rOUMATION or TARTARIC, MALIC, AND CITRIC ACIDS. 141 



in the fruit — either directly from the elements of carhonic acid and wa- 

 ter with the evolution of oxygen gas — or from the gum and sugar al- 

 ready present in the sap aided by the absorption of oxygen from the at- 

 mosphere. Thus 



A. 4 of Carbonic Acid = C4 O3 

 2 of Water . . = Ha O2 



Tartaric Acid. 



Sum . . = C4 H2 Oio or C4 H2 O5 + 50. 

 That is, one equivalent of tartaric acid may be formed from 4 of carbon- 

 ic acid absorbed by the leaf or fruit, and 2 of the water of the sap, while 

 5 of oxygen are at the same time given off by the leaf. Or, 



B. If Grape Sugar be C12 H^a O12 



i of Grape Sugar = C4 H3 O3 

 3 of Oxygen . . = O3 



Tartaric Acid. Water. 



Sum . . =64 H3 Oe or C4 H2 O5 -j- HO. 



Thpt is, by the absorption from the air of a quantity of oxygen equal to 

 that which it already contains, ^rape sugar may be converted into tar- 

 taric acid and water. 



In the sorrels and other sour-leaved plants, which contain tartaric acid 

 in their general sap, the acid may be formed by either of the processes 

 above explained. In the sunshine their green parts absorb carbonic 

 acid and evolve oxygen. If any of these green parts give off only | of 

 the oxygen contained in the carbonic acid they drink in, tartaric acid 

 may be produced (A.) In the dark they absorb oxygen and give off 

 carbonic acid. If the bulk of this latter gas which escapes be less than 

 that of the oxygen which enters, a portion of the sugar or gum of the 

 sap may, as above explained (B.), be converted into tartaric acid. 



We have as yet no experiments which enable us to say by which of 

 these modes the tartaric acid is really produced in such plants — or 

 whether it may not occasionally be compounded by both methods. 



In green fruits also, in the sour grape for example, it may, in like 

 manner, be produced by either method. The only experiments we yet 

 possess, those of De Saussure, though not sufficient to decide the point, 

 are in favour of the former explanation (A.) In the estimation of this 

 philosopher, the proportion of the oxygen of the carbonic acid which is 

 retained by the fruit, is sufficient to account for the acidity it gradually 

 acquires. 



2°. Malic and citric acids. — These acids are represented (p. 127) by 

 the common formulae C4 H2 O4. They may be produced from water 

 and carbonic acid, if three-fourths only of the oxygen of the latter be 

 given off. Thus 



4 of Carbonic Acid = C4 O3 

 2 of Water . . = Ho Oo 



Malic Acid. 



Sum . . =C4 H2O10 =€4 H2O4 -f 60. 



That such a retention of one-fourth of the oxygen of the carbonic acid 

 occasionally takes place in the green fruit, is consistent with the obser- 

 vations of De Saussure. The lime and the lemon are fruits on which 

 the most satisfactory experiments might be made with the view of fi- 

 nally determining this point. 



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