the presentation of metals by electro-chemical means. S39 
paratively more rapid than that of the iron, and the copper 
was rendered more highly negative, as was shown by a 
slight deposition of earthy matter upon it. 
A solution of potassa, or of alkaline substances possessing 
the electro-positive energy, has nearly the same effect on 
saline solutions as if they were deprived of air ; and when 
mixed with sea water impedes the action of metals upon 
them ; but if used in quantity in combinations such as these 
I have just described, in which iron is the protecting metal, 
it destroys the result, and renders the iron negative. Thus, 
if iron and copper in contact, or fastened to each other by 
wires, be in two vessels of sea water connected by moist 
cotton or asbestos, all the various circumstances of protec- 
tion of the two metals by each other may be exhibited by 
means of solution of potassa. By adding a few drops, of 
solution of potassa to the water in the glass containing the 
iron, the negative powers of the copper in the other glass 
are diminished ; so that the deposition of the calcareous and 
magnesian earths upon it is considerably lessened ; by a little 
more solution of potassa the deposition is destroyed, but still 
the copper remains clean. The corrosion of the iron, which 
before was rapid, is now almost at an end ; and a few drops 
more of the solution of potassa produces a perfect equilibrium : 
so that neither of the metals undergoes any change, and the 
whole system is in a state of perfect repose. By making 
the fluid in the glass containing the iron still more alkaline, 
it no longer corrodes ; and the green tint of the sea water 
shows that the copper is now the positively electrified metal ; 
and when the solution in the glass containing the iron is 
strongly alkaline, the copper in the other glass corrodes with 
MDcccxxv. Y y 
