Mallet, John William (1832–1912), chemist, was born 10 October 1832 in Dublin, eldest among three sons and two daughters of Robert Mallet (qv), engineer and seismologist, of Ryder's Row, Dublin, and Cordelia Mallet (née Watson; d. 1854), daughter of a Dublin bookseller. He was educated initially at home and, from the age of nine, at a private school run by J. P. Sargent, where he was instructed in classics, chemistry, and physics. Mallet supplemented this education at an early age by reading scientific works by men such as Joseph Black (qv), Lavoisier, Davy, and Fourcroy in his father's library. At the age of 16 he attended a course of lectures given by James Apjohn (qv) at the RCSI in Dublin. Apjohn noted Mallet's interest in science and arranged for him to take additional training in his private laboratory. Mallet assisted with his father's investigations into the velocity of transmission of gunpowder explosions through the wet sand of Killiney (1849) and the granite of Dalkey (1850). He entered the University of Dublin in 1849 and graduated in 1853 as first senior moderator and gold medallist in experimental physics. Mallet was a precocious student. While still an undergraduate he published (1850) his first paper, on a chemical examination of ‘killinite’, a mineral discovered in 1817 near Killiney, Co. Dublin. In 1851 he went to Göttingen, where he studied under Friedrich Wöhler (1800–82), a pioneer in organic analysis, and was awarded a Ph.D. for a thesis on the chemical examination of a number of antiquities in the RIA collection. This work was later published in the Academy's Transactions (1855) and was the first sustained account of an investigation into the chemical composition of prehistoric Irish materials and objects. At Göttingen Mallet made friends with a fellow student, the American William Smith Clark (1826–86), and they toured Europe meeting leading figures of science including Alexander von Humboldt, Heinrich Rose, and Eilhard Mitscherlich.
Between 1852 and 1854 he helped his father to compile an extensive catalogue of the earthquakes that occurred between 1606
Mallet had deep sympathies with the confederate cause, and though remaining a British subject he enlisted in the confederate army as a private when civil war broke out (April 1861). He was soon commissioned lieutenant and became ADC to Gen. Robert E. Rodes. His scientific abilities were recognised by Gen. Josiah Gorgas, chief of confederate ordnance, and Mallet was appointed superintendent of the confederate ordnance laboratory at Macon, Georgia, in May 1862. At Macon Mallet carried out experiments on new types of gunpowder, shells, and rockets, and improved the production of weaponry. He erected foundries, furnaces, rolling-mills, and munitions plants, and greatly helped the confederate war effort. When the war ended in April 1865, Mallet held the rank of lieutenant-colonel in the artillery.
After the war Mallet spent some time exploring for oil in Louisiana and Texas and accepted the chair of chemistry at the University of Louisiana medical school, where he studied medicine and carried out investigations in physiological chemistry and hygiene. On leaving Louisiana in 1868 he was awarded an honorary MD. Mallet moved to the University of Virginia, where he established one of the first systematic courses in industrial chemistry offered in the US. He lectured on the processes involved in the production of metals and alloys, the manufacture of chemicals, and the chemistry of agriculture. His courses and teaching were highly regarded; he emphasised the importance of laboratory instruction, and he trained his students in the qualitative and quantitative analysis of ores and soils while encouraging them to embark on original research. Mallet taught special lecture courses at Johns Hopkins University (1873) and briefly held positions at the University of Texas (1883–4) and the Jefferson Medical College, Philadelphia (1884–5), but after 1885 he returned to Virginia, where he stayed till his retirement in 1908.
As a chemist Mallet is best remembered for his pioneering work on the atomic weights of lithium, gold, and aluminium. Mallet's estimation of the atomic weight of the last was accepted till after his death. Further investigations included the analysis of meteorites and rare terrestrial minerals, the occurrence of silver in volcanic ash, the density of solid mercury, and the molecular weight of hydrofluoric acid. During his career he published over 100 papers. In view of his chemical expertise, he was frequently called on to act as an expert witness in medico-legal and commercial cases. He served as a judge at the Centennial Exposition (1876) and was a member of the US federal assay commission in 1886, 1888, and 1896. He was elected FRS (1877), served as vice-president of the Chemical Society (1888–90), and was president of the American Chemical Society (1882), of which he was a co-founder in 1876. He received honorary doctorates from five universities: William & Mary College, the University of Mississippi, Princeton, Johns Hopkins, and the University of Pennsylvania. Mallet retired in 1908 and continued to carry out experiments till his death (7 November 1912) at his home in Charlottesville, Virginia.
He married first (21 July 1857) Mary Elizabeth (d. 1886), eldest daughter of John Ormond, former judge of the Alabama supreme court; they had two sons and a daughter, but the elder son died of tuberculosis (1884). He married secondly (1888) Mrs Josephine Burthe (née Pagés).
A complete list of Mallet's public lectures and published papers can be found in William H. Echols, ‘John William Mallet: scholar, teacher, gentleman’, University of Virginia Alumni Bulletin, 3rd ser., i, no. 4 (1913), 3–47. Notable published papers include ‘A report on the chemical examination of Celtic antiquities from the museum of the Royal Irish Academy’, Transactions of the Royal Irish Academy, xxii (1855), 313–42; ‘On the atomic weight of aluminum’, American Journal of Science and Arts, xxviii (1859), 349–54; ‘Revision of the atomic weight of aluminum’, Philosophical Transactions of the Royal Society, clxxi, pt 3 (1880), 1003–35; and ‘Revision of the atomic weight of gold’, Philosophical Transactions of the Royal Society, clxxx (1889), 395–441. The library in the chemistry department of the University of Texas, and Mallet Hall in the University of Virginia, were named in his honour.