406 



eqiiilibriiim state and the dissolved Mg(OH), lias still retained its 

 aietallic properties. Still its pro|)erties have altered, for tlie precipitated 

 iiiercnry is not able to activate it at once. x\n a|)parent explanation, 

 namely that the precipitated nieronry does not dissolve in the 

 naagnesinm, is not correct. 



For if we remove the magnesium electrode covered with fine 

 mercury, prepared as above, wash it with distilled water and than 

 dip it into pure MgSO, solution, the potential is at first less 

 negative, but it becomes increasingly negative, so that after a few 

 minutes it is stronger than that which attained before the negative 

 electrode was coated with mercury. 



This is shown in the following table 



Solution 



Mg-potential in relation to a 

 1-Ncalomel electrode 



0,1 gr. mol. MgS04 p. liter 



to 150 ccm. of the above men- 

 tioned solution is added 5 ccm. of 

 a saturated solution of HgClj 



- 1.902 V. 



1.740 V. 



The magnesium electrode was then washed with distilled water and dipped 

 into a pure solution of MgS04. 



1 gr. mol. MgSOj p. Liter 



— 1.898 V. 



— 1.956 V. after 5 minutes 



The above data show that I he magnesium electrode, though its 

 surface is strongly disturbed by corrosion, has dissolved some mercury. 



When we consider the great change brought about in a magnesium 

 electrode by corrosion it is no wonder that its other properties, 

 such as the power of dissolving mercury, are modified. 



The explanation of the results in the above table now is clear. 

 The activating influence e.xerted by the small quantity of dissolved 

 mercury is not sufficient to decrease the retardation, exercised by 

 the Mg(OH), in such a wa3% that the electrode becomes insensible 

 to the corrosive action of water and sublimate. When this solution 

 has been substiluled by one of pure MgSO, the influence of the 

 sublimate disappears and that of the small quantity of mercury 

 becomes manifest. 



It might be supposed at first sight that in the experiment under 

 consideration solid MgvOH), depositing on the magnesium electrode 

 might diminish the contact between the nuignesium and the mercury, 



