Joly, John

John Joly (1857-1933)

Instrumentation, Colour Photography, Nuclear Medicine, Ascent of sap in plants.

John Joly was born in 1857 at Hollywood House, near Bracknagh, King’s County (now County Offaly), the son of a Church of Ireland rector with Belgium and Italian ancestors.   When John was not yet four months old, his father died and the family moved to Dublin.   He entered Trinity College Dublin in 1876, graduated in engineering in 1882 and became assistant to the professor of civil engineering; he had also access to the physics laboratory.   Joly invented many scientific instruments; the best known include a grease-spot photometer, a steam calorimeter and a meldometer for measuring the melting points of a very small quantities of minerals.   He was remarkably inventive and wrote about 270 scientific papers and several books.

In 1897 he was appointed professor of geology and he attempted to measure the age of the Earth from the amount of salt in the oceans.   However, with the discovery of radioactivity he realised that the ages of the rocks cold be estimated from their residual radiation.   Moreover, the presence of radioactive elements in the Earth meant that Lord Kelvin’s estimates for the age of the Earth were too low.

In 1894, Joly invented a system of colour photography based on taking and viewing photographs with plates ruled with many narrow lines in three colours.   The same year, he and his friend and colleague Henry Dixon explained how sap rises in plants (the cohesion theory).   Joly collaborated with a surgeon, Dr Walter Stevenson, to use radioactivity to treat cancer.   Their technique, known as the ‘Dublin Method’, involved capturing radon gas in thin glass tubes which were then fitted inside steel needles and inserted into the cancerous tumour.

Joly was an able administrator and served Trinity College and the Royal Dublin Society in many ways.  He died on 8 th December 1933 in Dublin.

Further reading:  

Charles Mollan, (2007) It’s Part of What We Are, Dublin, Royal Dublin Society

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