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Rh the history, to endeavor to ascertain several facts which were disputed; and this led me, by degrees, into a large field of original experiments, in which I spared no expense that I could possibly furnish. These experiments employed a great proportion of my leisure time; and yet, before the complete expiration of the year in which I gave the plan of my work to Dr. Franklin, I sent him a copy of it in print." It was this work which first brought him into notice as an experimental philosopher and procured for him the title of Doctor of Laws from the University of Edinburgh, and led to his being made a member of the Royal Society, from which he received its greatest honor, the Copley Medal. He married, while at Warrington, in 1763, the daughter of Mr. Isaac Wilkinson, an iron-master. Although not settled as a clergyman at Warrington, he kept up the habit of preaching, and was here ordained. Dr. Priestley stammered so badly that he sometimes thought he must give up preaching, but he at length measurably conquered the defect by the daily habit of slow reading in a loud voice. He recognized that this defect of delivery saved him from the temptation of trying to be an orator.

In 1767, Dr. Priestley went to Leeds and took charge of a chapel, and here he engaged keenly in the study of theology, and produced a great number of controversial works. He commenced his investigations on airs, and published a history of the discoveries in relation to vision, light, and colors, as the first part of a general history of experimental philosophy, which was not continued, because it failed to pay expenses. Here, likewise, he commenced the publication of a periodical, the Repository, devoted to theological subjects. Among numerous other things, he wrote an "Essay on Government," an enlarged "English Grammar," a "Familiar Introduction to the Study of Electricity," a "Treatise on Perspective and Chart of History," and, at the request of Drs. Franklin and Fothergill, an "Address to Dissenters on the Subject of the Difference with America."

It was in 1769, while at Leeds, that Dr. Priestley came into conflict with Blackstone, the celebrated author of the commentaries on the laws of England. Having, in that work, approved the statutes of Edward VI. and Queen Elizabeth, denouncing the penalties of confiscation and imprisonment against all who speak in derogation of the Book of Common Prayer, and justified the continuance of these penalties, Dr. Priestley replied to him, pointing out the injustice of such statutes, and the illiberality of those who undertake to defend them. He also convicted Dr. Blackstone of inaccuracy in the statement of historical facts. To this the learned lawyer made a reply, disavowing the sentiment that "the spirit, the principles, and the practices of the sectaries, are not calculated to make men good subjects;" and generously promised to cancel the offensive paragraphs in the future editions of his work. Dr. Priestley addressed him a handsome letter, and the controversy was brought to an amicable conclusion.