no 



E. J. LUND. 



The above illustrates the following facts: (a) The manipula- 

 tion described above permits of uniform titration of the free 

 Ba(OH) 2 . (6) The amounts of CO 2 absorbed in duplicate blank 

 bottles after allowing time for absorption, are the same, (c) 

 The CO 2 absorbed from the outdoor air during manipulation 

 and from the volume of air enclosed in the bottle may vary from 

 day to day with different conditions but is uniform over brief 

 periods of time. It is therefore clear that whenever necessary, 

 proper and adequate controls to determine this amount of CO 2 

 can readily be provided in experiments. 



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

 source of variation must be taken into account, viz., possible 

 differences in amounts of CO 2 in the water in the dish. This 

 may be made uniform by taking equal, and as small volumes as 

 possible, of the medium in which the organisms live, and still 

 further reduced to small and uniform amount by using tap water 

 as a medium as for example with Paramecium, Lund ('18). 

 That the variation exists and is quite definite is indicated in the 

 following Table I. 



TABLE I. 



The native medium was tap water in which Paramecia had been living for several 

 hours and from which they were removed with the centrifuge. Twenty c.c. Ba- 

 (OH)2 added to each bottle. 



Volume of Native Medium in each Dish. 



With air breathing organisms this source of variation is of 

 course absent. Escape of such organisms from the dish may be 

 prevented by a wire gauze cap placed over the small dish. 



In order to determine the rapidity and completeness of CO 2 

 absorption by the Ba(OH) 2 , simple and critical tests were carried 



