1826.] Preservation of Metats hi/ Electro-chemical Means. 251 



very minute inspection of the sheet with the largest quantity of 

 protecting metal, that there was not any adhesion of alkaline or 

 earthy substances to its surface. 



Having observed, in examining the results of some of the 

 experiments on the effects of single masses of protecting metal 

 on the sheeting of ships, that there was in some cases in which 

 sheets with old fastening had been used, tarnish or corrosion, 

 which seemed to increase with the distance from the protecting 

 metal, it became necessary to investigate this circumstance, and 

 to ascertain the extent of the diminution of electrical action in 

 instances of imperfect or irregular conducting surfaces. 



With single sheets or wires of copper, and in small confined 

 quantities of sea water, there seemed to be no indications of 

 diminution of conducting power, or of the preservative effects 

 of zinc or iron, however divided or diffused the surface of the 

 copper, provided there was a perfect metallic connexion through 

 the mass. Thus, a small piece of copper containing about 32 

 square inches, was perfectly protected by a quantity of zinc 

 which was less than ^-jj-'^-o- part of the whole surface ; and a 

 copper wire of several feet in length was prevented from tarnish- 

 ing by a piece of zinc wire which was less than -j^Vs- P^''^ of its 

 length. In these cases the protecting metal corroded with great 

 rapidity, and in a few hours was entirely destroyed ; but when 

 applied in the form of wire and covered, except at its transverse 

 surface, with cement, its protecting influence upon the same 

 minute scale was exhibited for many days. A part of these 

 results depend upon the absorption of the oxygen dissolved in 

 the water when its quantity is limited, by the oxidable metal, 

 and of course the proportion of this metal must be much larger 

 when the water is constantly changing ; but the experiments 

 seem to show that any diminution of protecting effect at a dist- 

 ance, does not depend upon the nature of the metallic, but of 

 the imperfect or fluid conductor. 



This indeed is shown by many other results. 



A piece of zinc and a piece of copper in the same vessel of 

 sea water, but not in contact, were connected by different lengths 

 of fine silver wire of different thickness. It was found that 

 whatever lengths of wire of -j-i-j;^ of an inch were used, there was 

 no diminution of the protecting effect of the zinc ; and the expe- 

 riment was carried so far as to employ the whole of a quantity 

 of extremely fine wire, amounting to upwards of forty feet in 

 length, and of a diameter equal only to yl"" -^ of an inch, when 

 the results were precisely the same as if the zinc and copper had 

 been in immediate contact. 



Pieces of charcoal, which is the worst amongst the more 

 perfect conductors, were connected by being tied together, 

 and made the medium of communication between zinc and 

 copper, upon the same principles, and with the same views 



