=Joseph Black= was born in 1728 at Bordeaux, where his father was engaged in the wine trade. A student of the University of Glasgow, he became its Professor of Chemistry in 1756. In 1766 he was transferred to the Chemical Chair of the University of Edinburgh, and died in 1799. Black published only three papers, the most important of which is entitled _Experiments upon Magnesia Alba, Quicklime, and Other Alkaline Substances_. He proved that magnesia is a peculiar earth differing in properties from lime. Lime is a pure earth, while limestone is carbonate of lime. He showed that magnesia will also combine with carbonic acid, and he explained that the difference between the mild and caustic alkalis is that the former contain carbonic acid, whereas the latter do not. He also explained how lime is able to convert the mild alkalis into caustic alkalis. Simple and well known as these facts are to-day, their discovery in 1755 excited great interest, and marked an epoch in the history of chemistry. Black’s name is associated with the discovery of latent and specific heat, and he made the first determinations of the amount of heat required to convert ice into water. [Illustration: JOSEPH PRIESTLEY. From a mezzotint after Fuseli in the possession of the Royal Society.] =Joseph Priestley=, the son of a clothdresser, was born in 1733 at Fieldhead, near Leeds. When seven years of age, on the death of his mother, he was taken charge of by his aunt, and was educated for the Nonconformist ministry, eventually becoming a Unitarian. He was first attracted to science by the study of electricity, of which he compiled a history. At Leeds, where he had charge of the Mill Hill congregation, he turned his attention to chemistry, mainly from the circumstance that he lived near a brewery and had the opportunity of procuring large quantities of carbonic acid, the properties of which he carefully studied. He abandoned the ministry for a time to become librarian and literary companion to Lord Shelburne, with whom he remained seven years. During this time he industriously pursued chemical inquiry, and discovered a large number of æriform bodies—viz., nitric oxide, hydrogen chloride, sulphur dioxide, silicon fluoride, ammonia, nitrous oxide, and, most important of all from the point of view of chemical theory, oxygen gas. Priestley’s work gave a remarkable impetus to the study of pneumatic chemistry. It exercised great influence on the extension of chemical science, and—in other hands than his—on the development of chemical theory. The most important of his contributions to science are contained in his _Experiments and Observations on Different Kinds of Air_. This work not only gives an account of the methods by which he isolated the gases he discovered, but describes a great number of incidental observations, such as the action of vegetation on respired air, showing that the green parts of plants are able in sunlight to decompose carbonic acid and to restore