6 Practical Farming 



each square inch, and each acre of land bears a pressure 

 of 46,200 tons. The decreased pressure of the air causes 

 a decided change of feehng as we ascend 

 The Work {lOYd the sea level to the summit of a high 

 Atmosphere mountain, though we hardly realize the pres- 

 sure at the sea level, and when it is removed 

 do not directly think of this change as causing our new and 

 strange sensation. Pure air is composed of oxygen and 

 nitrogen, two gases mixed together but not chemically 

 combined. The nitrogen exists as a diluent to enable us 

 to breathe the oxygen, for it comprises a much larger per- 

 centage than the oxygen — 79.051 per cent, of nitrogen to 

 20.949 per cent, of oxygen. Exactly how deep is the 

 ocean of air, at the bottom of which we live, cannot be 

 known; but it is usually estimated at about forty-five 

 miles. While the air is composed primarily of these two 

 elements, oxygen and nitrogen, it also carries with it other 

 matter useful to the farmer. A small percentage of car- 

 bon dioxide, commonly known as carbonic acid, which in 

 large quantities is deadly to animal life, is essential to 

 vegetation. Plants get their carbon from this combina- 

 tion, as will be explained later. The air also contains 

 some nitrogen in combination, in the form of ammonia, 

 as well as the free nitrogen gas and a minute portion of 

 oxidized nitrogen or nitric acid. Still another constituent 

 is that extremely active part of oxygen known as ozone, 

 which is esteemed of great value to animal life. From 

 this air and the matters it contains plants get more than 

 97 per cent, of their material for building the tissues, and 

 less than 3 per cent, from the soil. But for the air and its 

 constant pressure there would be no heat retained on the 



