VI.] THE ATMOSPHERE. 85 



Nitrogen .... 0-9713 



Oxygen i'io56 



Carbonic-acid gas . . i'5203 



The term specific gravity is used to denote the weights of 

 equal bulks of different kinds or species of matter, compared 

 with some known standard. Air is the standard used in the 

 comparison just made, and it is seen from the figures that 

 if a given bulk of atmospheric air weighs 100 pounds, then 

 the same bulk of nitrogen weighs 97 pounds ; the same 

 volume of oxygen no pounds, and of carbonic acid 152 

 pounds. Hence it might be assumed that the atmosphere 

 would consist of three strata or layers (like the mixture of 

 quicksilver, wa,ter, and oil)^ with the nitrogen as the top 

 layer, and the carbonic acid at the bottom. As a matter of 

 fact, however, this is not the case. All gases tend to inter- 

 mingle with each other, so that when different gases are 

 mixed they soon produce a uniform mixture, in spite of 

 differences in their relative weights ; in fact, the particles of 

 the heavy gas rise and the particles of the light gas fall, 

 until they are completely diffused through each other. In 

 consequence of this property, the composition of the 

 atmosphere is kept practically uniform, although local 

 variations, within narrow limits, may be detected. 



In addition to the oxygen, nitrogen, and carbonic acid, the 

 atmosphere always contains other constituents, but only in 

 subordinate and variable proportions. The gas called am- 

 monia, well known as the pungent gas which escapes from 

 " spirit of hartshorn," is constantly present in the air, being 

 indeed evolved from decomposing animal and vegetable 

 matter. Yet the proportion of ammonia is always excessively 

 small ; for example, twenty grains have been obtained from a 

 railUon cubic feet (or 536 million grains) of air. This ammonia 



