40 CARBONIC ACID GAS IN AIR. 



third gas or kind of air, which is not simple, like 

 oxygen and nitrogen, but a compound of charcoal 

 (by chemists named carbon) with oxygen, and called 



CARBONIC ACID GAS (108). 



38. It is known that all things containing carbon 

 will produce a quantity of this gas whilst burning ; 

 and hence we can have no difficulty in accounting for 

 its presence in the air. Indeed, we might at first sup- 

 pose that it must be always increasing in quantity ; 

 this, however, is not the case, for we always find ex- 

 actly the same quantity in any portion of air that we 

 analyze. The cause of this is, that all plants con- 

 tain substances which have a very strong affinity for 

 carbon, but which cannot combine with it in its solid 

 forms, because they are unable to come in contact 

 with it ; but which, when the carbon has combined 

 with oxygen and become a part of the air, are able, 

 in consequence of their having a more powerful at- 

 traction for it, to seize upon the carbon of the car- 

 bonic acid gas thus diffused throughout the air and 

 cause it to relinquish the oxygen, with which it was 

 previously combined (697, 708). 



39. These facts show us a new use of plants, for 

 we learn that the objects which we have only admired 

 for their beauty, or valued for their utility as pro- 

 ducing articles of food ; that even weeds themselves, 

 and things we usually consider as wholly useless, are 

 all constantly, by the agency of attraction or chemi- 

 cal affinity, decomposing carbonic acid gas, and thus 

 keeping the air in an uniform and healthy state (745). 



