STUDIES IN NATURE 



rate that they took in oxygen and gave out 

 carbonic acid gas. One of the reasons why 

 many plants and trees will not grow in large 

 towns is that their pores or breathing holes get 

 filled up with soot and dirt, and the plant, being 

 unable to breathe, is consequently choked. In 

 the day-time, however, and most of all when 

 the sun's rays fall directly on them, green plants 

 also perform the reverse process of breathing 

 to a much greater extent. They absorb car- 

 bonic acid from the air, and, using the energy 

 of the sun's rays, divide it up, keeping the carbon 

 to help make their leaves and stems, while the 

 pure oxygen passes out again into the open air. 

 Here, again, we see how the sun helps in all the 

 important affairs of life, and we also learn how 

 necessary it is to have plenty of trees and plants 

 in the world. We now also understand how 

 the carbon got into the coal, for coal, as we said 

 before, is nothing but the remains of the forests 

 and woods of bygone ages which have been 

 slowly covered up, perhaps with the fine mud 

 and sand that some great river brought down in 

 the dim past. 



We thus learn that air is principally a mixture 

 of three gases, nitrogen, oxygen, and carbonic 

 acid, and, in the open air, the amounts of these 

 gases are always very nearly the same. But 

 the quantity of water-vapour, which we also 

 know to be there, since it comes out with our 

 breath like the carbonic acid gas, is always 



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