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THE AMERICAN NATURALIST [Vol. LIV 



Compared with that of the standards. It is obvious that since 

 the indicator solution is the same in each case and the starting 

 point is the same, the same color indicates the same amount of 

 CO, added. It is only necessary, from the relative volumes of 

 water and air in the apparatus and the absorbtion coefficient 

 under the conditions of the experiment, to calculate the total 

 amount of C0 2 produced by the organism to the time of the ob- 

 servation. Since the absorption of the C0 2 by the solution lags 

 a little behind its production, it is well not to consider the time 

 from the starting point to the first tube; from the first to the 

 second, however, and the second to the third, etc., this factor is 

 approximately the same in each case and therefore does not ap- 

 preciably affect the results. 



As to the delicacy of the method, the indicator solution and 

 the air in the tube, if the temperature is 16° C. and the absorp- 

 tion coefficient therefore equal to 1, each contain 0.0006 mg. of 

 C0 2 per c.c. A ten per cent, increase causes a slight visible 

 change in the color of the indicator solution, so a production of 

 0.0003 mg. in the 5 c.c. tube ought theoretically to be detectable. 

 In practise, however, it is desirable to work with somewhat larger 

 amounts, e.g., 0.001 mg. or more. In measuring the time, it is 

 well instead of trying to determine the point at which the two 

 tubes exactly match to take the average between the last observa- 

 tion where the unknown tube is pinker and the first where it is 

 yellower than the comparison tube. To meet a possible objec- 

 tion, it may be said that when the carbon dioxide has increased 

 one hundred per cent., the oxygen in the tube has decreased only 

 approximately 0.2 percent.: consequently changes in the amounts 

 of oxygen available for the organism during an ordinary experi- 

 ment are hardly significant. 



In conclusion, it may be stated that the method has been 

 tested by comparing it with that of Lund 3 on the same organism 

 (a firefly) and the differences obtained were only of the order of 

 magnitude observed where two successive observations were 

 made on the same individual by the latter method alone; that is, 

 within the limits of uncontrollable normal variations of the 

 species in question. 



M. H. Jacobs 



