CHAP. XII. §. I. IN LEAVES DURING VEGETATION. ] 65 



similar!. This circumstance may be adduced as 

 shewing, that the effect upon the needle cannot be 

 entirely referred to the changes which occur in the 

 sap from exposure to the air ; and the question now 

 arises, To what actions can we refer these effects? 

 Is the sap acid, or does the sap contain a cation ? 



I shall not enter into any discussion as to the 

 functions of leaves, whether they may be considered 

 as organs of respiration, digestion, or absorption, 

 but refer my readers to works on physiological 

 botany. That oxygen gas or carbonic acid gas is 

 given off by the leaves of plants, may be considered 

 as having been experimentally proved. 



If we break off the petiole of a leaf, and just touch 

 a piece of litmus paper" with the divided surface, in 



' It is probable that the circumstance of not obtaining any 

 result upon some of the leaves, might depend upon their 

 not being always in the same state of action. Some experi^ 

 ments on the Sespiration of the Leaves of Plants, by Me. Pepys, 

 bearing upon this question, will be found related in the Phil. 

 Trans, for 1843, p. 339. 



"■ In the employment of litmus as ». test we must bear in 

 mind, that a compound might shew an acid reaction, without 

 possessing at the same time an excess of acid. According to 

 EerzeHus, " the colour of litmus is naturally red, and it is only 

 rendered blue by the colouring matter combining with an 

 alkali. If an acid be added to the blue compound, the colouring 

 matter is deprived of its alkali, and thus, being set free, 

 resumes its red tint. Now on bringing litmus paper in 

 contact with a salt, the acid and base of which have a weak 

 attraction for each other, it is possible that the alkali contained 

 in the litmus paper may have a. stronger affinity for the acid of 

 the salt, than the base has with which it was combined ; and in 



