SECT. XXVI. CHEMICAL ACTION OF LIGHT. 249 



Plants grow on the borders of hot springs : they form the oases 

 wherever moisture exists among the burning sands of Africa ; 

 they are found in caverns almost void of light, though generally 

 blanched and feeble. The ocean teems with vegetation. The 

 snow itself not only produces a red lichen, discovered by Saussure 

 in the frozen declivities of the Alps, found in abundance by the 

 author crossing the Col de Bonhomme from Savoy to Piedmont, 

 and by the polar navigators in the Arctic regions, but it affords 

 shelter to the productions of these inhospitable climes against the 

 piercing winds that sweep over fields of everlasting ice. Those 

 undaunted mariners narrate that under this cold defence plants 

 spring up, dissolve the snow a few inches round, and the part 

 above, being again quickly frozen into a transparent sheet of ice, 

 admits the sun's rays, which warm and cherish the plants in 

 this natural hothouse, till the returning summer renders such 

 protection unnecessary. 



The chemical action of light is, however, absolutely requisite 

 for the growth of plants which derive their principal nourish- 

 ment from the atmosphere. They consume the carbonic acid gas, 

 nitrogen, aqueous vapour, and ammonia it contains ; but it is the 

 chemical agency of light that enables them to absorb, decompose, 

 and consolidate these substances into wood, leaves, flowers, and 

 fruit. The atmosphere would soon be deprived of these elements 

 of vegetable life were they not perpetually supplied by the 

 animal creation; while, in return, plants decompose the moisture 

 they imbibe, and, having assimilated the carbonic acid gas, they 

 exhale oxygen for the maintenance of the animated creation, and 

 thus preserve a just equilibrium. Hence it is the combined and 

 powerful influences of the whole solar beams that give such 

 brilliancy to the tropical forests, while, with their decreasing 

 energy in the higher latitudes, vegetation becomes less vigorous. 

 On that account it is vain to expect that the fruit and flowers 

 raised in our hothouses can ever have the flavour, perfume, or 

 colouring equal to that which they acquire from the vivid light 

 of their native skies. 



By far the greater number of the known species of plants are 

 indigenous in equinoctial America ; Europe contains about half 

 the number ; Asia, with its islands, somewhat less than Europe ; 

 Australia, with the islands in the Pacific, still less; and in 

 Africa there are fewer known vegetable productions than in any 



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