18 PHYSIOLOGY 



If a small amount of clear lime water, which is a test 

 for carbon dioxide, is now put into these two jars, it 

 turns milky in both, showing that in the one case oxygen 

 united with carbon from the splinter to form carbon 

 dioxide, and that in the other the carbon dioxide was 

 unchanged. Carbon dioxide does not burn because it 

 can not hold any more oxygen; it does not allow any- 

 thing to burn in it because it will not allow the oxygen 

 which it holds to unite with anything else. It is there- 

 fore inactive. 



Oxygen is very active. It burns, or unites with, al- 

 most everything with which it comes in contact if the 

 temperature is raised to the kindling point. Even iron 

 burns in it, forming iron oxide or rust. 



Heat and Chemical Action. — There is a very close 

 relation between heat and chemical action. We use a 

 burning match to light the fire because heat is necessary 

 to bring about chemical action, and we sit near the fire to 

 get warm because chemical action gives rise to an in- 

 crease in heat. Heat, then, becomes an evidence that 

 a chemical action which may be invisible is taking 

 place. Heat will be given off whether the action takes 

 place quickly or slowly, but if the action takes place 

 slowly, as in the rotting of wood, or in the rusting of 

 iron, it is not evident to the senses. 



Manufacture of Starch. — When starch is made, six 

 parts of carbon dioxide and sh^ parts of water unite 

 chemically to form one part of starch. Twelve parts of 

 oxygen are left over. (6CO 2 +ta 2 O=C 6 H 12 O 6 +120). 

 The process takes place in the leaf in the presence 

 of sunlight, through the activity of chlorophyll, the sub- 



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