160 FIRST YEAR COURSE IN GENERAL SCIENCE 



necessary to life, water is useful to man in a greater number 

 of ways than any other liquid. 



The amount of carbon dioxide in the air is practically con- 

 stant. Respiration of plants and animals, the decay of 

 plant and animal substances, and combustion are con- 

 stantly giving carbon dioxide to the air. Green plants are 

 constantly removing carbon dioxide from the air. 



As the plant grows, it uses, in preparation of its food 

 materials, the carbon which' it gets from the carbon dioxide, 

 and sets free the oxygen. As the oxygen which the 



Animals 



Pbnts 



Oxygen 



FIG. 79. RELATION OF CARBON DIOXIDE TO LIVING THINGS 



1. Arrow points indicate the direction of reading. Begin with animals 

 and write fully what this diagram tells. 2. Begin with oxygen and write 

 the short story. 



plant releases is the only source of renewed supply, it is 

 plain that if it were not for the work of plants on carbon 

 dioxide, the quantity of oxygen in the air would be 

 exhausted. 



179. Acids. Acids are compounds which in water solu- 

 tions have a sour taste. They always contain hydrogen, 

 and when they act chemically on metals, the hydrogen is 

 usually given off. The dissolving of a metal by an acid is 

 not a mere physical change, as when water dissolves sugar, 

 but is a chemical change. The metal takes the place of 

 the hydrogen in the acid-molecule, forming a new compound 

 called a salt. 



A simple test for an acid is that it turns blue litmus paper 



