EXERCISE 13 



Ordinary air is a mixture of gases made up approximately of the following : 



Nitrogen about 79%, or | of volume 



Oxygen about 20%, or \ of volume 



Carbon dioxid about ^% 



Problem. What is the relation of each of the principal gases found in air to burning ? 

 What to use. Wide-mouthed bottles for containing gases ; glass covers ; splints ; matches ; nitrogen ; 

 oxygen ; carbon dioxid ; water. 



The nitrogen will be prepared by the teacher or a committee of students and supplied as needed. 



To prepare carbon dioxid, see Exercise 12. 



To prepare oxygen, place a small piece of sodium peroxid 1 (sold 

 Z^Q ^ under the trade name " Oxone ") under the mouth of an inverted 



/A jfJ' *lf: bottle full of water, or heat a mixture of one part manganese dioxid 



and three parts potassium chlorate in a large tube connected with the 

 gas collector. 



What to do. Plunge a lighted splint into a bottle 'of air, noting 

 the results. Do the same with a bottle of nitrogen ; a bottle of 



carbon dioxid ; a bottle of oxygen. Repeat in each case until you are sure of the results. Repeat 

 with a glowing splint. 



, Record. Describe completely what happened in each case. Answer the question in Problem as 

 concisely as you can. 



Questions. 1. What would be the result of a great change in the proportion of the gases in our 

 atmosphere ? 



2. How does blowing into a flame make it burn more briskly ? 



3. What can be done to intensify combustion besides increasing the air supply ? 



1 It is not safe to use pulverized peroxid. 

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