I. 

 Recent Researches on Plant Nutrition. 



IT was well-known before 1771 that a candle, in burning, vitiates the 

 air, and on that account will not continue to burn in a closed air- 

 space ; but Priestley, in his " Experiments and Observations on 

 different kinds of Air," records for the first time experiments made 

 about this date which show that the restoration of air thus vitiated can 

 be brought about by the intervention of plants. 



Perhaps it was, to some extent, by accident that he was led to 

 try the effect of placing a sprig of mint under a glass jar standing 

 over water. The sprig he observed continued to grow for some 

 months, and after the experiment the air inside the jar was found to 

 be capable of supporting the combustion of a candle, and a mouse 

 placed in it showed no signs of inconvenience. This was considered 

 a remarkable fact, since it was to be expected that as air was known 

 to be necessary for the support both of vegetable and animal life, 

 the effect on it by the life-processes would be the same in both cases — 

 in one case, deterioration was known to take place, and in the 

 other, the same results, it was thought, ought to follow. The fact 

 that a candle would burn in air which had supported plant-life for 

 some time seems to have suggested to him the idea that possibly 

 air injured by processes of combustion may be restored by the action 

 of vegetation, and he accordingly devised experiments to test this 

 hypothesis. 



Sprigs of mint and spinach were enclosed in air in which a 

 candle had been allowed to burn itself out, and it was found that the 

 air was restored to its original condition, so far as combustion was 

 concerned, in from two to eight days. 



The processes of combustion and respiration were not understood 

 at the time. Priestley seems to treat them as quite distinct, and 

 again goes on to flatter himself that he has " hit upon one of the 

 methods employed by Nature" for the restoration of air vitiated by 

 animal respiration. 



That some great operation was continually being carried on in 

 Nature for the restoration of air rendered noxious by breathing was 

 obvious, and Priestley, like many others, had tried various experiments 

 in order to elucidate the matter — absorption by the earth, sea, and 

 fresh-water, condensation by pressure, action of various gases, e.g., 



