BANCROFT, — POTENTIAL OF METALS. 99 



by cutting off a piece ; but experiment shows that this change passes 

 away more quickly than that caused by rubbing. In order to obtain 

 accurate figures the electrodes should not be jarred. In some experi- 

 ments the electrodes can be moved about without any very notice- 

 able effect ; but in most cases the slightest tremor has a very great 

 influeuce on the electromotive force. As a rule, the electromotive 

 force of this type of cell increases on standing, more or less quickly, 

 till it reaches a definite maximum, where it remains constant. It is this 

 maximum which I have given in the tables, because it is always the 

 same in different experiments, while the value first observed is very 

 irregular. In two cases only have I done differently. With magne- 

 sium one can get almost any desired value, according to the way the 

 measurement is made. Immediately after setting up, the value for 

 MglZn is about 0.56 volt. If the magnesium is left in the solution 

 and the liquid stirred slightly, the value becomes 0.713 volt. If the 

 liquid be stirred vigorously, the reading rises to 0.76 volt, and even 

 higher. In the last two cases there is a violent evolution of hydrogen. 

 I have taken the first value as the most probable, and consider it as 

 possibly too high. In the literature the recorded figures for MglZn 

 lie between 0.53 and 0.80 volt. The reason for this discrejiancy is, 

 as I have just said, in the varying conditions. With lead it is a little 

 different. One gets at first a definite value, which remains constant 

 quite a while, and then increases slowly to a maximum which I have 

 not determined. As this " stationary " value is an easy one to deter- 

 mine, I have measured it ; it evidently corresponds to a well defined 

 condition, and is therefore just as good for most purposes of compari- 

 son as the highest value. The probable error of the measurements is 

 not over 0.01 volt in most cases, and has no effect on the general 

 relations. 



I now come to the experimental data. The concentrations are 

 given in chemical units (gram molecules per litre). The single values 

 are the averages of five to twenty observations. Table II. shows the 

 effect of the concentration, Table III. the effect of the negative ion 

 when the electrodes are any two of the metals Mg, Zn, Sn, Pb, or Bi. 

 The measurements of ZnlBi and CdIBi had one marked peculiarity. 

 The electromotive force CdIBi was at first about 0.315 volt. This 

 potential difference increased slowly but regularly till it reached the 

 value given in the table. In all the cases which I have studied Zn|Bi 

 and CdIBi are the only ones in which there were signs of any rational 

 connection between the change of the electromotive force and the time. 

 I wished to include aluminium in the list ; but the purest Neuliausen 



