EFFECT OF VEGETATION ON AIR. 365 



remembered that from a number of other 

 sources carbonic acid is discharged into the 

 atmosphere, little difficulty as to the existence 

 and constant supply of a sufficiency of this gas 

 in the atmosphere to account for all the wood 

 upon the earth's surface will be experienced. 



Such is the chemical history of the formation 

 of wood from the air. Let us now inquire what 

 becomes of the other element entering into the 

 composition of carbonic acid gas namely, oxy- 

 gen. Is it condensed and solidified, so as to 

 form a part of the vital structures of the plant? 

 or is it again rejected, and again returned to 

 the air ? Upon the answer we are able to give 

 to this question depends another important 

 point, Do plants purify, or do they vitiate 

 the air ? If they retain the oxygen of the car- 

 bonic acid they decompose, they rather tend to 

 vitiate the air than otherwise, by removing one 

 of its most essential ingredients. But if, on 

 the contrary, they reject the oxygen, retaining 

 only the carbon, they purify the air in a double 

 sense ; for they not only remove from it a dan- 

 gerous ingredient, but add to it a salutary one, 

 in the element of oxygen. 



The opinion popularly held is not altogether 

 correct. It seems to be a general impression, 

 that the presence of plants in a room, or to be 

 long in the air of a conservatory, is unwhole- 



