MacAdam, James (1801–61), manufacturer and geologist, was born in High Street, Belfast, the first child and the elder of two surviving sons of James MacAdam (1775–1821), merchant, and Jane MacAdam (née Shipboy) (1774–1827), a Belfast native. Two siblings died in infancy while a third died in 1826, aged 14. MacAdam was educated at Belfast Academical Institution (BAI; latterly Royal Belfast Academical Institution), which his father, one of the community's leading progressive presbyterian citizens, had helped to found. The school's Irish class list of May 1819 prepared by William Neilson (qv) includes a ‘J. MacAdam’. Despite his academic abilities in both classics and science, and his personal inclinations, he entered his father's hardware business, taking it over on his father's early death and successfully developing it with his brother, Robert Shipboy MacAdam (qv). In 1833 the brothers moved to 18 College Square East which remained James's home until his death. A year later they opened the Soho Foundry at 28 & 30 Townsend Street, Belfast, manufacturing machinery for the textile industry, as well as gates, window frames, and other ironware items. An extremely important invention by the brothers in the early 1850s was the MacAdam flax roller. The original hardware shop and home were sold in 1844 and 1846 respectively.
MacAdam graduated with a BA from TCD in 1836. He was assistant secretary of the board of his old school, BAI (1843–52), and also managed the library. A candidate for the chair of mineralogy and geology in QCB on its opening (1849), he subsequently served two years as the college's first librarian, responsible for establishing its nucleus of books. One of eight foundational members (1822) of the Belfast Natural History Society (renamed in 1842 the Belfast Natural History and Philosophical Society (BNHPS), and latterly the Belfast Society), he served as corresponding secretary (1825–54), joint honorary secretary (1846–61), and president (1860–61). From the society's foundation until 1851 he read over thirty papers to its members, mainly on geology, his chief interest. His extensive geological collection, mainly from Ulster, included minerals and fossils, many of which he acquired during construction of the railway system. These formed part of the BNHPS museum in College Square North (opened 1831), the first provincial museum in Ireland and the basis of the collection now in the Ulster Museum. Two of MacAdam's papers published in the Journal of the Geological Society of Dublin were translated into German and published in Heidelberg. He was a fellow of both the Dublin and London geological societies.
MacAdam was prominent in other aspects of the cultural life of Belfast. He was secretary of the city's Flax Improvement Society, a committee member of the Linen Hall Library, and a founder of the botanic gardens (for which he obtained by correspondence plants and seeds from around the world). He was elected (1831) to the Belfast Literary Society, to which he read several papers, including ‘On the cosmogonic opinions of the ancients’ (3 Mar. 1834); he served as the society's president (1836–7). Elected to the Statistical and Social Inquiry Society of Ireland in its second session (1848), in the following year he read a paper on ‘Schools of design in Ireland’, published in Transactions of the Dublin Statistical Society, i (1847–9). He acted as correspondent for the Athenaeum at several meetings of the British Association for the Advancement of Science, and organised with his brother a major exhibition for the association's first visit to Belfast in 1852.
Throughout his life, MacAdam, who was unmarried, was a member of Rosemary Street non-subscribing presbyterian church. He died 1 June 1861 in Belfast, and was buried with his Shipboy and MacAdam relations in Knockbreda Church of Ireland graveyard, Belfast. Business papers of the ironware firm are in PRONI, and one of his unpublished papers is in QUB. A small portrait hangs in the librarian's office, QUB. Another, painted from a photograph by Richard Hooke (1820–1908) two years after MacAdam's death, is in the Ulster Museum art department; the museum's history department holds a painting in oil.