16 APPLIED BIOLOGY 



expelled by prolonged boiling. The following experiment 

 will show one simple method of testing the changes produced 

 in air which has been breathed : 



(D) Pour some lime-water or barium-water into a small bottle 

 and blow air from the human lungs through a small glass tube (or 

 straw) into the water. What happens? The change in the lime- 

 water is due to a gas, called carbon dioxide, which is added to the 

 air while it is in the lungs. 



The same gas is formed by ordinary burning. Wrap a piece 

 of wire around a small candle, light it, and then lower it into a tall, 

 wide-mouthed bottle. After a time the flame will flicker out. 

 Then lift out the candle and put in a little lime-water. Compare 

 the lime-water with that changed by air from human lungs. 



Take a tall, wide-mouthed bottle or fruit-jar, wash and rinse 

 with fresh water, then lower into the jar a frog inclosed in a loose bag 

 of cheesecloth or mosquito-netting with a string attached so that 

 the frog may be lifted out quickly without inverting the jar or leav- 

 ing it uncovered more than for a moment. One of the preceding 

 experiments suggests that human breathing changes the air of rooms ; 

 and hence it is important that the jar be held at an open window where 

 fresh air may enter while the frog is being placed in the jar. A good 

 plan is to keep the jar full of water until the moment when ready to 

 place the frog in it. The water will prevent the jar becoming 

 filled with the air of the schoolroom. A second jar, treated exactly 

 like the first, but without a frog, should be kept beside the first for 

 comparison. Leave the frog in the jar, carefully covered, for two 

 hours, quickly lift it out, pour in 10 cc. of lime-water, replace cover, 

 and shake. Has any change occurred in the lime-water when it 

 came into contact with the air which the frog had been breathing ? 

 Test the air in the jar without a frog. 



The above experiments prove that some change occurs 

 in air when animals breathe it. Until a later lesson we 

 need not take time to consider just what this change is. 

 For our present purpose it is sufficient to have demonstrated 

 that animals do change air when they breathe it and that 

 lime-water makes it possible to demonstrate breathing of 

 animals in which we cannot see breathing movements. 



21. The living frog has the power of reproducing new 

 animals like itself. Frogs' eggs gradually develop into new 



