28 ELEMENTS OE AGRICULTURE 



CHAPTER V.— The Atmosphere 

 (Continued) 



28. Carbon Dioxide. — Though this gas makes up a 

 relatively small proportion of the atmosphere, it is very 

 important, as it supplies a large part of the food of all 

 growing plants. The air about cities usually contains 

 more of this gas than the air of the country. The many 

 fires which are constantly kept going in cities give off 

 great quantities of this gas. Ordinary air contains about 

 .035 per cent by volume, or about 0.06 per cent by 

 weight of carbon dioxide. This seems a very small 

 amount, but when multiplied by the vast oceans of air 

 it becomes a large sum. It is estimated that the air 

 covering every acre of ground contains about 28 tons 

 of carbon dioxide, which give iO^ tons of pure carbon. 

 The supply of carbon dioxide in the air remains always 

 about- the same. The amount taken up by plants 

 during their season of growth is replaced by the gas 

 from fires, the breath of man and beast, and other 

 sources. » 



29. Ammonia. — This compound, often called harts- 

 horn, is made up of nitrogen and hydrogen. It has a 

 very powerful odor, but the air contains so little of it 

 that it is not perceptible. This gas is very easily dis- 

 solved in water, and rain washes it from the air and 

 carries it to the soil, where it may become plant food. 



