866 MESSES. B. T. GLAZEBROOK AND T. C. F1TZPATKICK 



chloride in nitric acid. On immersing the cups in mercury, after heating to drive off 

 traces of the acid, amalgamation readily took place. 



After this process thoroughly good contact between the platinum and the mercury 

 was secured. To test this, the two platinum cups were placed in the same vessel of 

 mercury, and the ends of the copper rods connected with the bridga In this position 

 the resistance of the connexion was measured, and gave the same value before and 

 after the experiments, while no appreciable change could be noticed on taking one of 

 the platinum cups out of the mercury and again replacing it. When only about one- 

 third of each platinum cup was in the mercury, the resistance was increased by about 

 00004 B.A. unit. In use care was taken to place sufficient mercury in the terminals 

 to cover the cups entirely. The following additional experiment shows the goodness of 

 the contact : The platinum cups were placed in the mercury, and the resistance 

 measured as described ; then the copper rods, g, were removed from the interior of the 

 cups and placed in the same vessel of mercury, the other ends of the copper rods 

 being in connexion with the bridge, and the resistance was again measured. No 

 difference between these two measurements could be detected. 



Thus, the contact through the platinum was practically as good as if the copper 

 rods, g, had dipped directly into the mercury in the terminals. 



Tables VII. and VIII. give details as to the mercury employed. The general 

 method of treatment was as follows : Mercury from the ordinary stock in the 

 Laboratory was treated with nitric acid and potash, and then distilled in vacuo in the 

 Laboratory still. Ihis mercury after being once distilled was again mixed with nitric 

 acid, being allowed to stand overnight in contact with it. After this it was heated 

 with caustic potash, and then well washed and dried by being strongly heated, and 

 finally it was passed through a second still, newly set up for the purpose, in which 

 only mercury which had been previously distilled and treated as above was ever placed. 



