LEAVES ON THE ATAIOSPHERE. * 175 



I 



istry^ which is well worth the attention of those who 

 wish to enter more deeply into ail the various chemical 

 examinations respecting it than suits our purpose. It is 

 onlv necessary to add a short view of Dr. Darwin's hy- 

 pothesis which Dr. Thomson has not mentioned, proba- 

 bly on account of its insufficiency. That lively writer 

 thought the watery perspiration of leaves, acted upon by 

 light, gave out oxygen for the use of the plant itself, 

 such oxygen being immediately absorbed by the air- 

 vessels. This is by no means adequate to explainanyof 

 the phoenomena, but rather contradictory to most of 

 them, and is totally superseded by the observations and 

 experiments of other writers. (85) 



(85) [Some late inquiries, by Mr. Ellis of Edinburgh, go to 

 prove that vegetating plants at all times, both by day and 

 night consume oxygen from the atmosphere, and produce car- 

 bonic acid in its stead. This carbonic acid appears to be the 

 product of carbon, afforded by the vegetable, and oxygen, con- 

 tributed by the air. Oxygen is also given out in considerable 

 quantities, by plants vegetating in the sunshine ; but this pro- 

 duction, Mr. Ellis considers to be not the result of any vegeta- 

 tive function, but only a subordinate operation accomplished by 

 the direct agency of the sun's rays. The disengagement of ox- 

 ygen by the solar sight, is attended with the production of the 

 green colour in plants. It takes place only from leaves, and 

 other parts, which acquire this colour. Flowers, fruits, roots, 

 See. whether in the sunshine or in the shade, afford nothing but 

 carbonic acid. 



Mr. Ellis believes that the production of oxygen from plants, 

 is more than balanced by its consumption, and the formation of 

 carbonic acid; and thus that growing vegetables deteriorate the 

 atmosphere, in a degr^^e which greatly surpasses their power to 

 ameliorate and improve it.J 



