164 CARBONIC ACID. 



water, a gas named carbonic acid, and a small quantity of 

 organic matters. 



The heat is easily recognized : you know that your 

 breath is warm. The water usually comes out in the 

 form of invisible vapor. On a cold day, however, it is 

 seen as mist, streaming from the nostrils ; and any day 

 it can be made visible by breathing on a cold bright 

 object, as a mirror or knife-blade. 



The carbonic-acid gas and the organic matters given 

 out in the breath, are unfortunately not so easily made 

 apparent as the heat and the water-vapor. They are, 

 however, of very great importance. Carbonic acid is 

 one of the chief waste substances made by the body 

 and must be removed from it. The organic matters 

 poison the body, if air containing them be breathed 

 over and over again. 



4. How Carbonic Acid is made from Charcoal. You 

 remember that when a human body is incompletely 

 burned (Chap. I.) it forms a black mass of charcoal. 

 Now charcoal is a mixture of a substance named carbon 

 with some minerals. We may call it impure carbon. 

 When it is burned, its carbon combines with oxygen and 

 makes carbonic-acid gas. Just as rust is oxidized iron, 

 so carbonic acid is oxidized carbon ; though it is a gas, 

 instead of being solid like iron-rust. 



5. How Carbonic Acid is made in our Bodies. All our 

 organs contain animal matter (p. 10), and all animal 



How may the heat be recognized ? How the water? Name a chief 

 waste matter produced by the body. What is the result of frequently 

 breathing air containing organic matters given out in the breath ? 



4. What is left when a human body is incompletely burned ? What 

 is charcoal? What may we call it? What happens when it is 

 burned ? How does carbonic acid resemble and differ from iron-rust? 



5. What do all our organs contain ? What do all animal matters 



