Joly , John

(1857–1933) Irish geologist and physicist

Joly was born the son of a clergyman from Hollywood, now in the Republic of Ireland. He entered Trinity College, Dublin, in 1876 where he studied literature and engineering. He taught in the engineering school from 1883 and was appointed professor of geology and mineralogy in 1897, a post he held until his death.

Joly's major geological work was in the field of geochronology. He first tried to estimate the age of the Earth by using Edmond Halley's method of measuring the degree of salinity of the oceans, and then by examining the radioactive decay in rocks. In 1898 he assigned an age of 80–90 million years to the Earth, later revising this figure to 100 million years. He published Radioactivity and Geology in 1909 in which he demonstrated that the rate of radioactive decay has been more or less constant through time.

Joly also carried out important work on radium extraction (1914) and pioneered its use for the treatment of cancer. His inventions in physics included a constant-volume gas thermometer, a photometer, and a differential steam calorimeter for measuring the specific heat capacity of gases at constant volume.