BIOGRAPHICAL SKETCHES. 53 his celebrated discovery of jxed air, or carbonic acid gas. We are informed by himself, that he was led to the examination of the absorbent earths, partly by the hope of discovering a new sort of lime and limewater, which might possibly be a more powerful solvent of the stone than that commonly used. The attention of the public had been directed to this subject for some years. Sir Robert, as well as his brother, Horace, afterwards Lord Walpole, were troubled with the stone. They imagined that they had received benefit from a medicine invented by a Mrs. Stephens; and, through their interest principally, she received five thousand pounds sterling for revealing the secret. It was accordingly published in the Londm Gazette on the 19th June 1739. This .had directed the attention of medical men to the employment of lime-water in cure of the stone. Upon the publication of the thesis, it immediately attracted the attention of chemical- philosophers ; and Dr. Black is now universally acknowledged to be the founder of pneumatic chemistry, and to have opened an iinmense field for observation and experiment to the philosophical world, which before his time had never been explored or even thought of. Dr. Cullen removing to Edinburgh in 1756, Dr. Black was appointed Professor of Anatomy and Lecturer on Chemistry ; but not conceiving himself so well qualsed for filling the anatomical chair, he obtained the concurrence of the University to accomplish an exchange with the Professor of Medicine. He brought to maturity his theory of latent heat, some time between 1759 and 1763 ; and he read, in April 1762, to a select society in Glasgow, the result of his experiments on the subject. Much about the same year he read the essay on latent heat before a society in Edinburgh, bearing the name of the Newtonian Society, instituted in 1759. The delicate state of his health was the cause of his never publishing an account of his doctrine, as the slightest exertion, if continued for any length of time, always brought on a spitting of blood ; and the excitement which a publication of this description would necessarily have produced, and the controversy and criticism that would have followed, was much more than his feeble frame could have borne. In 1764, it was fortunate both for Dr. Black and science, that Mr. James Watt, so justly celebrated for his improvements of the steam-engine, became his pupil, he being at that time employed in repairing the model of a steam-engine for the Natural Philosophy class in the University. In the year 1766, Dr. Cullen, the Professor of Chemistry in the University of Edinburgh, was appointed Professor of Medicine; and the chemical chair in the University thus becoming vacant, Dr. Black was immediately appointed to it, and he continued one of the chief ornaments of the University for a space of about thirty years. Dr. Black lived on very friendly terms with most of the many literary characters then resident in the northern metropolis. Amongst these we may mention his relative, Dr. Adam Ferguson, Mr. Home, author of the tragedy of DozcgZus, Dr. Alexander Carlyle, Sir George Clerk of Penycuick, his brother Mr. Clerk of Eldin, Dr. Roebuck, and Dr. Hutton.