NATURE AND FOR^[.\TION OF SOILS. H 



the carbon dioxide, and the calcium carbonate 

 is again precipitated. 



Other substances of the air which bear impor- 

 tant relations to agriculture are nitrogen and its 

 compounds, ammonia, nitrous and nitric acids, 

 and ozone. 



4. Alternations of Heat and Cold. — In dry, 

 hot countries, rocks become excessively heated 

 during the day and rapidly cooled at night. As 

 the outer layer cools it contracts upon the hot 

 and expanded interior, which tends to produce 

 snapping and crumbling of' the brittle min- 

 erals. 



Thus we see the work of the atmosphere is 

 constant ; it is universal ; it is not, however, 

 uniform.* Both the rapidity and the extent of 

 disintegration are ciependent upon the differ- 

 ences of climate in various latitudes and alti- 

 tudes, the differences in the rock substances 

 themselves, the differences of seasons and of 

 the amount of precipitation, and upon the pres- 

 ence or lack of protection from vegetation or 

 soil. 



* The composition of the air varies greatly in different locali- 

 ties, or under different conditions in the same localities; but, 

 under ordinary conditions, its constituents in a given volume are, 

 approximately: oxygen, 20.6 per cent.; nitrogen, 77.18 percent.; 

 water vapor, 1.4 per cent.; carbon dioxide, .04 per cent.; argon, 

 .78 per cent. The water vapor and carbon dioxide are the most 

 variable. And there is present a variable quantity of ammonia, 

 nitrous and nitric acids — a very small fraction of i per cent, alto- 

 gether. 



