│Thomas Graham│ was born in Glasgow on December 21, 1805, and, after studying under Thomas Thomson at the University of that city, attended the lectures of Hope and Leslie in Edinburgh. In 1830 he succeeded Ure as teacher of chemistry at Anderson’s College in Glasgow, and in 1837 was called to the Chair of Chemistry in the newly-founded University of London, in succession to Edward Turner. In 1854 he was made Master of the Mint. He died in London on September 16, 1869. Graham’s work was mainly devoted to that section of the science now known as physical chemistry. His contributions to pure chemistry are few in number. By far the most important is his discovery of _metaphosphoric_ acid and its relations to the other modifications of phosphoric acid. Ortho- or ordinary phosphoric acid was known to Boyle; pyrophosphoric acid was discovered by Clark. Graham’s work is noteworthy as first definitely indicating the inherent property of the acids to combine with variable but definite amounts of basic substances by successive replacement of hydroxyl groups—the property we now term _basicity_, and was of fundamental importance in regard to its bearing on the constitution of acids and salts. Graham’s fame chiefly rests upon his discovery of _the law of gaseous diffusion_ (1829–1831), upon his work on the _diffusion of liquids_, and upon his recognition of the condensed form of hydrogen he termed _hydrogenium_. Questions involving the conception of molecular mobility, indeed, constituted the main feature of his inquiries. We owe to him, among others, the terms _crystalloid_, _colloid_, _dialysis_, _atmolysis_, _occlusion_—all of which have taken a permanent place in the terminology of science. [Illustration: ALEXANDER WILLIAM WILLIAMSON.]