120 Studies on Solution. 



rheostat, bridge, and cell. Thus, by means of a rocking commu- 

 tator with mercury contacts, the positions of rheostat and cell relative 

 to the bridge could be interchanged so that both a and b could be read 

 directly. All copper wire in the external circuit was of such a gage 

 that the resistance was negligible. All connections were soldered. 



Cells. Because of the high resistance of the water used a special 

 "water cell" 1 having large electrodes was necessary. The electrodes 

 consist of two concentric platinum cylinders held in position by small 

 drops of fusion glass in such a manner that they are about 1 mm. apart . 



Because of the large conductivity of the solutions measured a differ- 

 ent type of cell was demanded. The cell finally adopted for this work 

 was that which has been previously used in this laboratory 2 for measure- 

 ments of the conductivity of concentrated solutions. It consisted of 

 a U-shaped tube made of difficultly soluble glass and fitted with ground- 

 g ass stoppers. A glass tube carrying a small platinum electrode is 

 sealed by means of sealing-wax into the hole bored in the center of each 

 stopper. The tubes were first held in position by tamping wet asbestos 

 between them and the walls of the stoppers. The distance between the 

 electrodes can be changed by removing the wax, adjusting the tubes, 

 and resealing. The platinum plates are coated with platinum black. 

 Numbers are etched upon the stoppers and the corresponding arms of 

 the U-tubes, so that the electrodes will always be placed in tha same 

 U-tube and in the same position. 



Constant Temperature Bath. A constant temperature was main- 

 tained by the application of a principle most clearly stated by Morse : s 



"If all the water or air in a bath is made to pass rapidly (1) over a continu- 

 ously cooled surface which is capable of reducing the temperature slightly 

 below that which it is desired to maintain, then (2) over a heated surface which 

 is more efficient than the cooled one but which is under the control of a thermo- 

 stat, and (3) again over the cooled surface, etc., it should be practicable to 

 maintain in the bath any temperature for which the thermostat is set, and 

 the constancy of the temperature should depend only on the sensitiveness of 

 the thermostat and the rate of flow of the water or air." 



The bath used is fully described by Davis and Putnam. 4 By means 

 of an improved toluene-mercury thermo-regulator 5 and an electrically 

 controlled gas valve 6 the temperature was maintained constant to 

 within 0.01. 



A Beckmann thermometer graduated to 0.05 was used in the bath. 

 Comparison was made with a thermometer recently standardized at 

 the Bureau of Standards, Washington. 



Glassware. Measuring flasks, burettes, and pipettes were recali- 

 brated by direct weighing. Jena glass bottles were used for keeping 

 the solutions. 



Carnegie Inst. Wash. Pub. No. 180, 89 (1913); Amer. Chem. Journ., 45, 282 (1911). 



2 Zeit. physik. Chem., 49, 389 (1904). "Carnegie Inst. Wash. Pub. No. 210, 119(1915). 



'Carnegie Inst. Wash. Pub. No. 198,50(1914). *>Ibid.,230, 13 (1915). "Ibid., 210, 121 (1915). 



