The Atmosphere and its Elements. 11 



humidity, amount and distribution of its raius, character and force 

 of the winds, intensity of light, and other general or local causes. 



T7ie Atmo&phere and, its Elements. 



47. The atmosphere from which the trees, through their foliage, 

 derive a part of their aliment, and to which they return certain 

 gaseous elements in the process of growth, consists of about one 

 part of oxygen to four parts of nitrogen by volume. It always 

 contains, besides these, a nearly constant proportion of carbonic 

 acid gas, and a variable amount of aqueous vapor. 



48. Oxygen. This gas is necessary to the existence of all animal 

 and vegetable life, and to combustion, respiration, fermentation, and 

 many other processes of nature. It has a wide range of affinities, 

 and forms a part of all organic and most mineral compounds. It 

 is absorbed or disengaged in various operations of tree-growth, and, 

 under certain conditions, it hastens decay. 



49. Nitrogen. This gas has a comparatively small range of affini- 

 ties, and in the air appears to dilute and moderate the action of 

 oxygen. Of itself, it does not sustain life. It forms a part of 

 some vegetables and of all animals, and, combined with hydrogen in 

 the proportion of 1 to 3 (NHg), it forms ammonia, which acts an 

 important part in the vegetation of trees, as well as of the culti- 

 vated grains. 



50. Carbonic Acid Gas. This is a compound made up of 2 atoms 

 of oxygen to 1 o^ carbon (CO2, or 72.73 of oxygen to 27.27 of 

 carbon by weight). It has been estimated that this gas forms one 

 thousandth part of the atmosphere, but recent experiments show that 

 the proportion is less, ranging from- two and a half to four ten- 

 thousandths, or even less. It is very uniform, yet is slightly varied 

 by local influences, being increased by combustion, respiration, and 

 other causes. 



51. It is from carbonic acid gas, either in the air or in the water 

 taken up by the roots, -that trees obtain the carbon that makes their 

 principal bulk. This gas was probably more abundant in former 

 times, as in the carboniferous period, since mineral coals are largely 

 made up of carbon. It also forms a part of all limestones and 

 marls, and of many minerals and ores. There is no evidence that 

 the proportion in the air has changed within the period of human 

 history. 



