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Rosalind Franklin was awarded her BA degree in 1941, and spent the following year honing her research skills during a fellowship at Cambridge, under R. G. W. Norrish's supervision. In 1942 World War II was raging across Europe and in the Pacific, and every able-bodied citizen was expected to contribute to the war effort. Franklin very much wanted to continue doing research, but knew she was unlikely to be assigned such work; men with science training were routinely drafted to do research, but women rarely were. At best, they might fill in as university lecturers. She was relieved to find a position as Assistant Research Officer with the British Coal Utilisation Research Association (BCURA), a new organization dedicated to researching the production, distribution, and use of coal and its derivatives. D. H. Bangham, BCURA's first director, had recruited several dozen recent physics and chemistry graduates to do original research on an array of carbon problems. Coal, charcoal, and other carbons were important strategic products during wartime, not just for fuel purposes, but for devices such as gas masks, which use special charcoal filters.
Despite numerous advances in carbon chemistry during the early twentieth century, many questions remained about the molecular structure of coals. For example, it was known that coals contained many fine pores, but why did some types prove more permeable to water, solvents, or gases than others? With the ample BCURA laboratory facilities and Bangham's support, Franklin carried out a series of original experiments that provided the answers. She tested a variety of different coals from the British Isles, grinding them to fine powders, then measuring their apparent density using water, methanol, hexane, or benzene. She compared these results with her density measurement using helium gas, reasoning correctly that helium, with its small molecule size, would provide a more "true" density. On the basis of the differences, she concluded that the pores in coals contain numerous fine constrictions, and the variation in the permeability of the pore space from one coal to another is related to a variation in the width of these constrictions, which is of the same order as the diameters of simple molecules. Coals containing between 89% and 93% carbon had the smallest constrictions. She then measured the densities for the same range of coal samples after heating them to carbonizing temperatures (600-1000 degrees Celsius). She found that the porosity increased with carbonization, even though reactivity decreased. At temperatures above 600°C, the large pore volume became inaccessible to n-hexane, and then, successively to methanol, carbon disulfide, and water. Finally, at temperatures over 1000°C even the helium could not penetrate completely. That is, substances were excluded in order of molecular size as the temperature--and the fine constrictions--increased. Franklin was the first to identify and measure these micro-structures, and this fundamental work made it possible to classify coals and predict their performance to a high degree of accuracy. Her four years of research at BCURA yielded a doctoral thesis--she received her PhD from Cambridge in 1945--and five scientific papers, three of which she was sole author.
Though she found the work at BCURA satisfying, Franklin was looking for something different after the war. Writing to French scientist Adrienne Weill, with whom she had studied at Newnham College, she said, "If ever you hear of anybody anxious for the services of a physical chemist who knows very little physical chemistry, but quite a lot about the holes in coal, please let me know." At a carbon research conference in London in the fall of 1946, Weill introduced Franklin to Marcel Mathieu, who headed the French government agency that supported much of the scientific research done in France. Mathieu was impressed, and found her a post with Jacques Mering at the Laboratoire Central des Services Chimiques de l'Etat in Paris. With Mering, she learned, and mastered, the techniques of x-ray crystallography in its applications to substances more complex than simple mineral or metal crystals.
X-ray diffraction had been first used to look at the crystal structure of simple table salt in 1912. With this technique, an x-ray beam bombards a small sample of the crystal substance being studied, which is mounted inside a camera. The rays are diffracted by electrons in the various constituent atoms of the sample and produce a pattern of spots on a photographic plate. Because crystals have a regular repeating structure, the density and location of spots made by the x-rays can be analyzed mathematically to discover the arrangements of the crystal's atoms and the size of its "unit cell," or basic repeating structure. For the analysis, two mathematical techniques, the Fourier transform and the Patterson function can be used. Each photo requires hundreds of calculations, which had to be done by hand until the advent of computer technology. The x-ray equipment available at the time was also challenging, frequently overheating and breaking down during the long exposures required for crystallography (some of Franklin's exposures were 100 hours or more!) Researchers often had to design, modify, and repair their own equipment.
Franklin became very skilled with crystallographic technique, though she always maintained that she was a chemist, not a crystallographer. (After her death, J. D. Bernal, himself a pioneer in the field, would note that Franklin took some of the most beautiful x-ray diffraction photos ever done.) In Paris she continued to work with carbon compounds, building on the work done at BCURA. She was able to divide coals and some other solid organic materials (including plastics) into two principal categories: those which, upon heating, yield non-graphitizing carbons, which have low density, large fine-structure porosity, and are very hard; and those which convert readily into graphite. She attributed the unique behavior of non-graphitizing carbons to the formation of a strong system of cross-links between the carbon crystallites which prevented the rearrangement of the material into graphite. The non-graphitizing carbons soon found important industrial applications as glassy or vitreous carbons used for laboratory crucibles, tubing, and other items that required reliable heat resistance. In the process of this work, Franklin also made many improvements in x-ray diffraction methods for imaging large complex molecules and in the accompanying mathematical techniques. Her extraordinary skills in this area later enabled her to quantitatively determine the structures of complex biological molecules and viruses.
Franklin enjoyed this demanding research, and also loved the camaraderie of the laboratory staff. The young researchers working with Mering were constantly engaged with each other, sharing meals, coffee breaks, and outings, and carrying on discussions and debates about their scientific work, politics, and social issues. Franklin felt at home in this environment, and she thrived. She forged many close friendships during her years in Paris, and--in an intellectual climate that genuinely respected and welcomed women--gained a solid sense of professional confidence. She had also begun to earn a reputation in her field through her publications and conference presentations. Although she could have stayed happily in France, she felt that she would have to return to England eventually. Her family wanted her home, and though she had become an expert on carbon structures, her friend Charles Coulson, a theoretical physicist, suggested that the crystallographic study of larger biological molecules would be a good career move. British researchers were then on the cutting edge of such work. Coulson, at King's College, University of London, advised her that John T. Randall's biophysics laboratory at King's would be a good place to work, if she could obtain a research fellowship. In 1950, she was awarded a three year Turner and Newall Research Fellowship to work at King's College, starting in January 1951.