36 



lettuce, and some others ; but with no better suc- 

 cess. His method was to put the roots of ihe plant 

 into a phial filled with earth and water, and then to 

 introduce it through water into the jar containing 

 the air on which he was making the experiment. In 

 some cases, the air was certainly meliorated, but Mr 

 Scheele, he adds, always found it injured : and he 

 concludes, that, upon the whole, it is jirobable that 

 the vegetation of healthy plants, growing in natural 

 situations, has a salutary effect on the air in which 

 they grow *. Thus we see, from Dr Priestley's own 

 representation of facts, related with his usual can- 

 dour, that nothing certain can be inferred from his 

 experiments in favour of this supposed purifying 

 power in plants : and that his conclusion was, even 

 at that period, in direct opposition to the experience 

 of the celebrated Scheele. 



29. Notwithstanding, however, the uncertain, and, 

 in many respects, contradictory evidence on which 

 this conclusion has been shewn to rest, few opinions 

 in modern science have obtained a more general be- 

 lief: and both physiologists and chemists seem, in 

 this instance, to have satisfied themselves with con- 

 templating at a distance the beauty of thejinal cause , 

 instead of approaching to a nearer examination of 

 the facts on which the opinion has been maintained. 

 Accordingly, this opinion still keeps its ground, and, 

 in no publication that we have seen, has its accura- 

 cy been even questioned. In the experiments, 

 however, which we are about to detail, it will be 



* Observations on Air abridged, vol. iii. p. 273. et seq. 



