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- 

 ing 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 small plant of 

 mint in a small flower-pot. (Mint was used by Priestley 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 with a rubber stopper through which a short glass 

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

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

 immersed and the jar filled, replacing the water in the tray as it 

 goes 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 containing asphyxial air, open 

 the clip 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. The water 

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

 so as to cover the bottom edge of the jar. 



For accuracy, a sample of the gas should be taken out of the 

 jar after it has stood in the dark for an hour or so, and analysed. 

 If a narrow rubber tube be attached to the tube of the jar, this 

 tube will become filled with the gas by the pressure of the mercury 

 when the clip is opened for a moment, and a pure sample can be 

 drawn into the measuring tube by lowering the reservoir. 



Place the jar in sunlight or bright daylight for a day or more. 

 Possibly the heat of the sun may expand the gas so far as to drive 

 some of it out through the mercury at the bottom, but since this 

 mercury prevents air entering when the jar cools, it does not matter. 



To obtain a pure sample of the final gas mixture, the tube con- 

 necting the jar with the analysis tube must be filled with the gas. 

 This may be done by sucking into the analysis tube 10 or 20 c.c., 

 and then driving the contents out through the funnel B. The tube 

 is then filled to a convenient level by lowering the reservoir again, 

 taking care that more mercury is added to the tray, if necessary. 

 On analysis, the oxygen will be found to have increased, the carbon 



