STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



29 



bonate allowed to settle out completely. When the supernatant 

 liquid was perfectly clear, this vessel was placed in the water ther- 

 mostat kept at 25°. The stopper was replaced by a dip-conductivity 

 arrangement, as shown in figure 5. The electrodes are essentially 

 the same as those used in the Cain method^ for the electrometric 

 determination of carbon in steel. A Leeds & Northrup combination 

 bridge and galvanometer operating on a 110-volt A. C. 60-cycle 

 current gave highly satisfactory results. 

 For the successful operation of this 

 method a number of jfacts had to be 

 determined which were not available in 

 satisfactory form in the chemical liter- 

 ature. It was necessary to choose a 

 barium-hydroxide solution of a con- 

 centration such as would completely 

 absorb the carbon dioxid in the air- 

 stream at the maximum rate emitted 

 by the leaves and for the longest period 

 of observation. Such a solution should 

 also show the maximum change of re- 

 sistance for a given change in concentra- 

 tion. After preliminary experiments in 

 which these limits were established, a 

 solution was decided upon with an initial 

 concentration of 0.12 normal, which 

 could wdth safety be decreased, through 

 the precipitation of barium carbon- 



FlGUKE 5. 



Conductivity cell used to determine the strength of 

 the barium-hydroxide solution. The barium carbon- 

 ate is allowed to settle and the resistance of the clear 

 supernatant solution is determined at 25°. 



ate, to 0.05 normal. It was necessary then to obtain experimental 

 data from which a curve of specific resistance could be drawn for 

 definite concentrations of the barium-hydroxide solution. The 

 concentrations of the solutions used for these determinations were 

 obtained by means of titration with hydrochloric acid, and the 

 resistances were determined at 25° by means of the cell shown in 

 figure 5. The values thus obtained are given in table 7. Fortu- 

 nately there were available two determinations of the specific con- 

 ductivity of barium hydroxide by A. A. Noyes^ at concentrations 

 very close to both extremes of the concentrations used by us. Cal- 



1 Cain, J. R., and L., C. Maxwell. Jour. Ind. and Eng. Chem., 11, 852 (1919). 



2 NoYEs, A. A. Tables annuellea constantes et donnees num^riques de chimie, de physique et 



de technologie, vol. i, p. 463 (1912). 



