ON SAP. 



81 



ed in the water of the sap. The carbonic acid, we have 

 already observed, is decomposed in the leaves, the carbon 

 is retained, and the oxygen thrown off; but what be- 

 comes of the carbon contained in the animal and vegeta- 

 ble matter which the sap holds in solution ? 



Caroline. I suppose it is assimilated to the substance 

 of the plant, together with the other nutritive ingredients 

 which the sap holds in solution. 



Mrs. B. No, that cannot be, for, in order to render 

 carbon fit to be assimilated, it appears to be necessary 

 that it should previously be combined with oxygen, and 

 afterwards separated from it. 



Caroline. Is there not something paradoxical in this ? 

 How can it be necessary that the carbon should be com- 

 bined with oxygen, merely for the purpose of being sep- 

 arated from it ? 



Mrs. B. It is very possible that this chemical process 

 may produce a more minute subdivision of the particles 

 than any mechanical operation could effect, and thus 

 prepare it for being assimilated to the plant. 



Caroline. Oh, then, now I guess it. During the night 

 the leaves absorb oxygen, to combine with this carbon, 

 and convert it into carbonic acid ; and, when the sun ri- 

 ses, this acid is decomposed, the carbon deposited in a 

 state fit to be assimilated, and the oxygen escapes. 



Mrs. B. You are right ; and as the decomposition of 

 the carbonic acid, which existed in that state in the sap, 

 takes place at the same time, these two operations, being 

 both similar and simultaneous, are confounded together. 

 But, so far as regards the purification of the atmosphere, 

 it is necessary to distinguish them ; for, in the first in- 

 stance, the oxygen exhaled is a mere restoration to the 

 atmosphere of oxygen which had been taken from it dur- 

 ing the night ; whilst, in the latter, the oxygen evolved, 

 being drawn from the soil with the sap, is so much clear 

 gain to the atmosphere. 



Caroline. Well, I breathe freely again, since I know 

 that the atmosphere positively acquires oxygen from the 



440. With what query does Mrs. B. conclude her reply to the above 

 question 1 441. How does Caroline answer it, at first! 442. 

 Why is not thte a correct answer! 443. What is the correct answer 

 which she afterwards does give! 444. In what respect are these two 

 operations confounded together! 445. In what respect are they to 

 be distinguished from each other! 



