[97] A common cause of adhesions of weeds or shell-fish, is the oxide of iron formed and deposited round the protectors. In the only experiment in which zinc has been employed for this purpose in actual service, the ship returned after two voyages to the West Indies, and one to Quebec, perfectly clean. The experiment was made by Mr. Lawrence, of Lombard Street, who states that the rudder, which was not protected, had corroded in the usual manner.
"The nails used in ships are an alloy of copper and tin, which I find to be slightly negative with respect to copper, and it is on these nails that the first adhesions uniformly take place: a slightly positive and slightly decomposable alloy would probably prevent this effect, and I have made some experiments favourable to the idea."
He next proceeds to state some circumstances, in addition to those he had formerly noticed, by which the electrical relations of copper are altered. "I found," says he, "copper hardened by hammering, _negative_ to rolled copper;--copper (to use the technical language of manufacturers) both _over-poled_ and _under-poled_,[98] containing, in one case, probably a little charcoal, and in the other a little oxide, _negative_ to pure copper. A specimen of brittle copper, put into my hands by Mr. Vivian, but in which no impurity could be detected, was negative with respect to soft copper. In general, very minute quantities of the oxidable metals render the alloy positive, unless it becomes harder, in which case it is generally negative."
[98] The _poling_ of copper is an operation, the theory of which is involved in a great deal of mystery. Copper, when taken from the smelting furnace, is what is termed _dry_, that is, it is brittle, has an open grain and crystalline structure, and is of a purplish red colour. The following is the process by which it is refined, or _toughened_, by the process of _poling_. The surface of the melted metal in the furnace is, in the first place, covered with charcoal. A pole, commonly of birch, is then plunged into the liquid metal, which produces a considerable ebullition from the evolution of gaseous matter, and this operation is continued, fresh charcoal being occasionally added, so that the surface may be always kept covered, until the refiner judges from the assays that the metal is malleable. The delicacy of the operation consists in the difficulty of hitting the exact mark: if the surface should by accident be uncovered, it will return to its _dry_ state; and should the process be carried too far, it will be _over-poled_, by which the metal would be rendered even more brittle than when in a _dry_ state. When this is found to be the case, or, as they say, _gone too far_, the refiner directs the charcoal to be drawn off from the surface of the metal, and the copper to be exposed to the action of the air, by which means it is again brought back to its _proper pitch_, that is, become again malleable. Now the question is, what are the changes thus produced in the copper? Is the metal in its _dry_ state combined with a minute portion of oxygen, of which _poling_ deprives it, and thus renders it malleable? and does the _over-poling_ impart to it a minute portion of carbon, and is copper, like iron, thus rendered brittle both by oxygen and carbon? Or, is the effect of the pole merely mechanical, that of closing the grain, and of altering the texture of the metal? Something might be said in support of all these opinions. Mr. Faraday, who has attentively examined the subject, is unable to detect any chemical difference between _poled_ and _unpoled_ copper. On the other hand, when the metal is _over-poled_, it is found to oxidate more slowly, and its surface when in the furnace is so free from oxidation, that it is like a mirror, and reflects every brick in the roof. This certainly looks very much like carbonization.--See "An Account of Smelting Copper, as conducted at the Hafod Copper-works; by J. H. Vivian, Esq."--_Annals of Philosophy_, vol. v. p. 113.