8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8/ 



being bent up so that the end, M, is shghtly higher than the electrode, 

 H. The cell is a few centimeters of small-bore glass tubing bent up- 

 ward at the two ends to prevent bubbles of gas collecting on the 

 walls. The electrodes, H and K, are made of platinized platinum. 



As the sensitivity required for this work was greater than that for 

 which the apparatus was originally developed it was necessary to 

 improve the temperature control and eliminate to a greater degree the 

 effects of polarization. For temperature control a special thermostat 

 was developed, whose bulb was a cylinder of annular section. This 

 has the advantage of securing a maximum of exposed surface with a 

 minimum of heat capacity. This thermostat was of the liquid ex- 

 pansion type, using carbon tetrachloride as the working material. The 

 temperature fluctuation was limited to ±0.005° C. Accurate control 

 of the temperature is necessary, since the conductivity of the cell 

 changes rapidly with temperature. 



The conductivity of the solution was determined by making the cell 

 one arm of a Wheatstone's bridge. To reduce the amount of polariza- 

 tion when using direct current on the bridge the direction of the cur- 

 rent through the cell was reversed several times a second by a special 

 commutator running in oil. There were also contacts on the commu- 

 tator to short-circuit the galvanometer during the time of reversal. 

 A'ery consistent results were obtained in this manner when the speed 

 of the commutator was held constant. 



For obtaining a zero point means were provided to recirculate the 

 air inside the apparatus by connecting the exhaust with the intake. 

 Thus in a short time the carbon dioxide was completely absorbed from 

 the recirculated air and the conductivity cell filled with standard KOH 

 solution. 



Since small concentrations of carbon dioxide were used, a calibra- 

 tion of the apparatus was determined from the available conductance 

 data and a measurement of the volume of air associated with each 

 drop of solution. At a pressure of 760 mm and a temperature of 

 25° C. the volume of air is 3.125 cc and the volume of each drop is 

 .0991 cc. The volume of carbon dioxide under the above conditions, 

 necessary to neutralize .0991 cc of .02N KOH, is .02444 cc. Thus 

 .02444^3.125 gives .782 per cent, the concentration of carbon dioxide 

 in the air necessary to neutralize the KOH solution and fill the cell 

 with -JKaCO.,. From the conductance data we find the equivalent 

 conductance of .02N KOH and ^KoCOa to be 225 and 109.2 re- 

 spectively. The resistance of the cell full of KOH is 26450 ohms, then 

 the resistance of the cell full of -IKsCOs is 225/109.2x26450 or 

 54513 ohms. For zero concentration of carbon dioxide the conduc- 



