Joly, John (1857-1933)

Irish geologist and physicist. As professor of geology and mineralogy at Trinity College, Dublin, from 1897 onwards, he made influential contributions to the contemporary debate about the age of the Earth. He was instrumental in founding the Radium Institute in Dublin, in 1914, where he pioneered the use of radiotherapy in the treatment of cancer.

Born in Offaly, King's County, Joly was educated at Trinity College, Dublin, where he gained his first appointment, as assistant to the professor of engineering, in 1882. In this role, he devised several pieces of scientific apparatus, including the meldometer for producing artificial crystals, a hydrostatic balance for density determination, a steam calorimeter to measure thermal properties, and a photometer to measure luminous intensity. In the 1890s, he estimated the age of the Earth (80-90 million years) by measuring the sodium content of the sea. Joly was the first geologist to recognize the significance of radioactive atoms in maintaining the heat of the Sun and in directly moulding the history of the earth (1903), and realized that pleochroic haloes (different areas of colour) in some minerals were the product of radioactivity and could be used for dating (1907-14). He also developed a practical method of colour photography.

Joly's publications include An Estimate of the Age of the Earth 1899, Radium and the Geological Age of the Earth 1903, Radioactivity and Geology 1909, and The Surface History of the Earth 1924.