METEOROLOGY. 311 



the River Belem), man has ever boldly ventured onward to- 

 ward the discovery of unknown regions. 



The second external and general covering of our planet, the 

 aerial ocean, in the lower strata, and on the shoals of which 

 we live, presents six classes of natural phenomena, which man- 

 ifest the most intimate connection with one another. They 

 are dependent on the chemical composition of the atmosphere, 

 the variations in its transparency, polarization, and color, its 

 density or pressure, its temperature and humidity, and its elec- 

 tricity. The air contains in oxygen the first element of phys- 

 ical animal life, and, besides this benefit, it possesses another, 

 which may be said to be of a nearly equally high character, 

 namely, that of conveying sound ; a faculty by which it like- 

 wise becomes the conveyer of speech and the means of com- 

 municating thought, and, consequently, of maintaining social 

 intercourse. If the Earth were deprived of an atmosphere, as 

 we suppose our moon to be, it would present itself to our im- 

 agination as a soundless desert. 



The relative quantities of the substances composing the 

 strata of air accessible to us have, since the beginning of the 

 nineteenth century, become the object of investigations, in 

 which Gay-Lussac and myself have taken an active part ; it 

 is, however, only very recently that the admirable labors of 

 Dumas and Boussingault have, by new and more accurate 

 methods, brought the chemical analysis of the atmosphere to 

 a high degree of perfection. According to this analysis, a 

 volume of dry air contains 20 "8 of oxygen and 79 2 of nitro- 

 gen, besides from two to five thousandth parts of carbonic 

 acid gas, a still smaller quantity of carbureted hydrogen gas,* 

 and, according to the important experiments of Saussure and 

 Liebig, traces of ammoniacal vapors,t from which plants de- 

 rive their nitrogenous contents. Some observations of Lewy 

 render it probable that the quantity of oxygen varies percep' 



* Boussingault, Recherche s sur la Composition de V Atmosphere, in tlie 

 Annales de Chimie et de Physiqiie, t. Ivii., 1834, p. 171-173; and Ixxi. 

 1839, p. IIG. According to Boussingault and Lewy, the proportion of 

 carbonic acid in the atmosphere at Audilly, at a distance, therefore, from 

 the exhalations of a city, varied only between 0-00028 and 00031 in 

 volume. 



t Liebig, in his important work, entitled Die Orgnnische Chemie in 

 ihrer Anwendung auf Agricultur und Physiologic, 1840, s. G2-72. On 

 the influence of atmospheric electricity in the production of nitrate of 

 ammonia, which, coming into contact with carbonate of lime, is changed 

 into carbonate of ammonia, see Boussingault's Economic Rurale cortr- 

 »id6r6e dans ses Rapports avec la Chimie et la M6l^orologie, 1844, t. ii., 

 p 2 17, 2G7, and t. i., p. 84. 



