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Marriage

By mid-summer 1854, Lister realised that he was attracted to Agnes Syme and he started to court her. Lister wrote to his father and mother about his love but both his parents were concerned about the union, particularly concerning the fact he was Quaker and Agnes had no indication that she was going to change her denomination. During that time, when a Quaker married a person of another denomination, it would be considered as marrying out of the society. Lister was determined to marry Agnes and sent a further letter to his father, asking his father if his financial support would continue should Lister and Agnes become married. Lister's father replied that Agnes not being in the Society of Friends would not affect his pecuniary arrangements. Joseph Jackson offered his son extra money to buy furniture and suggested that Syme would offer a dowry and that he would negotiate with Syme directly on it. His father suggested that Lister should voluntarily resign from the Society of Friends. Lister made up his mind and subsequently left the Quakers to become a protestant, later joining the congregation of the Saint Paul's Episcopal Church, in Jeffrey Street, Edinburgh. In August 1855, Lister became engaged to Agnes Syme. On 23 April 1856, Lister married Agnes Syme in the drawing room of Millbank, Syme's house in Morningside. Agnes's sister stated that this was out of consideration of any Quaker relations. Only the Syme family were present. The Scottish physician and family friend John Brown toasted the couple after the reception.

On their honeymoon, the couple spent a month at Upton and the Lake District, followed by three months on a tour of the leading medical institutes in France, Germany, Switzerland, and Italy. The couple returned in October 1856. By this time, Agnes was enamoured with medical research and became Lister's partner in the laboratory for the rest of her life. When they returned to Edinburgh, the couple moved into a rented house at 11 Rutland Street in Edinburgh. The house was situated over three floors with a study on the first floor, that was converted into a consulting room for patients and a room with hot and cold taps on the second floor that became his laboratory. The Scottish surgeon Watson Cheyne, who was almost a surrogate son to Lister, stated after his death that Agnes had entered into her work wholeheartedly, had been his only secretary, and that they discussed his work on an almost equal footing.

Lister's books are full of Agnes' careful handwriting. Agnes would take dictation from Lister, taken at hours at a stretch. Spaces would be left blank amongst the reams of Agnes' handwriting for small diagrams, that Lister would create using the camera lucida technique and Agnes would later paste in.