Unpublished paper, 'On the frequent occurence of indigo in human urine and on its chemical and physiological relations' by Arthur Hill Hassall Hassall explains that he was led to his investigations after examining urinary deposits under the microscope and frequently observing in the field of vision particles of a deep blue colour. He shares details of his experiments and observations on the presence of indigo in human urine. Subject: Biology / Chemistry / Physiology Communicated by Professor [William] Sharpey. Whilst the Royal Society declined to publish this paper in full, an abstract of the paper was published in volume 7 of the Proceedings of the Royal Society as 'On the frequent occurence of indigo in human urine, and on its chemical and physiological relations'. 1853