that when confined in this acid, plants will die. 

 This, however, is no more than what we have seen 

 to take place in nitrogen gas (29.), and in vacua, 

 and cannot therefore be received as proof of the 

 positive operation of carbonic acid. From the ex- 

 periments of the late Dr Percival, and Mr Henry, 

 we learn also, that although plants, when wholly 

 confined in carbonic acid, certainly died, yet that 

 where a small portion of oxygen gas was admitted, 

 they as certainly lived and nourished *. Neither do 

 plants, confined in a given bulk of atmospheric air, 

 die till all the oxygen gas disappears (30.) ; nor, con- 

 sequently, does the quantity of carbonic acid form- 

 ed prove fatal so long as any oxygen gas remains. 

 So far, indeed, is carbonic acid from being fatal 

 to vegetation, that many have deemed it highly salu- 

 tary f, and some even contend that it is essential to 

 that process, alleging that it is absorbed by the 

 leaves of plants during the day, decomposed within 

 their vessels, its oxygen being afterwards emitted, 

 while its carbon is retained as food J. 



42. Against this opinion of the absorption and 

 emission of gases by the leaves of plants, when grow- 

 ing naturally in air, we have already, both on phy- 

 siological and on chemical grounds, been induced to 

 enter our protest : but the importance of the que- 

 stion, together with the high authorities by which it 

 has been supported, will, we trust, plead in excuse 



* Manchester Memoirs, vol. ii. p. 341. et seq. 



f Ibid. vol. ii. p. 331. 



J Thomson's Chemistry, vol. iv. p. 283. et seq. 



