ganic matter introduced into natural waters is got rid of in nature, especially in porous soils, by means of oxidation, nitrogenous matter being partially converted into nitrates. This theory he supported by numerous subsequent experiments. In 1849 he examined various problems connected with sewage, and made important suggestions, which are still under discussion, with regard to its canalisation and treatment.
In 1851 Smith began his most extensive research. The fact that the ratio between the amounts of oxygen and nitrogen present in the air varies exceedingly little under the most varied conditions of time and place had led to the impression that chemical analysis was unable to discover the impurities of town air which were made evident by their effect on human health, and even in certain cases by smell. Smith set himself systematically to combat this notion, and began by making a series of determinations of the sulphur compounds introduced into the air by the combustion of coal (Brit. Assoc. Report, 1851, pt. ii. p. 52). He followed this work up later by numerous determinations of other impurities—e.g. ammonia and carbonic acid. In 1856 Smith published a memoir of John Dalton (1766–1844) [q. v.], which embraced a history of the atomic theory from early times. The book displays erudition, common-sense, and impartiality of judgment wherever the issues were simple; but Smith had not sufficient clearness of mind or of style (in spite of occasional happiness of expression) to make a first-rate historian, and he failed to explain the genesis of Dalton's ideas (see Roscoe and Harden's New View of the Atomic Theory). In 1857 he was elected F.R.S. In 1859 he lectured on the organic impurities of the air before the Royal Institution, and described an ingenious method for a comparison of the relative amounts in different places. In 1864 Smith contributed to the report of the royal mines commission an elaborate examination of the air of mines and a comparison with that from various districts in large towns, and a physiological investigation of the effect of carbonic acid. In the same year Smith was elected chief inspector, under the Alkali Act of 28 July 1863, which provided for the inspection of alkali-works and other classes of factories (extended by the act of 1872), and for the infliction of fines when excessive amounts of acid vapours, likely to damage health and vegetation, were emitted. Smith performed his duties with tact and skill, insuring the co-operation of the previously hostile manufacturers in the working of the act, which he showed to be to their financial benefit. His twenty annual reports (continued till his death) contain a large amount of information on the condensation of hydrochloric acid and kindred subjects.
In April 1865 Smith proposed an ingenious ‘minimetric’ method of estimating carbonic acid in the air. In 1869 he published a book on ‘Disinfectants and Disinfection,’ containing a summary of other work, together with experiments of his own performed for the cattle plague commission. In it he recognised the fact that Pasteur's work on germs would revolutionise the subject, but it was only later that he became practically acquainted with Pasteur's methods. Smith's work led to the manufacture on a large scale by his friend Mr. Alexander McDougall of a useful disinfectant powder, consisting of a mixture of calcium sulphite and calcium phenate. In 1872 Smith published his ‘Air and Rain, the beginnings of a Chemical Climatology,’ in which he collected a large amount of experimental material from his previous papers. Less attention has been paid to this work than it deserves, partly because of its defects in composition (of which Smith was conscious), partly because Pasteur's work has diverted attention from the inorganic impurities of air. In the same year he published a study on peat-formation (Memoirs of Manchester Lit. and Phil. Soc.  iii. 281).
After going in the autumn of 1872 to Iceland in the yacht of his friend, the chemist, James Young (1811–1883) [q. v.], he wrote an essay ‘On some Ruins at Ellida Vatu and Kjarlanes,’ and a book, ‘To Iceland in a Yacht’ (privately printed in May 1873). In the same year he paid a visit, also with Young, to the island of St. Kilda, which he described in ‘Good Words’ for 1875, and in a pamphlet, ‘A Visit to St. Kilda’ (privately printed in 1879). In 1876 he edited ‘The Chemical and Physical Researches of Thomas Graham [q. v.]’, with a useful analysis of the separate memoirs, and an introduction on Graham's place as a chemist. The book was privately printed at the expense of Young for distribution among chemists. In 1884 the introduction was republished, together with many of Graham's letters and explanatory notes by Smith, under the title ‘An Account of the Life and Works of T. Graham.’ In 1879 Smith, who was passionately devoted to archæology, and especially to Scottish archæology, published anonymously a book on ‘Loch Etive,’ where he had spent many vacations, and on the legend of the ‘Sons of Uisnach;’ a second edition appeared with his name, posthumously, in 1885. The work, which is written in dialogue form, is valuable