122 who made their first ascent from the garden of the Conser- vatoire des Arts et Metiers, on August '24, 1804. In this ascent an altitude of 4000 metres was attained, but unex- pected difliculties were encountered, and the results were not decisive. Not satisfied with the expedition, Gay—Lussac got a larger balloon provided with every requisite, and made an ascent by himself on September 16 of the same year. On this occasion the balloon rose to a height of 7016 metres, an altitude greater than any which had been formerly reached, and surpassed only by a few later ascents. At this great elevation of nearly 23,000 feet, and with the thermo- meter at 95° 0. below freezing, Gay—Lussac remained for a considerable time making observations on temperature, on the moisture of the air, on magnetism, and other points. He observed particularly that he had considerable difficulty in breathing, that his pulse was quickened, and that by the absence of moisture in the air his mouth and throat became so parched that it was painful to swallow even a piece of bread. The experiments on magnetism for which the ascent was primarily made were imperfect, but they led him to the conclusion that the magnetic effect at all attainable elevations above the earth's surface remains constant.‘ Having col- lected samples of air at different elevations he, on his return to Paris, proceeded to analyse them; and in conjunction with Alexander von Humboldt, whom he had associated with himself in this investigation, he published several papers on eudiometric analysis and related topics. The memoir, which was read to the Institute on October 1, 1804, con- tained the germ of what was afterwards Gay-Lussac’s most important generalization. The authors observed 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. Prior to this the numerous experiments on the volume composition of water had always brought out various complicated ratios, though approaching the simple one more or less closely. It was not, however, till 1808, that Gay—Lussac announced the law of combination by volume in its general form. Shortly after these investiga- tions were completed, Gay-Lussac got leave of absence to accompany Von Humboldt on a scientific journey to Swit- zerland, Italy, and Germany. Provided with physical and meteorological instruments, they left Paris March 12, 1805, and travelled by Lyons, Chambery, and Mont Cenis to Genoa, and thence to Rome, where they arrived on July 5. After a short stay at Rome in the residence of VVilliam von Humboldt, during which Gay-Lussac made a few chemical analyses, they departed for Naples in company with Leopold von Buch, afterwards so eminent as a geologist. During this visit Gay—Lussac had the opportunity of study- ing on the spot volcanic eruptions and earthquakes. Vesuvius, which was in violent action, he ascended six times. After this the pa.rty Went back to Rome, and then started for Florence on September 17, 1805. A few da.ys having been spent there, they went on to Bologna and thence to Milan, which they reached on October 1, and there they had the pleasure of meeting Volta. The party crossed the St Gotthard on October 14-15, in the midst of a storm which prevented their seeing anything, and after some delay reached Giittingen, where they were received with much attention by Blnmenbach, the famous naturalist. On November 16 they arrived at Berlin, where the winter and the following springwere spent. In this way Gay—Lussac became acquainted with the best society in Berlin, and was especially intimate with Klaproth and Ermun. In spring he hurriedly returned to Paris. The death of an Acade- mician had left a vacant place, and he was hopeful that he might be elected to fill it. Arago remarks that it is curious G-Y—LUSSAC that Gay-I.ussac should have found it necessary to be on the spot to ensure success. What he had already done for science might ha.ve been considered sufficient, apart from personal considerations, but there were prejudices which might have acted unfavourably, if he had not been present to meet them. These were, however, success- fully overcome, and he entered the Academy in 1806. In the following year was inaugurated the Société d’Arcueil, a small group of scientific men who used to assmble at Berthollet’s house. Gay—Lussac was an original member of this society, which is of interest chiefly on account of its having been the means of publishing some papefs which have since proved of great historical interest. The results of his magnetic observations made along with Humboldt were published in vol. i. of its Jllémoircs (1807); and vol. ii. (1809) contains the important memoir on gaseous com- bination, in which he pointed out that, when gases combine with one another by volume or by measure, they do so in the very simplest proportions, 1 to 1, 1 to ‘.2, 2 to 3, and so on, and that the volume of the product in the gaseous state bears a very simple ratio to that of the constituents. This law, which, along with Humboldt, he had shown to be true of water, be extended to several other gases, and he even deduced from the vapour density of compounds that of certain elements, more particularly, carbon, mercury, and iodine, which had not been ascertained by direct experiment. It would take too much space to give in detail the criti- cism which the enunciation of the principle evoked, more particularly from Dalton, who would not accept Gay- Lussac’s position, and afiirmed his belief that “ gases do 11ot unite in equal or exact measures in any one instance 3 when they appear to do so, it is owing to the inaccuracy of our experiments.” There was at that time the difliculty that the specific gravity of gases and vapours had been imper- fectly determined, and the necessary consequence of Gay- Lussac’s law, that the specific gravity and combining weight of elements should be expressed by the same number, could not be experimentally confirmed. Moreover, Dalton rested combination on atomic and not on combining weights, and the numbers he employed were in almost every instance very different from those which more accurate analysis has since determined. But the imperfect character of the then available data, and the amount of seemingly adverse experimental evidence, only throw a stronger light on the genius of Gay—Lussac in divining a law which, as science has progressed, ha.s been duly confirmed, and which not only forms the most important control of the combin- ing weight of chemical substances, b11t, when interpreted by the kinetic theory of gases, shows that the physical mole- cules (that is to say, the portions of the substance which are not broken up into smaller -parts during the motion which we call heat) exist in equal numbers in equal volumes of different gases at the same temperature and pressure. This la.w, which has as high a claim as the other to bear the name of Gay—Lussac, is also sometimes deprived of that honour, and called the law of Avogadro, who, long afterwards, by his more extended researches, caused the importance of the law to be recognized by chemists. The next events in Gay-Lussac’s scientific career are con- nected with what may be called his rivalry with Davy, who in matter of age (b. December 17, 1778) was almost exactly his contemporary. In 1808 when Davy, having isolated potassium and sodium, was awarded N apoleon’s prize for the most important discovery in voltaic electricity, the emperor is said to have asked how it was that these discoveries were made abroad and French prizes were carried away. Having I The numerous observations made in both ascents are recorded in the Jnurnal dc I‘h3/sique for 1804, vol. 59. ’ New System of Chemical Phz'losnph3/, Manchester, 1810, part ii. ,
p. 559.