METHOD FOR MEASURING CARBON DIOXIDE. 



107 



the cardboard disk. A drop of phenolphthalein is added through 

 the hole in the cardboard disk and then the Ba(OH) 2 is rapidly 

 titrated with a weak solution of HC1 of known and appropriate 

 concentration, say n/ioo. As control a similar bottle is set up 

 in exactly the same way but without the animals. The Ba(OH)2 

 is titrated at the end of the experiment, that is, at the same time 

 as the free Ba(OH)2 in the experimental bottle. The difference 

 in the amounts of HC1 necessary to neutralize the remaining 

 free base in the bottles is equivalent to the quantity of CO 2 pro- 



FlG. 2. 



duced by the organisms during the period of the experiment. 

 This description explains in brief the principle and procedure of 

 the method, but the following sources of variation and error 

 suggest themselves and will be considered in the same order as 

 follows : 



I. Temporary exposure to the CO 2 in the air of the open bottle 

 containing the Ba(OH) 2 may result in absorption of varying 

 amounts of CO 2 in different individual bottles. This variation 



