298 Sir H. Davy on Copper Shealhing. [April, 



that even ^g_ part of cast iron saved a certain proportion of 

 the copper. 



The sheeting of boats and ships, protected by the contact of 

 zinc, cast and malleable iron in different proportions, compared 

 with those of similar boats and sides of ships unprotected, exhi- 

 bited bright surfaces, whilst the unprotected copper underwent 

 rapid corrosion, becoming first red, then green, and losing a part 

 of its substance in scales. 



Fortunately, in the course of these experiments, it has been 

 proved that cast iron, the substance which is cheapest and most 

 easily procured, is likewise most fitted for the protection of the 

 copper. It lasts longer than malleable iron, or zinc ; and the 

 plumbaginous substance, which is left by the action of sea water 

 upon it, retains the original form of the iron, and does not 

 impede the electrical action of the remaining metal. 



I had anticipated the deposition of alkaline substances in 

 certain cases upon the negatively electrical copper. This has 

 actually happened. Some slieets of copper that have been 

 exposed nearly four months to the action of sea water, defended 

 by from -~-^ to -^-^ of their surface of zinc and iron, have become 

 coated with awhite matter, which, on analysis, has proved to be 

 principally carbonated lime, and carbonate and hydrate of mag- 

 nesia. The same thing has occurred with two harbour boats, 

 one of which was defended by a band of zinc, the other by a 

 band of iron, equal to about -J^ of the surface of the copper. 



These sheets and boats remained perfectly clean for many 

 weeks, as long as the metallic surface of the copper was ex- 

 posed ; but lately, since it has become coated with carbonate 

 of lime and magnesia, weeds have adhered to these coatings, 

 and insects collected on them ; but on the sheets of copper, de- 

 fended by quantities of cast iron and zinc, bearing a proportion 

 below -j-^, the electrical power of the copper being less nega- 

 tive, more neutralised, and nearly in equilibrio with that of the 

 menstruum, no such effect of deposition of alkaline matter or 

 adherence of weeds has taken place, and the surface, though it 

 has undergone a slight degree of solution, has remained perfectly 

 clean : a circumstance of great importance, as it points out the 

 limits oi protection ; and makes the application of a 't;e;7/ sm«// 

 quantity of the oxidable metal more advantageous in fact than 

 that of a larger one. 



The wear of cast iron is not so rapid, but that a mass of two 

 or three inches in thickness will last for some years. At least 

 the consumption in experiments which have been going on for 

 nearly four months does not indicate a higher ratio. This must 

 however depend on the relation of its mass to that of the cop- 

 per, and upon other circumstances not yet ascertained (such as 

 temperature, the relative saltness of the sea, and perhaps the 



