JOSEPH LOUIS GAY-LUSSAC (1778-1850), French chemist and physicist, was born at S t Leonard, in the department of Haute Vienne, on the 6th of December 1778. He was the elder son of Antoine Gay, procureur du roi and judge at Pont-deNoblac, who assumed the name Lussac from a small property he had in the neighbourhood of St Leonard. Young Gay-Lussac received his early education at home under the direction of the abbe Bourdieux and other masters, and in 1794 was sent to Paris to prepare for the Ecole Polytechnique, into which he was admitted at the:end of 1797 after a brilliant examination. Three years later he was transferred to the Ecole des Ponts et Chaussees, and shortly afterwards was assigned to C. L. Berthollet, who wanted an able student to help in his researches. The new assistant scarcely came up to expectations in respect of confirming certain theoretical views of his master's by the experiments set him to that end, and appears to have stated the discrepancy without reserve; but Berthollet nevertheless quickly recognized the ability displayed, and showed his appreciation not only by desiring to be Gay-Lussac's "father in science," but also by making him in 1807 an original member of the Societe d'Arcueil. In 1802 he was appointed demonstrator to A. F. Fourcroy at the Ecole Polytechnique, where subsequently (1809) he became professor of chemistry, and from 1808 to 1832 he was professor of physics at the Sorbonne, a post which he only resigned for the chair of chemistry at the Jardin des Plantes. In 1831 he was elected to represent Haute Vienne in the chamber of deputies, and in 1839 he entered the chamber of peers. He died in Paris on the 9th of May 1850.
Gay-Lussac's earlier researches were mostly physical in character and referred mainly to the properties of gases, vapourtensions, hygrometry, capillarity, &c. In his first memoir (Ann. de Chimie, 1802) he showed that different gases are dilated in the same proportion when heated from o° to ioo° C. Apparently he did not know of Dalton's experiments on the same point, which indeed were far from accurate; but in a note he explained that "le cit. Charles avait remarque depuis 15 ans la meme propriete dans ces gaz; mais n'ayant jamais public ses resultats, c'est par le plus grand hasard que je les ai connus." In consequence of his candour in thus rescuing from oblivion the observation which his fellow-citizen did not think worth publishing, his name is sometimes dissociated from this law, which instead is known as that of Charles. In 1804 he had an opportunity of prosecuting his researches on air in somewhat unusual conditions, for the French Academy, desirous of securing some observations on the force of terrestrial magnetism at great elevations above the earth, through Berthollet and J. E. Chaptal obtained the use of the balloon which had been employed in Egypt, and entrusted the task to him and J. B. Biot. In their first ascent from the garden of the Conservatoire des Arts on the 24th of August 1804 an altitude of 4000 metres (about 13,000 ft.) was attained. But this elevation was not considered sufficient by Gay-Lussac, who therefore made a second ascent by himself on the 16th of September, when the balloon rose 7016 metres (about 23,000 ft.) above sea-level. At this height, with the thermometer marking 9 degrees below freezing, he remained for a considerable time, making observations not only on magnetism, but also on the temperature and humidity of the air, and collecting several samples of air at different heights. The magnetic observations, though imperfect, led him to the conclusion that the magnetic effect at all attainable elevations above the earth's surface remains constant; and on analysing the samples of air he could find no difference of composition at different heights. (For an account of both ascents see Journ. de phys. for 1804.) On the 1st of October in the same year, in conjunction with Alexander von Humboldt, he read a paper on eudiometric analysis (Ann. de Chim., 1805), which contained the germ of his most important generalization, the authors noting that when oxygen and hydrogen combine together by volume, it is in the proportion of one volume of the former to two volumes of the latter. But his law of combination by volumes was not enunciated in its general form until after his return from a scientific journey through Switzerland, Italy and Germany, on which with Humboldt he started from Paris in March 1805. This journey was interrupted in the spring of 1806 by the news of the death of M. J. Brisson, and Gay-Lussac hurried back to Paris in the hope, which was gratified, that he would be elected to the seat thus vacated in the Academy. In 1807 an account of the magnetic observations made during the tour with Humboldt was published in the first volume of the Memoires d'Arcueil, and the second volume, published in 1809, contained the important memoir on gaseous combination (read to the Societe Philomathique on the last day of 1808), in which he pointed out that gases combining with each other in volume do so in the simplest proportions-1 to 1, 1 to 2, 1 to 3 - and that the volume of the compound formed bears a simple ratio to that of the constituents.
About this time Gay-Lussac's work, although he by no means entirely abandoned physical questions, became of a more chemical character; and in three instances it brought him into direct rivalry with Sir Humphry Davy. In the first case Davy's preparation of potassium and sodium by the electric current spurred on Gay-Lussac and his collaborator L. J. Thenard, who had no battery at their disposal, to search for a chemical method of obtaining those metals, and by the action of red-hot iron on fused potash - a method of which Davy admitted the advantages - they succeeded in 1808 in preparing potassium, going on to make a full study of its properties and to use it, as Davy also did, for the reduction of boron from boracic acid in 1809. The second concerned the nature of "oxymuriatic acid" (chlorine). While admitting the possibility that it was an elementary body, after many experiments they finally declared it to be a compound (Mem. d'Arcueil, 1809). Davy, on the other hand, could see no reason to suppose it contained oxygen, as they surmised, and ultimately they had to accept his view of its elementary character. The third case roused most feeling of all. Davy, passing through Paris on his way to Italy at the end of 1813, obtained a few fragments of iodine, which had been discovered by Bernard Courtois (1777-1838) in 1811, and after a brief examination by the aid of his limited portable laboratory perceived its analogy to chlorine and inferred it to be an element. Gay-Lussac, it is said, was nettled at the idea of a foreigner making such a discovery in Paris, and vigorously took up the study of the new substance, the result being the elaborate "Memoire sur 1'iode," which appeared in the Ann. de chir p. in 1814. He too saw its resemblance to chlorine, and was obliged to agree with Davy's opinion as to its simple nature, though not without some hesitation, due doubtless to his previous declaration about chlorine. Davy on his side seems to have felt that the French chemist was competing with him, not altogether fairly, in trying to appropriate the honour of discovering the character of the substance and of its compound, hydriodic acid.
In 1810 he published a paper which contains some classic experiments on fermentation, a subject to which he returned in a second paper published in 1815. At the same time he was working with Thenard at the improvement of the methods of organic analysis, and by combustion with oxidizing agents, first potassium chlorate and subsequently copper oxide, he determined the composition of a number of organic substances. But his last great piece of pure research was on prussic acid. In a note published in 18 r.1 he described the physical properties of this acid, but he said nothing about its chemical composition till 1815, when he described cyanogen as a compound radicle, prussic acid as a compound of that radicle with hydrogen alone, and the prussiates (cyanides) as compounds of the radicle with, metals. The proof that prussic acid contains hydrogen but no oxygen was a most important support to the hydrogen-acid theory, and completed the downfall of Lavoisier's oxygen theory;, while the isolation of cyanogen was of equal importance for the subsequent era of compound radicles in organic chemistry.
After this research Gay-Lussac's attention began to be distracted from purely scientific investigation. He had now secured a leading if not the foremost place among the chemists of the French capital, and the demand for his services as adviser in technical problems and matters of practical interest made great inroads on his available time. He had been a member of the consultative committee on arts and manufactures since 1805; he was attached to the "administration des poudres et salpetres" in 1818, and in 1829 he received the lucrative post of assayer to the mint. In these new fields he displayed the powers so conspicuous in his scientific inquiries, and he was now to introduce and establish scientific accuracy where previously there had been merely practical approximations. His services to industry included his improvements in the processes for the manufacture of sulphuric acid (1818) and oxalic acid (1829); methods of estimating the amount of real alkali in potash and soda by the volume of standard acid required for neutralization, and for estimating the available chlorine in bleaching powder by a solution of arsenious acid; directions for the use of the centesimal alcoholometer published in 1824 and specially commended by the Institute; and the elaboration of a method of assaying silver by a standard solution of common salt, a volume on which was published in 1833. Among his research work of this period may be mentioned the improvements in organic analysis and the investigation of fulminic acid made with the help of Liebig, who, gained the privilege of admission to his private laboratory in 1823-1824.
Gay-Lussac was patient, persevering, accurate to punctiliousness, perhaps a little cold and reserved, and not unaware of his great ability. But he was also bold and energetic, not only in his work but also in support and defence of his friends. His. early childish adventures, as told by Arago, herald the fearless aeronaut and the undaunted investigator of volcanic eruptions (Vesuvius was in full eruption when he visited it during his tour in 1805); and the endurance he exhibited under the laboratory accidents that befell him shows the power of will with which he would face the prospect of becoming blind and useless for the prosecution of the science which was his very life, and of which he was one of the most distinguished ornaments. Only at the very end, when the disease from which he was suffering left him no hope, did he complain with some bitterness of the hardship of leaving this world where the many discoveries being made pointed to yet greater discoveries to come.
The most complete list of Gay-Lussac's papers is contained in the Royal Society's Catalogue of Scientific Papers, which enumerates 148, exclusive of others written jointly with Humboldt, Thenard, Welter and Liebig. Many of them were published in the Annales de chimie, which after it changed its title to Annales de chimie et physique he edited, with Arago, up to nearly the end of his life; but some are to be found in the Memoires d'Arcueil and the Comptes rendus, and in the Recherches physiques et chimiques, published with Thenard in 1811.
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