HOW PLANTS ARE NOURISHED BT CARBONIC ACID ? 135 



be represented by carbon and water, (p. Ill,) and of which, as we have 

 seen, the solid parts of plants are chiefly made up. 



There are two ways in which we may suppose the oxygen given off 

 by the leaf to be set free, and the starch, sugar, and gum, to be subse- 

 quently formed. 



A. The action of light on the leaf of the plant may directly decompose 

 the carbonic acid after it has been absorbed, and cause the oxyen to sep- 

 arate from the carbon, and escape into the air ; — while at the same in- 

 stant the carbon thus set ^ra^ may unite with the water of the sap in 

 different proportions, so as to produce either sugar, gum, or starch. 

 Suppose 12 atoms of carbonic acid (12 COo) to be thus decomposed, and 

 their carbon to unite with 10 of water (10 HO), we should have 

 from 12 of Carbomc Acid . = C^a 



which united to 10 of Water . . . z= Hm Oio 



would give 1 of Gum or of Cane Sugar = C^o H^o ^m 

 while 24 of oxygen would be given off, the whole of which would have 

 been derived from the carbonic acid absorbed by the plant. 



B. Or the action of the sun's rays may be directed, in the leaf, to the 

 decomposition, not of carbonic acid, but of the water o?i\\Q sap. The oxy- 

 gen of the water may be separated from the hydrogen, while at the same 

 instant the latter element (hydrogen) may unite with the carbonic acid 

 to produce the sugar or starch. The result here is the same as before, 

 but the mode in which it is brought about is very differently represented, 

 and appears much more complicated. Th^is, suppose 24 of water 

 (24 HO) to be decomposed, and to give off their oxygen into the air, 24 of 

 oxygen would be evolved as in the former case, the whole oi which icould 

 he derived from the decomposition ofwatcx, while there would remain 

 24 of Hydrogen . . = H ^ 



Let this act on 12 of Carbonic Acid = C^a ^24 



and we have as the result C12 H24 O24 ; 



Starch, &c. Water. 



or C,2 H,o 0,0 + 14HO. 



According to this mode of representing the chemical changes, water i« 

 first decomposed and its oxygen evolved, then its hydrogen again com- 

 bines with the carbon and oxygen of the carbonic acid, and forming two 

 products — water and sugar or starch. This view is not only more com- 

 plicated, but it supposes ihe same action of light to be — continually, at 

 the same tiine, and in the same circumstances — both decomposing wa- 

 ter and re-forming it from its elements. While, therefore, there can be 

 no doubt, for other reasons not necessary to be stated in this place, that 

 the light of ihe sun really does decompose water in the leaves of planis, 

 and more in some than in others — yet it appears probable that the oxygen 

 evolved by the leaf is derived in a great measure from the carbonic acid 

 which is absorbed; and that the principal part of the solid substance of 

 living vegetables, in so far at least as it is derived from the air, is pro- 

 duced by the union of the carbon of this acid with the elements of the 

 water in the sap.* 



* I ought not to pass unnoticed the opin.on of Pcrsoz (Chemie Moleculai'-e), that the 

 starch, gum, &c., of plants are formed by the union of carbonic oxide (CO) with .he neces- 



