or four days prior to the setting in of very severe frosty weather. This was manifest at the latter end of the year 1794, at the coming on of the severe season that ensued. In the river Severn, about a mile and a half to the westward of this place, were seen and taken many species of water-birds, that generally confine themselves to the more northern regions. Far more pleasant is it to see during the continuance of hard frost, the return of those birds which had left us at the beginning. These are pleasant omens, and most certainly forbode a thaw. To recapitulate the substance of my observations. I have first adduced some arguments in support of migration, the fact itself not being generally admitted by naturalists of celebrity, and also against the hypothesis of a state of torpor, or what may be termed the hibernating system. I have represented that the swallow tribe, and many other birds that absent themselves at stated periods, return annually to the same spot to build their nests; and at the same time that any inference drawn from this fact in support of a state of torpor would be fallacious upon physiological principles. That certain periodical changes of their organs are the inciting causes of migration I have stated many facts, hitherto, I believe, unnoticed, chiefly with respect to the cause which excites the migrating bird, at certain seasons of the year, to quit one country for another, and the need of a country where they can for a while be better accommodated with succours for their infant brood than in that from which they depart. It is also attempted to be shown, that their departure from this country is not in consequence of any disagreeable change in the temperature of the air, or from a scarcity of their common food, but the result of the accomplishment of their errand, i. e. the incubation, and rearing of their young. That successive arrivals of migrating birds are attributable to the progressive developement already noticed in the male and female; that progressive developements are wise provisions of nature; that premature arrivals and departures are frequently to be accounted for on the same principle With respect to the winter birds of passage, I have stated that they quit their homes (this country) in spring, in quest of a country better suited to their intended purpose than their own; that they are actuated by the same impulse in quitting this country that causes the spring birds to come to it, and that want of food cannot be the inducement; that the emigration of the winter birds is less complete than that of the others (the spring migrators); that some species breed here, especially the wild-duck and wood-pigeon; that the redwings and fieldfares are the most regular and uniform in their appearance and disappearance, and, most probably, never risk the trial of incubation here; that they quit the country temporarily in severe and long continued frost through want of food, and return to it again at the approach of more temperate weather; that the arrival of water birds forebodes the approach of intense frost, the usual return of the winter birds, a thaw; that examinations of the latter prove them to have taken long flights before their return, and sets the fact of temporary migration beyond the reach of doubt. On the Corrosion of Copper Sheeting by Sea Water, and on Methods of preventing this Effect; and on their Application to Ships of War and other Ships. By Sir HUMPHRY DAVY, Bart. President of the Royal Society. [1824.] THE attention of Sir Humphry Davy having been directed to the rapid decay of the sheeting of our ships of war, by the Commissioners of the Navy Board, he commenced his investigation of this subject by some experiments on the causes of the action of sea water on copper. When a piece of polished copper is suffered to remain in sea water, the first effects observed are, a yellow tarnish upon the copper, and a cloudiness in the water, which take place in two or three hours: the hue of the cloudiness is at first white; it gradually becomes green. In less than a day a bluish-green precipitate appears in the bottom of the vessel, which constantly accumulates; at the same time that the surface of the copper corrodes, appearing red in the water, and grass-green where it is in contact with air. Gradually carbonate of soda forms upon this grass-green matter; and these changes continue till the water becomes much less saline. The green precipitate, when examined by the action of solution of ammonia and other tests, appears principally to consist of an insoluble compound of copper (which may be considered as a hydrated sub-muriate) and hydrate of magnesia. - On pursuing his enquiry with various specimens of copper, taken from different ships, some of which had been considered as remarkable for their durability, and others for their rapid decay, which difference some persons had supposed to result from the different states of the metal in regard to purity, he found, that although differently alloyed, they offered but very inconsiderable differences in their action on sea water, and, consequently, he inferred that the changes - they had undergone must have depended on other causes than the relative purity of the metal. Sir Humphry Davy had, as early as the year 1806, advanced the hypothesis, that chemical and electrical changes may be identical, or dependent on the same property of matter. He also showed that chemical attractions may be exalted, modified, or destroyed by changes in the electrical states of bodies; that substances will combine only when they are in different electrical states; and that, by bringing a body naturally positive, artificially into a negative state, its usual powers of combination are altogether destroyed. By reasoning on this hypothesis, he was' led to the discovery which is the subject of this paper. I at He says, "Copper is a metal only weakly positive in the electro-chemical scale; and, according to my ideas, it could only act upon sea water when in a positive state; and, consequently, if it could be rendered slightly negative, the corroding action of sea water upon it would be null; and whatever might be the differences 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? first thought of using a Voltaic battery; but this could be hardly applicable in practice. I next thought of the contact of zinc, tin, or iron: but I was for some time prevented from trying this, by the recollection that the copper in the Voltaic battery, as well as the zinc, is dissolved by the action of diluted nitric acid; and by the fear that too large a mass of oxidable metal would be required to produce decisive results. After reflecting, however, 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, I resolved to try some experiments on the subject. I began with an extreme case. I rendered sea water slightly acidulous by sulphuric acid, and plunged into it a polished piece 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 remained perfectly clean, whilst the tin was rapidly corroded no blueness appeared in this liquor; though, in a comparative experiment, when copper alone and the same fluid mixture was used, there was a considerable corrosion of the copper, and a distinct blue tint in the liquid. : "If 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 upon the loosely attached oxygen of common air, perfectly null; and on trying part of tin, I found the effect of its preventing the corrosion of the copper perfectly decisive. "This general result being obtained, I immediately instituted a number of experiments, in most of which I was assisted by Mr. Faraday, to ascertain all the circumstances connected with the preservation of copper by a more oxidable metal. I found, that whether the tin was placed either in the middle, or at the top, or at the bottom of the sheet of copper, its effects were the same; but, after a week or ten days, it was found that the defensive action of the tin was injured, a coating of submuriate having formed, which preserved the tin from the action of the liquid. "With zinc or iron, whether malleable or cast, no such diminution of effect was produced. The zinc occasioned only a white cloud in the sea water, which speedily sunk to the bottom of the vessel in which the experiment was made. The iron occasioned a deep orange precipitate: but after many weeks, not the smallest portion of copper was found in the water; and so far from its surface being corroded, in many parts there was a regeneration of zinc or of iron found upon it. "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, were found fully adequate to preserve 40 or 50 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 60 square inches, was cut in such 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, undefended, 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 40 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 piece of copper and a piece of zinc soldered together at one of their extremities, were made to form an arc in two different vessels of sea water; and the two portions of water were connected together by a small mass of tow moistened in the same water: the effect of the preservation of the copper took place in the same manner as if they had been in the same vessel. "As the ocean may be considered, in its relation to the quantity of copper in a ship, as an infinitely extended conductor, I endeavoured to ascertain whether this circumstance would influence the results; by placing two very fine copper wires, one undefended, the other defended by a particle of zinc, in a very large vessel of sea-water, which water might be considered to bear the same relation to so minute a portion of metal as the sea to the metallic sheeting of a ship. The result of this experiment was the same as that of all the others; the defended copper underwent no change; the undefended tarnished, and deposited a green powder. "Small pieces of zinc were soldered to different parts of a large plate of copper, and the whole plunged in sea water: it was found that the copper was preserved in the same manner as if a single piece had been used. "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 the copper and the iron remained unimpaired." Additional Experiments and Observations on the Application of Electrical Combinations to the Preservation of the Copper Sheathing of Ships, and to other Purposes. By Sir HUMPHRY DAVY, Bart. [1824.] SIR HUMPHRY DAVY, in this paper, presents the results of his progressive experiments, made in our naval establish |