Irish geologist and physicist who, soon after 1898, estimated the age of the Earth at 100,000,000 years. He also developed a method for extracting radium (1914) and pioneered its use in cancer treatment.

Joly was educated at Trinity College, Dublin, where he became professor of geology and mineralogy (1897) after having served as demonstrator in civil engineering (1883) and physics (1893). He first sought to estimate the age of the Earth from the salt content of the oceans, then from rocks containing radioactive zircon and alanite. He also tried to explain the formation of the Earth's crust by convection of heat generated by radioactive decay in the Earth's interior.

Joly is also noted for his inventions of a thermometer, a steam calorimeter for measuring heat energy, and a photometer for measuring light frequencies. The recipient of many honours, Joly was elected a fellow of the Royal Society of London (1892).