NATURE AND SOURCES OF FOOD. 91 



has been thus treated, and evinces that charcoal is the principal 

 constituent in the material out of which a plant is constructed. 

 The carbon found in plants is derived from the atmosphere, 

 and from the decomposing vegetable matter in the soil. It has 

 been shown how plants take in carbon from the atmosphere, 

 through the pores of their leaves, in the form of carbonic acid 

 gas. But the atmosphere is not the only source ; the soil also 

 contains an immense quantity, for carbonic acid is given ofiF not 

 only by the lungs of animals, but by burning bodies, and by 

 the decaying animal and vegetable matter in the soil. 



When we burn a plant and thus effect a separation between 

 its organic and inorganic constituents, restoring the former 

 to the atmosphere and isolating the latter under the form of 

 ashes, the process of combustion is only the result of the rapid 

 union of the oxygen of the air with the carbon in the leaf, and 

 the consequent formation of carbonic acid gas. Now precisely 

 the same process occurs in nature when plants decay and disap- 

 pear from the earth's surface. 



The decay of vegetable bodies in the soil, as Liebig has 

 shown, is only a slower process of combustion, being produced 

 by precisely the same cause, viz., the union of the oxygen of 

 the air with the carbon in the plant, with the conBequent pro- 

 duction of carbonic acid gas. 



Hence we see the reason why wood when it gradually 

 decays becomes brown and ultimately black, presenting pre- 

 cisely the same appearance as if it had been burnt with fire. 



In the process of decay, or as it is termed chemically, 

 eremacausds, that is slow burning, the oxidation of the vege- 

 table is so slow that neither heat nor light is evolved ; hence 

 the products of the vegetable decomposition are aqueous as 

 well as gaseous, or the body, popularly speaking, putrefies. 



