14 AIR AND SUNLIGHT 



now present in the air. However, there is a never failing supply 

 of carbon dioxide in the atmosphere, coming from the processes of 

 combustion, decay, and fermentation. Another source is found in 

 the respiration of animals, since they breathe in oxygen and give off 

 carbon dioxide. In some localities large quantities of carbon di- 

 oxide are given off from mineral springs and volcanoes. This gas is 

 also found in the pores of the soil, especially in regions where the 

 soil is of limestone origin. The acids of the soils coming in contact 

 with the limestone or calcium carbonate decompose this substance 

 and liberate carbon dioxide. An illustration of this may be seen 

 when a little hydrochloric acid is poured on some small pieces of 

 marble. There is a brisk evolution of gas which on testing we 

 find to be carbon dioxide. Its presence may be proved by passing 

 the gas through clear limewater, which it soon clouds and makes a 

 milky white. Its presence in the air we exhale may be detected 

 in the same way. Carbon dioxide also has the property of ex- 

 tinguishing flames by shutting off the supply of oxygen necessary 

 for combustion. 



Since there are so many courses of carbon dioxide, there is no 

 danger of the supply ever becoming exhausted. There is more than 

 enough present to meet all demands of vegetation, and there would 

 be no advantage gained by increasing the amount usually found in 

 the atmosphere. On account of the principle' known as diffusion 

 in gases and the stirring action. of winds the proportion of carbon 

 dioxide in the air is kept remarkably constant. The ventilating 

 power of the wind can scarcely be appreciated by any one who has 

 not given any thought to the matter. Professor Storer of Harvard 

 University says, " Air moving no faster than two miles an hour, 

 which is almost imperceptible, if allowed to pass freely through an 

 open shed, will change the air of the place 528 times in an hour. 

 Hence, having regard to their respective requirements, carbon 

 dioxide is, to all intents and purposes, supplied as freely to plants 

 by the air as oxygen is supplied to animals." 



The carbon in the atmosphere can be assimilated by plants only 

 in the presence of light and through the chlorophyll or green color- 

 ing matter in their leaves. In the absence of light plants exhale 

 carbon dioxide instead of oxygen. According to Professor Storer, 

 plants exposed to the dull light of a cloudy day will sometimes 



