John Joly (1857-1933) - physicist, geologist, engineer

John Joly is considered one of the most distinguished Irish scientist of his time. He made important contributions to geology, physics, engineering, medicine and  photography.

John Joly FRS (1857-1933) was born at Holywood, Co. Carlow, the son of the Rev. John Plunkett Joly and Julia Anna Maria Georgiana nee Comtesse de Lusi. Both parents had aristocratic lineages in France, Germany and Belgium. His great grandfather, Jean Jasper Joly, came to live at Carton House near Dublin, the residence of the second Duke of Leinster.  A detailed lineage and list of his publications may be found in John Joly’s obituary [1]. His life and work are described in some detail in an article by John Nudds [2] on which this is based.  Joly’s father died while he was an infant and his mother brought John and his two brothers to Dublin.  He attended Rathmines School, where his nickname was ‘Professor’ [2] and he entered Trinity College in 1876, aged 21. He sat his degree examinations in October 1882 and shortly thereafter he was appointed assistant to the professor of Civil Engineering in Trinity.  In 1891 he left the Department of Engineering and joined George Francis Fitzgerald, the Erasmus Smith’s professor of experimental and natural philosophy in what was then the Department of Natural and Experimental Philosophy and is now the School of Physics. In 1897 the chair of Geology and Mineralogy in Trinity became vacant. Joly’s undergraduate degree in engineering included work in mechanics and experimental physics and mining, chemistry, geology and mineralogy.  He applied for the chair and was successful, beginning as Head of the Department of Geology in Michaelmas Term 1897, aged 39. Joly had a very wide range of research interests and applied his knowledge as a prolific inventor.  He had previously invented instruments useful in geology such as a steam calorimeter for measuring specific heats of minerals by surrounding them with steam and weighing the resulting condensation and a meldometer for measuring the melting points of crystals. Now he began working in physical geology and in particular he began considering methods for estimating the age of the earth. 

McDowell and Webb in their history of Trinity College [3] said of Joly that he was, ‘Certainly the most versatile and productive, and perhaps the greatest of the scientists that the College has ever produced.’  It has also been said that Joly was essentially a physicist.  He brought the fundamental understanding and analytic thinking of the physicist to many practical and fundamental problems, the former as an inventor of technologies such as colour photography and the latter in his deep thinking about problems such as the age of the earth and radioactive decay in minerals.

Joly is perhaps best known as the inventor of the ‘Joly Process of Colour Photography’, which he patented in 1894.  It was the first successful method for producing colour photographs on a single plate. Black and white photography had begun around 1839 [4]. In 1861 James Clerk Maxwell had made a colour image of a tartan ribbon using three lanterns with red, green and blue filters, making slides and then projecting the three slides onto a screen to make a colour image.  The Joly process used a single filter on which fine red, green and blue lines (of width less than 0.1 mm) had been ruled.  Patents for the Joly process were acquired by a group of American businessmen and the Natural Colour Photo Co. Ltd. was formed in Great Brunswick St., Dublin. However, the process had limited commercial success and Joly was forced to fight patent battles in the United States, after which his priority was established.  The Autochrome, invented by Auguste and Louis Lumiere [4]

was the first commercially successful screen process from 1907.

Joly devoted much of his effort in the ensuing 25 years to estimating the age of the earth. His first paper on the subject was published in 1899 [5].  At that time, estimates of the age of the earth lay in the range 10-20 million years, based on the rate of cooling of the molten earth. Lord Kelvin was a leading proponent of this estimate.  Joly based his estimate on the amount of sodium in the oceans and the rate at which sodium is transported to the oceans by rivers, etc. Assuming that that rate had remained constant over time, Joly arrived at an estimate of 90-100 million years.  A few short years after the discovery of radium by Pierre and Marie Curie, Joly had turned his attention to implications of radioactive heating within the earth. His paper entitled, ‘Radmium and the geological age of the earth,’ [6] criticised the rate of cooling arguments by Kelvin. Since they omitted heat released withing the earth by radioactive decay, they would inevitably underestimate the age of the earth and, Joly noted in closing, ‘The hundred million years which the doctrine of uniformity requires may, in fact, yet be gladly accepted by the physicist’. Here he was referring to his estimate based on the salinity of the oceans and his assumption that conditions had remained uniform over that time.

In 1906 Ernest Rutherford used a method for estimating the age of the earth based on radioactive decay of two uranium isotopes to two lead isotopes. Based on their ratios in a rock containing uranium, he gave an estimate of the age of the earth of 3.4 billion years, more than an order of magnitude greater than Joly’s estimate.  If initially dismayed by the new estimate, Joly realised that ‘halos’ that he had observed in minerals such as biotite were caused by radioactive decay of heavy elements present in the mineral.  Joly’s work with Rutherford led to an estimate of rocks from the Devonian period of at least 400 million years [8].

During the first world war Joly contributed many ideas and inventions to the war effort which he sent to the British Admiralty.  At that time, Joly also continued his work with radioactivity.  Working with Walter Stevens in Dublin, he realised that the practice of that time of applying one or two radioactive sources to a tumour, it was better to use more, smaller sources to reduce injury to the patient by radioactivity as well as saving precious radium.  This became known as the ‘Dublin method’ [9]. Later Joly and Stevenson used glass capillary tubes filled with radioactive radon gas for cancer treatment.

Joly was such a prolific scientist that it is difficult to even mention all of his contributions.  He never married. As noted above, his father died while he was an infant and his mother died when he was in his late twenties.  He had an enduring friendship with Horatio Dixon, University Professor of Botany on Trinity College. With Dixon he wrote ‘The cohesion theory of the ascent of sap in plants’, explaining plant transpiration.  He was active in climbing and walking and enjoyed holidays in the Swiss Alps.  He also had a keen interest in sailing and maritime affairs.  He was elected a Commissioner of Irish Lights [10] in 1901 and served for 32 years as commissioner and later senior commissioner.  He received many honours for his work. He was elected a Fellow of the Royal Society in 1892, aged 34 and received the Royal Medal of the Royal Society in 1910. He was awarded the Boyle medal in 1911, Ireland’s most prestigious scientific recognition.  He received the Murchison Medal of the Geological Society in London in 1923 as well as honorary degrees and lectureships.  Through Joly’s efforts, Lord Iveagh chairman of Guinness brewery donated £24,000 for the erection of a Physical and Botanical laboratory in 1906 and a further £10,000 to equip the Geological laboratory in Trinity College in 1912.  According to Nudds [2], Joly remained active in Trinity for the rest of his life, delivering his last lecture just six days before he died, aged 76.



  1. John Joly 1857-1933,
  2. The life and work of John Joly, J. R. Nudds, Irish Journal of Earth Sciences 8, 1986, 81-94.
  3. Trinity College Dublin 1592-1952: An Academic History, Cambridge University Press (1982)
  5. An estimate of the geological age of the earth, J. Joly, Scientific Transactions of the Royal Dublin Society, Series 2, 7, 23-66 (1899)
  6. Radmium and the geological age of the earth, J. Joly, Nature, London, 68, 526 (1903)
  7. The Dating Game: One Man’s Search for the Age of the Earth, C. Lewis, Cambridge University Press (1982)
  8. The age of pleochroic haloes, J. Joly and E. Rutherford, Philosophical Magazine 25, 644-657 (1913)
  9. On the local application of radium in therapeutics, J. Joly Scientific Proceedings of the Royal Dublin Society 14, 290-296 (1914)
  10. The body responsible for maintaining light houses in Ireland.