33 
gaged, and, among these, oxygen gas itself may be 
either directly or indirectly set free. Thus M. de 
Saussure confined two plants of mtntha aquatlca in 
two equal jars of common air, and placed them to 
vegetate in sunshine for three weeks. By the side 
of each plant he put a piece of withered dead mint ; 
but in one case the mint was covered by water, while, 
in the other, it was suspended in the air of the ves- 
sel. The plant, which vegetated in the neighbour- 
hood of the suspended mint, had not ameliorated its 
atmosphere, but that, which grew in the vessel with 
the moistened dead leaves, had added many times its 
volume of oxygen gas to the common air which sur- 
rounded it *. Dr Priestley himself, indeed, observes, 
that, " in repeating his experiment, care must be 
taken to remove all the dead leaves from about the 
plant, lest they shou'o putrefy, and affect the air t ; 
and yet he inadvertently draws his conclusion from 
an experiment where this process of putrefaction was 
constantly going on. 
245. There are other experiments, however, of 
this philosopher, made in a less exceptionable mari- 
ner, and to hich no apparent objection occurs. He 
had several instances, he says, of vitiated air being 
meliorated, and of common air being considerably 
improved, by the shoots of strawberries, and of some 
other plants, which he could, by bending, introduce 
into jars of air, while the roots continued in the earth. 
This he considered to be the fairest method of trial. 
* Recherches Chim. sur la Vegetation, p. 22$. 
t Obs. abridged, vol. iii. p, 251. 
G 
