368 THE CHEMISTRY OF CREATION. 



strengthened; and in this very process the air 

 is purified, and rendered more suitable for the 

 existence of man and the animal world. 



But this process only takes place under cer- 

 tain conditions. The popular belief about plants 

 is less inaccurate at night. Plants cease almost 

 entirely to decompose carbonic acid at night. 

 When the sun sinks below the horizon, and the 

 shades of evening lengthen out ; when 



" twilight gray 

 Has in her sober livery all things clad ;" 



and when man, beast, and bird are seeking re- 

 pose for the night, the vegetable world sleeps 

 too. The leaves cease their daily task, and 

 pour out carbonic acid back into' the air unal- 

 tered. Whether a little carbonic acid is not 

 decomposed during the night is not altogether 

 decided, but it is certain that a very large pro- 

 portion of the carbonic acid inhaled by plants 

 during the night is given back to the air in its 

 original state. At night plants actually absorb 

 a certain measure of oxygen gas from the air, 

 which is appropriated, on the return of day, to 

 the formation of oils, acids, and other vegetable 

 ingredients. 



We must refer to the influence of sun-light 

 for the explanation of the cessation of this pro- 

 cess at night. It has . already been noticed that 



