and on the electrical effects of chemical transformations demonstrated Priestley's early and ongoing interest in the relationship between chemical substances and electricity. Based on experiments with charged spheres, Priestley was among the first to propose that electrical force followed an inverse-square law, similar to Newton's law of universal gravitation. He did not generalise or elaborate on this, and the general law was enunciated by French physicist Charles-Augustin de Coulomb in the 1780s. Priestley's strength as a natural philosopher was qualitative rather than quantitative and his observation of "a current of real air" between two electrified points would later interest Michael Faraday and James Clerk Maxwell as they investigated electromagnetism. Priestley's text became the standard history of electricity for over a century; Alessandro Volta (who later invented the battery), William Herschel (who discovered infrared radiation), and Henry Cavendish (who discovered hydrogen) all relied upon it. Priestley wrote a popular version of the History of Electricity for the general public titled A Familiar Introduction to the Study of Electricity (1768). He marketed the book with his brother Timothy, but unsuccessfully. Leeds (1767–1773) Perhaps prompted by Mary Priestley's ill health, or financial problems, or a desire to prove himself to the community that had rejected him in his childhood, Priestley moved with his family from Warrington to Leeds in 1767, and he became Mill Hill Chapel's minister. Two sons were born to the Priestleys in Leeds: Joseph junior on 24 July 1768 and William three years later. Theophilus Lindsey, a rector at Catterick, Yorkshire, became one of Priestley's few friends in Leeds, of whom he wrote: "I never chose to publish any thing of moment relating to theology, without consulting him." Although Priestley had extended family living around Leeds, it does not appear that they communicated. Schofield conjectures that they considered him a heretic. Each year Priestley travelled to London to consult with his close friend and publisher, Joseph Johnson, and to attend meetings of the Royal Society. Minister of Mill Hill Chapel When Priestley became its minister, Mill Hill Chapel was one of the oldest and most respected Dissenting congregations in England; however, during the early 18th century the congregation had fractured along doctrinal lines, and was losing members to the charismatic Methodist movement. Priestley believed that by educating the young, he could strengthen the bonds of the congregation. In his three-volume Institutes of Natural and Revealed Religion (1772–74), Priestley outlined his