to his ideas, it could only act upon salt water when in a positive state, could it be rendered slightly negative the corroding action of sea water upon it would be null; and whatever might be the difference of the kinds of copper sheeting, and their electrical action upon each other, still every effect of chemical action must be prevented, if the whole surface were rendered negative. But how was this to be effected? Reflecting for some time on the slow and weak action of sea water on copper, and the small difference which must exist between their electrical powers, and knowing that a very feeble chemical action would be destroyed by a very feeble electrical force, he began his experiments of prevention with an extreme case of plunging into salt water acidulated by sulphuric acid a polished plate of copper to which a piece of tin was soldered equal to about one-twentieth of the surface of the copper. Examined after three days the copper was perfectly clean, whilst the tin was much corroded; no blueness appeared in the liquid; though in a comparative experiment, when copper alone and the same fluid mixture were used, there was a considerable corrosion of the copper, and a distinct blue tint in the liquid.

This was a fundamental experiment, and decisive of the justness of the views he had formed, and almost decisive of the efficacy of the preventative means he contemplated. "If (he immediately remarks) one-twentieth part of the surface of tin prevented the action of sea water, rendered slightly acidulous by sulphuric acid, I had no doubt that a much smaller quantity would render the action of sea water, which depended only on the loosely attached oxygen of common air, perfectly null; and on trying 200 part

of tin, I found the effect of its preventing the corrosion of the copper perfectly decisive."

Exemplifying the happy and wonderful success of this simple means, I shall extract a few more details. from his first paper, which can hardly fail to excite the interest even of readers who have not devoted their attention to matters of science, and are but little acquainted with the mysteries it discloses.

"In pursuing these researches, and applying them to every possible form and connection of sheet copper, the results were of the most satisfactory kind. A piece of zinc as large as a pea, or the point of a small iron nail, was found fully adequate to preserve forty or fifty square inches of copper; and this wherever it was placed, whether at the top, bottom, or in the middle of the sheet of copper, and whether the copper was straight, or bent, or made into coils. And where the connection between different pieces of copper was completed by wires, or thin filaments of the fortieth or fiftieth of an inch in diameter, the effect was the same, every side, every surface, every particle of the copper remained bright, whilst the iron or the zinc was slowly corroded.

"A piece of thick sheet copper, containing on both sides about sixty square inches, was cut in such a a manner as to form seven divisions, connected only by the smallest filaments that could be left, and a mass of zinc of the fifth of an inch in diameter was soldered to the upper division. The whole was plunged under sea water; the copper remained perfectly polished. The same experiment was made with iron; and now, after a lapse of a month, in both instances, the copper is as bright as when it was first introduced, whilst similar pieces of copper unde

fended, in the same sea water, have undergone considerable corrosion, and produced a large quantity of green deposit in the bottom of the vessel.

"A piece of iron nail about an inch long was fastened by a piece of copper wire nearly a foot long to a mass of sheet copper containing about forty square inches, and the whole plunged below the surface of sea water. It was found, after a week, that the copper was defended by the iron in the same manner as if it had been in immediate contact.

"A small piece of zinc was fastened to the top of a plate of polished copper, and a piece of iron of a much larger size was soldered to the bottom, and the combination placed in sea water. Not only was the copper preserved on both sides, in the same manner as in the other experiments, but even the iron; and after a fortnight, both the polish of copper and the iron remained unimpaired."

His discovery, the result of these researches, he thus announced to me, in a letter written just after his first paper was read, and when not even a shadow of doubt appears to have crossed his mind of any possible failure in its application to the grand object of naval economy, for which the inquiry was instituted, and when he indulged, in consequence, in most sanguine expectation of perfect success :—

"Firle, Jan. 30. 1824.

*

"MY DEAR JOHN,

"I have lately made a discovery of which

you will for many reasons be glad. I have found a complete method of preserving the copper sheeting of ships, which now readily corrodes. It is by rendering it

negatively electrical. My results are of the most beautiful and unequivocal kind; a mass of tin renders a surface of copper 200 or 300 times its own size sufficiently electrical to have no action on sea

water.

"I was led to this discovery by principle, as you will easily imagine; and the saving to government and the country by it will be immense. I am going to apply it immediately to the navy. I might have made an immense fortune by a patent for this discovery, but I have given it to my country; for in every thing connected with interest, I am resolved to live and die at least sans tâche."

"I am, my dear John, very sincerely,

"Your affectionate Friend and Brother,
"H. DAVY."

In his second paper, which followed the first after an interval of five months, he relates the results of the experiments then in progress, on the protection of copper sheeting by his method in our naval establishments at Chatham and Portsmouth, applied to the bottoms of boats and vessels at rest. Up to that time his expectations had been fully answered; indeed, he says, the influence of the protectors (as the more oxidable metals attached were called) in many instances even surpassed his expectations.

to Tổō,

"When the metallic protector was from there was no corrosion nor decay of the copper; with smaller quantities, such as from 0 to 0, the copper underwent a loss of weight, which was greater in proportion as the protector was smaller; and as a proof of the universality of the principle, it was found that

even part of cast iron saved a certain proportion of the copper."

In some instances, as he had anticipated, there was a deposition of carbonate of lime and magnesia on the copper, followed, contrary to his anticipations, by the adhesion of sea insects and weeds. This happened when the copper was protected by a larger proportion than usual of zinc or iron; it did not occur when they were in a proportion less than ; when "the electrical power of the copper being less negative, more neutralised, and nearly in equilibrio with that of the menstruum," its decomposing effect as part of a voltaic combination was less; no earthy matter was deposited, and no weeds or insects adhered; and the surface, though it had undergone a slight degree of solution, remained perfectly clean; "a circumstance (he observes) of great importance, as it points out the limits of protection, and makes the application of a very small quantity of the oxidable metal more advantageous in fact than that of a larger one."

In his third paper, communicated a year after the preceding, he continued the inquiry, in connection with the result last mentioned, the knowledge of which had rendered the application of protecting means more difficult than was at first expected.

He renewed the investigation by carefully examining the circumstances of copper sheathing exposed to the action of salt water without artificial protection.

He found that the copper sheathing remained clean so long as the wear of its surface was equal and uninterrupted; its freedom from weeds and shells being owing, not to any poisonous quality of the copper, but to a loss of substance by solution and scaling off, preventing their adhesion.