300 RESPIRATION AND EXCRETION 



2. Remove the two upper corks from one end. How long 

 does the candle burn ? 



3. Remove the upper corks from both ends. Result ? 



4. Remove one upper and one lower cork from one end. 

 Result ? 



Conclusions. 1. Explain the result when all the corks are 

 in place. 



2. Explain the result when the upper corks in both ends are 

 removed. 



3. Explain the course of the air (oxygen) when the upper 

 and lower corks at one end are removed. Which of the three 

 trials is the most successful ? Why ? 



4. Make cross-section sketches and explain the different 

 trials as above. Use dotted lines and arrows to represent 

 the course of the air. Draw a horizontal dotted line to repre- 

 sent the level of the candle tip (person's head). Label inlet 

 i and outlet o. 



b. Dust 



Method. Allow only a beam of light to enter the room on a 

 sunny day. Shake various articles near this beam. Gather a 

 pail of clean snow and melt it. Let a pan of water stand un- 

 covered on the window sill or other outer exposure. 



Observations. 1. What is given off from the shaken arti- 

 cles ? 



2. Is the water from the melted snow clear and pure ? 



3. What do you notice on the surface of the water ? 

 Conclusion. What is your conclusion ? 



Questions 



1. How can you show that plants give off oxygen ? 



2. Name some of the impurities of the air. 



3. Why should people sleep with the windows open ? 



4. How should you ventilate a room heated with a stove ? 



5. Why should rugs be used in place of carpets ? 



