10 Prof. Daniell on Sulphuretted Hydrogen 



gen ; for as you will see by reference to the experiments upon 

 the table, copper is much more acted upon by this substance 

 than iron, the latter being protected by the former; and the 

 fact is, that a piece of iron attached to copper increases the 

 corrosion of the latter. 



Zinc, on the contrary, protects the copper not only from 

 the action of the chlorides in sea water, but also from the sul- 

 phuretted hydrogen. This I have ascertained by the experi- 

 ments before you, and you will find that the results are in 

 perfect accordance with the electric order of these two metals 

 in solutions of the hydrosulphurets, as given by Dr. Faraday 

 in his last beautiful number of Experimental Researches in 

 Electricity. 



In the table which he gives, iron stands far above copper 

 in electro-negative order, and zinc below it ; lead is also above 

 zinc ; while in the usual acid solutions both zinc and iron stand 

 below that metal*. 



Now I have long been of opinion that the experiment of 

 voltaic protection in the Navy was much too lightly abandoned 

 upon the first appearance of an unforeseen difficulty, and that 

 under circumstances otherwise the most encouraging. 



This abandonment, you are aware, arose from what might 

 be called over-protection, by which the attachment of weeds 

 and zoophytes to the ships' bottoms was found to be encou- 

 raged. Earthy deposits were formed, and to these the weeds 

 and shell-fish attached themselves. 



The remedy for this appears to me to be obvious: instead 

 of keeping the protectors always in contact with the copper, 

 let them be insulated, and let them only be brought into me- 

 tallic contact when occasion may require. This might readily 

 be done by means of a bolt or bar, forming in one position a 

 continuous conductor between the two metals, and in another 

 breaking the connexion : this might always be at the com- 

 mand of the proper officer of the ship. Nothing could then 

 be easier than to throw off the protection when the ship is in 

 harbour, or in situations particularly liable to deposits ; or to 

 restore it upon going to sea, or arriving in latitudes where 

 sulphuretted hydrogen might be found to exist. 



But the protectors should invariably be of zinc, which 

 would preserve the copper not only from the effects of sea 

 water generally, but from the more destructive agency of 

 sulphuretted hydrogen, which I shall presently give you my 

 reasons for concluding not only prevails upon the western 

 coast of Africa, but in other situations where it has never yet 

 been suspected. Indeed I incline to believe that it would 

 * Phil. Trans., 1840, p. 113. 



