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 of his friend Wollaston, who had just died. His own strength gradually declined, until on 2 July 1852 he died, while residing near the Holy Loch.

Thomson married, in 1816, Agnes Colquhon, the daughter of a distiller near Stirling, and left a son, Thomas Thomson (1817–1878) [q. v.], well known as a botanist and explorer, and a daughter, who married Robert Dundas Thomson.

As a chemist Thomson is best known for the warm and effective support which he accorded to Dalton's atomic theory. He visited Dalton in Manchester on 26 Aug. 1804, and received from him an account of the new theory which he introduced into the third edition of his ‘System’ (pp. 425 et seq.) published in 1807. This was the first detailed public announcement of the theory, for Dalton did not publish his ‘New System of Chemical Philosophy’ until 1808. After the publication of the second part of the first volume of Dalton's work in 1810, Thomson issued a long series of papers (Annals of Phil. 1813–14) in which the atomic theory was applied to elucidate the composition of a very large number of compounds. These contributed largely to making the theory known, especially on the continent of Europe.

In 1819 Thomson commenced a series of experimental researches with the view of testing, or rather of confirming, the theory of William Prout [q. v.], that the atomic weights of all the elements are exact multiples of that of hydrogen. The results of the many thousands of experiments which he conducted with this object were extremely favourable to the theory and were published in 1825 under the title ‘An Attempt to establish the First Principles of Chemistry by Experiment,’ in two volumes, primarily intended for the use of his students. The analyses recorded had not been carried out with sufficient care to justify the claim of high accuracy made for them by the author, and the work was very severely criticised, especially by the Swedish chemist Berzelius, himself an analyst of extraordinary skill, who went so far as to accuse the author of having done ‘much of the experimental part at the writing table’ (, Jahresbericht, 1827, vi. 77). The statements which induced this suspicion are explained by Walter Crum as follows: ‘The results which appear so perfect in the First Principles are not to be understood as the actual results of any one experiment, or even as the mean of several experiments, but as results which might fairly be deduced from them, and which, being in round as well as more perfect numbers, were more suitable for a school book’ (Proc. Phil. Soc. Glasgow, vol. iii. 1855). It has been claimed for Thomson that he introduced the use of symbols into chemistry (Edinb. New Phil. Journal, 1852–3, liv. 86). This claim is, however, unfounded, for symbols were in constant use among the earlier chemists; while Dalton introduced the modern atomic symbol, although he used signs instead of letters.

Besides the works already mentioned Thomson was the author of: 1. ‘Elements of Chemistry,’ 1810. 2. ‘History of Chemistry,’ 2 vols. 1830–1. 3. ‘An Outline of the Sciences of Heat and Electricity,’ 1830. 4. ‘Chemistry of Inorganic Bodies,’ 1831. 5. ‘Outlines of Mineralogy,’ 1836. 6. ‘Chemistry of Organic Bodies,’ 1838. 7. ‘Chemistry of Animal Bodies,’ 1843. 8. ‘Brewing and Distillation,’ 1849. No fewer than 201 scientific papers, including numerous articles in the ‘Annals of Philosophy’ and the ‘Records of Science,’ are placed to Thomson's credit in the Royal Society's catalogue; these deal chiefly with the atomic theory, analyses and preparation of salts, and with subjects connected with mineralogy, geology, and agriculture, in all of which he took an active interest. He was also the author of a pamphlet, ‘Remarks on the “Edinburgh Review” of Dr. Thomson's System of Chemistry, by the Author of that Work,’ Edinburgh, 1804. Thomson's portrait figures in the engraving, by Walker & Son, of the distinguished men of science of Great Britain living in the years 1807–8.

[A Memoir by W. Crum is given in Proc. Phil. Soc. of Glasgow, 1855, vol. iii. and by R. Dundas Thomson in Edinburgh New Philosophical Journal, 1852–3, liv. 86.]  THOMSON, THOMAS (1817–1878), naturalist, born in Glasgow on 4 Dec. 1817, was eldest son of Thomas Thomson (1773–1852) [q. v.], professor of chemistry in the university of Glasgow, by his wife Agnes Colquhon, daughter of a distiller near Stirling. Thomas was educated at the high school and the university of Glasgow. Throughout his college career he specially devoted himself to science, and when only seventeen discovered and described the celebrated beds of fossil mollusca on the Firth of Clyde, drawing conclusions that showed remarkable powers of generalisation.

Intending at first to adopt chemistry as a profession, he passed some years in the university laboratory, and spent a winter at Giessen under Liebig, when he discovered pectic acid in carrots. On entering the medical classes at Glasgow he concentrated