20 THE PRINCIPLES OF AGRICULTURE. 



is produced as if the iron or wood were burned, or oxi- 

 dized more rapidly. The process is so slow that the 

 heat is not noticeable. 



The decomposition and decay of all substances, in gen- 

 eral, is largely a process of oxidation. Articles of food 

 are preserved from decay by separation from the oxygen 

 of the air. The decay of soft vegetable substance, as well 

 as the crumbling of the hardest rock, shows the power 

 of oxygen in transforming the products of nature. 



3. Respiration. Respiration or breathing in animals 

 is similar to combustion and oxidation. The oxygen of 

 the air, taken into the lungs, enters the blood, where it 

 unites with carbon. The carbonic acid gas thus pro- 

 duced escapes with the breath into the atmosphere. 



The heat which results from this union serves to keep 

 up the temperature of the body of the animal. The 

 amount of heat per day produced in the system of a man 

 by breathing is about equal to that obtained by burning 

 a pound of coal. 



Plants breathe to a slight extent, taking in oxygen 

 through the pores of their leaves, and giving off carbonic 

 ,cid gas. 



Hydrogen is the lightest substance known. It weighs 

 only one sixteenth as much as oxygen. It combines 

 most readily with oxygen and chlorine. When burned 

 or combined with oxygen, it forms water. It is an es- 

 sential part of plants and animals. It is produced by the 

 decay of animal and vegetable matter, or from molecules 

 of water, by separating it from the atoms of oxygen, 



Nitrogen, in a free state, forms four fifths of the air. 

 It is an odorless and harmless gas. Its purpose in the 

 air seems to be to dilute the oxygen with which it is 

 mixed and diminish its force. 



