84 BOTANICAL GAZETTE [august 



shown in fig. 2c. In this U-cell the minimum obtained with the 

 telephone was distinct, the amount of resistance inserted in the 

 resistance-box varying from 500 to 2,000 ohms. The diameter 

 of the tube was 6"^™, and 5^*^ of solution was amply sufficient to 

 cover the electrodes. 



In order to extract a small amount of juice from the roots, 

 stems, or leaves of plants a special form of press was necessary. 

 This press and the method of obtaining the extract have been 

 previously described.^ 



METHOD. 



The capacity of the electrolytic cell must first be determined. 

 This is done by introducing in the cell a solution of known spe- 

 cific conductivity. For this purpose a ^^ normal solution of 

 potassium chloride was used, and the thermostat was kept at a 

 temperature of 25° C. This was the temperature at which all 

 subsequent measurements were made, the uniform temperature 

 being necessary, since the conductivity of electrolytes varies 

 with the temperature. After closing the circuit and starting the 

 coil, the movable contact {Jig, /, d) is pushed along the gradu- 

 ated resistance ab until the minimum for the telephone is found. 

 The resistances are then in the foUowinof ratio : 



R \ bd \\ s : ad . 



For the resistance, 



R • ad 



s 



bd ' 



and for the conductivity, 



I bd 



s R ' ad ' 



The solution used was one of known specific conductivity /, so 

 we have the following relation : 



, , bd , , ad ' R 



ad ' R bd 



m 



I := 0.002765,7 and from the readings made the value of /^, or 



■^Jour, Appl. Micr. and Lab. Methods 5: 1679. March 1902. 



7 LeBlanc, Electrochemie (2te Auflage), p. 77, gives the specific conductivity of 

 -J^ normal KC! at 25'' C. as 0.002765. 



