BIOGRAPHY OF SIR. IIUMPHRY DAVY, CONTINUED.

CHAPTER FIFTH.

As minute chemical detail is wholly inconsistent with the plan of a popular memoir, the author must leave the task of following Sir HUMPHRY DAVY, through his numerous inquiries and reflections, to him whose business it may hereafter become to write a scientific life.

His Bakerian Lecture of 1808, entitled " An Account of some new Analytical Researches on the nature of certain Bodies, particularly the Alkalies, Phosphorus, Sulphur, Carbonaceous matter, and the Acids hitherto undecomposed ; with some general observations on CHEMICAL THEORY," abounds in elaborate experiments with the voltaic apparatus, made with the hope of extending our knowledge of the principles of bodies, by the new powers and methods arising from the application of electricity ; but although it furnishes results of great interest, and re- cords phwnomena which may serve as guides in future inquiries, still it cannot lie said to have astonished the chemical world by any bril- liant discovery ; and yet it announced the decomposition of Boracic Acid, and the development of its inflammable base at the negative surface of the battery,—a discovery which at any other period would have produced great excitement in the chemical world ; but the fact is, that the splendour of his former lecture, like the blaze of the sun, left our organs of perception incapable of receiving a just impression from any lesser light.

In reviewing his several memoirs upon Voltaic Electricity, we have illustrated the original, cautious, and yet intrepid advances of his mind ; we are now about to notice a series of papers, in which the reader may discover the zeal and industry with which he pursued the track of others. The similarity of the laws of Electrical and Magnetical At traction had long excited the attention of the philosopher, and nume- rous had been the attempts to establish the existence of an identity, or intimate relation, between these two forces ; but little light had been thrown upon the subject before the year1— gl 9, when M. OERSTED,

Secretary to the Royal Society of Copenhagen, published an account of some experiments exhibited in his lecture before the University, by which it was demonstrated that the magnetic needle was moved from its position by the action of the galvanic apparatus. And it maybe here necessary to state, that these experiments, unlike all preceding

ones, were made-with the two ends of the pile in communication with each other; -te, -which circumstance are to be attributed the novel

results that followed. In pursuing the investigation with a more pow-

erftit battery, IV!. OEP.STED fully ascertained that the phaenomena exhibited by the needle did not depend upon electrical attraction and

repulsion, for its movements were wholly at variance with such an explanation ; they must depend, then, upon the production of a new energy, generated by the action of the two electricities thus brought into conflict, and which, if not identical with, must be

nearly related to magnetism! It moreover appeared probable from f the motions of the magnet, when differently placed with regard to the conjunctive wire, or that wire by which the opposite ends of

the battery were connected, that this energy circulated, or performed a circular movement around the axis of the conductor, and thus drove the magnetic pole towards the east or west, according to the direc- tion of the needle with reference to such a current. No sooner

had this extraordinay discovery been announced in this country, than Sir HUMPHRY DAVY proceeded to repeat the experiments, and with his characteristic talent, to vary and extend them. The nature and limits of this memoir will not allow us to follow him : it is-sufficient to say, that he obtained new results, and expanded the views which OERSTED had opened. He particularly investigated the magnetizing powers 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 be- come 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 plimnomena," says Sir Humanity, " 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 e'ectricity, and the variation of the needle to the alterations in the electrical currents of the earth, in con- sequence of its motions, internal chemical changes, or its relations to solar heat ; and whether the lnminous effects of the Auroras at the poles are not shown.by these - new facts to depend on electricity.".,It is certainly evident, thid, if strong electrical currents be supposed to follow the apparent eourse 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 con- structed, in which arrangements were made for passing the electrici- ties, from the two ends of the battery, in the direction of the ecliptic, upon which the poles were found to become magnetic.

In accordance with the plan originally proposed for the review of DAVY'S labours, we shall next offer an account of his method for pre- venting the corrosion of the copper sheeting of ships by sea-water ; which 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, fully impressed with the evil arising from the destructive influence of sea-water upon the copper sheeting of his Majesty's ships of war, applied to the Council of the Royal Society, in the hope that some plan might be suggested for ar- resting, if not for 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 22d January 1824, and which was continued in another communication dated 17th June 1524, and concluded in a third, read 9th June 1825. We shall endea- vour to put the reader in possession of the principal facts elicited by this inquiry. We have already stated, that DAVY had advanced the hypothesis, that chemical and electrical changes were identical, or de- pendent upon the same property of matter ; and that he had shown that chemical attractions may be exalted, modified, or de- stroyed, by changes in the electrical states of bodies ; that sub- stances will only combine 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 sheeting. Let us see how dexterously he grappled with 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 sm-face, and a cloudiness in the water, which take place in two or three murs : the hue of the cloudiness is at first white, and it gradually be- :tomes green. In less than a clay 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 topper (a sub muriate) and hydrate of magnesia. Reasoning upon these phmnomena, DAVY arrived at the conclusion that copper could 3nly act upon sea-water when-in a positive state ; and since that metal s only weakly positive in the electro-chemical scale, he considered' hat, if it could be only rendered slightly negative, the corroding ac- :ion of sea-water upon it would be null. But how was this to be ef- 7ected ? At first, he thought of using a voltaic battery ; but this .ould 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 diffe- rence 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, were 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 connexion between the different pieces of copper was com- pleted by wires, or thin filaments of the fortieth or fiftieth of an inch m 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 sea-water ; 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 sug- gested them. It remained only that the experiments should be con- ducted 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 1-1000th 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 sheeting of boats and ships, protected by the contact of zinc, cast and malleable iron in different proportions, compared with those of similar boats and sides of ships unprotected, exhibited bright surfaces, whilst the unprotected copper 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 electro-positive 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,—for the fact is notorious, and has called forth many animadversions to the disparagement of DAVY,—how has it happened that this scheme of protection has not been adopted? The fact is simply this, that in overcoming one evil, another has been created ; by protecting the copper, the accumulation of sea-weeds and marine 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 depo- sition of saline and calcareous matter, arising from the decomposition of marine salts. Had DAVY'S health remained unimpaired, his genius would, without doubt, have suggested a remedy; but he unfortunately declined in health, at the very moment his energies were most required. Future philosophers may propose successful expedients to obviate the evil, but the glory of the discovery will justly belong to him, who first developed the principle. Indeed we have just learnt, that a patent has been granted for substituting iron for copper sheeting, and protecting it from rust by the application of slips of zinc, in the proportion of two per cent. Should the plan succeed, to whom ought the public to award the palm ?

Whether or not the principle discovered by Sir HUMPHRY DAVY can be rendered subservient to the protection of copper sheeting, we must at least admit that the results obtained by him are of the most interesting description, and capable of various useful applications ; se- veral of which lie has himself suggested, whilst others have been dis- covered by the ingenuity of contemporary chemists. 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 pre- vention of the wear of the paddles, or wheels, which are rapidly dis- solved by salt-water. Mr. PEPYS has extended the principle, for the preservation of steel instruments, by guards of zinc : we have seen razors and lancets thus defended with perfect success. A knowledge of this principle, moreover, enables us to explain many ordinary phm- nomena which were befor • perfectly unintelligible. The writer of the present article can fairly state, that he has never derived so much pleasure from the application of any principle in natural philosophy, for the explanation of common occurrences, as from that of the one under discussion : and he will beg leave to illustrate it by one or two examples,—in the first place, for the_ purpose of showing that, when- ever a principle or discovery involves or unfolds a law of nature, its applieations are almost inexhaustible, and that, however abstracted it may appear, it is sooner or later employed for the common purposes of life ; and inlhe next place, for the sake of convincing the reader, that there does not exist any source of pleasure so extensive and so permanent as that derived from intellectual exertion. The saunterer stumbles over the stone that may .cross his path, and vents only his indignation at the interruption ; but there is not a body, animate or inanimate, with which the philosopher can come into contact, which does not yield its treasures at his approach, and contribute to exalt the pleasures of his existerce. The author remembers, several years ago, that in passing through Deptford to a house he then occupied at Blackheath, he was daily sur- prised at the brilliancy of a portion of the gilded sign of an inn in that town, whilst the other parts had entirely lost their metallic lustre. At length, his curiosity became so painful, that he actually alighted from the carriage, obtained a pair of steps, and ascended to the sign, in order, if possible, to solve the problem that had so much excited his curiosity. The mystery vanished, f1 an iron nail appeared in the centre of the spot, which had protected The copper leaf, for several inches round, from the corrosive action of the elements. The reader may satisfy him- self of the efficacy of such protection in his rambles through the metro- polis : he may frequently have noticed gilded, or rather coppered, sugar-loaves, suspended over the shops of grocers ; if he inspect them he will generally find the parts into which the iron supports have entered, preeminently brilliant. The rusting of a common piece of iron, if carefully inspected, will furnish an interesting object of con- templation. The oxide formed by the contact of moisture becomes ne- gative with respect to the metallic surface, and thus forming a galvanic circuit, exalts the oxidability of the mass ; and the rust consequently extends in a circle. We could extend these illustrations, but those already offereatare sufficient to sustain our position. Here then we conclude our history of the discoveries of Sir HUMPERY DAVY as they relate to the subject of Voltaic Elec- tricity. Without dwelling any longer upon their immense import- ance, we shall conclude by observing, that they afford without any exception the most perfect specimen of philosophical induction ever recorded. He commenced by the discovery of a simple prin- ciple, which was pursued fthrough all its relations and bearings ; and during the whole progress of the inquiry, we do not find that he had any occasion to retrace his steps for the purpose of correction. Justly has he observed, in his last Bakerian Lecture of 1826, that not- withstanding the various novel views which have been brought for- ward in this and other countries, and the great activity and extension of science, it is peculiarly satisfactory to find that he has nothing to alter in the fundamental theory laid down in his original communica- tion ; and which, after the lapse of twenty years, has continued as it was in the beginning, the guide and foundation of all his researches.