loss be attributed entirely to the abstraction of car- 

 bonic acid, the quantity of acid produced will ra- 

 ther exceed that of oxygen gas lost. But on pass- 

 ing into carbonic acid, this gas suffers (11.) a small 

 degree of condensation ; and whatever future expe- 

 rience may determine this to be, it must of course 

 be deducted from the whole diminution which the 

 air suffers, and consequently will diminish some- 

 what the quantity of carbonic acid formed. 



35. Having thus endeavoured to trace the na- 

 ture and extent of the changes which the air 

 suffers in vegetation, let us next direct our atten- 

 tion to the manner in which they take place. 

 Through the whole of the preceding experiments 

 (30. et seq.) we have seen that oxygen gas is essen- 

 tial to vegetation, and that it gradually and com- 

 pletely disappears, when, in a given bulk of air, 

 that process is made to take place. What then be- 

 comes of it ? By many it is supposed to be absorbed 

 by a necessary function of the plant. It may be 

 worth while, therefore, to inquire what is that struc- 

 ture of the plant by which such an absorption can 

 be effected ? Grew and Malpighi speak of sfiiral ves- 

 sels which they discovered in the wood, and which 

 later inquirers have found to extend to the minutest 

 branches, and to spread through every leaf. From 

 these vessels being always found empty, they sup- 

 posed them to be air-vessels, and called them tra- 

 c/iete ; but Dr Darwin observes, that if the end of 

 a vine-stalk, two or three years old, be cut horizon- 

 tally, these vessels may, by the help of a common 

 lens, be seen to be full of juice, which, in a minute 



