56 THE FOOD OF PLANTS CHAP. 



of the phosphorus ; but at the same time heat has 

 been given out and has escaped. Chemical changes 

 then never result in loss of mass, but often result in 

 the giving out of heat. 1 



Oxygen set Free by the Plant in Sunlight 



We can now understand what was happening 

 while the charcoal was burning. It was combining 

 with the oxygen of the air. 



The compound thus formed is known as carbonic 

 acid gas, and is formed during the breathing of 

 animals and the burning of animal or vegetable stuff. 



During sunlight, as we have seen, it is absorbed 

 by the plant, the charcoal retained, and the oxygen 

 restored. 



That oxygen is thus formed can be easily shown 

 by the following experiment : 



Take a tall glass cylinder and fill it with tap 

 water (which contains a good deal of carbonic acid 



1 This experiment can also be performed \vitli greater safety by 

 floating a watch-glass in some water in a dish, placing a piece of 

 phosphorus in the watch-glass, removing the stopper from the bell- 

 jar used before, lowering the bell-jar over the floating phosphorus 

 into the water, lighting the phosphorus with a piece of hot wire, 

 and inserting the stopper. The way given in the text is neater, 

 and gives a more accurate measurement of the volume of oxygen, 

 but should only be performed by a skilled manipulator. 



