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MANUAL OF NATURE STUDY. 
united in the way mentioned above, are together 
called carbon dioxide and when dissolved in water it 
is called carbonic acid. It is carbonic acid that es¬ 
capes from our lungs. We have no more use for this 
acid gas, and hence throw it out with our breath. The 
plants are glad to take it up. This they do as just 
stated, by breathing it through the stomata of their 
leaves. The plant-substance factory is not in 
operation at night, so carbonic acid gas is allowed 
to escape to the atmosphere until morning. 
Throughout the day, if the factory is running at 
full speed, carbon dioxide is separated as rapidly as 
it arrives, the oxygen returning to the air for more 
carbon, securing which, it makes another visit to 
the factory; and thus the process goes on through¬ 
out the entire plant growth. 
The vapor that we collected on the inner surface 
of the tumbler in our experiment with the 
green leaves a while ago was given off by the 
leaves through the stomata. This process of 
giving off vapor is called transpiration. Again 
experiment with the plant a few days by putting 
some in a dark place, others in a room of medium 
light, and others in a strong light for the same 
length of time. Note the difference in color and 
in growth. Account for the difference. Take 
another body of plants and separate it into groups 
