THE LEAF 77 



visible by taking advantage of the property which it 

 possesses of combining with lime to form chalk. If a 

 solution of lime in water that is, clear lime-water is 

 brought into contact with carbon dioxide, chalk is formed, 

 and, being insoluble in water, becomes at once apparent 

 by the milky or turbid appearance it gives to the 

 water. 



Plants and the Atmosphere. 



On breathing into lime-water it soon becomes 

 cloudy, owing to the carbon dioxide present in our 

 breath. Plants can easily be shown to produce a similar 

 effect. We see, therefore, that both animals and plants 

 breathe out carbon dioxide, and, as it is also formed in 

 the burning of wood, coal, and all other substances con- 

 taining carbon, it follows that carbon dioxide is con- 

 tinually being added to the air in large quantities. But 

 carbon dioxide, when present to a certain degree, is 

 injurious to life, and yet for countless ages the actual 

 amount of carbon dioxide in the atmosphere has not 

 increased. It follows, therefore, that there must be some 

 agency at work whereby its accumulation in the air is 

 prevented, or all life would become impossible. Plants 

 are the means whereby this accumulation is hindered. 

 When carbon dioxide comes in contact with the living 

 substance of the plant, under certain conditions, it is 

 split up into its constituent parts, carbon and oxygen. 

 The carbon is kept by the plant and built up into its 

 tissues, and the oxygen set free. The conditions referred 

 to above are the presence of (i) the green colouring 

 matter (leaf-green or chlorophyll) which gives the 



