Emily Hart, also called Emma Hart, was the mistress of politician Charles Greville in the 1780s. Without telling the Eighth Doctor or Scarlette, Juliette Vierge conducted secret alchemical experiments with Emily's help to increase her understanding of the nature of time.
Later, after the death of the wife of Greville's uncle, who was the British Envoy in Naples, Emily was "passed on" to him as a mistress. She settled down with him and returned to her given name Emma; later, in 1798, she became famous in Britain for being the mistress of a noted Admiral from the Napoleonic Wars. (PROSE: The Adventuress of Henrietta Street)