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say, that the green leaf affords oxygen, as that it be- 
comes green when that gas is expelled ; and thus it is 
that the decomposition of carbonic acid by solar light 
gives rise at once to the production of oxygen gas, 
and to the formation of the green colour in plants. 
379. The relation which we have thus traced be- 
tween these operations, enables us to explain why 
the expulsion of oxygen appears to take place only 
from the green parts of plants, since it is only when 
oxygen is expelled that those parts acquire a green 
colour; why light is equally necessary to the expulsion 
of oxygen and to the production of the green colour, 
because alkaline matter cannot be rendered predo- 
minant, and produce this colour, unless carbonic acid, 
which affords the oxygen, be first decomposed ; and, 
lastly, why no oxygen is produced, and no green 
colour is formed in darkness, because no carbonic 
acid is then decomposed, and its presence suspends 
the action of the alkaline matter. The mode, how- 
ever, in which these operations are carried on in the 
leaves, the circumstances in which they take place, 
and the agents by which they are effected, all con- 
spire to prove that the processes are purely chemical, 
and proceed in a manner entirely independent of 
those functions, which contribute to the evolution, the 
growth, and nutrition of the plant. 
380. Such is the view, which, in the progress of 
our researches, we have been led to form of the 
causes which influence and produce the green colour 
of plants. To us they appear sufficient to account 
for the phenomena ; and they are, indeed, so fami- 
liarly and precisely illustrated by the well-known 
