Stokes, George Gabriel (1819–1903), 1st baronet, mathematician, physicist, and scientific administrator, was born 13 August 1819 in Skreen, Co. Sligo, youngest child among five sons and three daughters of Gabriel Stokes (1761–1834), rector of Skreen, and his wife Elizabeth, daughter of John Haughton, rector of Kilrea, Co. Londonderry. A brother and sister died in infancy. His three surviving elder brothers all became clergymen; one was archdeacon of Armagh. George Gabriel Stokes was a great-grandson of Gabriel Stokes (qv) (d. 1768), and was thus related to many distinguished clergymen, doctors and scholars, including William Stokes (qv) (d. 1878), who was his second cousin. His early education was at home; he was taught by his father and by the parish clerk. From 1831 he attended Dr R. H. Wall's school in Hume St., Dublin, proceeding in 1835 to Bristol College in England. Teachers who greatly influenced him there included the Irish-born headmaster Joseph Henry Jerrard (d. 1853) and Francis Newman (brother of John Henry Newman (qv)), both able mathematicians.
Academic career and scientific work Stokes entered Pembroke College, Cambridge in 1837, graduating as senior wrangler and first Smith's prizeman (1841). After graduation he was elected a fellow of Pembroke, a position he held until 1857 when, under college statutes, his marriage rendered him ineligible for fellowship. When a new statute permitted fellows to marry, he was reelected in 1869 and held the position until his death. In 1849 he was appointed Lucasian professor of mathematics at Cambridge and supplemented his rather limited income from the post by contributing scientific articles to encyclopaedias and by holding a lectureship in physics at the Government School of Mines, London (1854–60). Unlike his predecessor in the Lucasian chair, Stokes lectured regularly. For several decades, Britain's ablest students of mathematics and physics profited from Stokes's rigorous lectures and experiment demonstrations. He was convinced that physical experiments and observations were of primary importance in science, and throughout his career retained an interest in the practical applications of physics, acting as scientific consultant to the optical-instrument maker Howard Grubb (qv). He was also a consultant on lighthouse illuminants for Trinity House and was advisor to the Trigonometrical Survey of India.
His early work was in hydrodynamics, and he gained recognition in 1845 for his paper on the internal friction, or viscosity, of an incompressible fluid, ‘On the theories of the internal friction of fluids in motion, and of the equilibrium and motion of elastic solids’, deducing the equations of motion for elastic solids. Arguing from the law of continuity, he held that the equations of motion he obtained for an elastic solid were the same for viscous fluids; he was later to extend this insight in his work on the luminiferous ether. His report on recent developments in hydrodynamics for the British Association for the Advancement of Science in 1846 confirmed his reputation as a promising young talent. In one of his most important papers on hydrodynamics, ‘On the effect of the internal friction of fluids on the motion of pendulums’, presented in 1850, he applied his theory of the internal friction of fluids to the behaviour of pendulums in liquids and also in air, indicated how the friction of the air would affect measurements of the pendulum's motion, and derived an expression for the drag force on a sphere or spheres moving relative to a fluid of known viscosity.
This expression now forms part of the set of Navier–Stokes equations, which bear the name of Claude-Louis Navier, a French scientist, as well as that of Stokes; Navier had been engaged in earlier studies of the physics of flow and viscosity, but Stokes's formulations improved on his work. The Navier–Stokes equations are a set of differential equations that are of fundamental importance in many applications of hydrodynamics. They are used in meteorology, for instance, to explain how clouds, composed of tiny water droplets, are able to form in the atmosphere, because the drops are so small that the friction of the air operates against gravity to keep them in suspension. Stokes dealt with the formation of clouds in his original paper, but many other economically important applications have been discovered since. The flow of liquids in pipes, clouds of dust, and pollutants, airflow over aircraft wings, and even blood flow, are all phenomena that can be analysed and predicted using Navier–Stokes equations.
An 1849 paper by Stokes on the variation of gravity was also based on a further development of his work with pendulums. In it he discussed the changes in the value of the force of gravity over the earth's surface, and pointed out that these could be measured without needing to make any assumptions about the conditions pertaining in the earth's interior. This suggested methodology was the major impetus to the modern science of geodesy. Stokes also worked on the physics of water waves, and on summer holidays in Ireland in the early 1880s he waded into the sea at Portstewart Strand to take measurements. These enabled him to validate a hypothesis about the greatest possible angle of a wave crest.
Stokes devoted a great deal of attention to the wave theory of light, and to investigation of the properties of the ether. Physicists of the day accepted the concept of the ether (or aether) as a basis for understanding fundamental aspects of the laws of nature; the ether was a medium of unascertained composition, within which light waves could propagate and through which heavenly bodies were able to move. Stokes's knowledge of the properties of fluids led him to regard the ether as an incompressible elastic medium, perhaps jelly-like, partaking of the properties of both liquid and solid states. This description accounted for much of what was then known about the behaviour of light, and contemporaries were impressed by Stokes's theory. The conceptualisation enabled him to make important additions to the mathematical formulations of the theory of the diffraction and polarisation of light, which have survived the almost complete rejection by the scientific community of the concept of the ether.
In 1851 he became interested in the blue shimmer elicited from near the surface of an otherwise colourless and transparent solution of sulphate of quinine when it was illuminated (a phenomenon that had been reported by Sir William Herschel in 1845). He carried out experiments which showed that this phenomenon contradicted the hitherto unchallenged Newtonian principles of the prismatic analysis of light, as the blue colour appeared even when it was not a constituent of the exciting incident light. He realised that the blue shimmer was caused by the quinine solution absorbing invisible ultraviolet rays, which were then emitted at longer wavelengths, visible as blue light. He named this emission of light ‘fluorescence’. He later discovered that quartz, rather than glass, prisms could be used to study the ultraviolet segment of the spectrum (glass absorbs the UV wavelengths); and suggested that fluorescence would be useful in chemical analyses. For this fundamentally important research, published (1852) in the Philosophical Transactions of the Royal Society as ‘On the change of the refrangibility of light’ he was awarded the Rumford medal by the Royal Society in the same year.
In discussions around 1854 with his close friend William Thomson (qv), Stokes speculated on the possibility that the Fraunhofer lines in the spectrum of sunlight could be used to elucidate the chemical composition of the sun. Since he did not publish on this topic, he was unwilling to claim that he had anticipated the discovery of the principles of spectrum analysis in 1859 in the work of Gustav Robert Kirchhoff and Robert Wilhelm Bunsen. However, Stokes is associated with, and his name perpetuated in, an important device for measuring sunshine hours; the Campbell–Stokes recorder was invented by John Francis Campbell, but greatly improved by Stokes in 1879; it is a simple device which focuses the sun's rays to burn a trace on a card, and it was for at least a hundred years the most widely used sunshine recorder in the world. His final mathematical study of light – an important paper on the dynamical theory of double refraction – was presented to the British Association in 1862.
Though only a few of his papers are purely mathematical, they are of great importance. His insights into the asymptotic forms of functions were highly original and very relevant to applications such as the explanation of diffraction effects in the rainbow. Their importance is recognised in the use of the term ‘Stokes phenomena’ for such manifestations. However, the ‘Stokes theorem’ or ‘Stokes formula’ used in vector calculus, and of great importance in mechanics, was first formulated by William Thomson in a letter to Stokes; it came to be associated with Stokes because he set it in the Cambridge tripos examinations.
Stokes is remembered, rather unusually for a mathematical physicist, for an important discovery in physiology. This came about as a result of his work in spectroscopy. A German biochemist, Ernst Hoppe-Seyler, had in 1862 described the absorption spectrum of the red pigment in blood, which in 1864 he named ‘haemoglobin’; when he repeated Hoppe-Seyler's experiment, Stokes was intrigued by the resulting spectrum. It occurred to him that it would be interesting to try to produce the change of colour from arterial to venous blood, by adding a reducing agent to a dilute solution of blood. When he added alkaline ferrous tartrate to blood, the blood darkened in colour, but when the resulting solution was shaken in air, it appeared again to have the scarlet colour of oxygenated blood. Crucially, the absorption spectra at different stages of the experiment clearly indicated the role of haemoglobin in carrying oxygen in the blood. Stokes's 1864 publication was thus a breakthrough in physiology and ultimately in biochemistry and medicine.
Science administration and public life In 1851 Stokes was elected a fellow of the Royal Society, and served as secretary (1854–85) and president (1885–90). Thirty years as secretary, dealing with all aspects of the Society's work at a time when it was becoming an increasingly professional and complex institution, with a developing public and international role, left little time for Stokes's own research and publishing. Among many other tasks, he undertook the responsibility for the Society's Philosophical Transactions, and personally reviewed and corrected over one hundred papers between 1852 and 1900, as well as engaging in a far-flung and voluminous correspondence with authors and other referees. When he retired after serving on the council of the Royal Society from 1853 to 1892, he was awarded the Copley medal (1893), honouring his service to the Society as well as his scientific achievements. In 1887 he was elected unopposed as Conservative MP for Cambridge university – the first scientific man to represent the university in parliament since Sir Isaac Newton – and sat in the house of commons until 1891. In 1859 he was secretary to the Cambridge University commission, and in 1888–9 he sat on the royal commission on the university of London. For many years he was involved with the work of the Solar Physics Laboratory in London.
From childhood on, Stokes was a profoundly religious man, though his beliefs were not founded on an unexamined childhood faith; he was very much influenced by the thought of the day and by his own scientific training, and he became a leading advocate of natural theology. Stokes and fellow thinkers hoped that it would be possible to develop a theology derived from the operation of reason and supported by science, rather than deriving from a system of belief relying on revelation or appeal to the miraculous. He served (1886–1903) as president of the Victoria Institute in London, founded to examine the relationship between Christianity and contemporary thought. He was the first Burnett lecturer at Aberdeen (1883–5); the lectureship was founded with the intention of providing evidences of divine goodness. Stokes's Burnett lectures on the nature of light were published in 1887 (second ed. 1892); he did not otherwise publish a full-scale consideration of optical topics. He was Gifford lecturer at Edinburgh (1891–3); the prestigious Gifford lectureships were also founded to promote the knowledge of God. Stokes's theological essays were collected and published as Natural theology in two volumes (1891, 1893). In his Conditional immortality: a help to sceptics (1897), Stokes tried to deal with what he saw as the horrifying prospect of eternal punishment; he believed that an eternity in torment was not part of the Bible's message, and that immortality would only be granted to those who merited eternity in God's presence.
Public reputation On the occasion of the jubilee of his professorship in Cambridge in 1899, fifty years after taking up the post, Stokes was celebrated by the contemporary scientific world as one of the outstanding thinkers of the age; the university marked the occasion with two days of academic festivities, attended by distinguished guests from all over the world, and he received the Arago medal from the Institute of France, as well as a gold medal from his own university. In October 1902 he was elected master of Pembroke. He was an honorary member of scientific societies in Edinburgh, Uppsala, Göttingen, France, Vienna, Washington, Glasgow, Rome, Berlin, Moscow, Belgium, and Turin, and received honorary doctorates from the universities of Oxford, Cambridge, Edinburgh, Glasgow, Aberdeen, Dublin, Victoria, and Christiania. He was awarded the Helmholtz medal by the Prussian Academy of Science. He was a knight of the Prussian order Pour la Mérite (an honour accorded also to his kinsman William Stokes), and was president of the British Association for the Advancement of Science at its Exeter meeting of 1869.
In the early twenty-first century, more than a hundred years after his death, he was commemorated in the naming of a research institute in the university of Limerick, in an important annual summer school held in his honour in Skreen, Co. Sligo, and in the name of a large-scale science investment programme funded by the Irish government (2007). His name is also perpetuated in a number of scientific terms familiar to scientists in a variety of disciplines worldwide. The ‘Stokes line’ is a spectrum line observed in fluorescence; its wavelength is longer than that of the exciting radiation, and thus it is in agreement with ‘Stokes's law’, formulated in 1852, which holds that the wavelength of emitted fluorescent light is always greater than the wavelength of the exciting light. This effect is measured in Stokes and anti-Stokes Raman scattering in spectroscopy; the associated ‘Stokes shift’ is the name given to the conversion in fluorescence of shorter wavelengths of light into longer visible wavelengths. ‘Stokes parameters’ are used in the description of the state of polarisation of an electromagnetic wave, such as a beam of light. ‘Stokes's solution’ (or ‘Stokes's reagent’) is ferrous tartrate, a reducing agent used in the biochemical study of blood; a ‘stokes’ is the unit of kinematic velocity, and there are craters on Mars and on the moon bearing his name.
Private life Stokes met Mary Susanna Robinson (d. 1899), daughter of Thomas Romney Robinson (qv), at a meeting of the British Association; like several of her relatives, including Arthur A. Rambaut (qv), Mary Robinson was interested in science. However, the apparently coldhearted emphasis on his commitment to mathematical studies in Stokes's letters to her during their courtship, and presumably also when they met, was almost too much for her, and at one stage she thought of calling off their wedding. His legendary shyness and intellectual intensity and his lack of emotional experience were probably to blame. However, plans went ahead and they were married in Armagh on 4 July 1857, when Stokes was 38. They were to have two sons and three daughters in what turned out to be a very happy marriage, though there were great sadnesses when two of their daughters died in infancy, and the younger son, a doctor, died before his father, in 1893, of an accidental overdose of morphine. George Gabriel Stokes was created a baronet by Queen Victoria in July 1889; his eldest son, Arthur Romney Stokes, succeeded as 2nd baronet when George Gabriel Stokes died in his daughter's home in Cambridge, 1 February 1903. He was accorded the traditional funeral dignities of a master of Pembroke, and was buried at Mill Road cemetery in the town.
His collected scientific papers were published between 1880 and 1905 in five volumes, the first three edited by himself, and two posthumous volumes edited by Joseph Larmor (qv). His unpublished papers are in the library of Cambridge University. The voluminous correspondence between Stokes and his friend William Thomson, Lord Kelvin, edited (2 vols, 1990) by David B. Wilson, is a very valuable source of information on the history of science. Kelvin was very fond of Stokes, and like other recipients, gratefully acknowledged the intellectual stimulus and mathematical and scientific assistance provided in untold numbers of letters which Stokes somehow found time to write to scientific friends. He knew almost all of the great nineteenth-century scientists of the UK, and corresponded and collaborated fruitfully with many of them. The importance of Stokes's influence on the science of his day as administrator, arbiter, consultant, experimentalist, facilitator, teacher, theologian, and theorist can scarcely be overstated.