SECT. VI. ELABORATION OF OXYGENE. 155 



tion or contamination of the air of the atmosphere 

 effected by the vegetating plant. For if this air was 

 purer than that of the atmosphere, then it seemed 

 to afford a proof that the phlogiston of the atmos- 

 pheric air had heen retained by the plant as its true 

 food, and the pure part liberated, agreeable to the 

 hypothesis by which he supposed phlogiston to be 

 the pabulum of plants. 



In order, therefore, to ascertain the fact he plung- 

 ed into water a number of phials containing sprigs 

 of Mint, so as that the air discharged might be re- 

 tained in them, the bottoms being a little elevated. 

 The sprigs thus placed continued to vegetate and also 

 to evolve air, so that in the course of a few days he 

 procured an ounce measure of it, which proved to 

 be so pure that when mixed with equal measures of 

 nitrous gas the mixture occupied but the space of 

 one measure. In repeating the experiment he 

 found that many of his phials became lined with a 

 green vegetable matter, Conferva minima, which also 

 gave out bubbles of pure air when exposed to the 

 light of the sun, but never except in such exposure. 



From the above experiments, made in the month 

 of June 1788, Priestley inferred that the air of the 

 atmosphere is ameliorated through the process of 

 vegetation, and purged of the impurities with which 

 it is loaded by the putrefaction of vegetable and 

 animal substances, the noxious part being assimi- 

 lated to the substance of the plant, and the remain- 

 ing part evolved pure ; so that the atmosphere even 



