BY LEAVES. 203 



giving it out again ; still less of their affecting 

 any change in its properties. 



Dr. Priestley was the first who suggested 

 this last-mentioned quality in vegetables. He 

 ascertained their power of absorbing carbonic 

 acid gas, denominated by him fixed air, and 

 giving out oxygen gas, or pure respirable 

 air. It was also his opinion that leaves im- 

 bibed the former by their upper, and gave 

 6ut the latter by their under surface. He 

 found some aquatic or marsh plants extremely 

 powerful in this respect, especially the Wil- 

 low-herb or Epilobium, and the Conferva, a 

 minute branching cotton-like vegetable which 

 grows in putrid water, and the production of 

 which, in water become foul from long keep- 

 ing on ship-board, Dr. Priestley judged to 

 operate principally in restoring that fluid to 

 a state fit for use. 



Dr. Ingenhousz, pursuing Dr. Priestley's 

 inquiries, found light to be necessary to 

 these functions, and that in the dark leaves 

 gave out a bad air. He observed moreover 

 that fruits and flowers almost invariably gave 

 out a bad, or carbonic, air, but more espe- 

 cially in the dark. He probably carries his 



