on the Preservation of the Copper Sheathing of Ships. 205 



I had anticipated the deposition of alkaline substance in cer- 

 tain cases upon the negatively electrical copper. This haiS 

 actually happened. Some sheets of copper that have been 

 exposed nearly four months to the action of sea-water, de- 

 fended by from ^^th to J^th of their surface of zinc and iron, 

 have become coated with a white matter, which on analysis has 

 proved to be principally carbonated lime and carbonate and 

 hydrate of magnesia. 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 ^'jth 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 

 defended by quantities of cast iron and zinc, bearing a pro- 

 portion below jjo th, the electrical power of the copper being 

 less negative, more neutralized, and nearly in equilibria with 

 that of the menstruum, no such effects of deposition of alkaline 

 matter or adherence of weeds have taken place; and the surface, 

 though it has undergone a slight degree of solution, has re- 

 mained perfectly clean ; a circumstance of great importance, 

 as it points out the limits of protection, and makes the appli- 

 cation of a very small quantity of the oxidable metal more ad- 

 vantageous 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, howevei", depend on the relation of its mass to 

 that of the copper, and upon other circumstances not yet as- 

 certained (such as temperature, the relative saltness of the sea, 

 and perhaps the rapidity of the motion of the ship) ; circum- 

 stances in relation to which I am about to make decisive ex- 

 periments. 



Many singular facts have occurred in the course of these 

 researches. I shall mention some of them that I have con- 

 firmed by repeated experiments, and which have connexions 

 with general science. Weak solutions of salt act strongly 

 upon copper ; strong ones, as brine, do not affect it ; and the 

 reason seems to be that they contain little or no atmospheric air, 

 the oxygen of which seems necessary to give the electro-posi- 

 tive principle of change to menstrua of this class. 



I had anticipated the result of this experiment, and upon 

 the same principle of some others. 



Alkaline 



