist Thomas Young (1773–1829) demonstrated the phenomenon of interference which could only be explained by the wave theory of light. He also produced results from his transmission diffraction grating to assign wavelengths to the colors of the solar spectrum. In 1814, the German optician Joseph von Frauenhofer (1787–1826) independently invented the diffraction grating. With his gratings, he made a detailed study of the dark lines in the solar spectrum and produced some astonishingly accurate wavelength determinations.
John Frederick William Herschel (1792–1871) published the results of some early experiments with flames in 1823. On a trip to the continent in 1924, Talbot met John Herschel, son of Sir Frederick William Herschel, in Munich, Germany. They shared many scientific interests and became good friends. Shortly after this meeting Talbot turned his attention to optics (3). Three years later we had Talbot's first paper on flame spectra, which was published by the Scottish physicist, David Brewster who was also experimenting with light spectra at this time.
In 1835, Charles Wheatstone (later to be knighted for his many scientific contributions) provided a very short communication concerning his observations of electrically excited samples (6):
The spectrum of the electro-magnetic spark taken from Mercury consists of seven definite rays only, separated by dark intervals from each other. . . . The spark taken in the same manner from Zinc, Cadmium, Tin, Bismuth, and Lead . . . gives similar results; but the number, position and colours of the lines varies in each case; the appearances are so different, that, by this mode of examination, the metals may be readily distinguished from each other.
In a paper published in 1840, Herschel investigated the action of spectral lines in the UV region on photographic film. He noted that his photographic apparatus was less sensitive than the experimental set-up used by Talbot and adopted Talbot's process.
Perhaps also significant are the many honors Talbot received during his long life. He was elected a member of the newly founded Royal Astronomical Society in 1822 and was elected a Fellow of the Royal Society in 1831 for his mathematical work. He received the Royal Medal in 1838 and the Rumford Medal in 1842. Talbot was well-known, well-connected, and respected. He had contact with at least two, but probably more, of the scientists who were at the same time pursuing investigations into spectrum analysis.
Talbot observed that molecular compounds emit light with which they can be identified. What he didn't realize was that it was the atoms of the molecules dissociated by the heat of the flame that were emitting the light because these observations were of the visible part of the spectrum, and he observed lines, not the bands usually associated with molecules. Nevertheless, he recognized that characteristic spectral lines exist that may be used to identify a material.
He gives a good account of the continuous spectrum. However, he is not quite on the mark with his attempted explanation of absorption.
Similar results to his had been produced by others working around the same time. However, his thinking or theorizing was ahead of the game. We conclude that he was a major contributor to the founding of spectrochemical analysis and as such deserves more credit that traditionally afforded him by the spectrochemical community.
However, to equate his many achievements with those of Kirchhoff would be erroneous. R.A. Sawyer noted that "There seems no doubt that while others . . . may have been near the truth or parts of it, it was Kirchhoff who had the vision and ability to state the general laws so clearly and convincingly as to attract the attention of the scientific world" (4).
Talbot invented the "calotype" photographic process, which helped lay the foundation for modern photography. He produced the earliest known photograph on paper in 1834. We will not belabor this field because it is well covered in the available biographies. In the collection of "Fathers of Photography" at the South Kensington Museum there is a portrait of William Henry Fox Talbot.
By 1824, Talbot had published six papers on mathematical topics, primarily on elliptic integrals. The Talbot curve is named after him.
In optical physics we find the Talbot effect, a near-field diffraction effect first observed by him in 1836.
In the field of astronomy, he published a paper in 1842 entitled, "On the Improvement of the Telescope." Some years later in 1871 he produced the paper "On a New Method of Estimating the Distance of some of the Fixed Stars." A crater on the moon is named after him.
Talbot published papers on archaeology and, together with Sir Henry Rawlinson and Dr. Edward Hinks, was one of the first to decipher the cuneiform writings from Nineveh.
He also published several books on classical scholarly subjects. Perhaps the most notable is The Pencil of Nature (1844–1846, published in six parts) which is the first book ever illustrated by photographs (6). Other works include Legendary Tales in Verse and Prose (1830), Hermes, or Classical and Antiquarian Researches (1838–1839), The Antiquity of the Book of Genesis (1839), English Etymologies (1847), and Assyrian Texts Translated (1856).
A brilliant and versatile man, Talbot made many contributions in several different fields of endeavor. He was a key figure in the founding of both photography and spectrochemical analysis. Talbot died on September 17, 1877, in Lacock Abbey (near Chippenham), Wiltshire, England.
(1) R. Porter and M. Ogilvie, Eds., The Hutchinson Dictionary of Scientific Biography, two volumes, third edition (Helicon Publishing, 2000).
(2) Wikipedia (www.wikipedia.org
(3) J.J. O'Connor and E.F. Robertson, The MacTutor History of Mathematics Archive (http://www-history.mcs.st-andrews.ac.uk/index.html).
(4) R.A. Sawyer, Experimental Spectroscopy, (Prentice-Hall, New York, 1944).
(5) N.H. Nachtrieb, Principles and Practice of Spectrochemical Analysis (McGraw-Hill, New York, 1950).
(6) F. Twyman, The Spectrochemical Analysis of Metals and Alloys (Chemical Publishing Co., New York, 1941).
Volker B.E. Thomsen a physicist by training, has some 30 years of experience in elemental spectrochemical analysis (OES and XRF). He is currently a consultant in this area from his home in Atibaia, São Paulo, Brazil. His other interests include mineralogy and history of science. Occasionally, he still plays the blues harmonica. He can be reached at email@example.com
Volker B.E. Thomsen