CHAPTER X. 

 INTERNAL RESPIRATION. 



Composition of the Air. Air has about 20 per cent oxy- 

 gen and 80 per cent nitrogen, or one fifth oxygen and four 

 fifths nitrogen. There is a very small amount of carbon 

 dioxid, and usually there are traces of other gases. 



Experiments illustrating Internal Respiration. MATERIAL NECES- 

 SARY. A piece of candle an inch or two long, two tumblers, a tube 

 eight inches long (a straw will serve), a nail, and lime water. The lime 

 water should be prepared the day before by putting a piece of fresh 

 quicklime as big as a hen's egg in a quart of water. The next morning 

 carefully pour off the clear water for use in experiment. 



EXPERIMENT i. Light the candle and hold a cold tumbler inverted 

 a little above it. The moisture that dims the inside of the tumbler is 

 water that has been produced by the burning of the candle. The oxy- 

 gen of the air unites with something in the candle and forms water. 



EXPERIMENT 2. Breathe into a cold tumbler. The tumbler is 

 dimmed by the water in the air we breathe out. 



EXPERIMENT 3. Lower a tumbler over the burning candle till the 

 tumbler rests on the table. Observe that the flame is soon put out. 

 Carefully lift the tumbler and slip one hand under it so that the palm 

 tightly covers the mouth of the tumbler. Invert the tumbler. Lift one 

 edge of the hand and pour in about two tablespoonfuls of lime water. 

 Thoroughly shake the tumbler, keeping it tightly closed. The lime 

 water is turned milky by the carbon dioxid produced by the burning 

 candle. There is carbon in the material of the candle, and the union of 

 oxygen with this carbon produces carbon dioxid. 



EXPERIMENT 4. Pour about two tablespoonfuls of lime water into 

 a tumbler and breathe through it by means of a tube. The lime water 

 is turned milky by the carbon dioxid in the breath. There is carbon in 



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