MIXTURES OF ELECTROLYTES. McINTOSH. 123 



Temperature. 



As the laboratory temperature varied considerably from day 

 to day, the electrolytic cell was placed in a bath whose tempera- 

 ture was regulated by a thermostat of the kind described by 

 Ostwald in his Physico-chemical Measurements, p. 59. The 

 resistances measured were so small that a sharp " minimum " was 

 obtained when a water bath was used. There was no necessity 

 therefore, for a petroleum bath. The bath was stirred by a 

 current of air from a small suction pump, and the temperature 

 kept as near to 18C as possible. When this temperature could 

 not be exactly obtained, measurements of resistance were made 

 at several near temperatures, and the temperature co-efficients 

 found. The co-efficient was always about 2 per cent, per degree. 



The thermometer used was graduated to 0.1 degree centri- 

 grade, and could be easily read to 0.05 degree. This meant a 

 possible error of 0.1 per cent, in the determination of the 

 resistance. The errors of the thermometer had recently been 

 determined at the Physikalisch-Technische Reichsanstalt, Berlin. 



The Platinizing of the Electrodes. 



The electrodes after having been boiled in alkali and acid, 

 were placed in a very dilute solution of chloroplatinic acid 

 (H 2 Pt C1 ) and connected with a small battery, the direction of 

 the current being frequently changed. When the electrodes had 

 become covered with a black velvety coating, they were removed 

 from the cell, and in order to get rid of the chloroplatinic acid 

 which adheres strongly to the platinum black, they were washed 

 several times with boiling water. On one occasion, in the course 

 of the experiments, the minimum point was found to be indistinct 

 The plates were accordingly replatinized and distinctness found 

 to have been regained. The experiments previously made (those 

 on potassium chloride) may have been affected by a slight error 

 due to defective platinizing. 



The Salts and Acids. 



The potassium and sodium chlorides, obtained as chemically 

 pure from Eirner and Amend of New York, were further purified 



