58 



gas does not depend so much on the power of the 

 leaves, as on the quality of the water in which they 

 are immersed ; for if the water be previously boiled, 

 little or no oxygen gas is collected. River water af- 

 fords very little gas, but pump water is the most pro- 

 ductive of all : arid Sennebter also proved, that if 

 the air be previously deprived of all its air by boil- 

 ing, the leaves of plants immersed in it do not emit 

 a particle of air *. But not only must the water 

 previously contain air, but it must contain carbonic 

 acid ; for Sennebier has shewn, that no oxygen gas 

 is yielded by leaves when plunged in water destitute 

 of carbonic acid : that the quantity of oxygen afford- 

 ed is proportional to the quantity of carbonic acid 

 which the water contains ; and that when the water 

 loses the power of affording oxygen gas, all the car- 

 bonic acid which it contained has disappeared. These 

 experiments prove, that the oxygen gas, which is se- 

 parated when the leaves of plants are immersed in 

 water, depends altogether upon the presence of car- 

 bonic acid ! 



46. But surely it cannot be maintained by any 

 one, who for a moment considers the structure and 

 livingfunctions of vegetables, that the leaves of plants, 

 when, as in these experiments, they are separated 

 from the branches, and wholly immersed in water, 

 absorb and decompose this carbonic acid as a natu- 

 ral and healthy function, and afterwards emit its oxy- 

 gen and retain its carbon. Not only, in these cir- 

 cumstances, must the circulation of their fluids be 



Thomson's Chemistry, vol. iv. p. 28 4. f Ibid. 



