INTERCHANGE IN NATURE. 



343 



speak, and nourishment from water, ammonia, and carbonic acid, all 

 compound bodies, but inorganic. 



Water consists of oxygen and hydrogen, carbonic acid of carbon and 

 oxygen, and ammonia of hydrogen and nitrogen. Water and ammonia 

 are present in the air ; so are oxygen and nitrogen. Water falls in the 

 form of rain, dew, etc. So in the atmosphere around us we find nearly 

 every necessary for plant-life ; and in the ground, which supplies some 

 metallic oxides for their use, we find the remainder. From the air, then, the 

 plant derives its life. 



The vegetable kingdom in turn gives all animals their food. This 

 you will see at a glance is true. Certainly animals live on animals. Man 

 and wilder animals live on the beasts of the field in a measure, but those 



Fig. 325 Drawing the oxygen from air by combustion. 



beasts derive their nourishment from vegetables the vegetable kingdom. 

 So we live on the vegetable kingdom, and it separates the carbonic acid 

 from the air, and absorbs it. What we do not want it takes. What we 

 want it gives. Vegetables give out oxygen, and we consume it gladly. 

 We throw away carbonic acid, and the plants take it greedily ; and thus 

 the atmosphere is retained pure for our use. We can, if desirable, prove 

 that plants absorb carbonic acid and give out oxygen by placing leaves 

 of a plant in water, holding the acid in solution, and let the sun shine upon 

 them. Before long we shall find that the carbonic acid has disappeared, 

 and that oxygen has come into the water. 



Carbonic acid is sufficiently heavy to be poured from one vessel to 

 another ; and if we have obtained some in a glass, we can extinguish a 

 taper by pouring the invisible gas on to the lighted taper, when it will be 

 immediately extinguished. 



