78 , ATMOSPHERE, [LesSWl XVI. 



':. I" ' - '.. I I ' P I 



weight of about 74 nitrogen, and 26 oxygen ; and 

 is soluble in about 30 times its bulk of water : 

 10O cubic inches weigh 3 1 grains. On the surface 

 of the earth it is compressed by the weight of the 

 superincumbent atmosphere: its density, therefore, 

 diminishes according to its height above the earth. 

 It is dilatable by heat : at 60 of temperature, its 

 bulk is increased about its l-82d part. The con- 

 st^ituent principles of atmospheric air are rendered 

 evident by the following experiment: quicksilver 

 being enclosed in a proper vessel of atmospheric 

 air, on heat being applied the air will be diminish- 

 ed, and the quicksilver will lose its splendour, and 

 gradually change to a reddish powder ; acquiring, 

 at the same time, an augmentation of weight. 

 When neither the air nor the quicksilver suffers 

 any farther change, the separation of the princi- 

 ples has taken place : the one, the gas remaining 

 in the receivers, is now unfit for supporting flame,, 

 or maintaining respiration, and is nitrogen gas : 

 the other is absorbed by the quicksilver, while re- 

 ducing to the state of an oxide, and may be ex- 

 tricated from it on the application of heat ; when 

 the powder, to which the quicksilver is reduced, 

 will be restored to its metallic state> but will have 

 lost the weight it had gained during its oxidation } 

 this deficiency being exactly equal to the weight 

 of the evolved gas, which is oxygen gas. These 

 separated gases, thus differing in their properties 

 from each other, and, from atmospheric air being 

 again mix,ed, form atmospheric air of the or^i- 

 nary degree of purity. 



The- 



