48 PLANT RESPIRATION 



pressure) has changed. If c is less than 79.1 there has been an 

 increase in the total volume. If c is greater than 79.1 the total 

 volume at the end of the experiment is less than at the start. 

 The increase in total volume signifies that there has taken place 

 a production of CO2 without the corresponding absorption of 

 oxygen. In the opposite case one is justified in supposing that 

 there has been an excess absorption of oxygen. In both cases 

 the value obtained for the oxygen absorbed requires a correction 

 in order that it be directly comparable with the percentage of 

 CO2 found. The original content of oxygen corresponding to 



the nitrogen content c would be equal to 20.9 • Accord- 



20.9 • c 

 ingly the amount of oxygen absorbed is equal to — '- 



and ^ = ' or if we represent — '— by q, the following 



Oo 20.9 , ^ 79.1 



-c — 



79.1 



equation results: 



CO2 a 



O2 cq — b 



The oxygen content of the surrounding air. which varies 

 between 20.80% and 20.96%, is determined by a series of 

 analyses and value of q is calculated accordingly. 



In some experiments one may not be content with the deter- 



mination of ^ " but must also know the absolute amounts of 

 U2 " 



the oxygen absorbed and the CO2 Hberated. For this purpose 

 it is necessary to determine the total volume of the gas mixture 

 in the container as well as the percentages of the single constit- 

 uents. Since a direct cahbration of the research vessel together 

 with the experimental material contained therein is in most 

 cases not feasible, the volume of the inclosed space is determined 

 as follows. Before the experiment is started a gas sample v, 

 measured at atmospheric pressure H in a calibrated eudiometer, 

 is drawn from the flask. The lowering of the pressure in the 

 flask which results from the removal of the test portion of gas 

 is then noted with the aid of the manometer tube. Let // be 

 the gas pressure in the flask before, and //' the pressure after 



