MOSELEY, HENRY GWYN JEFFREYS

(b. Weymouth. Dorsetshire, England, 23 November 1887; d. Gelibolu, Gallipoli Peninsula, Turkey, 10 August 1915)

physics.

Like his friends Julian Huxley and Charles Galton Darwin. Harry Moseley came from a family long distinguished for its contributions to science. His paternal grandfather, Canon Henry Moseley (1801–1872), F.R.S., the first professor of natural philosophy at Kings College, London, was an international authority on naval architecture. His maternal grandfather, John Gwyn Jeffreys (1809–1885), F.R.S., was the dean of England’s conchologists. His father, Henry Nottidge Moseley (1844–1891), F.R.S., a protégé of Charles Darwin’s, was the founder of a strong school of zoology at Oxford. Harry inherited a full share of his ancestors’ energy, intelligence, and singleness of purpose.

H. N. Moseley died just before his son’s fourth birthday. The family then moved to Chilworth, near Guildford, in Surrey, where Harry and his two older sisters received their elementary education. The elder sister died in childhood; the other. Margery, four years Harry’s senior, was his constant companion, confi dante, and collaborator in neighborhood bird’s-nesting expeditions. Harry’s mother and such old family friends as E. R. Lankester encouraged the children in their natural-historical interests, which they never discarded. Margery later published a paper about protozoa in the Journal of the Royal Microscopical Society, and Harry, who became an enthusiastic gardener, was to fill his last letters from Gallipoli with observations of the flora near his campsites. At the age of nine Harry was sent to Summer fields near Oxford, a preparatory school that specialized in training King’s Scholars, winners of scholarships to Eton. He and six of his fellow students won seven of the dozen scholarships available for 1901.

Although Edwardian Eton tended to produce over cocky, undereducated sportsmen, those who preferred not to conform, to be “saps” like most King’s Scholars, could obtain a thorough grounding in almost any respectable branch of learning. After passing through the required curriculum with a prize for Latin prose, Harry specialized under T. C. Porter, the discoverer of Porter’s law of flicker and one of the first Englishmen to experiment with X rays. Harry fought with most of the stodgy science masters, but found Porter’s unconventional approach—a minimum of books and a maximum of challenging experiments—exactly to his taste. Before leaving Eton he had begun quantitative analysis, had measured the freezing point of saline solutions of diverse concentrations, and had discovered, to his “astonishment,” that helium, despite its lower boiling point, is denser than hydrogen. For the rest Harry shared quietly in Eton life. He won a place in the college boats, participated in college debates, and found nourishment for his natural patrician aloofness.

In 1906 Harry entered Trinity College, Oxford, on one of the few science scholarships then available in the university. He was the only important early contributor to atomic physics who attended Oxford, which he chose over Cambridge in order to be near his mother, who had returned there in 1902. He read for honors in physics, rowed for his college, and worked in the Balliol-Trinity laboratory where, among other things, he built a primitive Wilson cloud chamber. The official curriculum provided no opportunity for the kind of independent work to which Porter had exposed him; and partly from tension over his course of study he obtained a second-class degree in the bookish Oxford honors schools. Nonetheless with the help of testimonials gathered before the examinations and a visit to Rutherford, he obtained a demonstratorship in the physics department at the University of Manchester for the fall of 1910.

Harry found the students dense and provincial and taught them without enthusiasm. But he worked happily as much as sixteen hours a day on the research problem Rutherford had given him—to determine how many β particles are expelled in the disintegration of one atom of radium B (Pb²¹⁴) or radium C (Bi²¹⁴). No one at Manchester doubted that the answer would be one; but it was an important fact to establish, and gave the neophyte opportunity to master the techniques of high vacuums and radioactivity. Difficulty with the apparatus prevented completion of the work until 1912. Meanwhile Harry also collaborated on small projects that trained him to handle α particles and γ rays. These exercises, typical of Rutherford’s laboratory, came to an end in the spring of 1912, along with Harry’s teaching duties. In the fall he was to enjoy a fellowship established by a Manchester industrialist and the congenial obligation of devoting himself entirely to research.

As Moseley passed from being Rutherford’s apprentice to independent journeyman, news of the success of Laue’s experiment on the diffraction of X rays by crystals reached Manchester. Moseley decided to exercise his new independence in investigating how the wavelike properties of the rays, established by Laue, could be reconciled with the phenomena from which W. H. Bragg had persuasively deduced their particulate nature. Anticipating that he would need the help of a mathematical physicist. Moseley asked C. G. Darwin, then also between researches, to collaborate. They approached Rutherford for permission. The master refused, apparently on the ground that he knew nothing about the subject and consequently could not offer his usual paternal guidance. Moseley insisted, won, and went off to Leeds to W. L. Bragg, who had generously agreed to instruct him. It appears that Moseley’s habit of having his own way and his Etonian experience with circumventing inconvenient masters made possible the researches that made his reputation.

To explain his diffraction patterns Laue assumed that the radiation striking the crystal contained precisely six groups of effective monochromatic rays. Darwin and Moseley rejected the assumption, which indeed was scarcely plausible, and concluded instead that only certain planes through the crystal—namely, those “rich” in atoms—gave rise to sensible interference. W. L. Bragg reached the same conclusion earlier and undertook to confirm it by reflecting X rays from the atom-rich cleavage surface of mica. He found, as he had anticipated, that in reflection the crystal did behave like a pile of semitransparent mirrors, causing interference of reflected radiation of wavelength » incident upon the surface at glancing angle θ in accordance with the formula nλ = 2d sin θ, where n is the order of the interference and d the separation of the atomrich planes. Moseley adopted Bragg’s method with the important exception, typical of Rutherford’s school, of substituting an ionization chamber for a photographic plate as detector of the reflected radiation. In their first research, in which the Braggs again anticipated them, Moseley and Darwin determined that the reflected radiation had the same penetrating power as the incident. They next under took to measure the intensity of reflection I as a function of θ their results, obtained with the sensitive ionization chamber, quickly became standard. The narrowness of their collimation, however, caused them to miss places where I(θ) had sharp peaks. The Braggs found the peaks, which Darwin and Moseley then examined carefully. All agreed in referring the maxima to monochromatic radiation characteristic of their platinum anticathodes and identical to the L rays earlier identified by Barkla.

On the publication of their paper in July 1913, the collaborators separated, Darwin to consider theoretical problems relating to I(θ) and Moseley to map the characteristic K and L spectra of the elements. It appears that Moseley aimed to test the doctrine of atomic number, then recently suggested by van den Broek of Amsterdam, and doubtless also independently invented at Manchester where, besides Moseley, Bohr, Russell, and Hevesy also worked in 1912. According to measurements made at the Cavendish Laboratory in 1911 by R. Whiddington, the minimum velocity w that a cathode ray requires to stimulate the emission of a K ray from a target of atomic weight A is w = A · 10⁸ cm/sec. The doctrine of atomic number, however, would suggest a dependence on Z, rather than on A; and apparently Moseley’s initial intent was to discover, by examining those places in the periodic table where the sequence of weights inverts the chemical order, whether the frequency v_K followed A or Z. Very likely he expected v_K to grow roughly as Z²; for assuming that the entire energy of Whiddington’s electron goes into the K ray,

v_K ≈ (m/2h) A² · 10¹⁶ = 2.7Z² · 10¹⁵ sec^-1,

where the approximation Z = A/2, derived from Rutherford’s famous analysis of α scattering, has been used. By no means did Moseley expect the coefficient of Z² to be exactly constant independent of Z. Among other considerations, the atomic model of his friend Niels Bohr, which assumed that the population of the innermost ring changed with Z, ruled out any such regularity.

Moseley, began his experiments in the fall of 1913, using a clever device that enabled him to bring different anticathodes into position without interrupting the vacuum in his X-ray tube. He photo graphically recorded the position of constructive interference and found the K rays to consist of a soft, intense line, which he called K_α and a harder, weaker satellite K_β. The L rays appeared to be more complicated, a soft intense line L_α and several weaker companions. Measurements of Co and Ni immediately showed, as expected, that v_Kα followed Z. In addition, and most unexpectedly, the frequencies for ten elements from Ca to Zn satisfied to a precision of 0.5 percent the astonishingly simple relation

v_Kα\R = (3/4)(Z – 1)²,

where R stands for the Rydberg frequency. Emboldened by this formula, and especially by the appearance of the Bohr-Balmer coefficient 3R/4, Moseley guessed on very little evidence that

v_Lα/R = (5/36) (Z – 7.4)².

These results were published in the Philosophical Magazine for December 1913, about one month after the photographic measurements began.

Moseley quickly perceived that if these formulas held exactly, they could be used to test the periodic table for completeness; he need only obtain v_Kα and/or v_Lα for the known elements and determine which values of Z (if any) were not represented. The task offered no difficulties of principle. The procure ment of rare substances, however, presented a severe problem which could not be confronted with any particular advantage in Manchester. Since at this juncture (December 1913) Moseley considered that Rutherford had no more to teach him, he decided to resign his fellowship and to migrate to Oxford. There he could be near home and might build up local support for a possible candidacy for the professorship of experimental physics, then still held by its first incumbent, R. B. Clifton, whose overdue resignation was expected momentarily. Moseley obtained permission to work in a private capacity in the new electrical laboratory of Townsend, where he carried his examination of the elements into the fuzzy family of the rare earths.

Chemists then officially recognized thirteen “lanthanides” stretching from cerium to lutetium, the most recently accepted elemental earth, which had been independently isolated in 1906/1907 by Auer von Welsbach and Urbain. There were a great many other putative earths, including Auer’s thulium II and Urbain’s celtium; and it was this confusion that Moseley proposed to resolve with his X-ray machine. After hasty examination he announced with his usual confidence that three elements remained undiscovered between aluminum and gold. Two of these, numbers 43 and 75, corresponded to spaces the chemists had long recognized. The other, 61, lay in the middle of the lanthanides which, according to Moseley. began at cerium 58 and terminated with Auer’s two thuliums, 69 and 70, and Urbain’s ytterbium 71 and lutetium 72.

This assignment, which Moseley deduced without examining any samples of the heaviest earths, annoyed Urbain, as it left no room for celtium and established the second thulium of his rival Auer. In May 1914 Urbain brought his best specimens to Oxford, where the X rays revealed (to his mixed satisfaction) neither celtium nor thulium II nor any trace of element 72 and demonstrated that the known lanthanides terminated at lutetium, which Moseley had given a Z one unit too high. Elements 43 and 61 do not appear to exist naturally; number 75 was found among manganese ores as expected; and hafnium, which is not a rare earth, came to light in Copenhagen in the fall of 1922 after Urbain, in violation of one of Bohr’s theories, had again tried to install celtium as element 72.

Moseley’s formulas not only terminated what Urbain called the “romantic age of chemical discovery”; they also seemed to lift the veil guarding the atom’s innermost recesses. Moseley himself argued that the factors

3R/4 = (1/1² – 1/2²)R

and

5R/36 = (1/2² – 1/3²)R

showed that Bohr’s rules of behavior for the single electron of hydrogen also governed the deep-lying electrons of the heavy elements; in particular, according to Moseley, his formulas confirmed the principle of quantization of the angular momentum. The factor (Z-1)² gave him more trouble; and, in the event, neither he nor Bohr was able to design a plausible model of X-ray emission that satisfied Bohr’s principles and agreed even qualitatively with the Kα formula. Nonetheless Moseley expressed his confidence in a letter to Nature, for which he was roundly and unfairly cudgeled by F. A. Lindemann. In the fall of 1914 W. Kossel supplied a successful qualitative model of X-ray emission. No one, however, has managed to derive Moseley’s formulas from Bohr’s initial principles of atomic structure.

In June 1014 Moseley and his mother left England for a leisurely trip to Australia, where the British Association for the Advancement of Science was to meet in September. When news of the outbreak of war reached him, he determined to return home immediately upon concluding his obligations to the associalion—participation in a discussion on atomic structure and delivery of a paper on the rare earths. He practiced semaphore and Morse code aboard ship and upon arrival rushed to procure a commission in the Royal Engineers. Neither the solicitations of his family nor the initial refusal of the engineers deterred him from what he and his former classmates at Summer fields and Eton considered their duty. His obstinacy eventually secured the commission, and after eight months’ training he was shipped to the Dardanelles as signal officer of the thirty-eighth brigade of the new army. He participated in some minor skirmishes near Cape Helles before his unit went to reinforce the last desperate effort to reach the critical ridge of Sari Bair. He was killed in a furious counterattack led by Kemal Ataturk.

BIBLIOGRAPHY

I. Original Works. Moseley’s important papers are “The Number of β Particles Emitted in the Transformations of Radium,” in Proceedings of the Royal Society of London, 87A (1912), 230–255; “The Attainment of High Potentials by the Use of Radium,” ibid., 88A (1913), 471–476; “The Reflexion of X Rays,” in Philosophical Maga zine, 26 (1913), 210–232, written with C. G. Darwin; “The High-Frequency Spectra of the Elements,” ibid., 1025–1034, and 27 (1914), 703–713, A full list of Moseley’s pub lished writings is in Poggendorff and in J. B. Birks, ed., Rutherford at Manchester (London, 1962), 349. Moseley’s correspondence, scientific and personal, has been published in J. L. Heilbron, H. G. J. Moseley. The Life and Letters of an English Physicist, 1887–1915 (Berkeley, 1974).

II. Secondary Literature. For Moseley’s career and work see the Life and Letters; Rutherford in Proceedings of the Royal Society of London, 93A (1916), xxii-xxviii; C. G. Darwin, “Moseley and the Atomic Number of the Elements,” 17–26 in the volume edited by Birks; and (for high-schoolers) B. Jaffe, Moseley and the Numbering of the Elements (New York, 1971).

J. L. Heilbron