Page:An introduction to physiological and systematical botany (1st edition).djvu/235

 Rh more or less pure. The Nymphæa alba, ''Engl. Bot. t.'' 160, affords an extraordinary abundance of it. Dr. Ingenhousz observed plants to be very various in their mode of emitting these bubbles, but it was always uniform in the same species. Air collected from water placed in similar circumstances without plants, proved not oxygen, but much worse than common air, viz. carbonic acid gas, which following chemists have confirmed, and which we have already mentioned. Ingenhousz also found the air collected from plants under water in the dark worse than common air, especially that from walnut-leaves; which confirms the common opinion, above alluded to, respecting this tree.

Plants purify air very quickly. A vine-leaf in an ounce phial of carbonic acid gas, that immediately extinguished a candle, placed in the sun, without water, changed it to pure respirable air in an hour and half. Dr. Priestley found plants to alter even unmixed inflammable air, or hydrogen, especially the Epilobium hirsutum, if I mistake not, and Polygonum Hydropiper.

Succulent plants are found to afford most