from Water exposed to Light. 225 



That the fresh leaves of certain vegetables, exposed in 

 water to the action of the sun's rays, cause a certain 

 quantity of pure air to be produced, is a fact which has 

 been put beyond all doubt ; but it does not appear to 

 me to be by any means so clearly proved, that this air is 

 ''' elaborated in the plant by the powers of vegetation " ; 

 — "phlogisticated or fixed air being first absorbed, or 

 imbibed by the plant as food, and the dephlogisticated air 

 being rejected as an excrement " : for, besides that many 

 other substances, and in which no elaboration, or circula- 

 tion, can possibly be suspected to take place, cause the 

 water in which they are exposed to the action of light to 

 yield dephlogisticated air as well as plants, and even in 

 much greater quantities, and of a more eminent quality, 

 the circumstances of the leaves of a vegetable which, ac- 

 customed to grow in air, are separated from its stem, 

 and confined in water, are so unnatural that I cannot 

 conceive that they can perform the same functions in 

 such different situations. 



Among many facts which have been brought in sup- 

 port ot the received opinion of the elaboration of the air 

 in the vessels of the plants in the experiments in ques- 

 tion, there is one upon which great stress has been laid, 

 which, I think, requires further examination. 



The fresh, healthy leaves of vegetables, separated from 



the plant, and exposed in water to the action of the sun's 



rays, appear, by all the experiments which have hitherto 



been made, to furnish air only for a short time ; after a 



day or two, the leaves, changing colour, cease to yield air ; 



and this has been conceived to arise from the powers of 



vegetation being destroyed ; or, in other words, the death 



of the ■plant ; and from hence it has been inferred, with 



some degree of plausibility, not only that the leaves 

 VOL. I. 15 



