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Attempts to unravel the cause of cancer really began in the eighteenth century; perhaps the first milestone was the work of the British surgeon Percival Pott, who in 1775 made the observation that cancer of the scrotum was more common in men who had worked as chimney-sweeps in boyhood. The guess chat some substance in soot was causing this cancer on rne skin has been borne out by work in the following two centuries. Pott's work also illustrated another important principle. The cancer occurred in men who had worked as chimneysweeps years before, demonstrating the delayed effect of expo-save so some cancer-generating substances. Also in the late eighteenth century, physicians described possible links between snuff-taking and tobacco-smoking and cancers occurring on the note or on the lip, all of which have been borne out by subsequent observation. The nineteenth century saw further efforts to detect underlying causes for cancers. Physicians and surgeons studied the occurrence of uterine cancers and related these to the reproductive and sexual histories of the patients. They studied the relationship between cancer in the bladder and occupational exposures to chemicals in the dye industry, and they studied the links between industrial exposure of some miners and lung cancer. These valuable studies generated insights into the causes of cancer. The observations were mainly of strong associations. That is to say the risk of suffering from the cancer was greatly enhanced by the particular exposures that were considered. The number of patients included in these studies was usually relatively small and would not have served as a basis for detecting less obvious but important factors in the cause of cancer.