324 ASSIMILATION. 



process might also restore the air that 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 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. 1 



" Several times I divided the quantity of air in which the candle 

 had burned out, into two parts, and putting the plant into one of them, 

 left the other in the same exposure, contained also in a glass vessel 

 immersed in water, but without any plant; and never failed to find 

 that a candle would burn in the former, but not in the latter. I gen- 

 erally found that five or six days were sufficient to restore this air, when 

 the plant was in its vigour; whereas I have kept this kind of air in glass 

 vessels immersed in water many months without being able to perceive 

 that the least alteration had been made in it." 



853. Iiigenhousz in 1779 showed that light is necessary to 

 assimilation. He proved experimentally that the purification 

 of air does not go on in darkness, but that light is essential. 

 His statements are here given : 



" Plants not only have a faculty to correct bad air in six or ten days, 

 by growing in it, as the experiments of Dr. Priestley indicate, but they 

 perform this important office in a complete manner in a few hours. 

 This wonderful operation is by no means owing to the vegetation of 

 the plant, but to the influence of the light of the sun upon the plant. 

 . . . This operation of plants diminishes towards the close of the day, 

 and ceases entirely at sunset, except in a few plants which continue 

 this duty somewhat longer than others. This office is not performed 

 by the whole plant, but only by the leaves and the green stalks that 

 support them. Acrid, ill-scented, and even the most poisonous plants 

 perform this office in common with the mildest and the most salutary." 2 



1 Priestley thought that this effect upon the air is due to the growth of the 

 plant, an idea which will be shown in Chapter XII. to be wholly erroneous. On 

 pages 50 and 52 of the volume quoted above are the following statements : 

 " One might have imagined that since common air is necessary to vegetable, as 

 well as to animal life, both plants and animals had affected it in the same man- 

 ner ; and I own 1 had that expectation when I first put a sprig of mint into 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. ... This 

 restoration of the 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 quan- 

 tity 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." 



2 Experiments upon Vegetables, discovering their great Power of purifying 

 the Common Air in the Sun-shine, 1779, p. xxxiii. 



