THE CHEMISTRY OF THE ATMOSPHERE. 93 



116. Quantity of Carbonic Anhydride in the Atmosphere. 

 As the air encircling the earth is from 45 to 50 miles in 

 height, the quantity of carbonic acid, although proportion- 

 ably small, is really in the whole very great. It has been 

 estimated that there is seven tons' weight of this gas over 

 every acre of the earth's surface. 



117. Chief Use of Nitrogen in the Air. Oxygen, you have 

 seen, is a very active substance, supporting life and com- 

 bustion every where. It is so active that it needs to be 

 diluted in the air with four times its bulk of nitrogen gas. 

 If it were not thus diluted the world would be one vast 

 scene of continued conflagrations. Combustible substances 

 would take fire five times as easily as now, and when once 

 on fire it would be difficult to put them out. Iron would burn 

 as readily as wood now does. So also the operations of 

 life would be attended with five times the amount of heat 

 that they now are, and the tendency in every animal would 

 be to fever and inflammation. As oxygen is so stimulat- 

 ing, it has sometimes been used successfully in reviving 

 persons who have been drowned or suffocated. In such 

 cases, the more of this gas you can introduce into the lungs 

 for a little while the better. It is easily introduced by a 

 pipjB from a bladder filled with it. The remedy, however, 

 has seldom been used, because it is not at hand unless the 

 accident occur near the laboratory of a chemist. 



118. Analysis of the Air. We will describe two modes of analyz- 

 ing the air. The first is this : A certain volume of air is allowed to pass 

 slowly through a tube containing potassium hydrate. This has a very 

 strong attraction for one of the ingredients of the air, carbonic anhydride, 

 and has none for either the oxygen or nitrogen. It takes, therefore, the 

 carbonic anhydride, and the weight of the tube compared with its weight 

 before the experiment shows how much there is of this ingredient in the 

 volume of air employed. And now the air, thus deprived of its carbonic 

 anhydride, is made to pass through a tube filled with red-hot copper-filings. 

 The copper in this state attracts to itself the oxygen, but having no tendency 



