(b. Holywood, King’s County [now Offaly], Ireland, 1 November 1857; d. Dublin, Ireland, 8 December 1933)
geology, experimental physics, chemistry, mineralogy.
Joly was the third son of Rev. J. P. Joly, who was of French extraction, and Anna Comtesse de Lusi, who came from a mixed German-Italian family. He was educated at Rathmines School and Trinity College, Dublin. He graduated in 1882 with a degree in engineering, physics, chemistry, geology, and mineralogy; he then became assistant to the professor of engineering. In 1897 Joly was appointed professor of geology at Trinity College, a post he held until his death. He had both a fertile mind and the ability to apply the fundamental principles of physics and chemistry to the explanation of new facts; and he often devised new forms of apparatus for his researches.
One of Joly’s earliest inventions was the steam calorimeter, which he used to determine the specific heats of minerals. Using this apparatus, he also determined, for the first time, the specific heats of gases at constant volume. In 1895 he devised a new method for the production of photographs in natural colors. Using an ordinary camera with an isochromatic plate, Joly placed a glass screen ruled with closely spaced alternating lines of red, blue, and violet between the lens and plate. The resulting negative, viewed through a similar screen with red, blue, and violet lines, gave a picture reproducing natural colors.
In 1899 Joly estimated the age of the earth by a method originally suggested by Halley, based on the rate of increase in the sodium content of the oceans. His estimate of 80-90 million years represented the time elapsed since moisture first condensed upon the earth. Although now of only historic interest, at the time his estimate was of some importance because it supported the views of geologists and evolutionists who were unwilling to accept the much lower estimates of contemporary physicists, notably Lord Kelvin, which were based on the supposed rate at which the earth had cooled by radiation, assuming only the sources of heat then known.
The discovery of radioactivity, and particularly the possibility of its application to the solution of geological problems, aroused Joly’s interest. In 1903 he drew attention to the probable importance of radioactivity as a source of terrestrial heat and the effect it would have on calculations of the age of the earth made by Kelvin’s method. In 1907 Joly, using his knowledge of mineralogy, made a discovery that proved of great importance in connection with the new method of calculating the earth’s age by radioactive methods. Mineralogists had long known that certain rock-forming minerals, especially biotite, when viewed under the microscope, were often characterized by the presence of small circular dark spots or concentric rings, known as pleochroic halos, which were centered on minute inclusions of other minerals, for example, zircon. Joly demonstrated that these halos were spherical in form and proved that they had been formed by radioactive emanations from the mineral at their center. Subsequently he and others carried out exact measurements of halos present in rocks of differing geological ages, establishing by this means that the rate of decomposition of radioactive minerals must have been constant throughout geological time, an assumption necessary in all subsequent calculations of the age of geological formations by radioactive methods. Joly’s studies in radioactivity were incorporated in his books Radioactivity and Geology (1909) and The Surface History of the Earth (1925).
Joly’s interest in radioactivity also extended to its use for therapeutic purposes. It was on his suggestion that the Royal Dublin Society founded a radium institute; and, in collaboration with Walter Stevenson, he invented a hollow needle for use in deep-seated radiotherapy (the “Dublin method”), which came into worldwide use.
Joly carried out much experimental research into the physical and chemical properties of minerals, publishing many papers on the subject. He was a pioneer in the microscopical study of rock-forming minerals in relation to their suitability as road metal.
During his lifetime Joly did much to improve the facilities for teaching science and for carrying out scientific research at Trinity College. He was elected fellow of the Royal Society in 1892 and was awarded its Royal Medal in 1910. He received the Boyle Medal from the Royal Dublin Society in 1911 and the Murchison Medal from the Geological Society of London in 1923.
A complete list of Joly’s numerous publications, mainly contributions to learned societies and journals, is in Obituary Notices of Fellows of the Royal Society of London, 1 , no. 3 (1932-1935), 259. His two books are Radioactivity and Geology (London, 1909); and The Surface History of the Earth (Oxford, 1925).
No biography has been published. The most complete account of Joly’s life and scientific work is contained in the obituary notice cited above, which also reproduces his portrait. The following obituary notices are also worth consulting Nature, 133 (1934), 90; and Quarterly Journal of the Geological Society of London, 90 (1934), “Proceedings,” Iv-Ivii.
V. A. Eyles