RHEOTAXIS OF ISOPODS 203 



which will give the first precipitate of barium carbonate when 

 introduced into a chamber in which a perfectly clear drop of 

 barium hydroxide is exposed. It was previously found by work 

 with known amounts of very dilute carbon dioxide that the 

 minimum amount of carbon dioxide which gives the first pre- 

 cipitate is 0.000,000,1. gram. Thus, by determining the mini- 

 mum number of cubic centimeters of a gas from a respiratory 

 chamber of known volume, we can calculate very accurately 

 the amount of carbon dioxide given off by the tissue or animal 

 under observation. 



The details of the method are as follows: An animal is left 

 in the respiratory chamber (15 cc. capacity) for a respiration 

 period of ten minutes. Then this air is driven into a gas pipette. 

 After cleaning and washing the apparatus one cubic centimeter of 

 this gas is introduced into the barium hydroxide chamber but gives 

 no precipitate; .25 cc. more is introduced with no result; .25 cc. 

 more gives a precipitate. The total volume introduced, 1.5 cc. 

 is the minimum volume that will give the first precipitate. 

 Since 0.000,000,1 gm. of carbon dioxide is the minimum amount 

 which gives the first precipitate, it is certain that 1.5 cc. of 

 respired air must contain 0.000,000,1 gm. of carbon dioxide. 



15 



The total respiratory chamber must have contained ■ x 



0.000,000,1 or 10 x 10 -7 grams carbon dioxide. 



Not all of the work to be reported in this paper was done 

 in this detailed quantitative manner since in some cases only 

 comparative results were needed. For the comparative work a 

 piece of apparatus called a Biometer> (Tashiro, '13.) was used. 

 This consists essentially of two chambers of equal size which 

 are prepared for a determination in exactly the same manner 

 and the animals to be tested for relative carbon dioxide pro- 

 duction are inserted. At the start each chamber contains a 

 perfectly clear drop of barium hydroxide. The chamber which 

 first shows a precipitate of barium carbonate and which later 

 shows more precipitate evidently has had carbon dioxide pro- 

 duced at a higher rate than the other chamber. Hence it is 

 easy to find which of two isopods is producing the greater amount 

 of carbon dioxide. 



' Quantitative determinations can also be made with the Biometer by using 

 one chamber as a respiration chamber and the other for the determination. 



