60 CHEMISTRY, GEOLOGY 



the lungs, by a very simple experiment. Put a 

 piece of lime into a pitcher of water, and let it 

 stand a day. Take some of the clear lime-water, 

 put it into a tumbler, and pass the air from the 

 lungs through it, by breathing through a straw 

 or a clean pipe-stem. In a little while, the 

 water, which was clear, becomes milky, and soon 

 carbonate of lime (chalk) falls to the bottom. 

 (See Fig. 18.) 



The water contained a clear solution of lime ; 

 the lime combined with the carbonic acid in the 

 air breathed from the lungs, and formed the car- 

 bonate of lime ; and, as that will not dissolve in* 

 water, it is visible to the eye, and soon settles 

 on the bottom of the tumbler. 



Almost all the nitrogen contained in the food 

 is retained in the excrements of animals, mixed 

 with a smaller quantity of carbon than they took 

 in with their food ; and the larger proportion of 

 nitrogen contained in the dung of animals is one 

 of the causes of their greater value as a manure. 



The nitrogen, in the process of the decompo- 



Questions. How can you prove that carbonic acid is 

 thrown out in the act of breathing ? What becomes of the 

 nitrogen contained in the food of animals ? 



