Most instances of chemical carcinogenesis were identified through clinical observations. In 1775, English physician Percival Pott noted an unusually high incidence of scrotal cancer in men who were employed as chimney sweeps in childhood. He correctly surmised that the cancer was directly attributable to soot exposure and suggested that chimney sweeps bathe daily as a means of prevention. More than a century later, Japanese pathologists induced skin tumors in rabbits by the repeated application of coal tar to the ears. Such early experiments gave rise to the notion that environmental factors can be responsible for the development of cancer, especially with repeated exposures over time.