W. J. V. OSTERHOUT 21 



studied by means of the apparatus here described. In this case A is 

 partly filled^* with liquid/^ and due allowance must be made for this in 

 calibrating the apparatus with respect to absolute amounts of CO2 

 required to produce a given change of pH value. This calibration 

 must be revised if any reagent is added which has a pronounced 

 buffer action; this should also be done when A is filled with gas if 

 any volatile substance with a pronounced buffer effect (or with an acid 

 or alkaline reaction) is used. 



In general it would seem to be desirable to have A large enough to 

 contain a number of organisms, so as to reduce as much as possible the 

 irregularities which may occur when small numbers are used. But 

 if the amount of CO2 given off is small, it may be necessary to reduce 

 the size of A by substituting a U-tube for the bottle. The size of F 

 may also be reduced by substituting a smaller bulb, or by using in 

 place of a bulb a tambour provided with valves. 



Whenever it becomes desirable to sweep out the exhaled CO2 and 

 to fill the apparatus with ordinary air, this can be easily and quickly 

 accomplished, without disturbing the organism. It is only necessary 

 to remove the U-tube and to close the screw clamp, D, (which is usu- 

 ally open) and open the screw clamps, C and E, (which are ordinarily 

 closed) and then to start the motor and allow the gas to circulate, 

 passing out of the apparatus at E. Air will then enter at C and will 

 quickly wash out the excess of CO2. When this is accomphshed (as 

 shown by the color of the indicator), C and E are closed and D is 

 opened. 



Ordinarily it will be found desirable to remove the excess of CO2 at 

 frequent intervals. In this case phenolsulfonephthalein will be found 

 useful as an indicator.' If, however, COo is allowed to accumulate, 

 other indicators or mixtures of two or more indicators may be used 

 to measure the lower pH values. 



^'* Except in cases where the organisms respire normally when merely moistened 

 and kept in a saturated atmosphere. Ordinarily the air will become partly satu- 

 rated with the moisture taken up from the indicator solution. If this is not 

 desirable, in the case of non-aquatic organisms, it can be obviated by adding 

 something to the indicator solution to lower vapor tension or by drying the gas on 

 its way from B to A. 



^^ The tube by which the gas enters A is drawn to a point and brought down to 

 the bottom of A to permit the gas to bubble through any liquid contained in A. 



