COMPOSITION OF AIR 113 



of wood having a spark at the end. It bursts into flame. Does the 

 same thing take place when the stick with the spark upon it is 

 held in a bottle of air ? 



Hold a lighted match at the mouth of another of X _ 



the bottles containing oxygen. Does the gas itself 

 burn as illuminating gas does when a match is 

 applied to it? If the oxygen in the air were in- 

 creased or decreased, it would have a great effect 

 upon combustion. Attach a piece of sulphur to 



a short piece of picture wire. Ignite it and place Fig. 50. 



the wire in a bottle of oxygen (Fig. 50). Does the 



sulphur burn strongly ? How about the wire ? Does it burn too ? 



The oxygen is the most important part of the air to 

 animals, for without it they could not live. They breathe 

 in oxygen and breathe out carbon dioxide. All their 

 heat and energy is due to the power they have of com- 

 bining oxygen with carbon and forming carbon dioxide. 

 Plants also need it. 



Plants need carbon dioxide as much as animals need 

 oxygen. By far the greater part of plants is made from 

 the carbon which they get from this gas. The growth of a 

 plant is due to the power it has of tearing apart the carbon 

 dioxide by the help of the sun and building the carbon 

 into its structure. It returns the oxygen to the air to be 

 used again by the animals and plants. 



Experiment 69. Get two or three bottles of carbon dioxide from 

 the chemical laboratory, or prepare it by pouring dilute hydrochloric 

 acid upon pieces of limestone in a bottle and collecting the gas over 

 water. Does the appearance of this gas differ in any way from that 

 of air? Smell of one of the bottles that has stood over water for 

 some time. The gas has no odor. Plunge a lighted match into one 

 of the bottles containing the carbon dioxide. What happens? Does 

 the gas burn or support combustion ? Slowly overturn a bottle of 

 the gas above a lighted candle. The candle is extinguished. The 

 gas falls out when the bottle is overturned, thus showing that it is 

 heavier than air. If the amount of carbon dioxide in the air were 

 largely increased, what effect would it have upon combustion ? 



