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46 and criticism, on history and general policy, on civil law, and on anatomy. A goodly number of subjects, but Priestley was a most versatile man, as the catalogue of his 108 books published in 1794 bears witness. During his six years' residence at Warrington, he married the daughter of Isaac Wilkinson, an ironmaster of Wrexham, and the union proved a happy one. In 1766 he became acquainted with Benjamin Franklin (who lived in Craven Street, Strand, London). The acquaintance ripened into friendship. Franklin encouraged Priestley in his philosophical pursuits, and no doubt this was the origin of his electrical researches, which were subsequently published in his History of Electricity. This work greatly enhanced his fame, and gained for him an F.R.S., and the degree of LL.D. of Edinburgh University.

In 1767 Priestley left Warrington for Leeds, having been appointed minister of Millhill Chapel. In the Yorkshire town he commenced his chemical investigations, at the same time freely expressing his opinions on theology and metaphysics. Living near a brewery, he made experiments on the "fixed air" (carbon dioxide) of Black, and this led to investigations with "airs," or, as we should say, with gases. Priestley's chemical work was essentially in the domain of pneumatic chemistry. In 1772 he published a work on the method of impregnating water with fixed air—hence the birth of the mineral water