206 Natural Theology. 



such substances as hydrogen and carbon, both to- 

 gether giving light and heat, and the products of 

 both taking the invisible forVn. 



While carbon in the form of charcoal, or mineral 

 coal, defies the action of oxygen at common tempera- 

 tures, it has no such power in its organic combina- 

 tions. In animal and vegetable tissue it seems to 

 be in a state of unstable equilibrium, so that por- 

 tions of plants and animals readily return to the form 

 of carbonic acid. Who can help admiring this beau- 

 tiful adjustment of affinities, by which carbon can 

 become fixed and remain for ever unchanged until 

 used by man, and also be so joined to other elements 

 in animal and vegetable tissues, that when they are 

 exposed upon the surface of the earth after death, 

 they rapidly decompose, giving up their carbon as 

 carbonic acid, needed to renew the face of the earth ? 



The properties of carbonic acid itself are emi- 

 nently worthy of attention. It is heavier than air, 

 but according to the law of diffusion of gases, it is 

 rapidly mingled throughout the atmosphere. It is 

 thus ever present, where vegetation is found, to sup- 

 ply it with the needed carbon. It is highly soluble 

 in water, so that both rain and dew bring it down to 

 be absorbed by the leaves and roots of plants. It 

 gives to the waters percolating the earth greater 

 solvent power for certain substances, so that they 

 set free more readily the mineral salts needed for 

 vegetation. W 7 hen this gas has reached the tissue 

 of the leaves, the strong affinity of its elements is 

 overcome by the magic power of light, the plant is 



