OF ZINC, CADMIUM, LEAD, COPPER, AND LITHIUM 51 



pressed in percentage, and not in parts per unit of mercury, as it is in the 

 original paper. 39 



The results obtained by Spencer were not so consistent with theory, 

 but, as will be seen, are more like our own experience. He found great 

 difficulty in getting constant readings of potential. His results are given 

 in table 13 ; it will be observed that he used far more dilute amalgams. 



TABLE 13. Potentials of Copper Amalgams measured by Spencer. 



It will be noticed that at first the actual potential is larger than theory, 

 and afterwards smaller. The reason will soon become clear. 



The next step of the present research was to repeat these experiments 

 in order to discover the difficulty. Commercial " C. P." copper sulphate 

 was carefully recrystallized three times with centrifugal filtration, and the 

 resulting copper salt was used in the experiments. A copper amalgam 

 was then made by electrolysis, using mercury as the cathode, the amount 

 of copper deposited being estimated by means of a silver coulometer in 

 series. The amalgam was found to contain 0.2311 per cent of copper. On 

 drawing this amalgam into the pipette, preparatory to its being sealed in 

 hydrogen, a pasty residue was left which would not enter the fine tip of 

 the pipette. Hence it was clear that the above amalgam was not a solu- 

 tion, but rather a suspension of copper or of some copper-mercury com- 

 pound in mercury. The amalgam which had been drawn into the pipette 

 was used to fill a cell in the ordinary manner. 



This cell proved two important points : first that neutral copper sulphate 

 could not be used as electrolyte, because the amalgam acted on it, giving 

 Cu 2 O ; and secondly, that a very dilute amalgam, made by diluting the 

 original sixteen times, gave only an exceedingly small potential with the 

 original amalgam. Thus it appeared that the diluted amalgam was still 

 saturated and there could be no doubt that crystals of the solid had not 

 all been left behind in the pasty mass mentioned above. 



It then became necessary to find the solubility of copper in mercury. 

 Saturated solutions of copper in mercury were made either by allowing 

 amalgamated copper wire to stand in mercury for a week, or by carefully 

 filtering a partially solid amalgam, prepared electrolytically, several times 

 through leather. 



Zeit. phys. Chem., 7, 477. 



