Chap. 3.] Bad Air correfted by Vegetation. 23 



filled with water .in fuch a manner as to receive the 

 air- bubbles as they arife. All plants, however, do not 

 emit this air with the fame facility; there are fome 

 which emit it the moment the rays of the fun act 

 upon them, and this is the cafe with lavender. Some 

 aquatic plants afford vital air with great facility, fome 

 more (lowly, but none later than in eight or ten mi- 

 nutes, provided the fun's light is ftrong. The air is 

 almoft entirely furniihed by the inferior furface of the 

 leaves of trees; herbaceous plants afford it from 

 almoft the whole of their furface. The leaves afford 

 more air when attached to the plant, than when ga- 

 thered, and the quantity is greater, the frefher and 

 founder they are. Young leaves afford but a fmall 

 quantity of vital air; thofe which are full grown afford 

 more, and the more the greener they are. The epi- 

 dermis, the bark, and petals do not afford it, and in 

 general vital air proceeds only from thofe parts of 

 plants which are of a green colour. Thus gr.~en corn 

 and green fruits afford this air, but it is not produced 

 by thofe which are ripe ; and flowers in general render 

 the air noxious. Thefe facts may tend to explain the 

 manner in which the light of the fun operates in ma- 

 turing fruits, viz. by expelling the fuperfluous oxygen, 

 and thus changing them from a harfh and four, into a 

 mild and fweet fubftance. Aquatic plants, and fuch as 

 grow in moift places, are remarkable not only for 

 affording a large quantity of vital air, but alfo for ab/ 

 ibrbing inflammable gas, and are therefore in all re* 

 fpects calculated for purifying the air of marmy fitua- 

 tions. A very extraordinary power of abforbing in-' 

 flammable air was obferved in the willow by Dr. 

 Prieflley ; and this fact feems connected with the ra- 

 pid growth of that plant in marmy fituations, where 

 much inflammable air is produced. M. Sennabier 

 C 4 found 



