Friday, May 24, 2013

Something to Bragg about

This year isn’t only the diamond anniversary of Watson & Crick’s DNA structure but also the centenary of the technique that enabled them to get it: X-ray crystallography, devised by William and Lawrence Bragg. I have been asked by the University of Leeds, where Bragg Snr did that work, to write a piece for their magazine celebrating that achievement. The final piece will look somewhat different to this, since it will include material on contemporary research at Leeds using XRD; but this basically historical view is how it looked at the outset. It owes much to John Jenkin’s biography William and Lawrence Bragg, Father and Son (OUP, 2008).

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The invention of X-ray crystallography by William Henry Bragg, working at the University of Leeds, and his son W. Lawrence Bragg in Cambridge, was one of the culminating episodes in perhaps the most extraordinary three decades that the physical sciences have ever experienced. Between 1890 and the end of the First World War, scientists discovered X-rays and radioactivity, devised the theories of relativity and quantum mechanics, and decoded the inner secrets of atoms. During this period Marconi developed radio telecommunication, the cathode-ray tube that led to the discovery of electrons and X-rays was morphing into the first television, and the Wright brothers made their first flights. These were, in other words, the formative decades of the modern age.

It is not often appreciated how important to that incipient modernity the Braggs’ work was. By showing how X-rays reflected from substances could reveal the arrangements of their atoms, William and Lawrence paved the way to countless scientific and technological breakthroughs. By understanding the crystal structures – the orderly stacking of atoms – of metal alloys, it became possible to develop new and better materials. X-ray crystallography became the chemist’s most reliable tool for deducing the shapes of molecules, and when applied to the molecules of life it ushered in the age of molecular biology and genetics – most notably as the technique that revealed the chemical constitution of DNA to James Watson and Francis Crick in 1953. The Braggs’ early work on liquid crystals illuminated this puzzling state of matter, seemingly poised between the living and the inorganic, and laid the foundations for their use in today’s display technologies. Crystallography now tells us about the nature of the rocks in the deep Earth and the iron at its core. From drugs to earthquakes, microchips to meteorites, the understanding and technological capabilities that X-ray crystallography has provided are surely unmatched by any other scientific technique.

For their achievements, William and Lawrence Bragg were awarded the Nobel prize for physics in 1915, the year that William left Leeds for University College London. Astonishingly, that work had not even begun when the Braggs arrived in England from Australia six years earlier. Such immediate recognition is rare for Nobel prizes, and testifies both to the evident importance of their work and the clarity with which they explained and demonstrated its potential in many areas of science.

William Henry Bragg was born in Cumbria, but after studying at Cambridge University he moved to Australia in 1886 to take up a post in physics at the University of Adelaide. This was an adventurous decision: although Adelaide possessed many modern amenities and some splendid civic buildings, the British colonies here still had something of a raw, outback character. But William thrived, not least by meeting his future wife Gwendoline Todd, daughter of the astronomer and eminent government figure Charles Todd. They had two sons, Robert and William Lawrence (known to the family as Willie), and a daughter Gwendolen (Gwendy).

In the 1890s William established a solid international reputation for his work on radioactivity and the nature of the new invisible ‘emanations’ from matter: X-rays, gamma rays and alpha particles, the latter two being types of radioactivity. This work brought him in contact with the New Zealander Ernest Rutherford, who left the Antipodes in 1894 to work at Cambridge. Rutherford was fast emerging as the leading expert on atomic physics and radioactivity, and his supportive correspondence with Bragg led to a close friendship and collaboration. When William Stroud resigned his chair of Cavendish Professor of Physics at Leeds in 1907, Rutherford’s colleague the English chemist Frederick Soddy recommended Bragg as his successor, saying to the Dean of Science Arthur Smithells that, when he visited Adelaide “I was much struck with the spirit he has created around him.” Smithells took the advice, and Bragg was offered the post, which he accepted.

In January 1909 Bragg boarded the coal-fired Watarah in Adelaide for the journey to England, arriving in Plymouth in March. By this time, Rutherford was working at Manchester – doubtless one of the reasons the Leeds offer seemed so attractive – and Bragg wasted no time in visiting him. But he was less delighted with what he and his family found in Leeds: the industrial city was smoky and grimy, and poverty was all around, poor labouring families being housed in cramped, cold terraces. The Braggs were, however, able to rent accommodation in fashionable Headingley, near the university, before eventually settling in a country cottage called Deerstones, near Bolton Abbey 20 miles north of the city. Lawrence now went to study, as his father had, at Trinity College in Cambridge.

Rutherford was evidently as pleased at their proximity as Bragg was, and his booming voice became a regular counterpoint to Bragg’s reserve at Deerstones. “Rutherford was continually turning up at our home with an enthusiastic ‘D’you know, Bragg’”, recalled Gwendy. While Rutherford was clarifying the nature of alpha particles and using them to probe inside the atom, Bragg was interested in the character of X-rays, discovered in 1985 by the German Wilhelm Röntgen, which Bragg believed to consist of particles too: a kind of electron with its negative electrical charge neutralized by “a quantity of positive electricity”. Thus William entered a vigorous debate being conducted in Germany over whether X-rays were ‘corpuscles’ or ‘pulses’ – particles or waves. They are in fact electromagnetic waves, like light, which were then still widely believed to travel through an invisible medium called the ether. But as Albert Einstein argued in 1905, even light can be considered to be like a stream of particles too, called photons: this ‘wave-particle duality’ was one of the first fruits of the nascent quantum theory.

It is no surprise, then, that William was very interested in the news mentioned in a letter from the Norwegian physicist Lars Vegard which he received in June 1912. “Recently”, Vegard wrote, “certain new, curious properties of X-rays have been discovered by Dr Laue in Munich.” Max von Laue was one of the most brilliant students of the physicists Max Planck (who first proposed the quantum hypothesis in 1900) and Arnold Sommerfeld in Munich. Earlier that year he and his colleagues found that when a beam of X-rays was fired at a crystal, the reflected rays imprinted a geometric pattern of bright spots on photographic emulsion placed around the sample.

Vegard explained that Laue “thinks that the effect is due to diffraction of the Röntgen rays by the regular structure of the crystal, which should form a kind of grating, with a grating constant of the order of 10**-8 cm, corresponding to the supposed wavelength of Röntgen rays… he is, however, at present unable to explain the phenomenon in its details.”

What did that mean? Laue was referring to the way waves are reflected from an array of evenly spaced objects (a ‘grating’). At some angles, the peaks and troughs of the reflected waves coincide and reinforce each other, producing an intense beam – an effect called constructive interference. At other angles the waves are perfectly out of step, so that the peaks and troughs cancel one another out and the beam vanishes – destructive interference. The resulting patterns of weak or strong reflected waves depend on the wavelength of the waves, which is why this kind of interference of light bouncing off the pitted ‘grating’ of a CD produces different colours (each with its specific wavelength) at different viewing angles. This interference effect is called diffraction.

The effect only happens when the wavelength of the waves is roughly equal to the size and separation of the objects that make up the grating. That is why Laue talked about the ‘grating constant’, meaning the spacing of elements in the grating. He supposed that this grating is produced by the regular arrangement of atoms in the crystal, which are packed together like oranges on the greengrocer’s stall. At that time, very little was known about the nature of atoms and how they were ordered in crystals, but scientists did at least have a fairly good sense of their size – which, as Laue attested, was around a ten billionths of a centimetre, or 10**-8 cm. This was about the same as the wavelength of X-rays, which most researchers (if not Bragg) regarded as ether waves.

On the face of it, then, Laue seemed to have a pretty good explanation for the pattern of bright X-ray spots recorded in the emulsion: it was the result of diffraction of X-rays. But Vegard was right to say that the details were still not clear. By assuming that the diffraction was caused by a simple grid of atoms, Laue could explain why he saw X-ray spots, but he predicted too many of them, and couldn’t account for their elliptical shape. What was it, then, that acted as the diffraction grating: the atomic array, or something else? Many chemists believed that atoms in a crystalline substance such as copper sulphate (used for the first experiments in Munich) were clustered into groups called molecules – in which case, how could they form regular arrays anyway?

Lawrence Bragg, home from Cambridge for the summer, recalled that he and his father discussed Laue’s report intensely “when we were on holiday at Cloughton on the Yorkshire coast”. At that stage, still faithful to William’s ‘corpuscular’ view of X-rays, the Braggs at first sought to interpret Laue’s findings on the basis that X-ray ‘particles’ were being channelled along ‘avenues’ between the rows of atoms, an idea that they described in a paper published that October in the prestigious science journal Nature.

But when that same month Lawrence returned to university, he hit on a better explanation while strolling through the meadows of the Cambridge Backs. “The idea suddenly leapt into my mind”, he later wrote, “that Laue’s spots were due to the reflection of X-ray pulses by sheets of atoms in the crystal.” Sheets? What Lawrence understood was that, if you stack atoms regularly, you end up with series of layers, which become evident only when you look along a certain direction parallel to each set of layers. William explained this in 1915 in the Leeds student magazine The Gryphon with reference to the rows of vines in a vineyard – not the obvious reference for someone surrounded either by the Yorkshire dales or the Cambridgeshire fens, but wine had been cultivated in the Adelaide Hills since the early nineteenth century. As you walk through the rows, every so often the vines line up in rows and you can see them stretching away in parallel formation: a grating.

On this assumption that it was the regularly spaced sheets of atoms that caused X-ray diffraction, Lawrence worked out how the reflection angles at which spots appear depends on the distance between sheets and the wavelength of the X-rays. He included this formula in a paper that he presented to the Cambridge Philosophical Society in November, which was reported in Nature in December. Here he also showed how, by assuming a particular kind of stacking of atoms in crystals of zinc sulphide, he could account perfectly for the X-ray pattern produced from this substance by the Munich group.

Lawrence carried out this work before he had even graduated from Cambridge, which he did in mid-1912. He was already starting to conduct research in the Cavendish Laboratory at Cambridge, and here he was able to test and verify his diffraction law with experiments on X-ray reflection from the mineral mica. His ideas about the atomic arrangements in zinc sulphide had been influenced by the picture of crystal structures then being developed by William Pope, Professor of Chemistry at Cambridge. At Pope’s suggestion, Lawrence began to carry out diffraction experiments on alkali halide salts, such as common salt sodium chloride: a particularly fortunate choice, because these substances have very simple crystal structures in which the atoms are arranged at the corners of stacks of cubes. Those experiments, Lawrence wrote to his father at Leeds, “turned out toppingly”.

William was immensely proud of his son’s achievements, which he mentioned to Rutherford in December. But he was no bystander in all this. The Braggs were constantly discussing the phenomenon of X-ray diffraction, and William was exploring it at Leeds too. Here he had rather better technical support than Lawrence did at the Cavendish, where the ‘sealing wax and string’ approach to experimentation pursued by J. J. Thomson, the discoverer of the electron in 1897, was perpetuated by Rutherford when he became director in 1919. William, in contrast, enjoyed the services of an excellent workshop led by the head mechanic Jenkinson, who built for him an instrument that could be used both to look at how X-rays were absorbed by substances (the technique of X-ray spectrometry, which William studied) and reflected by them (X-ray diffraction).

Lawrence was able to use this instrument for some of his Cambridge studies, and in April 1913 he and his father described their diffraction technique in a joint paper read to the Royal Society in London. Called simply “The reflection of X-rays by crystals”, it was essentially an announcement of the birth of this new discipline. The crucial realisation was that, if the X-ray diffraction pattern recorded in photographic film could be accurately predicted from the crystal structure, then one could also work backwards, deducing from the experimentally measured pattern what the structure of the crystal is. This point was emphasized in a paper by Lawrence read to the Royal Society in June, and was demonstrated most dramatically in a joint paper later that year in which the Braggs reported the structure of diamond.

For that work they needed a particularly fine specimen of the gemstone, which was not easy to get. It came from the mineralogical collection at Cambridge, despite the fact that the Professor of Mineralogy William Lewis had strictly forbidden any loans of the precious specimen. The demonstrator, Arthur Hutchinson, had risked his neck to lend the gem to Lawrence behind Lewis’s back. “I shall never forget Hutchinson’s kindness in organising a black market in minerals to help a callow young student”, Lawrence wrote. “I got all my first specimens and all my first advice from him and I am afraid that Professor Lewis never discovered the source of my supply.”

Although it was Lawrence who had had the crucial insight that diffraction was caused by the atomic planes, and who was doing much of the key experimental work, it was William who was the eminent and senior scientist. And so it was William who was asked to deliver talks on this new science of X-ray crystallography. “It was my father who announced the new results at the British Association, the Solvay Conference, lectures up and down the country, and in America, while I remained at home”, Lawrence recalled ruefully. “It was not altogether an easy time… a young researcher is as jealous of his first scientific discovery as a kitten is with its first mouse… My father more than gave me full credit for my part, but I had some heart-aches.”

The Solvay meeting – a roughly triennial gathering of Europe’s top physical sciences in Brussels – was a particularly prestigious platform, and at the 1913 meeting on “The Structure of Matter” William Bragg discussed his work with Einstein and Marie Curie, along with several scientists, such as Leon Brillouin and Frederick Lindemann, who went on to make important contributions to the understanding of diffraction and crystal structure.

Under these circumstances, it is hardly surprising that many people assumed that X-ray crystallography was William’s discovery. It was from this time that Lawrence began to sign his papers as “W. Lawrence”, foregoing the childhood name of Willie, better to distinguish himself from his father. He was not being merely filial in his comments about his father’s generosity, however. When the Braggs put the seal on their achievements with their joint 1915 book X-Rays and Crystal Structure, William wrote the preface alone while Lawrence was away at war, and he said: “I am anxious to make one point clear, viz., that my son is responsible for the ‘reflection’ idea which has made it possible to advance, as well as for much the greater portion of the work of unravelling crystal structure to which the advance has led.”

All the same, the difference in status could not but cause tensions. “Father and son never managed to discuss the situation”, Gwendy later admitted, “WHB being very reserved and WL inclined to bottle up his feelings.”

Yet there could be no mistaking the joint effort when the two Braggs shared the Nobel prize in 1915. By that point, William had left Leeds, and the award must have only deepened the sense of loss in the physics department. In late 1914 William received an offer of a professorship at University College London. He declined it in March of 1915, but later accepted an improved offer that Leeds simply could not match. He completed the academic year, and started in London in September. “Professor Bragg has not left us for the honour of going to London”, Smithells was at pains to point out in The Gryphon, “…[he] has left us because he thinks that in London he can do better work.” He added that Bragg “has a perfect right to think so”, which can leave little doubt about what Smithells thought of the matter.

But it was not necessarily the tragedy for Leeds that it might have seemed at the time. William Bragg’s seminal work there inevitably left a legacy, and this was re-ignited when his student William Astbury, who worked with him at UCL and later at the Royal Institution in London, was appointed Lecturer in Textile Physics at Leeds in 1928. He remained in the department until his death in 1961, and during that time Astbury’s studies of the crystal structures of proteins, beginning with the keratin protein of wool, were central to the genesis of structural molecular biology. Astbury’s work on keratin guided the chemist Linus Pauling towards the discovery of the basic structural elements of all proteins, while his work on DNA was an important precursor to the breakthrough of Watson and Crick 60 years ago. In such ways, the influence of William Bragg’s time at Leeds continues to resonate at the forefront of science today.

1 comment:

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These were, in other words, the formative decades of the modern age.
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