41 
and examined some part of this air every two 
days, but he always found it foul * ; so that the ex- 
perience of Scheele is directly opposed to that of 
Prie rley. 
256. From what has been stated, however, we 
may collect, that both these eminent men were, to a, 
certain extent, correct, and that both erred only by 
pushing their conclusions too far. Little doubt can 
remain but that, in many of Priestley's experiments, 
the air was much ameliorated ; and it is equally cre- 
dible that, in all those of Scheele, it continued un- 
improved. Both philosophers seem to have been 
aware how much the state of the plant, and the a- 
gency of light, influenced the result ; but both failed 
in duly discriminating the composition of the air, 
which they were accustomed to denominate foul ; 
for while the air used by Priestley (247.) seems al- 
ways to have contained a portion of carbonic * acid, 
that employed by Scheele (247.) consisted only of 
nitrogen gas. It was this difference in the composi- 
tion of the air employed in the experiments, which 
gave rise to a difference in their results ; and the 
mode in which it contributed to do so will hereafter 
be distinctly stated. 
257. The experiments of Priestley and Scheele 
were repeated, and pursued to a great extent, by Dr 
Jngenhousz. Common air, says this author, is e- 
qually necessary to the life of animals and of plants. 
It is, however, only one-fourth part of this air that 
serves this purpose ; and hence a plant, confined in 
* On Air ?,nd Fire ; p, 1()3. 
