94 Mr. Burnett on the Development 



these effects have been too generally attributed to the same 

 cause, viz., the respiration of plants, which has been supposed 

 under certain circumstances to form, and under others to 

 decompose, carbonic acid ; but they are, in truth, distinct, and 

 performed by two separate systems the one being the result 

 of the digestive, the other of the respiratory function. 



Since the researches of Priestley and Ingenhouz directed the 

 attention of philosophers to the effects produced by plants on 

 atmospheric air, very different opinions have been successively, 

 and some that are incompatible, simultaneously inculcated ; 

 speculation being so far confused with facts, that the simplest 

 phenomena have been misunderstood, and the clearest indica- 

 tions have led theorists astray. Thus, when it had been shewn 

 that confined portions of atmospheric air in which a lighted 

 taper had been burned till it became extinguished, so that the 

 oxygen was converted into carbonic acid, and the air rendered 

 irrespirable became again purified in the course of a few 

 hours if growing plants were placed therein, so that the air 

 would again be respirable, i. e., again would support the com- 

 bustion of a taper, it was by some precipitately assumed that 

 this change resulted from the respiration of the plants : and 

 the opinion became prevalent, that vegetables breathe carbonic 

 acid, and convert it into oxygen by the retention of its carbon, 

 so that, by their agency, the deterioration of the atmosphere 

 caused by the respiration of animals, and by combustion, 

 which convert large quantities of oxygen into carbonic acid, 

 was counterbalanced, and the consumptive process of the one 

 neutralized by the restorative respiration of the other. This 

 idea was beautiful, and we can scarcely wonder that it was with 

 difficulty relinquished, even when other experiments seemed to 

 shew that it was untenable, viz., those which prove that, in the 

 shade and in the dark, or during the night, vegetables, so far 

 from improving the constitution of the atmosphere by con- 

 verting the superabundant carbonic acid into oxygen, dete- 

 riorate it by consuming a part of that already there, and 

 replacing it by carbonic acid, just as is found to take place 

 with animals: but there are quite wonders and beauties enough 

 in the truths of nature to excite our admiration, without 

 wishing to increase their number from the pages of romance. 



