MANURES. 80 



Tin's is the case with very many substances in or- 

 ganic nature, both solid and liquid : thej are liable 

 to assume a gaseous form, and become mixed with 

 the atmosphere. They are not destroyed, but are 

 changed in form. 



As an instance of this action, suppose an animal 

 to die and to decay on the surface of the earth. 

 After a time, the flesh will entii-ely disappear, but is 

 not lost. It no longer exists as the flesh of an ani- 

 mal, but its carbon, hydrogen, oxygen, and nitrogen, 

 still exist in the air. They have been liberated from 

 the attractions which held them together, and have 

 passed away ; but (as we already know from what 

 has been said in a former section) they are ready to 

 be again taken up by plants, and pressed into the 

 service of life. 



The evaporation of liquids may take place without 

 the aid of anything but heat ; but, in the case of 

 solids, it is often assisted by decay and combustion, 

 which break up the bonds that hold the constituents 

 of bodies together, and thus enable them to return 

 to the atmosphere, from which they M-ere originally 

 derived. 



It must be recollected that everything which has 

 an odor (or can be smelled) is evaporating. The 

 odor is caused by parts of the body floating in the 

 air, and acting on the nerves of the nose. This is 

 an invariable rule ; and when we perceive an odor, 

 we may be sure that parts of the material from which 

 it emanates are escaping. If we perceive the odor 

 of an apple, it is because parts of the volatile oils of 



