20 METHOD OF STUDYING RESPIRATION 



the original color. This is repeated until the normal time of respira- 

 tion is established (unless it is fairly constant the experiment should 

 be rejected). 



The reagent is now introduced into A (by means of the short tube) 

 and the time is ascertained which is required to produce the same 

 change in the color of the indicator. A comparison of this with the 

 normal time gives the relative rate of respiration under the influence 

 of the reagent. 



The amount of CO2 produced by the organisms is ascertained by 

 comparing' ° the color of the indicator with the colors of a series of 

 buffer solutions^' having the same concentration of indicator and 

 contained in Pyrex glass tubes of the same size as B. This determines 

 the pH value of the solution. In order to find out how much CO2 must 

 be evolved to produce a given change in the color of the indicator we 

 introduce known amounts of CO2 into the apparatus^- by means of a 

 device which has recently been described.'^ The gas is made to cir- 

 culate in the apparatus until the introduced CO2 is thoroughly mixed 

 with the air and with the indicator solution, so that equilibrium is 

 established between the latter and the CO2 in the apparatus. We 

 know that this has occurred when continued circulation fails to pro- 

 duce any further change in the color of the indicator. 



Instead of introducing gaseous CO2 we may introduce a solution 

 whose C02-content is known from its pH value. 



The change in the color of the indicator produced by a given amount 

 of CO2 will vary according to the volume of air in the apparatus (the 

 amount of indicator solution being constant). When determinations 

 have been made for a number of volumes, intermediate values may be 

 obtained by interpolation. As a rule there will be no changes in 

 volume except those produced by variations in the volume of the 

 organisms which are introduced into the apparatus. 



Ordinarily the respiration of aquatic organisms may be studied by 

 the method described by Haas^ but such organisms as impart a color 

 to the solution or give off acid (other than COo) or alkali may be 



^"This should be done, if possible, under a "Daylight" lamp. 

 " Cf. Osterhout, W. J. V., and Haas, A. R. C, Science, 1918, xlvii, 420. 

 ^^ This can be done by means of the short tube in the stopper of A. 

 "Osterhout, W. J. V., /. Biol. Chem., 1918, xxxv, 237. 



