6S ON VEGETABLE PHYSIOLOGY. 



obviate the injury which the atmosphere sustains 

 by the operations of the night. 



Thus we have seen that the leaves of plants 

 perform very different operations at different times ; 

 since during the day, they are giving out moisture, 

 absorbing carbonic acid gas, and emitting oxygen 

 gas; during the night, they are absorbing mois- 

 ture, giving out carbonic acid and nitrogen gases, 

 and taking up oxygen gas. By these operations 

 assisted by the agency of light, (which, indepen- 

 dently of its imparting colour to the leaf, contri- 

 butes essentially to its chemical changes,) the 

 sap receives all the primary principles which 

 constitute the plant namely, oxygen, hydrogen, 

 carbon and nitrogen; by the various combinations 

 of which, nourishment to the plant is not only 

 produced, but also through the agency of secre- 

 tion, those other substances are elaborated which 

 we know can be extracted from vegetables ; 

 and which, taken from one description of vege- 

 table or another, amount to no less than thirty-one 

 articles, exclusively of those which, belonging to 

 the mineral kingdom, have teen denominated 

 extraneous ; while, by a very beautiful process, 

 the purity of the atmosphere is so balanced within 

 the twenty-four hours, as to be fitted for all the 

 purposes of animal and vegetable economy. Thus 

 by a wonderful piece of mechanism that cannot 

 be too much admired and investigated, arid in the 

 construction of which there is still a wide field 



