35 



before Dr Priestley had discovered any correct means 

 of analyzing the air ; nor does he seem to have been 

 aware, that the effects produced on the air by the 

 living functions of animals are very different from 

 those which arise from the decomposition of organi- 

 zed bodies. However much, therefore, we may be 

 disposed to admire the beautiful provision which 

 this supposed reciprocity of action would establish 

 between the functions of animal and vegetable life, 

 we must suspend our belief in its truth until better 

 proofs are brought forward than these experiments 

 afford. Even the fluctuation of opinion on this point 

 which Dr Priestley himself experienced, may well 

 suggest doubts of the correctness of his views ; for 

 towards the end of the same year, some experiments 

 of the same kind, he observes, did not answer so 

 well, and the restored air relapsed to its former 

 noxious state, which led him to suspend his judg- 

 ment concerning this power in plants *. In 1772, 

 on resuming his experiments, he had the most indis- 

 putable proof of the restoration of putrid air by ve- 

 getation ; but hearing that Mr Scheele and others 

 had obtained different results, he again repeated them 

 in 1778. The experiments of this year were unfa- 

 vourable to his former hypothesis ; for whether the 

 air had been injured by respiration, the burning of 

 candles, or other means, it was not rendered better 

 by vegetation, but worse ; and the longer the plants 

 continued in the air, the worse it became. He tried 

 a great variety of plants, as sprigs of mint, spinach, 



Observations on Air abridged, vol. iii. p. 251. to 263. 

 C 2 



