268 EFFECTS OF LIGHT. 



action of air than those which are of a more porous 

 and open structure, and in the former, therefore, the 

 process of germination proceeds more slowly. 



699. In the next stage of the growth of plants, or 

 when they have leaves, they begin to absorb carbon 

 from the air instead of parting with it; and this they 

 do by decomposing the carbonic acid always present 

 in the air, a power which they possess when exposed 

 to the influence of light. 



700. The effects of light in increasing and dimin- 

 ishing chemical affinity are highly curious, and but 

 very imperfectly understood : there are a good many 

 substances which, although they have an aflfinity for 

 each other, cannot combine in the dark. In the 

 same way, that peculiar chemical change which goes 

 on when plants grow, cannot proceed well in the 

 dark ; the decomposition of carbonic acid and water, 

 and the combination of their elements in order to 

 form lignin or starch, &c., goes on very imperfectly 

 without light. 



701. The effect of Light, in causing chemical com- 

 bination and decomposition, is quite independent of 

 its brightness or illuminating power. The rays of 

 light which reach the earth from the sun, possess 

 three distinct classes of properties ; those which give 

 light, those which give heat, and those which cause 

 chemical action. In passing through different sub- 

 stances it is found that one or other of these proper- 

 ties is lost; the heating rays, for example, passing 



