on this eclipse, and those carried out by similar methods by Father Angelo Secchi (1818–1878) at Desierta de las Palmas, proved conclusively that the ‘red flames’ or ‘prominences,’ observed during eclipses, belong to the sun and not to the moon. ‘To De la Rue,’ says Lockyer (Contributions to Solar Physics, pp. 111, 112), ‘belongs the full credit of having solved this important question.’ In 1862 De la Rue communicated the results of the eclipse expedition to the Royal Society as the Bakerian lecture for the year. He now, in conjunction with Balfour Stewart [q. v.], the superintendent of, and Mr. Benjamin Loewy, observer to, the Kew Observatory, made a large number of observations of the sun and of sun-spots, the results being first published in three memoirs entitled ‘Researches in Solar Physics,’ printed privately in 1865–8, and later in the ‘Philosophical Transactions.’ In 1861 De la Rue obtained a stereoscopic view of a sun-spot, and this and further observations by himself and his colleagues strongly supported the suggestion of Alexander Wilson (1714–1786) [q. v.] of Glasgow, based on observations made in 1769–74, that sun-spots are depressions in the sun's atmosphere; the facular appendages were shown to occupy a higher position, and in most cases to lag behind the spots in their movement of rotation, the smaller velocity of rotation being accounted for on the supposition that they had been flung up from a considerable depth. From the study of over 660 sun-spots the three astronomers attempted, but with no decided success, to connect the frequency of sun-spots with planetary movements (Young, The Sun, p. 149). They confirmed R. Wolf's expression for the total area of sun-spots in terms of the number of groups of spots and of isolated spots, and the total number of spots visible. The Kew heliograph, after being used on the 1860 eclipse expedition and from May 1863 to 1872 at Kew, was transferred to the Greenwich Observatory, but is now again at Kew.
In 1873 De la Rue took an active part in the preparation for observing the transit of Venus in 1874, but, finding that night work had become too arduous for him, gave his telescope to the university of Oxford, removed from Cranford to Portland Place, and fitted up a private physical laboratory for himself and his friend Dr. Hugo Müller, with whom, although mainly occupied with astronomical work, he had carried out a number of chemical researches. The most important of these were on Rangoon tar (1859), glyceric acid (1859), and terephthalic acid (1861). The research on Rangoon tar led to a patent which proved very profitable financially. He continued in this laboratory with Dr. Müller an elaborate series of researches on the electric discharge through gases, which were begun in 1868 and continued to 1883. It cannot be said that the results led to any simple explanation of the complex phenomena observed, but they furnished a valuable series of data and have special interest in connection with the discharge of the aurora borealis. The experiments were carried out by means of a battery of constant cells, devised and gradually improved by the two experimenters, of which silver and zinc formed the electrodes, and fused silver chloride and a solution of zinc, sodium, or ammonium chloride formed the electrolytes. A similar cell had been described in 1853 in ‘Electric Telegraph in India’ (p. 14), by Dr. (afterwards Sir) William Brooke O'Shaughnessy [q. v.], whose priority De la Rue acknowledged (Phil. Trans. clxix. 55). The battery was gradually increased until in 1883 it contained fifteen thousand cells.
De la Rue, who had retired from business in 1869, returned to it on the death of a younger brother in 1870, but finally retired in 1880. He died on 19 April 1889. He had married, in 1840, Miss Georgiana Bowles, and left four sons and a daughter.
De la Rue received the gold medal of the Astronomical Society in 1862, a royal medal from the Royal Society in 1864, and the ‘prix Lalande’ for 1865 (Comptes Rendus de l'Académie des Sciences, lxii. 476) for his discoveries. He also received the honorary degrees of M.A. and D.C.L. at Oxford, was elected corresponding member of the French Academy of Sciences, was made commander of the legion of honour, and received many other honours from abroad. His application of photography to celestial objects, in which he displayed ‘unfailing fertility of invention,’ has been of the utmost service to physical astronomy. He gave money as well as his own time freely for the advancement of pure science, and showed exceptional kindness to younger scientific men. He was an original member of the Chemical Society, over which he presided from 1867 to 1869, and again from 1879 to 1880; he served first as secretary, and then from 1864 to 1866 as president of the Royal Astronomical Society, was for many years president of the London Institution, and from 1878 to 1882 secretary to the Royal Institution. He was also an early and active member of the Royal Microscopical Society.
The ‘Royal Society's Catalogue’ (continued to 1884) contains a list of fifty-five papers published independently by De la Rue (of which the majority appeared in the ‘Monthly