254 Sir Humphry Davy on the [April, 



increased, till with the six folds the protecting power appeared 

 to be lost : and in the case of the single leaf, there was this 

 difference from the result of immediate contact, that there was 

 no deposition of earthy matter. Showing that there was no 

 absolute minute contact of the metals through the moist paper; 

 which was likewise proved by other experiments; for a thin 

 plate of mica, as 1 have just mentioned, entirely destroyed the 

 protecting effect of zinc : and yet when a hole was made in it, so 

 as to admit a very thin layer of moisture between the zinc and 

 copper, the corrosion of the copper, though not destroyed, was 

 considerably diminished. 



The rapid corrosion of iron and zinc, particularly when used 

 to protect metals, only in very small quantities, induced me to 

 try some experiments as to their electro-chemical powers in 

 menstrua out of the contact, or to a certain extent removed 

 from the contact of the air, such as might be used for moistening 

 paper under the copper sheathing of ships : the results of these 

 experiments I shall now detail. A small piece of iron was 

 placed in one glass filled with a saturated solution of brine, 

 which contains little or no air ; copper, attached by a wire to 

 the iron, was placed in a vessel containing sea water, which was 

 connected with the brine by moistened tow. The copper did 

 not corrode, and yet the iron was scarcely sensibly acted upon, 

 and that only at the surface of the brine ; and a much less effect 

 was produced upon it in many weeks than would have been 

 occasioned by sea water in as many days. 



With zinc and brine in the same kind of connexion there was 

 a similar result ; but the solution of the zinc was comparatively 

 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 pt)ssessing 

 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 circum- 

 stances 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 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 



