238 The Rise of the Modern [lect. 



" necessary to vegetable as well as to animal life, both plants 

 " and animals had affected it in the same manner, and I own I 

 " had that expectation, when I first put a sprig of mint iDto a 

 " glass jar, standing inverted in a vessel of water : but when it 

 " had continued growing there for some months, I found that 

 " the air would neither extinguish a candle, nor was it at all 

 " inconvenient to a mouse which I put into it. The plant was 

 "not affected any otherwise than was the necessary consequence 

 " of its confined situation. 



" Finding that candles would burn very well in air in which 

 " plants had grown a long time, and having had some reason to 

 "think that there was something attending vegetation which 

 "restored air that had been injured by respiration, I thought it 

 " was possible that the same process might also restore the air 

 "which had been injured by the burning of candles. 



"Accordingly on the 17th of August, 1771, I put a sprig of 

 " mint into a quantity of air, in which a wax candle had burnt 

 " out, and found that on the 27th of that same month another 

 " candle burned perfectly well in it. This experiment I repeated, 

 " without the least variation in the event, not less than eight 

 " or ten times in the remainder of the summer. 

 ******* 



" This restoration of air, 1 found, depended on the vegetating 

 "state of the plant; for though I kept a great number of the 

 " fresh leaves of mint in a small quantity of air in which candles 

 " had burnt out, and changed them frequently, for a long space 

 " of time, I could perceive no melioration in the state of the air." 



About the same time, following up an experiment of Hales, 

 he prepared what he called nitrous air or nitrous acid, and he 

 made the remarkable observation that this nitrous acid in 

 producing certain effects on air acted only on air fit for 

 respiration. He says : 



" One of the most conspicuous properties of this kind of air 

 "is the great diminution of any quantity of common air with 

 " which it is mixed, attended with a turbid red, or deep orange 

 " colour and also a considerable heat. 



