nature of this action, or the changes which the air 

 undergoes in the process of vegetation. 



27. Dr Priestley, whose experiments and disco- 

 veries first opened the way to an investigation of the 

 reciprocal changes which take place between living 

 vegetables and the air, was led to conclude, that 

 plants not only lived in nitrogen gas, or what he 

 called phlogisticated air, but that, during their 

 growth, they restored to the air the pure part of which 

 it had been previously deprived by the processes of 

 combustion and respiration, and by the decomposi- 

 tion of organized bodies. He found, that if a sprig 

 of mint was put into a jar of air inverted over wa- 

 ter, and in which a candle had previously burned 

 out, the air was in a few days so far restored that 

 another candle would burn in it perfectly well. He 

 also put a sprig of mint to grow in air wherein mice 

 had died, and in eight or nine days a mouse would 

 again live very well in this air, though he died in 

 another portion of the same air in which no mint 

 had grown. In air strongly tainted by putrefaction, 

 sprigs of mint have sometimes presently died ; but 

 ii: this do not soon happen, they thrive in a surpri- 

 sing manner. These facts led him to conclude, 

 that plants, by their vegetation, reverse the effects 

 produced on the air by the several processes above 

 described, and thus become the chief means by 

 which the purity of the atmosphere is maintained *. 



28. The experiments from which this conclusion 

 was drawn, were made so early as the year 1771, 



* Observations on Air abridged, vol. iii. p. 251. et scq. 



