Joly, John (1857–1933), geologist, was born 1 November 1857 in Hollywood House (the Rectory), Bracknagh, King's Co. (Offaly), third and youngest son of John Plunket Joly, rector of Clonbullogue, and Julia Anna Maria Georgina Joly (born the comtesse de Lusi). Educated at Rathmines School, Dublin, and at TCD, he studied classics and modern literature and later engineering. In 1883 he was conferred with the degree of B.Eng., and was appointed assistant to the professor of civil engineering at TCD, where he was to spend all his academic life.
He soon began investigations on the density of gases and the detection of small pressures, and invented the steam calorimeter for measuring the specific heat of minerals. This piece of equipment later played an important role in the kinetic theory of gases. In 1891 he was appointed assistant to the professor of natural and experimental philosophy (physics), and in the next six years he invented and developed a mercury-glycerine barometer and an electrolyte ampere-meter, and published on the specific heat of gases. Together with his lifelong friend Henry Horatio Dixon (qv), the botanist, he explained the mechanism for the transportation of water in plants, and in the late 1890s invented a system of colour photography which is essentially the method used latterly in film cameras. In 1897 he was appointed professor of geology and mineralogy, and soon afterwards (1899) he published an estimate of 100 million years for the age of the earth, which he determined by estimating the volume of sodium in the oceans and the rate at which it entered them. He was a pioneer in the field of radioactivity and its connections to geology. In 1907 he demonstrated that pleiochroic halos found in biotite in some granites were formed as a result of the decay of radioactive zircon crystals. He played a major role in the foundation in 1914 by the RDS of the Irish Radium Institute, which exploited the medical advantages of radium. His thoughts on radioactivity and geology were summarised in a book under that title (1909), which was expanded to include global tectonics in The surface-history of the earth (1925).
He was also an educationist and encouraged and advanced the teaching of education through laboratory studies. He was warden of Alexandra College for a period, and also visited North America as part of a British education mission sent in 1918 to examine the university system. Joly wrote over 250 scientific papers and several books, including an autobiography, Reminiscences and anticipations (1920). He was elected FRS (1892), and received the society's Royal Medal in 1910. In 1919 he became the first fellow of TCD to be elected rather than gain this accolade through examination. He was president of section C (geology) when the British Association for the Advancement of Science visited Dublin in 1908, and served as president of the RDS 1929–32; he received its Boyle medal in 1911. The Geological Society of London awarded him its Murchison medal in 1923. He was a keen traveller, especially to the Alps, a notable yachtsman, and a commissioner for Irish lights. He died, unmarried, in Dublin on 8 December 1933 and is buried in Mount Jerome cemetery, Dublin. A bust (1930) by Oliver Sheppard (qv) and a posthumous portrait (1934) by Leo Whelan (qv) are at TCD, as are his papers, medals, and mineral collection. Forty years after his death, a crater on Mars was named after him, which was appropriate, given his 1897 paper on the nature and origin of Martian canals.