of the conjunctive wires, and the circumstances under which they became effective; he found, for instance, that if a small steel bar be attached to the conjunctive wire, and parallel to it, it does not become a polar magnet; but that, if it be attached transversely, it does become polar, and that it becomes north and south, or south and north, according to the direction of the supposed electric current traversing the conjunctive wire, as one or the other end of it may be positive or negative. "In viewing these phanomena," says sir Humphry, "a number of curious speculations cannot fail to present themselves to every philosophical mind; such as, whether the magnetism of the earth may not be owing to its electricity, and the variation of the needle to the alterations in the electrical currents of the earth, in consequence of its motions, internal chemical changes, or its relations to solar heat; and whether the luminous effects of the auroras at the poles are not shown by these new facts to depend on electricity." It is certainly evident, that, if strong electrical currents be supposed to follow the apparent course of the sun, the magnetism of the earth ought to be such as it is actually found to be; and to afford a popular illustration of this theory, sir Humphry directed a sphere to be constructed, in which arrangements were made for passing the electricities, from the two ends of the battery, in the direction of the ecliptic, upon which the poles were found to become magnetic.

Sir H. Davy's method for preventing the corrosion of the copper sheathing of ships by sea-water, VOL. LXXI.

being founded upon Voltaic principles, must be considered as properly falling under the head of his electrical researches. It appears that the Commissioners of the Navy, impressed with the evil arising from the destructive influence of sea-water upon the copper sheathing of ships of war, applied to the Council of the Royal Society, in the hope that some plan might be suggested for arresting, if not preventing, the decay of so expensive an article. Sir H. Davy charged himself with the inquiry; and presented its results in a paper which was read before the society on the 22nd of January, 1824, and which was continued in another communication dated 17th of June, 1824, and concluded in a third, read 9th of June, 1825. We shall endeavour to put the reader in possession of the principle facts elicited by this' inquiry. Davy had advanced the hypothesis, that chemical and electrical changes were identical, or dependent upon the same property of matter; and he had shown 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: it was, in short, by an application of this very principle that he decomposed the alkalies; and it was from the same energetic instrumentality that he now sought a remedy for the rapid corrosion of copper sheathing. Let us see how dexterously he grappled with 2 L

the difficulties of his subject. When a piece of polished copper is suffered to remain in sea-water, the first effects are, a yellow tarnish upon the surface, and a cloudiness in the water, which take place in two or three hours: the hue of the cloudiness is at first white, and it gradually becomes green. In less than a day a bluish-green precipitate appears in the bottom of the vessel, which constantly accumulates; this green matter appears principally to consist of an insoluble compound of copper (a sub-muriate) and hydrate of magnesia. Reasoning upon these phenomena, Davy arrived at the conclusion that cop per could act upon sea-water only when in a positive state; and since that metal is only weakly positive in the electro-chemical scale, he considered that, if it could be only rendered slightly negative, the corroding action of sea-water upon it would be null. But how was this to be effected? At first, he thought of using a Voltaic battery; but this could hardly be applicable in practice he next thought of the contact of zinc, tin, or iron; but he was prevented for some time from trying this, by the recollection that the copper in the Voltaic battery, as well as the zinc, was dissolved by the action of dilute 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, he

was encouraged to proceed; and 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 the 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 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 seawater; the copper remained perfectly polished. The same experiment was made with iron; and after the lapse of a month, in both instances, the copper was found as bright as when it was first introduced, whilst similar pieces of copper, undefended, in the same sea-water, underwent considerable corrosion, and produced a large quantity of green deposit in the bottom of the vessel. Numerous other experiments were performed, and with results equally conclusive of the truth of the theory which had suggested them. It remained only that the experiments should

be conducted on a large scale. The lords commissioners of the Navy accordingly gave sir Humphry permission to ascertain the practical value of his discovery by trials upon ships of war; and the results, to use his own expression, even surpassed his most sanguine expectations. Sheets of copper, defended by from 1-40th to 11000th part of their surface of zinc, malleable and cast iron, were exposed, for many weeks, in the flow of the tide in Portsmouth harbour, their weights having been ascertained before and after the experiment. When the metallic protector was from 1-40 to 1-110, there was no corrosion nor decay of the copper; with small quantities it underwent a loss of weight. The sheathing of boats and ships, protected by the contact of zinc, cast and malleable iron in different proportions, compared with that of similar boats and sides of ships unprotected, exhibited bright surfaces, whilst the unprotected cop per underwent rapid corrosion, becoming first red, then green, and losing a part of its substance in scales. Is it not, then, a fact, established beyond all controversy, that small quantities of electropositive metals will prevent the corrosion or chemical changes of copper exposed to sea-water; and that the results appear to be of the same kind, whether the experiments are made upon a minute scale, and in confined portions of water, or on large masses, and in the ocean? How, then, has it happened, that this scheme of protection has not been adopted? Simply, because, in overcoming one evil, another has been created; by protecting the copper, the accumulation of sea-weeds and ma

rine insects has been favoured, and the ships, thus defended by iron or zinc, have become so foul, as scarcely to continue navigable. This would seem to depend upon several causes, especially upon the deposition of saline and calcareous matter, arising from the decomposition of marine salts. Whether or not his principles can be rendered subservient to the protection of copper sheathing, it must at least be admitted, that the results obtained by him are of the most interesting description, and capa ble of various useful applications. By introducing a piece of zinc, or tin, into the iron boiler of the steam-engine, we may prevent the danger of explosion, which generally arises, especially where salt-water is used, as in those of steam-boats, from the wear of one part of the boiler, Another important application is in the prevention of the wear of the paddles, or wheels, which are rapidly dissolved by salt water. Mr. Pepys has extended the principle, for the preservation of steel instruments,, by guards of zine; and razors and lancets have been thus defended with perfect success,

In the year 1805, Mr. Davy was elected a member of the Royal Irish Academy; and towards the close of the year 1810, he delivered a course of lectures before the Dublin Society, and received from Trinity College, Dublin, the honorary degree of LL.D.

In 1812 Mr. Davy married. The object of his choice was Jane, daughter and heiress of Charles Kerr, of Kelso, esq., and widow of Shuckburgh Ashby Apreece, esq., eldest son of the present sir Thomas Hussey Apreece, bart.

By his union with this lady, Mr. Davy acquired a considerable fortune. On the 9th of April, only two days previously to his marriage, he received the honour of knighthood from the Prince Regent, being the first person on whom his Royal Highness conferred that dignity.

The frequency of accidents, arising from the explosion of the fire-damp, or inflammable gas, of the coal mines, mixed with atmospherical air, occasioned the formation of a committee at Sunderland, for the purpose of investigating the causes of these calamities, and of endeavouring to discover and apply a preventive. Sir Humphry received an invitation, in 1815, from Dr. Gray, one of the members of the committee; in consequence of which he went to the north of England, and visiting some of the principal collieries in the neighbourhood of Newcastle, soon convinced himself that no improvement could be made in the mode of ventilation, but that the desired preventive must be sought in a new method of lighting the mines, free from danger, and which, by indicating the state of the air in the part of the mine where inflammable air was disengaged, so as to render the atmosphere explosive, should oblige the miners to retire till the workings were properly cleared. The common means then employed for lighting the dangerous part of the mines consisted of a steel wheel revolving in contact with flint, and affording a succession of sparks: but this apparatus always required a person to work it, and was not entirely free from danger. The firedamp was known to be light car

buretted hydrogen gas but its relations to combustion had not been examined. It is chiefly pro

1

duced from what are called blowers or fissures in the broken strata, near dykes. Sir Humphry made various experiments on its combustibility and explosive nature; and discovered, that the fire-damp requires a very strong heat for its inflammation; that azote and carbonic acid, even in very small proportions, diminished the velocity of the inflammation; that mixture of the gas would not explode in metallic canals or troughs, where their diameter was less than one seventh of an inch, and their depth considerable in proportion to their diameter; and that explosions could not be made to pass through such canals, or through very fine wire sieves, or wire gauze. The consideration of these facts led sir Humphrey to adopt a lamp, in which the flame, by being supplied with only a limited quantity of air, should produce such a quantity of azote and carbonic acid as to prevent the explosion of the fire-damp, and which, by the nature of its apertures for giving admittance and egress to the air, should be rendered incapable of communicating any explosion to the external air. These requisites were found to be afforded by air-tight lanterns, of various constructions, supplied with air from tubes or canals of small diameter, or from apertures covered with wire-gauze, placed below the flame, through which explosions cannot be communicated, and having a chimney at the upper part, for carrying off the foul air. Sir Humphry soon afterwards found that a constant flame might be kept up from the

explosive mixture issuing from the apertures of a wire-gauze sieve. He introduced a very small lamp in a cylinder, made of wiregauze, having six thousand four hundred apertures in the square inch. He closed all apertures except those of the gauze, and introduced the lamp, burning brightly within the cylinder, into a large jar, containing several quarts of the most explosive mixture of gas from the distillation of coal and air; the flame of the wick immediately disappeared, or rather was lost, for the whole of the interior of the cylinder became filled with a feeble but steady flame of a green colour, which burnt for some minutes, till it had entirely destroyed the explosive power of the atmosphere. This discovery led to a most important improvement in the lamp, divested the fire-damp of all its terrors, and applied its powers, formerly so destructive, to the production of a useful light. Some minor improvements, originating in sir Humphry's researches into the nature of flame, were afterwards effected. Experiments of the most satisfactory nature were speedily made, and the invention was soon generally adopted. Some attempts were made to dispute the honour of this discovery with its author, but his claims were confirmed by the investigations of the first philosophers of the age. The coal-owners of the Tyne and Wear evinced their sense of the benefits resulting from this invention, by presenting sir Humphry with a handsome service of plate, worth nearly 2,000, at a public dinner at Newcastle, October 11th, 1817.

In 1813, sir Humphry was elected a corresponding member

of the Institute of France, and vice-president of the Royal Institution. He was created a baronet, October 20, 1818. In 1820, he was elected a foreign associate of the Royal Academy of Sciences at Paris, in the room of his countryman Watt; and in the course of a few years, most of the learned bodies in Europe enrolled him among their members.

Much of this period of his life was spent in visiting different parts of Europe for scientific purposes. He analysed the colours used in painting by the ancient Greek and Roman artists. His experiments were chiefly made on the paintings in the baths of Titus, the ruins called the baths of Livia, in the remains of other palaces and baths of ancient Rome, and in the ruins of Pompeii. By the kindness of his friend Canova, who was charged with the care of the works connected with ancient art in Rome, he was enabled to select, with his own hands, specimens of the different pigments that had been found in vases discovered in the excavations which had been lately made beneath the ruins of the palace of Titus, and to compare them with the colours fixed on the walls, or detached in fragments of stucco. The results

of all these researches were published in the Transactions of the Royal Society for 1815. On his examination of the Herculaneum manuscripts at Naples, in 1818-19, he was of opinion they had not been acted upon by fire, so as to be completely carbonized, but that their leaves were cemented together by a substance formed during the fermentation and chemical change of ages. He invented a composition for the solution of