51 



ficiency of oxygen gas. Now, it has been shewn (29. % 

 that although plants do not grow in nitrogen gas, 

 yet that gas suspends only, but does not destroy 

 the vegetating faculty : and precisely the same oc- 

 curs when they are placed in vacuo, where no direct 

 effect can be attributed to any air at all. In all the 

 preceding experiments also, vegetation did not cease 

 till all the oxygen gas had disappeared ; and ? there- 

 fore, the superabundance of nitrogen gas did not 

 prove fatal so long as any oxygen gas remained. 

 Mr Gough, indeed, has shewn (29.), that certain 

 plants retain the faculty of growth after being con- 

 fined for weeks in nitrogen gas. To the want or 

 absence, therefore, of oxygen gas, and not to the 

 presence of nitrogen gas in any proportion, is the 

 death of plants in a given bulk of air, as well as that 

 of seeds (21.), to be entirely attributed. How far 

 the proportions of nitrogen, and oxygen gas, as they 

 exist in atmospheric air, may be the best adapted to 

 the actual structure and constitutional habits of plants, 

 we have not experimentally attempted to determine : 

 but the general analogy to be derived from the case 

 of seeds (1'4, 15.), and the fitness of means to the 

 perfection of an end observed throughout all the 

 operations of nature, lead irresistibly to the belief, 

 that these proportions are in reality the best. 



41 . The only other gas present in the preceding ex- 

 periments, by which the growth of plants can be af- 

 fected, is carbonic acid : and the experiments of 

 Priestley *, Ingenhousz, and others, clearly evince, 



* On Air, vol. i. p. 36. and iii. p. 310. 

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