COMPOSITION OF AIR: ATOMIC THEOEY 65 



oxygen to maintain the burning of a flame, and therefore 

 the flame is extinguished. 



Carbon dioxide is about one and one-half times as heavy 

 as air, and therefore can be collected in an open bottle. With 

 care it may be poured from one bottle to another. 



It is familiar to us as the gas that bubbles up in our soda 

 water and in other effervescent drinks, and as the important 

 substance in chemical fire extinguishers. Sometimes it occurs 

 in wells, cisterns, sewers, and in mines, where it is called choke 

 damp. One may drown in it as truly as in water, since it fills 

 the lungs but does not supply any free oxygen for respiration. 

 There is a very small amount always present in the air,' and 

 this will be discussed later. 



66. The gases of the air. We have already found that the 

 air consists of several gases. These are merely mixed; they 

 do not form a compound. The most striking one of those that 

 we have been studying is oxygen. It constitutes about one 

 fifth of the volume of the air. Of carbon dioxide there are 

 only about 3 parts in 10,000. 



There is yet another gas present in large quantities in the 

 air. It is called nitrogen. We shall have more to say about 

 it in connection with the food of plants and animals. 



Several other gases have lately been discovered. Also, dust 

 particles are always present, and some of these are of great 

 importance. They will be discussed in Chapter VIII. 



Below is a table showing the gases commonly found in the 

 air, and the average amount of each, given in percentage. 

 The exact percentage cannot be given, because the amounts 

 vary slightly from time to time. 



COMPOSITION OF AIR 

 (Approximately) 



Nitrogen 78 per cent 



Oxygen 21 per cent 



Argon 1 per cent 



Carbon dioxide . . . . T $ 7 of 1 per cent 



