EXERCISE 69 



The air cells of the lungs expose a large surface to the air which is breathed in. Through the 

 membranes lining the air cells the exchange of gases takes place by osmosis. To keep up this gas 



exchange it is necessary that the membranes be kept constantly moist. 

 Problem. What effect has the amount of moisture in the surround- 

 ing air upon evaporation, or drying up ? 



What to use. Filter paper ; two preserving jars ; water. 

 What to do. Soak a piece of filter paper in water ; divide it into 

 two ; fasten each piece on the under surface of one of the jar lids (use 

 sealing wax or paraffin). In the bottom of one jar place water to a 

 depth of an inch ; leave the other jar dry ; close the jars with the lids 

 from which the wet paper is suspended. 



Record. What is the condition of the two papers at the end of 

 after forty-eight hours ? What effect has the presence of water in the bottom of 

 What is the answer to the problem ? 



twenty-four hours ? 



the jar upon the air in the jar ? 



Questions. 1. Why do clothes dry more readily on some clear days than on others? 

 2. What are the advantages (or disadvantages) of a dry atmosphere in warm rooms ? 



[74] 



