42 VITAL ACTIONS. 



is given off will be found to be carbonic acid, which 

 plants exhale at all times in small quantities ; oxygen, 

 however, which was before expelled, is inhaled. 

 Hence plants decompose carbonic acid during the 

 day, and form it again during the night, the oxygen 

 they inhale at that time entering again into combina- 

 tion with their carbon ; and, during the healthy state 

 of a plant, the decomposition by day, and recomposi- 

 tion by night, of this gaseous matter, is perpetually 

 going on.* The quantity of carbonic acid decomposed 



* [This absorption of oxygen and reeomposition of carbonic acid 

 during the night, might perhaps be left out of the account in a gene- 

 ral view of the subject, except as an explanation of the manner in 

 which plants are injured or destroyed by the protracted absence of 

 light According to the celebrated chemist from whom the following 

 remarks are cited, this process is not at all connected with the life 

 or growth of vegetables, but is entiiely chemical. — " It is true that 

 the decomposition of carbonic acid is arrested by the absence of light 

 But then, namely at night, a true chemical process commences, in 

 consequence of the action of the oxygen in the air upon the organic 

 substances composing the leaves, blossoms, and fruit . . . The 

 substances composing the leaves of different plants being known, it 

 is a matter of the greatest ease and certainty to calculate which of 

 them, during life, should absorb most oxygen by chemical action, 



when the influence of light is withdrawn Whilst the 



tasteless leaves of Agave Americana absorb only 0*3 of their volume 

 of oxygen, in the dark, during 24 hours, the leaves of the Pinui 

 Abies, which contain volatile and resinous oils, absorb 10 times; 

 those of Quercus Robur containing tannic acid 14 times; and the 

 balmy leaves of the Popvlv^ alba 21 times that quantity. This 

 chemical action is shown very plainly, also, in the leaves of the Co- 

 tyledon calycinuw^ the Cacalia ficoides, and others; for they are sour, 

 like sorrel, in the morning, tasteless at noon, and bitter in the even- 

 ing. The formation of acids is effected during the night by a true 

 process of oxidation: these are deprived of their acid propertiea 



