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A K A G 303 along with Dulong, a series of experiments on the elastic force of steam at different temperatures. This difficult and most hazardous task, where they were exposed to the bursting of a boiler at any moment, they accomplished with the greatest ability and coolness. Of these experi ments, as well as of others, on the density of air, on the velocity of sound, on the pressure of gases being propor tional to their density, &c., there will be found elaborate memoirs in the Comptes Rendus of the Academy, and in the Annales de Chimie et de Physique. It is, however, on his contributions to Magnetism and Optics that Arago s fame as an experimenter and discoverer rests. By employing the method of oscillating needles, he established the universal influence of magnetism on all substances, though it was left to Faraday to make the grander generalisations of diamagnetism. Arago found that a magnetic needle, made to oscillate over non-ferni- ginous surfaces, such as water, glass, copper, &c., falls more rapidly in the extent of its oscillations according as it is more or less approached to the surface. Over ice, for example, an oscillation of 53 on each side of the magnetic meridian, fell to 43 after 60 oscillations at a distance of 52^ milli metres from the ice ; while, at a distance of 7 mm., it fell by the same amount after 26 oscillations. Over glass there was a fall from 90 to 41 in 221 oscillations, at a distance of 4*01 mm., and in 122 oscillations at a distance of 91 mm. For this discovery Arago was awarded the Copley medal by the Royal Society of London in 1825 ; and he has the honour of being the first foreigner to whom this distinction was accorded. This discovery was followed by another. It occurred to him to try, con versely, the effect of oscillating or rotating a plate of non- ferruginous metal near a magnetic needle at rest. He caused a plate of pure copper to revolve by clock-work, which was also of copper, under a magnetic needle within a glass-receiver, and with a sheet of paper between the needle and the revolving plate to prevent air currents. Gradually the needle was drawn from the magnetic meridian, till finally, on a very rapid rotation of the plate, it fairly swung round and continued in constant rotation. An account of these experiments he read before the Academy of Sciences, on the 7th March 1825, and they were exhibited in London in the following April by Gay- Lussac, exciting universal interest among the philosophers of this and other countries. This phenomenon of rotatory magnetism, as it is termed, is indissolubly linked with the name of Arago ; but it was reserved for the genius of Faraday to offer the true explanation of it, after it had long occupied the attention of Herschel, Babbage, Barlow, Nobili, and other distinguished men, as well as of its dis coverer. Arago is also fairly entitled to be regarded as having proved the long suspected connection between the aurora borealis and the variations of the magnetic elements. He established, too, the fact of an hourly variation of these, traceable, no doubt, to the influence of the sun as an enor mous magnet at a distance. It is worth mentioning that Arago was the first to employ the galvanic current for the permanent magnetisation of steel, though in the field of electro-magnetism his discoveries and researches as a whole are not to be compared with those of the profounder Ampere. It is, however, his investigations in Optics that form the most marked feature and the crowning glory of Arago s scientific career. We owe to him not only important optical discoveries of his own, but the credit of stimu lating the genius of Fresnel, with whose history, as well as with that of Malus and of Young, this part of his life is closely interwoven. Shortly after the beginning of the present century the labours of the three philosophers last named were shaping the modern doctrine of the undu Jatory theory of light. Malus had made in 1808 hia discovery of the polarisation of light by reflection, identi fying it with the effects of double refraction; Young had devised his beautiful theory of interferences; and Fresnel had been simultaneously conducting elegant experiments in the very same field as the English philosopher. It was the communication to the French Academy of Sciences of a paper on this subject by Fresnel that led to his intimacy with Arago. Fresnel s arguments in favour of the undu- latory doctrine found little favour with Laplace, Poisson, and Biot, the champions of the emission theory ; but they were ardently espoused by Humboldt and by Arago, who had been appointed by the Academy to report on the paper. This was the foundation of an intimate friendship between Arago and Fresnel, and of a determination to carry on together further researches in this subject. These investigations were rewarded by a remarkable discovery, in 1811, the very year that Malus terminated his career so auspiciously begun. After Huyghens s time, when Iceland spar and rock crystal were alone supposed to have the pro perty of double refraction, mineralogists had suspected the same property to exist in a number of other crystals, though it was often almost impossible to determine its existence. But Arago discovered that a plate of any doubly refracting crystal, such as selenite or mica, when interposed between two similar polarising prisms or piles of glass plates, displays splendid tints, varying in colour with the thickness of the plate and with its inclination to the transmitted beam, and varying in intensity as the plate of mica or selenite is turned round in its own plane. By this simple means double refraction may be detected in the smallest plate of a crystal. The explanation of this remarkable phenomenon was a difficult problem, and for some time perplexed both English and Continental investi gators. Dr Young, in 1814, ascribed the cause generally to interference; but Arago saw that this explanation was incomplete. The clue to it was, however, found in the law, established by the joint labours of Arago and Fresnel, that two polarised rays cannot interfere unless polarised in the same or parallel planes. This involved the idea of transverse vibrations of the ether across the direction of propagation, an idea which at first seemed a stumbling- block to the theory of undulations, and which Fresnel and Arago did not venture for some time to embrace. Once expressed, however, this theory of transverse vibra tions reduced the whole phenomena of polarisation to a simple mechanical effect, a resolution of rectilinear motion into two rectangular components; and so far from upsetting the wave theory, it has by its very simplicity and com prehensiveness but helped to establish it on a sure founda tion. Arago applied his discovery to the construction of a polariscope, for estimating the feeblest amount of polarisa tion; and he used this instrument for some very interest ing experiments on the polarisation of the light of the sky (which is sunlight polarised by reflection from the atmos phere), and on that of different incandescent and reflecting surfaces. He also found that the moon and the tails of comets send light to the eye which is slightly polarised, thus betraying its borrowed origin. But that of the sun, being absolutely neutral, is only comparable (according to Arago) to the light arising from incandescent vapours, thus distinguishing the sun from a solid or liquid globe. "We cannot do more than allude to Arago s other optical papers and experiments. He was, probably, the only Frenchman of his time who was well acquainted with Young s discoveries. The explanation by the doctrine .of interference of the colours of Newton s rings received an

important confirmation from an experiment of Arago s,