216 GRAVITY, LIGHT, AND CALORIC. L,ECT. XI. 



natural conclusion to draw from this fact is, that the production 

 of the green matter in vegetables, and the extraordinary pro- 

 perty with which this substance is endowed, of decomposing 

 carbonic acid under the influence of light, of appropriating 

 to itself the carbon and of exhaling the oxygen, take place 

 under the chemical action of the solar rays. Nevertheless, 

 it follows from some of Draper's experiments, that the lu- 

 minous rays, properly so called T those which act more espe- 

 cially on the retina, viz, the yellow rays, are those under 

 whose influence principally the green matter of vegetables 

 decomposes carbonic acid. With respect to the absorption 

 of oxygen and the exhalation of carbonic acid in darkness, 

 it is admitted, that these are effected independently of the 

 condition of life. 



You perceive, by the little which I have been enabled to 

 state to you respecting this very important subject, how 

 very limited our knowledge is respecting it. What is- 

 really the immediate chemical principle which acts thus in 

 plants, and which is capable of accomplishing a chemical 

 action, whose intensity has no parallel in the most energetic 

 ordinary chemical affinities ? What share does the organism 

 take in this action ? 



In a balloon, filled with water acidulated with carbonic 

 acid, I exposed to the light some leaves which had under- 

 gone a very strong trituration, and I obtained no trace of 

 oxygen, whilst, in another similar apparatus, in which the 

 leaves w ? ere uninjured, I soon discovered its presence. I 

 may also add, that there is a great number of green vege- 

 table parts containing a substance analogous to that of the 

 leaves, and having no action upon carbonic acid in solar 

 light. It is very desirable that these experiments should 

 be extended and varied, in order to establish the influence 

 of organization on the respiration of plants. 



