96 Sir H. Davy’s further Researches 
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 sa- 
line 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 cop- 
per in contact, or fastened to each other by wires, be in two 
vessels of sea water connected by moist cotton or asbestes, all 
the various circumstances of protection 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 cop- 
per 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 re- 
pose. 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 cor- 
rodes with great rapidity, and the iron remains in the electro- 
negative and indestructible state. 
I began this paper by some observations upon the nature 
of the processes by which copper sheeting is destroyed by sea 
water, and on the causes by which it is preserved clean, or 
rendered foul by adhesions of marine vegetables or animals ; 
I shall conclude it by some further remarks on the same sub- 
ject, and with some practical inferences and some theoretical 
elucidations, which naturally arise from the results detailed in 
the foregoing pages. 
The very first experiment that I made on harbour-boats at 
Portsmouth, proved that a single mass of iron protected fully 
and entirely many sheets of copper, whether in waves, tides, 
or currents, so as to make them negatively electrical, and in 
such a degree as to occasion the deposition of earthy matter 
upon them: but observations on the effects of the single con- 
tact of iron upon a number of sheets of copper, where the 
junctions 
