Bernal, John Desmond (1901–71), scientist, was born 10 May 1901 at Nenagh, Co. Tipperary, eldest of three sons and two daughters of Samuel George Bernal, farmer, of Brookwatson, Nenagh, and his wife Elizabeth Miller, daughter of a Presbyterian minister in San José, California. Bernal's mother converted to catholicism before her marriage and he was raised as a catholic. He was educated in England from the age of 10, first at Hodder, preparatory school for the Jesuit public school Stonyhurst, and briefly at Stonyhurst itself, but transferred to Bedford School because science was in the curriculum there. He won (1919) a scholarship to Emmanuel College, Cambridge. A fellow student, H. D. Dickinson, introduced Bernal to socialist ideas; he embraced Marxism before the end of 1919, joined the Communist Party of Great Britain in 1923, and remained a committed Marxist for the rest of his life. He witnessed the conflict in Ireland while on holidays from Cambridge and expressed republican views, which were at variance with those of his family circle. Bernal took part I of the mathematical tripos in 1920, part I of the natural sciences tripos (chemistry, geology, and mineralogy) in 1922, and part II (physics) in 1923. While still an undergraduate, he used Hamiltonian quaternions to derive the 230 space groups of crystallography – the possible spatial arrangements of atoms in crystals. This thesis had little connection with his prescribed studies, but his tutor forwarded a copy to Sir William Bragg at the Royal Institution, who was sufficiently impressed to accept him as a research student (1923).
Bernal's early work at the Institution involved the use of X-rays in investigating the atomic structure of crystals: he successfully solved the structure of graphite and made considerable progress in determining the structure of δ bronze. In the same period he designed an instrument for recording X-ray diffraction, which was marketed commercially. Bernal returned to Cambridge (1927) as the university's first lecturer in structural crystallography and continued the study of metal alloys, investigating the structures of both ε bronze and η bronze, but his attention was increasingly drawn to newly-isolated biological materials. His study of the structure of sterols, particularly that of calciferol (vitamin D2), overturned existing ideas of sterol structure and demonstrated the utility of crystallography in investigating complex organic molecules. Turning to proteins, Bernal succeeded in obtaining clear X-ray diffraction patterns for crystalline pepsin, the first time that this had been done for a protein. A pioneering X-ray analysis of the tobacco mosaic virus allowed him to conclude that the viral particles were long and rod-shaped, a finding later confirmed by electron microscopy. In 1938 Bernal went to Birkbeck College, London, as professor of physics, but on the outbreak of war he joined the research department of the Ministry of Home Security, where he worked on the design of air-raid shelters. In 1943 he was attached to Combined Operations Command and worked on an abortive project for constructing floating airfields made of ice, and on the development of the artificial breakwaters and Mulberry harbours used in the Normandy landings.
After the war Bernal returned to Birkbeck College, where he directed research into the structure of proteins, viruses, cement, and inorganic oxides, while personally conducting research into the liquid state. Bernal's communist views made him suspect in official circles and he experienced growing difficulty in obtaining research funding as the cold war intensified; his defence of the discredited theories on acquired characteristics advanced by the Soviet biologist Trofim Lysenko also tended to alienate fellow scientists. He was a founding vice-president of the World Peace Council (1949) and visited the USSR frequently during the 1950s, becoming friendly with Nikita Khrushchev. Bernal was awarded the Royal Medal of the Royal Society (1945), the Medal of Freedom with palms of the United States (1945), and the Lenin peace prize (1953); he became FRS (1937), was elected to membership of the academies of science of Hungary (1954), Poland (1954), Romania (1957), Bulgaria (1958), USSR (1958), Czechoslovakia (1960), East Germany (1962) and Norway (1966), and received many academic distinctions. In 1922 Bernal married (Agnes) Eileen, daughter of Dr William Carr Sprague; they had two sons, Michael and Egan. He also had a son, Martin, by Margaret Gardiner and a daughter, (Susanna) Jane, by Margot Heinemann. Having experienced a stroke in 1963, Bernal became increasingly incapacitated and died in London 15 September 1971. His works include The social function of science (1939), Science in history (1954), and The origin of life (1967). A select bibliography of his publications is in Bibliographical Memoirs of Fellows of the Royal Society, xxvi (1980), 74. His scientific and personal papers are in Cambridge University Library.