39 
these experiments are altogether inadequate to decide 
the general issue of the present question. 
253. Lastly, Dr Priestley relates that his method, 
in making these experiments, was to put the roots of 
the plants into phials filled with earth and water, 
and then to introduce them, through water, into the 
jar containing the air on which he was making the 
experiment, a method which itself gives rise to new 
sources of error. It was observed long since by Dr 
Ingenhousz, that fresh vegetable mould, confined in 
common air, considerably depraved it, by forming, 
as he supposed, carbonic acid *. Dr Thomson con- 
firmed this fact, by finding, as M. Humboldt had al- 
so done, that newly turned up soil causes a removal 
of the oxygen gas of the air, and a production of 
carbonic acidf. And M. de Saussure found that 
pure earth, confined either in atmospheric air, or in 
oxygen gas, formed carbonic acid in proportion to 
the oxygen lost, without at all diminishing the volume 
of the air, farther than by the subsequent attraction 
of a small portion of the acid. The oxygen consumed, 
he adds, is found in a quantity rigorously equal in 
the carbonic acid produced : whence it follows, that 
the earth does not assimilate the oxygen to itself, but 
only affords carbon ' > unite with it J- From these 
facts, it is evident that the earth, in Dr Priestley's 
experiments, must have continually vitiated the air, 
and thus have introduced a constant source of fallacy 
into their results. 
* Exper. sur les Veget. torn. ii. p. 189. 
f Syst. Chem. vol. iv. p. 458. 1st edit. 
J Recherches Chim. p. ] 7.Q. 
