His first object of research was one which possessed high medical as well as chemical interest:—the nature and properties of magnesia. This substance had hitherto been confounded with lime: Dr. Black first showed it to be characterized by peculiar properties which demonstrate its distinct nature as a separate species of earth. The second point of his investigation was the difference between mild and caustic alkalis, between limestone and quick-lime, common and calcined magnesia, &c. The whole of this subject was at that period involved in complete obscurity. Dr. Black showed by simple and decisive experiments the real condition of these substances, and indicated the general law by which they are governed, viz.:—that the difference consists merely in the combination of the simple earth or alkali with a peculiar air, which is driven off by heat, and which was called _fixed air_ by him, and _carbonic acid gas_ by later chemists. He did not however prosecute the inquiry into the nature and properties of this gas. This discovery supplied the foundation on which all subsequent researches and theories have been built. He gave an account of these investigations in an inaugural dissertation, composed as an exercise on taking his Doctor’s degree, and in a paper entitled Experiments on Magnesia Alba, &c., first published in the Edinburgh Physical and Literary Essays in 1755. It was almost immediately after the publication of these researches that Dr. Cullen was elected Professor of Chemistry at Edinburgh. The reputation which Dr. Black had now acquired pointed him out as the proper person to succeed to the vacant chair at Glasgow, to which he was accordingly appointed in 1756. His department included chemistry and medicine; and he also practised as a physician. His lectures soon became highly popular from the clearness of his style and method, and the beauty and simplicity of his experimental illustrations. He did not however prosecute his inquiries, in that particular department of chemistry, in which he had already had so much success. But in another branch of science his power of original research was signally displayed. The relations of bodies to heat, especially in connexion with the changes of state they undergo, was a subject which had hitherto excited hardly any notice; and though some effects were such as might have been supposed obvious, still no one had as yet reasoned on them, or understood their nature.