Thomson entered upon his medical studies at the University of Edinburgh, and during the session of 1795-96 attended the lectures of the celebrated Dr. Black, who first awoke in him the latent taste for that science of which he was destined to become so bright an ornament. In 1796 he became connected with the _Encyclopædia Britannica_, for an early edition of which he wrote the articles--Chemistry, Mineralogy, Vegetable Substances, Animal Substances, and Dyeing Substances, &c. These articles formed the basis of his system of chemistry, which he published at Edinburgh in the year 1804, in four volumes, and afterwards greatly enlarged and improved as the demand for the book increased. Dr. Thomson commenced delivering a series of lectures on chemistry at Edinburgh in 1800, which were continued with increasing popularity until 1810. Meanwhile he invented the system of chemical symbols now generally adopted by all men of science (with variations as the time demands), and without which chemical language would be unintelligible. He was also the first to open a laboratory in Great Britain for practical manipulation in chemistry. In 1810 he published his 'Elements of Chemistry,' and in 1812 visited Sweden, and on his return wrote a description of that country. The following year to this Dr. Thomson started in London the 'Annals of Philosophy,' a scientific journal, which he continued to edit until the year 1822, and which a few years afterwards was merged in the 'Philosophical Magazine.' He also about this time conducted for the Board of Excise a series of investigations on brewing, which formed the basis of Scottish legislation on that subject. In the year 1817 Thomson was elected lecturer on chemistry in the University of Glasgow, and in the following year received the title of Professor. This chair he held until his death, being assisted in his latter years by his nephew and son-in-law, Dr. R. D. Thomson. When Dalton had worked out his grand discovery of the Atomic Theory, he communicated the result of his researches to Thomson, who at once perceived the value and importance of the discovery, and in the year 1807 was the first to publish it to the world. He gave a sketch of this grand theory in the third edition of his 'System of Chemistry;' and we are chiefly indebted to the labours of Professor Thomson, conjointly with Dr. Henry of Manchester, and Dr. Wollaston, for luminous views on this important subject. In 1825 Dr. Thomson wrote, in two volumes, 'An Attempt to Establish the First Principles of Chemistry by Experiment.' In 1830-31 he published his 'History of Chemistry,' a work which has been described as a masterpiece of learning and research. In 1836 appeared his 'Outlines of Mineralogy and Geology;' and in 1849 he issued his last work, 'On Brewing and Distillation.'