68 



Composition of the Atmosphere 



Finally, as an indication of the present-day conception of the com- 

 position of the atmosphere, the following, written by F. W. Clarke 1 in 

 1908, may be cited: 



In a roughly approximate way it is often said that air consists of four-fifths nitrogen 

 and one-fifth oxygen, and this is nearly true. The proportions of the two gases are almost 

 constant, but not absolutely so; for the innumerable analyses of air reveal variations 

 larger than can be ascribed to experimental errors. A few of the better determinations 

 are given in the subjoined table [table 49], stated in percentages by volume of oxygen. 

 They refer, of course, to air dried and freed from all extraneous substances. 



Some of these variations are doubtless due to different methods of determination, 

 but others can not be so interpreted. Hempel, comparing his analyses of air from Trom- 

 soe, Norway, and Para, Brazil, infers that the atmosphere is slightly richer in oxygen 

 near the poles than at the equator, an inference that would seem to need additional 

 data before it can be regarded as established. The most significant variation of all, 

 however, has been pointed out by E. W. Morley. 2 As oxygen is heavier than nitrogen 

 it has been supposed that the upper regions of the atmosphere should show a small 

 deficiency in oxygen, as compared with air from lower levels; although analyses of samples 

 collected on mountain tops and from balloons have not borne out this suspicion. It is 

 also supposed that severe depressions of temperature, the so-called "cold waves," are 

 connected with descents of air from very great elevations. Morley's analyses, conducted 

 daily from January, 1880, to April, 1881, at Hudson, Ohio, sustain this belief. Every 

 cold wave was attended by a deficiency of oxygen, the determinations, by volume, 

 ranging from 20.867 to 21.006 per cent, a difference far greater than could be attributed 

 to errors of measurement. Air taken at the surface of the earth seems to show a very 

 small concentration of the denser gas, oxygen. 



1 Clarke, Data of geochemistry, U. S. Geological Survey Bulletin 330, 190S, p. 38. 



2 Morley, American Journal of Science, 1879, 3d ser., 18, p. 168; 1881, 22, p. 417. 



