Popular Science Monthly/Volume 30/December 1886/Sketch of M. Arago
ON the 26th of February last the one-hundredth anniversary of the birth of François Arago was celebrated at Perpignan, France, his native city. A grand celebration of the day had also been planned at Paris, to be held under the direction of the scientific men and publicists of the capital, but the municipal subvention, on which its promoters depended for its expenses, was not granted, and it failed. Nearly seven years previous to this time, on the 21st of September, 1879, a statue of the philosopher and patriot, the work of M. Mercier, was inaugurated at Perpignan; and one year previous to it an eloquent eulogy on M. Arago was delivered in the Academy of Sciences by M. Jules Jamin.
Dominique François Arago was born at Estagel, near Perpignan, February 26, 1786. His father, who was a sub-treasurer at Perpignan, put him to school quite early in the college of that city. At seven-teen years of age he was admitted to the Polytechnic School after a brilliant examination, in which he exhibited a peculiar spirit of independence, rising to the point of chiding his examiner for unwillingness to question him on account of his delicate appearance. Some months afterward, when the proclamation of the empire was contemplated, circulars inviting the act were distributed and introduced into the school to be signed by the pupils. Arago refused to sign the paper, and was the leader among the pupils who took that position. General Lacuée, reporting the transaction to the first consul, demanded that the recusants be dismissed from the school. Bonaparte took the list, read it, and remarked: "We will only not send the first name up for promotion. We shall have to give these boys a little time to be converted. You others have turned too quickly."
Before the end of his course at the Polytechnic, Arago, whose abilities had impressed all of his teachers, was appointed a secretary in the Bureau des Longitudes, where he became associated with Biot, twelve years his senior, who was destined to be his co-worker or rival, according to circumstances, during his life. In 1806 he and Biot were appointed by the emperor to co-operate with the Spanish commissioners, Chaix and Rodriguez, in continuing the measurement of the arc of the meridian, for the establishment of the metric system. In this expedition he found arduous work, and underwent hard sufferings from the fortunes of war, the story of which we will let M. Jamin tell further along. He returned to France in the summer of 1809, and was received into the Academy of Sciences, in departure from its rules, at the age of twenty-three years. The emperor, who always manifested a remarkable esteem for him, considering how he had behaved when a school-boy, appointed him Professor of Analysis and Geodesy in the Polytechnic School, a position, or the equivalent of which, he held for twenty years. He also became director of the observatory and delivered lectures on astronomy, which were heard with equal interest by astronomers and by persons who knew nothing of mathematics, and were fully understood by the latter.
In 1830 he took the place of Fourier as perpetual secretary of the Academy, in which position it became his duty to pronounce eulogies upon deceased members, the felicity of the style and the scientific accuracy of which gained for him a world-wide reputation.
In 1830 M. Arago became a member of the Chamber of Deputies for the Pyrénées-Orientales. He took his seat on the Extreme Left, and became a conspicuous advocate of measures tending to the extension of public liberty and to electoral reform. He was also prominent in discussions relating to the marine canals, public instruction, and railroads. When the revolution that expelled the Orleans dynasty took place in 1848, M. Arago was made a member of the provisional government by popular acclamation, and was given charge of the bureaus of the marine and of war. He took part in all the events of that stirring epoch, sat among the moderate members, opposed the most radical republicans, while he always enjoyed their respect, was a member of the executive commission appointed by the Constituent Assembly, and marched to the barricades at the head of his troops during the bloody days of June. But so many struggles and shocks had broken his physical and moral energies, and he afterward sat in the Legislative Assembly without taking an active part. He declined to take the oath to the new government in 1852, as inconsistent with his past acts and professions, and was excused from it, and was allowed to keep his place in the observatory unsworn. He died in the next year, October 2, 1853.
The scientific, personal, and social aspects of Arago's life have been admirably portrayed by M. Jamin in his eulogy, and most of what follows on those points is drawn from that address. The first scientific labors coming under his notice were in association with Biot, the continuation of Borda's investigations of the indices of refraction of gases, and the determination of the relative densities of the air and mercury, from which they were able to calculate theoretically the constant of the barometric formula. The two friends next were interested in the scheme of continuing the measurement of the arc of the meridian from the shore-line at Barcelona, where Delambre and Mechain had left it, to Majorca, and thence to Formentera. In the execution of this enterprise it became necessary to set up a beacon-light in the Island of Iviça, and to observe it from two points in Spain, forty leagues off. Arago established himself upon a rock called Desierto de las Palmas, the summit of which hardly afforded room enough for his tent and instruments. On account of the distance, the frequency of fogs, and uncertainty as to the exact direction, it was six months before ho could get sight of the beacon. As soon as the measurements were made, Biot took the first results to France, leaving his colleague to finish the work at Majorca and Formentera. Then began for Arago a series of exciting adventures. War had been declared with Spain in 1808, and the Majorcans sought to arrest him as a spy. He had barely time to disguise himself, and, gathering up the papers containing his observations and his instruments, to take refuge on the vessel which had brought him to the island. Thence he escaped, with the aid of the ship-captain, to the citadel of Belver, where he passed several months, substantially a prisoner, in making his calculations. He was at last permitted to go to Algiers, where he embarked on a vessel of the regency under the name of a Hungarian merchant, with a false passport, along with a crowd of Mussulmans and renegades, to which were added two lions and a family of monkeys which the dey was sending to his ally the Emperor of France. The vessel was taken close upon Marseilles by a Spanish corsair, "which conducted its prize to Palamos and took Arago to a country where he was only too well known and from which he had nothing good to expect." He suffered great privations till the dey was informed of the state of affairs, and bullied Spain into setting the lions, monkeys, ships, and philosopher again on the way to Marseilles. They had reached that port when a mistral suddenly arose and sent the vessel to the coast of Sardinia, and thence to Bongie, in Algeria, where the new dey, not so friendly as his predecessor, was disposed to hold Arago as a prisoner. Finally, he was allowed to return to France, running a new danger from pursuit by an English cruiser, and at length to receive a welcome from his mother, "who thanked God for having preserved his life, after having had masses said for the repose of his soul; he returned with the triple consecration of having encountered danger, done his duty, and attained a scientific success beyond his anticipations."
In 1809 the phenomena of optics engaged especial attention, and Arago entered upon the study with the ardor of his nature and his age. He engaged in the investigation of the polarization of light, which no one had as yet been able to explain, but of which he reached a satisfactory and correct elucidation by the aid of the undulatory theory of light. At the time Arago approached this question, only a few of the facts hearing upon the undulatory theory had been determined, while the most of them were unknown, or hardly suspected. He had to begin by finding them out and classifying them, and so bringing himself gradually nearer to the primary ideas. "The work resembled the play of guessing at words, which all the company know but the person who has to find the word. The inquirer has to ply Nature with methodical questions, numerous and close, to extract her secret from her. No one was better suited to the performance of such a part than Arago; no one more obedient to experiment, no one more systematically rebellious to preconceived theories. He began by studying how natural matter becomes polarized, and found that it is when it is divided into two parts. If there is polarized light in one of these parts, an exactly equal quantity of it will be found in the other, both vibrating in perpendicular planes. This mode of division forms a physical law which is still known as Arago's law." From this law Arago drew two practical results. The first one is applicable to lakes and seas, the surface of which divides the light rays into two parts—the reflected part, which takes the color of the sky and vibrates horizontally; and a part which, having penetrated to the interior and having vertical vibrations, is returned to us with the color of the water. "Both parts are mingled, but a double-refracting crystal separates them, and we see in one of the images the reflected sky, and in the other the bottom of the lake and all that it contains." The second result is that the sun, reflecting only natural or unpolarized light, is a flame, an incandescent gas, and not an incandescent solid.
Arago next published his discovery of the phenomena of rotatory polarization, with the production of complementary colors, varying in properties according to the crystalline medium through which they are viewed. One of his experiments was applied to the edification of the public by the optician Soleil, who devised various fanciful designs on laminæ of gypsum, which, colorless in natural light, were transformed, under the working of the polariscope, into polychrome images having the most beautiful appearance. One of the favorite designs was the word "Arago" surrounded by a laurel-wreath.
"Rarely," says M. Jamin, "has an inventor ever reached the limits of his discovery. He looks for its consequences where they are not, he goes astray in the labyrinth where no thread guides him, he passes by the truth without perceiving it, and leaves to his successors to reap where he has sown. Like so many others before him, Arago left the great work he had labored at without completing it. He was endowed with unequaled clairvoyance, and divined discoveries before making them; but he had no patience for details: he opened mines without working them out, and began labors without pursuing them. His
�� � first curiosity once satisfied, he gave himself up to new curiosities. He resembled a traveler who glances over a virgin country, gives it a name, and hastens on to more distant horizons. All phenomena excited his imagination without holding him long. An experimenter by inspiration, a discoverer by instinct, he had too much passion, too little leisure, too fertile a spirit, but not enough of that obstinate perseverance that finishes what is begun.... Of theoretical ideas which include a whole science in a few general hypotheses, and leave an ineffaceable trace, he produced none, but sometimes repelled them, even when his own experiments led others to them." Biot, his former collaborator, took up Arago's experiments and worked them out in detail to the discovery of those more particular properties of polarization and the two rotatory powers which have been found of such useful application in the arts.
But neither Arago nor Biot was destined to work out the undulatory theory of light in all its significance and to the full explanation of the phenomena. That part fell to the young engineer Fresnel, who, rusticating in a village near Caen, in expiation of some political errors, passed his time in studying optics. He wrote to Arago, and received in return advice by which he profited so well that he shortly afterward published his memoir on diffraction. He and Arago then together made the experiments on interference, by which a theoretical explanation of polarization was obtained; but Arago, heartily with him at the beginning, was not able to follow him in all his conclusions, and left to Fresnel the honor of explaining the experiments which he had himself performed.
Oersted having discovered the power of the voltaic current to produce deviation of the magnet, and having thence deduced the theory of the relationship of magnetism and electricity, Arago took up his experiments. With a conductor of copper wire and a pile of iron filings, he learned that the current would also generate magnetism. He communicated his discovery to Ampere, and they made, with knitting-needles, those experiments in electro- magnetism which transformed a whole science, and cleared the way for the electric telegraphs, electric lights, electric clocks, and other instruments of to-day. One day an artisan of the engineers brought him a compass, which was nearly inert in its copper box, but lively enough in action when taken out of it. Experimenting with this apparatus to discover the cause of the compass's inaction, he discovered the magnetism of rotation—a discovery which Faraday complemented by showing how induction-currents are created in the copper.
The observation of a beautiful aurora borealis in 1817 gave Arago opportunity to verify the fact, which had already been observed and remarked upon, that the bands of light and the arch bore a relation to the magnetic meridian; to this he added the new observation that the magnetic needle was disturbed during the whole time of the prevalence of the phenomenon. On consulting the registers of previous observations, he discovered that similar perturbations had accompanied the aurora, even in places where it could not be seen on account of the weather, in the daytime, and in the polar regions. He recollected also that electricity is propagated in vague lights in vacuum-tubes and that these lights are deflected by the magnet; and he affirmed that auroras are electric sparks circulating in the higher parts of the atmosphere, oriented on the terrestrial magnet, and acting on the magnetic needle. This theory was attacked by Brewster, but Arago replied to his argument at length and convincingly.
The experiments which were entered upon for the purpose of measuring the force of the vapor of water were very important and very dangerous: important, because the safe working of steam-engines was dependent upon correct measurements of the force, and because all the properties of heat had to be passed in review; and dangerous, because they "imposed the task of confronting the unknown caprices of a formidable force. There were but two men to accept it and conduct it to success: Arago, who never shrank from a duty; and Dulong, already maimed by an explosion, whose previous studies had admirably prepared him for the new work." A rude manometer was extemporized, and a boiler, far less stanch than the steam-boilers of to-day, was set up, in which water was heated till the pressure was twenty-seven atmospheres. "They could not go further. At this extreme point, it leaked at all the joints, and the steam escaped through the fissures with a hissing that was of bad omen. But the observers, though aware of the danger, silent and resigned, finished without accident the measurements which they had begun." Telling M. Jamin the story, which was written out as above from his dictation, Arago said: " Only one being of our company preserved his serenity and slept quietly; it was Dulong's dog; they called him Omicron."
By the terms of the creation of the Bureau des Longitudes, the duties of the direction of the observatory and of delivering the lectures on astronomy were to be performed by the members in turn, a year at a time. Practically they fell continuously to Arago, and from 1813 to 1847 he delivered those lectures on popular astronomy which had a wonderful success, and of the life and vigor of which the tame rendering in the book of that name gives no idea. He did not write them out, but only prepared the outlines, and for the rest depended on the inspiration of the moment. They were attended by young men who went to learn, older men for the pleasure of hearing, and women, M. Jamin suggests, for the pleasure of seeing. "It was his habit, when he rose to speak, to select the least intelligent-looking face in the audience. He then never left it, but seemed to speak for it alone, and continued his demonstration, with various modes, till that face showed that its owner understood him ; a fortiori, all of the auditory must have understood him as well.
Arago published, in the "Annuaire" of the Bureau des Longitudes, popular papers on natural phenomena and on the applications of science to industry. One of these dissertations was on thunder and thunderbolts. Another, which appeared in 1829, was on the history of the steam-engine; others were on rain, the cold of night, the ruddy moon, and the influence of the moon on terrestrial phenomena. He also published a paper on eclipses of the sun; and a total eclipse occuring in July, 1842, which could be favorably seen at Perpignan, he went there to observe it. Notice had already been taken of the aureole which appears around the moon during an eclipse, and to what are now known as the protuberances, and he gave his special attention to them.
Having been made perpetual secretary of the Academy of Sciences in 1830, he was accustomed to come early to the meeting every Monday, where he received foreign savants read the correspondence, and, if it was his day, began the sitting with an analysis of the papers offered; and so clear and so much sought for were his analyses, that the memoirs sent up were frequently indorsed "For M. Arago's day." He also accompanied his analyses with a history of the questions discussed and a criticism of the proposed solution, the authority of which was never contested. In a short time this audience, though evidently illustrious, seemed to him too restricted. He desired to extend it. He had found a close Academy, working without witnesses, with doors closed or only half opened to a few privileged persons. He had them opened wide to all the world; and, in order that science might be spread more rapidly and further, he invited journalists to attend the meetings, and provided a place for them where they could take notes. He further, in 1835, induced the Academy itself to publish its proceedings under the supervision of the perpetual secretaries; and this was the origin of the famous "Comptes Rendus."
As a deputy and member of the political body, Arago proposed a scheme for damming one of the arms of the Seine and establishing a system of turbine pumps by which Paris should be fully supplied with water, which was defeated by a ridiculous jest. He induced the municipal council to bore the Artesian well of Grenelle, which was a great wonder in its day. He secured a public recompense for Vicat, who had invented an economical hydraulic cement. And when Daguerre came forward with his wonderful invention, which made it possible to take an exact portrait, by the aid of the sun, in fifteen minutes, Arago explained the method before the Academy and expounded its capabilities.
While a member of the Legislative Assembly, Arago was attacked with a malady which resulted in a gradual loss of sight, that became total in 1852. Being unable to make further researches, he endeavored to gather up and reduce to form the unfinished work of his past career. The visible results of this effort were seven conferences which 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.