David Hartley, the younger (1732 – 19 December 1813), statesman, scientific inventor, and the son of the philosopher David Hartley. He was Member of Parliament (MP) for Kingston upon Hull, and also held the position of His Britannic Majesty's Minister Plenipotentiary, appointed by King George III to treat with the United States of America. He was a signatory to the Treaty of Paris. Hartley was the first MP to put the case for abolition of the slave trade before the House of Commons, moving a resolution in 1776 that "the slave trade is contrary to the laws of God and the rights of men.
He was expert in public finance and spoke frequently in parliament in opposition to the war in America. Although a liberal on American policy, Hartley was a long-time friend of Lord North and strongly disliked the Prime Minister, Shelburne. He supported the Coalition by voting against Shelburne's peace preliminaries. It was probably owing to his friendship with Franklin, and to his consistent support of Lord Rockingham, that he was selected by the government to act as plenipotentiary in Paris, where on September 3, 1783 he and Franklin drew up and signed the definitive treaty of peace between Great Britain and the United States of America. He died at Bath on December 19, 1813, in his eighty-second year.
His portrait was painted by George Romney and has been engraved by J. Walker in mezzotint. Wraxall says that Hartley, "though destitute of all personal recommendation of manner, possessed some talent with unsullied probity, added to indefatigable perseverance and labour." He adds that his speeches were intolerably long and dull, and that "his rising always operated like a dinner bell" (Memoirs, iii. 490).
In his last years, Hartley studied chemistry and mechanics. in 1785 he published Account of a Method of Securing Buildings and Ships against Fire, by placing thin iron planks under floors and attaching them to the ceilings, partly to prevent immediate access of the fire, and partly to stop the free supply of air. He built a house on Putney Heath to verify the efficacy of his invention, and on the occasion of a fire at Richmond House, December 21, 1791 wrote a pamphlet urging the value of his fire-plates. Hartley edited his father's well-known Observations on Man, in London 1791 and (with notes and additions) in 1801.
In 1859 a number of Hartley's papers were sold in London. Six volumes of letters and other documents relating to the peace went to America and passed into the collection of L.Z. Leiter of Washington; others are in the British Museum.