28 CHEMISTRY, GEOLOGY 



Carbonic acid gas is the form of food they main- 

 ly derive from the air. This gas has no color, 

 but it has a peculiar smell. It is so heavy that 

 it can be poured from vessel to vessel, like water. 

 It extinguishes burning bodies, and destroys 

 animal life when breathed. It causes the spark- 

 ling of soda-water and the frothing of beer, and 

 forms nearly half the weight of all limestone 

 rocks. Animals throw carbonic acid gas out 

 in breathing, and plants drink it in. (See Ap 

 pendix, 15.) 



The quantity of carbonic acid gas in the at- 

 mosphere is very small, only about two gallons 

 in five thousand ; the air we breathe is almost 

 wholly oxygen and nitrogen : in five gallons of 

 the atmospheric air, there are four gallons of 

 nitrogen and one of oxygen. Although the 

 atmosphere contains only the comparatively small 

 quantity of carbonic acid named, yet plants 

 drink it in freely, from the abundance of their 

 leaves, which contain a great number of small 



Questions. In what form do plants take food from the 

 air ? What is carbonic acid gas ? Describe some of its 

 properties. Is there much carbonic acid in the atmosphere ? 

 In what proportion is it found ? How do plants obtain the 

 cai-bonic acid from the air ? 



