to the highest scientific positions, to buckle down to the tedious drudgery involved in exact measurements. The result was the classical series of papers published, mainly by the Royal Society, in 1881–1883, and reprinted in volume ii of Rayleigh's collected works.
Finding his life at Cambridge rather too exacting, Rayleigh resigned his professorship at the end of 1884 and retired to Terling to pursue his researches in his private laboratory. A few months later he accepted the secretaryship of the Royal Society, vacant through the resignation of Sir George G. Stokes. The duties of this office were not so onerous as they have since become, but that he found them sufficiently so is suggested by a sentence which he wrote in the obituary notice of his predecessor. Commenting on the marked decrease of scientific output resulting from Stokes's acceptance of the secretaryship, he remarked, ‘The reflexion suggests itself that scientific men should be left to scientific work and should not be tempted to assume heavy administrative duties, at any rate until such time as they have delivered their more important messages to the world’. No such falling-off occurred in Rayleigh's work, his output of work being consistently high throughout his eleven years of secretaryship. The tenure of this office gave him the opportunity to discover and rescue from oblivion the valuable memoir in which J. J. Waterston in 1846 had anticipated some of the important features of the kinetic theory of gases. During this period he began his experimental determinations of the densities of gases, of which the culminating success was the discovery of argon in 1894. In 1892 he had announced that two samples of nitrogen which he had prepared in chemically different ways had shown densities differing by as much as one part in 1,000, and had concluded that such a difference could be attributed only to a variation in the character of the gas. Gradually he was led to the view that what had so far been regarded as pure atmospheric nitrogen was a mixture of chemical nitrogen and some heavier atmospheric constituent. Sir William Ramsay [q.v.] joined the research in its later stages, the final outcome being the classical paper Argon, a new Constituent of the Atmosphere which was communicated to the Royal Society by Rayleigh and Ramsay jointly on 31 January 1895. About this time Rayleigh became deeply interested in physical optics. His researches in this subject will perhaps constitute his most enduring title to fame, the papers in which they are recorded, over 150 in number, are probably those in which his intellectual powers are displayed to best advantage, and he himself said, when late in life he thanked the Royal Society for the award of their Rumford medal, that they were those which had given the greatest pleasure to their author.
In his later years honours and responsibilities fell thick upon Rayleigh. He was one of the original recipients of the order of merit in 1902; he received a Nobel prize, jointly with Sir William Ramsay, in 1904, and he was made a privy councillor in 1905. In the same year he was elected president of the Royal Society, and in 1908 succeeded the Duke of Devonshire as chancellor of the university of Cambridge. Some of the most arduous, although probably also most pleasant, duties of his later life centred in his association with the National Physical Laboratory. He acted as chairman of the Treasury committee which reported in favour of its formation in 1898, and presided with unfailing regularity over the meetings of its executive committee until the onset of his last illness. In 1909 he was appointed president of the special government advisory committee on aeronautics, an appointment which led to his taking great interest in, and devoting much time to, problems of aviation.
The passing of Lord Kelvin left Rayleigh undisputed leader of British science on the physical side. It is no easy task to explain to the layman the grounds on which this supremacy was unanimously accorded him. His massive, precise, and perfectly balanced mind was utterly removed from that of the erratic genius who typifies the great scientist in the popular imagination. Of striking discoveries or inventions practically none stand to his credit with the single exception of the discovery of argon. His special aptitude was for arranging and levelling up existing knowledge rather than for taking giant strides into unexplored country. The outstanding qualities of his writings were thoroughness and clearness: he made everything seem obvious. These talents which would have been dangerous in a man of less sound judgement, or one less scrupulously careful not to lead others astray, were safe in Rayleigh's keeping. The inscription on his memorial in Westminster Abbey, ‘An unerring leader in the advancement of natural knowledge’, does not overstate the case. His researches covered almost the whole field of exact
516
