154 PURIFICATION OF AIR 



it appears from his work that he merely observed the 

 bubbles of air which cling to leaves, dead as well as 

 living, and indeed to any other body, when immersed 

 in water and exposed to the light of the sun. He 

 found these bubbles disappeared in the evening, and 

 returned again when the sun shone, and he faithfully 

 reports that by their attachment to the surfaces of 

 leaves, the latter were rendered more buoyant, and 

 rose in the water ; a sure proof that the air had not 

 previously existed, in the same volume at least, in the 

 substanceofthoseleav.es. Accordingly, Bonnet con- 

 cluded that the latter, in imbibing the surrounding 

 water, left the air which had been contained in the 

 water, and that this liberated air became visible from 

 being warmed and rarefied by the sun. This was as 

 near the truth as Bonnet could come, it not being then 

 known that light has a power of separating air of a 

 peculiar kind, carbonic acid gas, from water. I tind 

 no indications in his work of his having had any idea 

 of leaves absorbing air and giving it out again ; still 

 less of their effecting 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 re- 

 spirable air. It was also his opinion that leaves imbibed 

 the former by their upper, and gave out the latter by 

 their under surface. He found some aquatic or marsh 

 plants extremely powerful in this respect, especially 



