The birth of crystallography

William Henry Bragg giving the Royal Institution's Christmas lecture.

Structural biology, geology, engineering, chemistry, physics. Bragg’s law. Spectrometers. Diffractometers. Pioneering work done at Leeds a century ago still feeds into much of modern science.

This is how it started.

The birth of X-ray crystallography at Leeds in 1912-1913 through the work of Sir William Henry Bragg and his son Sir W. Lawrence Bragg was one of the culminating episodes in arguably the most  extraordinary three decades in the physical sciences.

Between 1890 and the end of the First World War, X-rays and radioactivity were discovered, the theories of relativity and quantum mechanics developed, and the constitution of atoms first explained. During this period Marconi developed radio telecommunication, the Wright brothers made their first flights and Max Planck proposed quantum theory.

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

It is not often appreciated how important to that incipient modernity the research by William and Lawrence Bragg was. William and Lawrence paved the way to countless scientific and technological breakthroughs by revealing the arrangement of atoms in crystals. Although it had been long suggested that crystals were made up of a regular pattern of atoms and molecules, there was previously no way of knowing precisely how these were arranged.

X-ray crystallography is the chemist’s most reliable tool for deducing the shapes and arrangements of molecules. It tells us about the nature of terrestrial and extra-terrestrial minerals. Through an understanding of crystal structures, it became possible to develop new and better materials. When applied to the molecules of life, it ushered in the age of molecular biology and genetics – most famously as the technique that revealed the structure of DNA to James Watson and Francis Crick in 1953.

For their achievements, William and Lawrence Bragg were awarded the 1915 Nobel Prize in Physics. Yet research on crystallography had not even begun when the Braggs arrived in England from Australia six years before the Nobel award. Such immediate recognition is rare for Nobel Prizes, and it testifies both to the importance of their work and the clarity with which they explained and demonstrated its potential in many areas of science.

William Bragg with his spectrometer

Cumbrian-born William came to Leeds from the University of Adelaide, Australia, where he had 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. As Professor of Physics and Mathematics, William had found in Adelaide a meagre laboratory, so he set about making his own equipment by apprenticing himself to a local instrument maker.

When Leeds needed a new Cavendish Professor of Physics in 1907, the English chemist Frederick Soddy recommended William to the University, saying that, when he had visited Adelaide, “I was much struck with the spirit he has created around him.” William was offered and accepted the Leeds post.

In January 1909 William and his family – wife Gwendoline, sons Lawrence and Robert and daughter Gwendy – boarded the coal-fired Watarah for the journey to England, arriving in Plymouth in March. They rented a house in fashionable Headingley as well as a weekend cottage near Bolton Abbey 20 miles north of the city. Lawrence enrolled, as his father had 28 years earlier, at Trinity College, Cambridge.

William and Lawrence paved the way to countless scientific and technological breakthroughs by revealing the arrangement of atoms in crystals.

William was deeply interested in the nature of X-rays. At the time a vigorous debate raged among physicists about whether X-rays were ‘corpuscles’ or ‘pulses’ – particles or waves – the latter widely believed to travel through an invisible medium called the ether. Although William preferred the particle interpretation, X-rays are in fact electromagnetic waves, like light, but of a very much smaller wavelength. Yet Albert Einstein argued in 1905 that light can also be considered to be like a stream of particles, called photons: this ‘wave-particle duality’ was one of the first fruits of the nascent quantum theory.

Given this interest, the Braggs, father and son, were fascinated by news of work in Munich by Max Laue, a student of Max Planck. Laue found that when a narrow beam of X-rays was directed at a crystal, the scattered rays formed a geometric pattern of bright spots on a photographic plate placed behind the sample. Laue attempted to interpret the pattern but could not account for all the spots.

Lawrence, still at Cambridge, recalled that he and his father discussed Laue’s findings intensely “when we were on holiday at Cloughton on the Yorkshire coast.” Over the summer and autumn of 1912, William and Lawrence collaborated in the Leeds Physics laboratory. Writing in 1961, Lawrence pointed out how adept William was in the laboratory. “My father was supreme at handling X-ray tubes and ionization chambers. You must find it hard to realize these days what brutes X-ray tubes then were.”

blue plaque for braggs, chris hammond

A 2013 celebration of a new blue plaque for Whin Brow, the house at Cloughton, North Yorkshire, where the father-and-son team started collaborting. From left to right - Dr Chris Hammond, Life Fellow in Material Science at the University of Leeds; Linda Pollard, pro-Chancellor of the University; Charles Bragg, the great-grandson of William Henry Bragg; Professor Michael Arthur, Vice-Chancellor of the University.

At Leeds, William had rather better technical support than Lawrence did in Cambridge, which still pursued a “sealing wax and string” approach to experimentation. William, in contrast, enjoyed the services of an excellent workshop led by head mechanic C H Jenkinson. It was Jenkinson who built, from William’s design, a revolutionary instrument that could be used both to measure X-ray wave lengths (the technique of X-ray spectrometry) and to measure reflections from crystal plains (X-ray diffraction).

The instrument, used both as a spectrometer and a diffractometer, partly superseded Laue’s photographic technique in that it enabled precise measurements of the angles and intensities of the diffracted beams. Adhering to William’s ‘corpuscular’ view of X-rays, the Braggs at first sought to interpret Laue’s bright X-ray spots on the basis that X-ray ‘particles’ were being channelled along ‘avenues’ between rows of atoms, an idea they described in a paper published in October 1912 in the journal Nature. But later that month, shortly after his return to Cambridge for the Michaelmas term, Lawrence hit on a novel explanation.

The Braggs' work was named the third most important British innovation of the 20th century in an online vote of 80,000 people.

“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.” What Lawrence understood was that the beam behaves as though it has been reflected by these sheets, or layers, as light is reflected by a mirror.

William explained the idea of sheets or planes of atoms in 1915 in the Leeds student magazine The Gryphon with reference to the rows of vines in a vineyard – not an obvious Yorkshire reference, but wine had been made in the Adelaide Hills since the early nineteenth century. As you walk through the rows of vines, every so often they align and you can see them stretching away in parallel formation.

On this basis, Lawrence worked out how the reflection angles of the spots depend on the distances between sheets and the wavelength of the X-rays. He expressed this in a formula now known as ‘Bragg’s law,’ which first appeared in a paper presented to the Cambridge Philosophical Society in November 1912 and was reported in Nature in December. Here he also showed that, by assuming a particular kind of arrangement of atoms in crystals of zinc sulphide, he could account perfectly for the X-ray pattern.

The Braggs’ crucial realisation was that, if the X-ray diffraction pattern could be accurately predicted from a crystal structure, then one could also work backwards, deducing from the experimentally measured pattern, the structure of the crystal itself.

William Lawrence Bragg
William Lawrence Bragg

Of all the crystals whose structures were worked out principally during 1913 (sodium chloride, potassium chloride, calcium fluoride, zinc sulphide and diamond) it was the structure of iron sulphide which gave Lawrence “the greatest thrill,” as he recorded long afterwards. This was the first structure in which the positions of the (sulphur) atoms were determined from the intensity (brightness) of the reflections.

Lawrence recalled that “I worked it out in the drawing room of our house in Leeds and was so excited that I had to tell my aunt who was sitting in a corner all about it, with indifferent success.

The collaboration between father and son continued throughout the whole of 1913 and until the outbreak of war in 1914. Lawrence spent part of the spring and summer terms in 1913 at Cambridge but the rest of the year at Leeds.

The Braggs’ work, for which they jointly were awarded the Nobel Prize, was published in a series of papers by the Royal Society in London, marking the birth of X-ray crystallography. William delivered talks on this new science around the country, at the British Association and in particular at the Solvay Conference.

William Bragg at 2013 Solvay Conference
William Bragg, third from the left, with Laue, Curie, Rutherford, Thomson, Einstein and others at the 2013 Solvay Conference

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

Late in 1914, William wrote a long letter to the Leeds Vice-Chancellor Michael Sadler, pointing out the University’s pre-eminence in X-ray diffraction. “The practical applications are likely to be of no less importance than the theoretical,” he wrote. Nearly a century later in 2013, the Braggs' work was named the third most important British innovation of the 20th century in an online vote of 80,000 people. 

Although his request for funds was supported, Leeds couldn’t match the offer in early 1915 of a professorship from wealthy University College London (UCL). William at first refused the offer, but by the time he accepted their second offer he had decided he needed to be in London at the centre of the war effort.

William’s departure was not necessarily the tragedy for Leeds that it might have seemed at the time. The Braggs’ seminal work here inevitably left a legacy. In 1929 William’s student William Astbury, who worked with him at UCL and later at the Royal Institution in London, came to Leeds as a ‘textile physicist.’ Textiles was the manufacturing base on which Leeds had grown prosperous, and the hope was that research on wool and other economically important fibres might one day improve the manufacturing process.

plaque for the Braggs on campus
A blue plaque on the Parkinson building celebrates the Braggs

X-ray crystallography had been initially applied to inorganic crystals, and the challenge of applying the same technique to the study of the large biological molecules found in fibres was considerable. Yet Astbury met the challenge and, thanks to a series of breakthrough papers on the structure of proteins and to his energetic proselytising, Leeds became famous as the “X-ray Vatican” and the home of molecular biology.

In 1938 one of Astbury’s research students, Florence Bell, produced the first X-ray diffraction image of DNA, a crucial step that led in 1953 to one of the most important discoveries of the 20th century: the double-helical structure of DNA.

Astbury and Bell's work is one example of the significance of the Braggs’ research not only within the confines of crystallography, but more broadly across science, engineering and societal boundaries. It has been fundamental to the development of various scientific fields within industry, including microelectronics, pharmaceuticals, aerospace and power generation. 

The influence of William Bragg’s time at Leeds continues to resonate at the forefront of science a century later.  

Watch a lecture by Sir William Henry Bragg explaining the structures of crystals.