THE ATMOSPHEllE. METEOROLOGY. 305 



at the bottom or on the shoals of which we live. The 

 atmosphere presents six classes of phsenomena which manifest 

 the most intimate connection with each other. These arise 

 from its chemical composition ; the variations in its transpa- 

 rency, polarisation, and colour ; its density or pressure ; its 

 temperature ; its humidity ; and its electric tension. The 

 atmosphere contains, in the form of oxygen, the first element 

 of the physical life of animals ; and we may here notice another 

 benefit scarcely less important, of which it is the instrument; 

 the air is the " conveyer of sound ;" the channel of the com- 

 munication of ideas, the indispensable condition of all social 

 life. The earth, deprived of its atmosphere, as we believe 

 our moon to be, presents to our imagination the idea of a 

 soundless desert. 



Since the beginning of the present century, the relative 

 proportion of the constituents of the accessible strata of the 

 atmosphere has been the object of researches, in which 8 a,y- 

 Lussac and myself took an active part : but the chemical 

 analysis of the atmosphere has only very recently attained a 

 high degree of perfection, through the meritorious labours of 

 Dumas and Boussingault by new and more accurate methods. 

 According to this analysis a volume of dry air contains 20.8 

 parts of oxygen and 79.2 of nitrogen, besides from two to 

 five ten-thousandth parts of carbonic acid gas, a still smaller 

 quantity of carburetted hydrogen, ( 375 ) and, according to the 

 important experiments of Saussure and of Liebig, traces of 

 ammoniacal vapours, ( 376 ) which furnish to plants the azote 

 they contain. Some observations of Lewy have rendered it 

 probable that the quantity of oxygen varies slightly, but per- 

 ceptibly, in different seasons of the year, and over the sea or 

 in the interior of continents. It is quite a conceivable case 



