214 Mr. R. Sabine en Motions produced by 



The latter, in rich amalgams, are distinctly visible to the eye, 

 producing a somewhat roughened surface. Electric currents 

 arc generated between the mercury and the particles of foreign 

 metal, through the acid. When the foreign metal is positive 

 to mercury, the latter (which by contact with the air is always 

 more or less oxidized) becomes deoxidized underneath the drop 

 of dilute acid and therefore cleaned. The drop has less affi- 

 nity for the clean than it had for the oxidized surface. Its 

 adhesion is therefore diminished ; and it draws itself together 

 in consequence, leaving a surrounding ring of deoxidized 

 mercury, in which the eye can detect few or no floating par- 

 ticles of foreign metal ruffling the smoothness of the surface. 

 Oxidation of this exposed clean portion of mercury, however, 

 gradually sets in; and when the mercury up to the boundary 

 line of the drop is sufficiently reoxidized, the acid reasserts its 

 affinity for the oxide, and the drop is enabled to spread out 

 again to its original dimensions. Interior deoxidation then goes 

 on again by means of the small surface couples ; and the play 

 of alternate contraction by reduced adhesion, due to deoxida- 

 tion, and spreading by affinity for the oxide formed by the 

 outer atmosphere, is kept up so long as positive metal and acid 

 last. 



The contraction due to deoxidation by the current would 

 necessarily be slower than the spreading due to chemical affi- 

 nity between the acid and oxidized surface.' 



In explanation of the acid-drop travelling bodily over the 

 amalgam surface, I would suggest that if, from any reason, the 

 oxidation were more energetic at one side of the drop than at 

 the other, it must lose equilibrium and go on travelling in the 

 direction of the more energetic action, because the mercury 

 surface on that side of the drop would be ready oxidized, whilst 

 on the opposite it would be continually left deoxidized. 



Supposing that the foregoing explanation is the right one, 

 there are two points well worthy of noting. First, it would 

 follow that the electropositive metal in the amalgam is to some 

 extent electrically independent of the mercury — asserts, in 

 fact, a certain electrical integrity. Secondly, it would be ne- 

 cessary that the electromotive force between the foreign metal 

 and mercury (say lead and mercury) when in very dilute acid 

 must be greater, or at least have a greater decomposing action, 

 when the one metal is in a state of minute subdivision and its 

 particles in intimate electric contact w r ith the other metal, than 

 we should be led to expect from the behaviour of the metals 

 in separate masses. 



An analogy appears to exist between this behaviour and the 

 augmentation of chemical affinity of the couples constructed 



