Ott VEGETABLE PHYSIOLOGY. 67 



absorption by the same surface of the leaf of 

 the carbonic acid gas of the atmosphere, and a 

 decomposition of some of the water left in the sap, 

 are taking place ; by which, in the former in- 

 stance, the carbon is separated and fixed in the 

 sap, and the oxygen gas is set at liberty ; while in 

 the latter, the hydrogen is communicated to the 

 sap, and its oxygen gas also becomes free ; by 

 which operations, the sap has acquired two of the 

 leading principles necessary to vegetables, the 

 carbon and the hydrogen ; while a double supply 

 of oxygen, or the vivifying principle, is restored 

 to the atmosphere, by which its purity is preserv- 

 ed against the deterioration to which it is uni- 

 formly exposed by animal respiration, combustion, 

 and mineral absorption. 



During the night, the under surface of the leaf 

 absorbs moisture from the air, or from the even- 

 ing dew, to make up in some degree the deficiency 

 of the previous day's evaporation, and takes up 

 oxygen from the atmosphere, by decomposing it 

 and setting part of the nitrogen at liberty : a por- 

 tion of the oxygen thus absorbed, is fixed in the 

 sap, and the other part, uniting with the super- 

 fluous carbon in the plant, forms carbonic acid 

 gas, which escapes from the leaf and mixes with 

 the atmosphere. This will serve to explain, why 

 the night air is less salubrious than that of the 

 day ; and the necessity of a large proportion of 

 oxygen being set at liberty during the day, to 



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