86 BOTANY 



oxygen, which we have seen to be associated with the 

 manufacture of sugar. It can, however, be detected by 

 experiments. To prove the absorption of oxygen, take 

 a flask and fit it with an india-rubber stopper through 

 which a glass tube bent twice at right angles can be 

 passed; let the end of the tube dip into mercury in a 

 small vessel standing side by side with the flask. Fill 

 the flask with healthy leaves. In addition to the leaves 

 place in the flask a test tube containing a solution of 

 caustic potash and cork it up so that the outlet tube 

 dips into the mercury. Keep it at a constant tempera- 

 ture for some hours. The mercury will rise gradually 

 into the tube, and will continue to do so for some time, 

 so showing that a diminution of the volume of the air in 

 the flask has taken place. Analysis of the air then left 

 in the flask will show that the volume of nitrogen is un- 

 changed, while that of oxygen has diminished. Exami- 

 nation of the caustic potash will show that it has 

 absorbed an amount of carbon dioxide. 



The reason for employing the potash is that the 

 carbon dioxide which is exhaled in respiration is about 

 equal in volume to the oxygen absorbed, so that unless 

 it is removed the diminution of the volume of the 

 oxygen will be compensated for by the addition of the 

 carbon dioxide and the mercury will not rise in the tube. 



We can show the exhalation of carbon dioxide by the 

 living plant by means of another experiment. Place a 

 number of leaves in a flask through the stopper of which 

 air can be made to enter by one glass tube and can be 

 removed by another. Make the air as it enters pass 

 through a bottle containing a solution of caustic potash 

 which will free it from all traces of carbon dioxide. 

 Pump the air through the flask by means of some form 

 of air pump or aspirator and lead it through a bottle of 

 lime-water. It will soon make the lime-water milky in 

 consequence of the formation of carbonate of calcium, 



