THE CULTURE OF FARM CROPS. 



CHAPTER VII. 



OF THE ATMOSPHERE. 



The earth is surrounded by a gaseous body known as the 

 atmosphere or the air, which is supposed to extend to a 

 height of forty-five miles above its surface. The actual 

 height however is indefinite and immeasurable, because be- 

 ing a gas, the air is capable of infinite expansion, according to 

 the pressure upon it, and while we know that the pressure 

 at the earth's surface of the vast mass of the air above it is 

 equal to 15 pounds to the square inch, yet as the height 

 above the surface increases, the pressure decreases ; and as 

 the pressure decreases, the air expands, therefore the exten- 

 sion of the atmosphere upwards cannot be marked by any 

 distinct boundary, but gradually fades to a limit which can- 

 not be precisely defined. The limit however of 45 miles is 

 sufficiently precise for all practical purposes. The physical 

 properties of the atmosphere however, are of the greatest 

 importance, as a great many results interesting to the far- 

 mer as affecting the growth of his crops depend upon them. 



Air, as has been already stated, consists of 79 parts by 

 bulk of nitrogen, and 21 parts by bulk of oxygen; or by 

 weight 77 parts of nitrogen and 23 of oxygen. These two 

 gases are mixed or diffused together according to a law by 

 which gases mixed together become evenly or uniformly 

 diffused or mingled with each other, without reference to 

 their weight. One may be much heavier than the others, 

 yet it will diffuse itself perfectly through every part of them. 

 If it were not for this law the air would not be fit to sup- 

 port life. In one place there would be masses of pure oxy- 

 gen which would be equally destructive of life as the masses 

 of pure nitrogen which has no vital qualities at all. 



This law of diffusion has another interesting application 

 as regards the behavior of the air, and moisture in the soil, 



