108 E. J. LUND. 



may be due to non-uniformity of manipulation and different 

 CC>2 content of the air in the room. The greatest source of error 

 comes from the CC>2 in the expired air of the experimenter and 

 from burning flames in the room. 



2. If the organisms are in water in the dish, Fig. I, d, then the 

 CC>2 dissolved in this water either as dissolved free CO2 or pos- 

 sibly partly in the form of bicarbonate, may vary unless pre- 

 cautions are taken to control this source of variation of CO 2 . 



3. Another possible source of variation is the excretion of CO 2 

 by bacteria in the liquid in the dish, if this is for example a hay 

 infusion containing Paramecium or from some other source 

 depending upon the material experimented upon. This source 

 of error is present and must be controlled in all other methods 

 for CC>2 determination as well as in the one described here. 



4. Finally perhaps the most important question is: how uni- 

 form, rapid and complete is the absorption of the CC>2 from the 

 air in the bottle, by a w/75 to 72/150 Ba(OH) 2 solution? 



Repeated tests have shown that all of the above four sources 

 of error can readily be either eliminated or controlled if the fol- 

 lowing precautions are taken. 



The first source of error mentioned under number one above 

 can be controlled and thus practically eliminated by working in a 

 well-ventilated room in front of an open window through which 

 fresh air from the out doors is entering, and by covering the mouth 

 of the bottle with a perforated cardboard disk during the addition of 

 the Ba(OH}* at the beginning, and titration with HCl at the end of 

 the experiment. It is best to aerate the open bottle by holding or 

 shaking it in the stream of air from the window just before cover- 

 ing it with cardboard disk and the rapid addition of the required 

 amount of Ba(OH)2 from the burette. The pasteboard disk 

 should be held tightly over the bottle during all the manipulation 

 and only removed just before the stopper with the small dish is 

 inserted. A similar procedure must be carried out at the end of 

 the experiment during titration of the remaining free Ba(OH)2 

 with HCl. 



The current of air from the window carries the exhaled air of 

 the experimenter away from the bottles and makes the CC>2 

 content of the air the same in the duplicate bottles. 



