170 THE WORLD'S LUMBER ROOM. 



when we have comparatively few scavengers to purify the 

 air for us ? 



When it is winter here, however, it is summer in the 

 South, and, thanks to winds and currents, as well as to 

 the movement of the gases among themselves, our breath 

 may go to feed the palms and sugar-canes of the tropics, 

 while English oaks and English wheat are in their turn fed 

 by the breath of Africans and South Americans. 



When the leaves have attracted and absorbed the gas, 

 the sunbeams with their rapid vibrations tear the atoms of 

 carbon and oxygen asunder ; the carbon is kept by the 

 chlorophyll or leaf-green, to which the leaves owe their 

 colour, and nearly the whole of the oxygen is given back, 

 to be breathed over again and to purify the air of those 

 organic impurities, which, besides carbonic acid, are con- 

 stantly being poured into it from the lungs of animals. 



Plants breathe by means of their leaves, and if. stripped 

 of them will die : but without the leaf-green they cannot 

 separate the carbon, and to make leaf-green they need 

 iron, in very small quantities, it is true, but if kept quite 

 without it, the plant, like the human being, grows pale 

 and sickly, it cannot digest its food, and finally dies of 

 starvation. But the leaf-green, however healthy, cannot do 

 its work without sunlight, and plants kept in the shade, or 

 even in rooms lighted only from one side, give out carbonic 

 acid instead of oxygen. 



At all hours of the day and night plants give off car- 

 bonic acid, but the quantity is so minute that in the sun- 

 light it almost escapes notice, whereas by night, or in 

 the shade, they give off carbonic acid only. Even then, 



