LABORATORY WORK 185 



of rubber tubing on the outer end in order to place a spring clip on 

 it after the last expired air has entered. 



Fill the tube C with mercury by raising the reservoir and open- 

 ino- the tap slowly. Any mercury that escapes is allowed to fall 

 into a cup and replaced in the reservoir. Holding the bladder 

 under the arm, press it gently while the clip is open and slip the 

 rubber tube on the tube C., the three-way tap being closed. Open 

 the latter and draw a sample of the gas into the measuring tube as 

 before Read its volume, absorb the carbon dioxide and the oxygen 

 in turn It will be found that there is an increase in carbon dioxide 

 a decrease in oxygen. Thus the combustion processes in the animal 

 body have consumed oxygen and replaced it by carbon dioxide 



We require now to allow this expired air to be subjected to the 

 action of a green plant in sunlight. We may take a sma 1 plant of 

 mint in a small flower-pot, (Mint was used by Priestley 1 in his 

 classical experiment) Place this pot in the middle of a shallow 

 earthenware tray and cover it with a bell jar which has an opening 

 at the top closed 7 with a rubber stopper through which a short glass 

 tube passes A piece of rubber tube with a pmchcock is. fitted on 



he glass tube. Suck up water from the dish until the plant is 



mmlrsed and the jar filled, replacing the water in the : tray as it 

 p-oes into the jar. The flower-pot may have muslin tied over the top 

 to prevent the soil being washed out. Close the rubber tube by a 

 clip Attach the football bladder ^*^ Hy^". P e " 



Jclip and fill the jar with the air, pressing the bladder so as to 

 drive some of the contents out at the bottom of the jar. 



is allowed to run into the sink. Finally pour mercury into 



S ^ It accuracy 6 asample of the gas should be taken out of the 

 jar after it has stood in the dark for an ^***^^^ 

 If a narrow rubber tube be attached to the tube oi the jar, t 

 tl: wiU bJcome filled with the gas by the pressure c mercury 

 when the clip is opened for a moment, and^a P^sample can 





