72 Hoiv a Tree Lives 



use up the oxygen, and breathe out the nitrogen to- 

 gether with a good deal of this poisonous carbonic acid 

 gas. In this same way all burning things use up oxygen 

 and make carbonic acid gas and moisture. This is the 

 reason why if you are for some time in a room with a 

 lot of people or gas jets the air soon becomes stuffy 

 and you will get a headache if you do not open a 

 window and let in some fresh air. 



Now as for so many years all the people, animals and 

 plants in the world, to say nothing of the fires, have 

 gone on making this poisonous gas and using up the 

 oxygen you might think that the air would have long 

 ago become unfit to breathe. This is prevented by the 

 leaves of trees and other green plants. All day long 

 they are busy making the air wholesome by taking from 

 it the carbonic acid gas which is to them a most useful 

 food. 



This carbonic acid gas is made up of two parts, 

 carbon and oxygen, bound together so tightly that it is 

 almost impossible to separate them. Green leaves have, 

 however, wonderful chemical powers. Helped by light 

 which is as necessary to them as heat is to a steam 

 engine, they split up this gas, take the carbon which is 

 what they need to build up the plant, and send the 

 oxygen that has been set free out into the air. You see 

 how enormously valuable they are to us, for they are in 

 this way not only taking the poisonous part from the air, 

 but are actually adding to it more of the wholesome 

 part that we want to breathe, the oxygen. I have said 

 that this useful work can only go on in the light. In 

 the dark it stops and the other process, that of using up 

 oxygen and breathing out carbonic acid gas, which has 

 been going on slowly all day, goes on alone. This is the 



