40 AGRICULTURE. 



ought to be presumed from the analysis of plants 

 and the necessary and well-known action of water 

 on vegetation." 



III. Of air, and its agency in vegetation. 



A seed deprived of air will not germinate ; and a 

 plant placed under an exhausted receiver will soon 

 perish. Even in a close and badly ventilated gar- 

 den, vegetables indicate their situation; they are 

 sickly in appearance and vapid in taste. These 

 facts sufficiently show the general utility of air to 

 vegetation : but air is not now the simple and ele- 

 mentary body that the ancient chymists described 

 it to be. Priestley first,* and Lavoisier after him, 

 analyzed it, and found that, when pure, it consisted 

 of about 70 parts of azote, 27 of oxygen, and 2 of 

 carbonic acid. In its ordinary or impure state, it 

 is loaded with foreign and light bodies ; such as 

 mineral, animal, and vegetable vapours, the seeds of 

 plants, the eggs of insects, &c. Is it to this aggre- 

 gate that vegetation owes the services rendered to 

 it by air ? And, if not, to how many and to which of 

 its regular constituents are we to ascribe them 1 ? 

 This inquiry will form the subject of the present 

 article. 



All vegetables in a state of decomposition give 

 azote; and some of them, as cabbages, radishes, 

 &c., in great quantity. This abundance, combined 

 with the fact that vegetation is always vigorous in 

 the neighbourhood of dead animal matter, led to the 

 opinion that azote contributed largely to the growth 

 of plants ; but experiments, more exactly made and 

 often repeated, disprove this opinion, and show that 

 in any quantity it is unnecessary, and that, in a cer- 

 tain proportion, it is fatal to vegetation. 



In hydrogen gas plants are found to be variously 

 affected, according to their local situation ; if in- 



* See Priestley's Experiments and Observations on different 

 kinds of Ait bogun in 1767. 



