124 CONSTITUTION OF TARTARIC AND CITRIC ACIDS. 



SO many of those we have hitherto had occasion to notice, be represented 

 by carbon and the elements of water alone. 

 It may be represented by 



4 of Carbon . . = C4 ) 



2 of Water . . = H2 O^ V or, 4C+2H4-30 

 and 3 of Oxygen . . = O3 5 



Tartaric Acid = C4 H2 O5 



And, though this mode of representation does not truly exhibit the con- 

 stitution of the acid, inasmuch as we have no reason to believe that it 

 really contains water as such — yet it serves to show very clearly that in 

 the living plant this acid cannot be formed directly from carbon and the 

 elements of water, as starch ant! sugar may, but that it requires also 

 three atoms of oxygen in excess to every five of carbon and two of water. 

 We shall, in the following lecture, see how nicely the functions of the 

 several parts of the plant are adjusted, — at one period to the formation of 

 this acid, and at another to its conversion into sugar during the ripening 

 of the fruit. 



HI. citric ACID, OR ACID OF LEMONS. 



This acid gives their sourness to the lemon, the lime, the orange, the 

 cranberry, the red whortleberry, the bird-cherry, and the fruits of the 

 dog-rose and the woody night-shade. It is also found in some roots, as 

 in those of the dahlia pinnata, and the asarum europa3um {asarrabacca), 

 and mixed with much malic acid, in the currant, cherry, gooseberry, 

 raspberry, strawberry, common whortleberry, and the fruit of the haw- 

 thorn. 



When extracted from the juice of the lemon or lime, and afterwards 

 purified, it forms transparent colourless crystals, possessed of an agreea- 

 ble acid taste ; effervesces like tartaric acid with carbonate of soda, and 

 like it, therefore, is much employed for effervescing draughts. With 

 potash it forms a soluble salt, which is a citrate of potash, and from lime 

 water it throws down a white, nearly insoluble, sediment of ci^ra^e q/" 

 lime, which re-dissolves when the acid is added in excess. In combi- 

 nation with lime it exists in the tubers, and with potash in the roots, of 

 the Jerusalem artichoke. 



When free from water, citric acid consists of 



Carbon .... 41-49 — 4 atoms. 



Hydrogen . . . 3-43 = 2 atoms. 



Oxygen . . . . 55*08 = 4 atoms. 



100 

 and is therefore represented by C4 Hg O4. 



This formula differs from that assigned to the tartaric acid only in 

 containing one atom of oxygen less, O4 instead of O5. In the citric 

 acid, therefore, there are 2 atoms of oxygen in excess, above what is 

 necessary to form water with the 2 of hydrogen it contains. 



IV. — malic acid. 

 The malic and oxalic acids are more extensively diffused in living 

 plants than any other vegetable acids. If acetic acid be more largely 



