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" 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 that had l>een injured by the 

 burning of candles. Accordingly, on the 17th of August, 1771,1 put a sprig of mint into 

 a quantity of air in which a wax candle had burned out, and found that, on the 27th of 

 the 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, I found, depended upon 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 burned out, and changed them frequently for a long space of time, I 

 could perceive no melioration in the state of the air. 



" This remarkable effect does not depend upon anything peculiar to mint, which 

 was the plant that I always made use of till July, 1772, for on the 16th of that month 

 I found a quantity of this kind of air to be perfectly restored by sprigs of balm, which 

 had grown in it from the 7th of the same month." 



We need not recount his experiments on " dephlogisticated air " — 

 what we now know as oxygen. He had, indeed, prepared that gas, 

 and knew many of its properties, but failed to recognise its true 

 meaning, so dominated were his conceptions by the phlogistic theory. 

 We may, however, in these days of inhalation of oxygen, record 

 Priestley's own observations on this subject : — 



" It may hence be inferred that a quantity of very pure air would agreeably 

 qualify the noxious air of a room in which much company should be confined, and 

 which should be so situated that it could not be conveniently ventilated, so that, 

 from being offensive and unwholesome, it would almost instantly become sweet 

 and wholesome. This air might be brought into the room in casks, or a laboratory 

 might be constructed for generating the air, and throwing it into the room as fast 

 as it could be produced. This pure air would be sufficient for the purpose of many 

 assemblies, and a very little ingenuity would be sufficient to reduce the scheme into- 

 practice. 



"From the great strength and vivacity of the flame of a candle in this pure air, 

 it may be conjectured that it might be peculiarly salutary to the lungs in certain 

 morbid cases, when the common air would not be sufficient to carry off the phlogistic 

 putrid effluvium fast enough. But, perhaps, we may also infer from these experi- 

 ments that, though pure dephlogisticated air might be very useful as a medicine, it 

 might not be so very proper for us in the usual healthy state of the body ; for, as a 

 candle burns out much faster in dephlogisticated than in common air, so we might, as 

 may be said, live out too fast, and the animal powers be too soon exhausted in this 

 pure kind of air. A moralist, at least, may say that the air which nature has provided 

 for us is as good as we deserve. My reader will not wonder that, after having ascer- 

 tained the superior goodness of dephlogisticated air, by mice living in it and the 

 tests aVxjve-mentioned, I should have the curiosity to taste it myself. I have gratified 

 that curiosity by breathing it, drawing it through a glass syphon, and by this means I 

 reduced a large jar full of it to the standard of common air. The feeling of it to my 

 lungs was not sensibly different from that of common air, but I fancied that my breast 

 felt peculiarly light and easy for some time afterwards. Who can tell but that, in 

 time, this pure air may become a fashionable article in luxury 1 Hitherto, only two mice 

 and myself have had the privilege of breathing it." , 



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