PLANTS AND THE ATMOSPHERE. 79 



carbon, forming the gas carbon dioxide and 

 the substance has then, we usually say, 

 * burnt away/ 



3. The presence of carbon dioxide can 

 readily be made visible by taking advantage 

 of the property which it possesses of combin- 

 ing 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. 



1. 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 containing carbon, it follows that 

 carbon dioxide is continually being added to 

 the air in large quantities. But carbon diox- 

 ide, when present to a certain degree, is in- 

 jurious to life ; it follows therefore that there 

 must be some agency at work whereby its 

 accumulation in the air is prevented, or all 



