28 OF THE COMPOSITION OF THE ATMOSPHERE. 



ined by many chemists with great care, and the results 

 of their researches have shown, that its principal 

 constituents are always present in the same propor- 

 tion. These are the two gases, oxygen and nitro- 

 gen, the general properties of which have been 

 already described. One hundred parts, by weight, 

 of atmospheric air contain 23*1 parts of oxygen, 

 and 76*9 parts of nitrogen ; or 100 volumes of air 

 contain nearly 21 volumes of oxygen gas. From 

 the extensive range of affinity v^^hich this gas pos- 

 sesses, it is obvious, that were it alone to constitute 

 our atmosphere, and left unchecked to exert its 

 powerful effects, all nature would be one scene of 

 universal destruction. It is on this account that 

 nitrogen is present in the air in so large proportion. 

 It is peculiarly adapted for this purpose, as it does 

 not possess any disposition to unite with oxygen, 

 and exerts no action upon the processes proceeding 

 on the earth. These two gases are intimately mixed, 

 by virtue of a property which all gases possess in 

 common, of diffusing themselves equally through 

 every part of another gas, with which they are 

 placed in contact. 



Although oxygen and nitrogen form the principal 

 constituents of the atmosphere, yet they are not the 

 only substances found in it. Watery vapor and 

 carbonic acid gas materially modify its properties. 

 The former of these falls upon the earth as rain, and 

 brings with it any soluble matter which it meets in 

 its passage through the air. 



Carbonic acid gas is discharged in immense quan- 

 tities from the active volcanoes of America, and 

 from many of the mineral springs which abound in 

 various parts of Europe; it is also generated during 

 the combustion and decay of organic matter. It is 

 not, therefore, surprising that it should have been 

 detected in every part of the atmosphere in which 

 its presence has been looked for. Saussure found it 

 even in the air on the summit of Mont Blanc, which 

 is covered with perpetual snow, and where it could 



