^10 PHILOSOPHICAL TRANSACTIONS. [aNNO 1787. 



sediment (the green matter of Dr. Priestley) beginning to be formed on the 

 bottom of the jar, it began to yield air in abundance, and continued to furnish 

 it in pretty large quantities till about the 18th day, when it appeared to be 

 exhausted. Why the water should turn green in this experiment, and not in 

 the last, I know not ; unless it was in consequence of the large surface of water 

 in the cylindrical jar, which was exposed to the air in this experiment; or in 

 consequence of the sun's shining directly on the bottom of the vessel where the 

 sediment was formed. 



p. s. Since writing the above, an interval of fine weather, and a moment of 

 leisure, have given me an opportunity of making a few more experiments, of 

 which I have thought it right to give a short account. And I must begin by 

 observing, that having never been thoroughly satisfied with respect to the origin 

 of the dephlogisticated air produced on exposing fresh vegetables in water to the 

 action of the sun's rays, according to the method of Dr. Ingenhousz, my doubts, 

 with respect to the opinion generally entertained of its being elaborated in the 

 vessels of the plant, instead of being removed, were rather confirmed by the 

 result of the foregoing experiments ; and however disposed I was to adopt the 

 beautiful theory of the purification of the atmosphere by the vegetable kingdom, 

 I was not willing to admit a fact which has been brought in support of it, till it 

 should appear to have been demonstrated by the most decisive experiments. 



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 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 circulation, 

 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 circum- 

 stances of the leaves of a vegetable, which, accustomed to grow in air, are sepa- 

 rated from its stem, and confined in water, are so unnatural, that I cannot con- 

 ceive, that they can perform the same functions in such different situations. 



Among many facts which have been brought in support of the received opi- 

 nion of the elaboration of the air in the vessels of the plants in the experiments 

 in question, there is one on 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 : 



