of preserving Irmijrom Corrosion in Sea- Water. 49 



it by iron. The 7th experiment finally, shewed evidently the 

 eminent property of zinc to protect iron from corrosion in sea- 

 water. 



I can, therefore, consistently with truth, by no means retract 

 any part of my memoir, being still strongly convinced, that tin 

 cannot be made use of to preserve the iron-boilers of steam- 

 vessels from corrosion by sea-water ; whilst, on the contrary, zinc 

 is perfectly adapted for that purpose. 



But I proceeded further, in asserting that tin in the iron-boil- 

 ers, instead of preserving the iron, would greatly increase the 

 danger, by promoting the speedy oxidation of that metal. This 

 fact is, as 1 suppose, entirely placed beyond doubt, by the re- 

 sults of my above-mentioned experiments ; for, if the compari- 

 son of the 1st and 2d experiment should still have left any 

 doubt on this head, the great quantity of oxide derived from the 

 small piece of iron in the 3d experiment, compared with the 

 far smaller quantity of the same matter, obtained at the same 

 time from the much larger iron-plates in my 1st and 2d expe- 

 riments, proves sufficiently the pernicious influence of tin in 

 this respect. "^ 



The 4th experiment shews evidently the powerful influence 

 of contact on mutual preservation of metals ; but we learn at 

 once that this influence only consists in the factof an electrical con- 

 duction between the two metals, so that the immediate contact 

 seems not to be absolutely necessary to preservaiion ; for, ac- 

 cording to the fifth experiment, the preservation of the tin by 

 the iron, and consequently the oxidation of the iron, was in a 

 great measure restored, by establishing, by means of a third me- 

 tal, the electrical conduction between the two metals which had 

 been separated by the plate of mica. 



Mr John Davy, fully avowing the fitness of zinc to protect 

 iron from corrosion in sea-water, has remarked, that, by using 

 this mode of preservation for iron steam-boilers, it is to be ques- 

 tioned whether danger may not arise from the inflammable gas. 

 mixing with the steam. But besides the very small quantity of 

 that gas, which can never increase indefinitely in the boiler, on 

 account of its escaping from time to time together with the 

 steam, I think that hydrogen gas, even in a great quantity, in- 

 cluded in a steam-boiler, will not be more dangerous there than 



VOL, XXm. NO. XLV. JULY 1837- i> 



