PURIFICATION OF THE ATMOSPHERE. 141 



The moisture of the air is necessary for the 

 existence of both animal and plant life, although, 

 perhaps, it is more necessary for the existence of 

 plant life. 



Every action of an animal results in the decay 

 and subsequent death of some part of its body. 

 Although this death does not take place immedi- 

 ately, yet the us*e of any part or member of the 

 body results in its waste and subsequent death. 

 In order to replace these dead parts some form 

 of nourishment is necessary. This nourishment 

 comes from the food of the animal, which, by the 

 process of digestion, goes to make up the blood. 

 The blood carries to the parts of the body which 

 require nourishment the materials needed for sub- 

 sequent growth, and, at the same time, takes away 

 or carries off the dead or decaying parts. 



The blood is forced through the different parts 

 of the body by the action of the heart, which acts 

 like a force-pump. The blood goes to these parts 

 of the body as bright red arterial blood. It leaves 

 them so clogged with dead and decaying parts, 

 that it becomes changed into a dark, bluish-black, 

 venous blood. 



The oxygen of the air is, in general, necessary 

 to the existence of animal life in order to burn 



