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266 he delivered in the Academy on photometry and polarization, and which were collated by Langier and published with his works. A final experiment, which was made under his direction a short time before his death, was to determine the relative velocity of light through the air and through water. Upon it was thought to hang one of the crucial arguments as between the corpuscular and undulatory theories. The result—determining that light moves more swiftly through the air—was in harmony with the undulatory theory.

Arago's contributions to scientific literature are to be found scattered as special papers recording his experiments, or brief treatises, in the "Memoirs of the Academy of Sciences," the "Mémoires de la Société d'Arcueil," the "Annales de Physique et de Chimie," the "Annuaire du Bureau des Longitudes," where his "Scientific Notices" appeared, and in the reports of his lectures of the observatory, which are embodied in the "Popular Astronomy." They were never arranged by him, but were collected after his death, and published in 1856-'57, in fourteen volumes, of which the "Popular Astronomy" forms the principal part. His chief work in science was in making special investigations, resulting often in brilliant discoveries, which served as the foundation and support of accepted theories. Besides this, he possessed a rare gift, which his friends and biographers claim to be an equal and a specific title of glory, of making scientific truths and conceptions comprehensible to the multitude, and this without sacrificing the dignity of science. He was a member of all the learned societies of Europe, and the particular friend of many foreign scientific men.

The best part of Arago's time was given, according to M. Jamin, to the duties of his professorship in the Polytechnic School. "In this often exacting position, he did not cease to be the object of affection for his good heart, and of admiration for the vigor of his teaching, the ease of his elocution, and the lucidity of his demonstrations. A former pupil himself, he loved his young comrades. It might be affirmed that he was an example to them by his thorough uprightness, judicial-mindedness, disinterestedness, and patriotism, qualities which he communicated, and which became as it were permanent characteristics of that admirable school. He defended it on every occasion, extolled it and looked at everything in its light. Whoever had come out from it was sure of kindness from him. On occasions of difficulty, when a general excitement threatened the discipline and future of the school, the pupils would go to the observatory for advice and protection, and were sure of getting them. He excited like admiration and found like esteem at the observatory." His weakness was, that he was subject to transitory fits of passion, which passed away and left no lasting trace.