Chapter XIII (2)

Amongst the party present were several members of the Diet (Storthing), which had recently refused the applications of the King for various grants of money. This subject excited much animated conversation, and the majority of the persons present expressed their approbation at so bold and independent a measure. This called forth a political toast relating to the situation of their country; when the whole company, elated with wine and conversation, simultaneously burst forth into the national chorus of Norway, which had been composed as a prize poem during the short struggle against the union of that country with Sweden, and which was much admired by the Norwegians, and on all occasions sung by them with the utmost enthusiasm of feeling; but, notwithstanding the liberal politics of the party, they drank Sir Humphry's toast--"THE KING OF NORWAY AND SWEDEN"--with much apparent loyalty.

A succession of toasts followed, the last of which recommended "THE BRITISH CONSTITUTION AS A MODEL FOR ALL THE WORLD." With this sentiment the festivities concluded--a momentary silence ensued; the custom of the country assigned to a stranger the honourable office of returning to the host and hostess the thanks of the company for their hospitable reception; all eyes were anxiously fixed upon the English philosopher; and as soon as he was made acquainted with the duty he was expected to perform, he rose from his seat, and in allusion to the sentiment so recently drunk in compliment to himself, he proposed as a concluding toast, "NORWEGIAN HOSPITALITY A MODEL FOR ALL THE WORLD."

From Arendal the vessel proceeded to Laurvig, where she stopped only a few hours; but Sir Humphry seized this opportunity to go on shore to view the country, and he afterwards weighed the copper plates which had been attached to the ship in Christiansand, as the vessel was now to cross that deep bay, at the bottom of which is situated Christiana, the capital of the kingdom. The few plates were found to be in good order; and the results, which however must be allowed to have been very incomplete, confirmed, as far as they went, the conclusions to which he had been led by former experiments, viz. that 1/200 of iron surface was the proportion best calculated to defend the copper, without so overprotecting it as to favour the adhesion of marine productions; while they moreover proved that there is a mechanical as well as a chemical wear of the copper, which, in the most exposed part of the ship, and in the most rapid course, bears a relation to it of nearly 2 to 4.55.

The country increased in fertility towards the eastern parts of it; but it possessed much less beauty than the neighbourhood of Mandal.

As soon as Davy perceived that the vessel had to pass near the mouth of the Glommen, the largest river of Norway, he directed that she should enter it. Steam-boats appeared to have been entirely unknown in that part of the country. The inhabitants of the town of Frederickstadt were alarmed by the belief that the vessel was on fire, and they ran down to the beach in multitudes. As the vessel proceeded up the river, the people every where left their work, looked on awhile in silent amazement, and then shouted with delight.

The vessel anchored a mile below the great fall of the Glommen, called _Sarpen_, and which Davy visited on the following day (July 15). Three Kings of Denmark have visited this fall, and a name commemorates the spot whence they viewed this grand scene of nature. The fall is not one perpendicular descent, but consists of three sheets of water closely succeeding each other; and, by means of a barometer, he ascertained the entire altitude to be little more than a hundred feet. In comparing the character of this waterfall with those of the others he had visited, he observes, that size is merely comparative; and that he prefers the Velino at Terni, on account of the harmony that exists in all its parts. It displays all the force and power of the element, in its rapid and precipitous descent; and you feel that even man would be nothing in its waves, and would be dashed to pieces by its force. The whole scene is embraced at once by the eye, and the effect is almost as sublime as that of the Glommen, where the river is at least one hundred times as large; for the Glommen falls, as it were, from a whole valley upon a mountain of granite; and unless where you see the giant pines of Norway, fifty or sixty feet in height, carried down by it and swimming in its whirlpools like straws, you have no idea of its magnitude and power. Considering these waterfalls in all their relations, he is disposed to think, that while that of Velino is the most perfect and beautiful, the fall of the Glommen is the most awful.

On both sides of this fall are extensive saw-mills, with machinery of very imperfect construction. Davy spent some time with the proprietors of these mills, who were acquainted with the English language, and showed him every attention in their power. As an angler, he spoke with regret of the immense quantity of sawdust which floated in the water, and formed almost hills along the banks, and which, he observed, must be poisonous to the fish, by sometimes choking their gills, and interfering with their respiration.

From the Glommen the steam-vessel passed through the Svinesund to Strömstadt, the first town in Sweden beyond the frontier of Norway, from which Charles XII. essayed to besiege the neighbouring fortress of Frederickstadt in Norway. From Strömstadt, Davy set out on the 17th of July, and reached Gottenburg by land in two days, where he remained for a short time, in consequence of a slight indisposition. On his journey, he had a conversation with Oscar, the Crown Prince of Denmark, who, under the direction of Berzelius, had diligently devoted himself to the study of chemistry. He conversed with our philosopher upon various subjects connected with that science; and Davy, on his return to England, declared that he had never met with a more enlightened person.

The Crown Prince expressed great surprise, as indeed did every body in Sweden, on hearing that it was not Davy's intention to visit Professor Berzelius at Stockholm; and his astonishment was still farther increased, when he was informed by himself, that he came to Norway and Sweden with no other view than to enjoy the diversion of hunting and fishing! He however did by accident afterwards meet Berzelius, but his interview was but of short duration.

From Gottenburg he hastened to Copenhagen, where he renewed his acquaintance with Prince Christian of Denmark, cousin of the King, and heir presumptive of the crown; in whose company he had some years before observed an eruption of Mount Vesuvius. He also visited Professor Oersted, and earnestly requested that he might see the apparatus by which that philosopher had made those electro-magnetic experiments which had rendered his name so celebrated throughout Europe.

He next proceeded to Neuburg and Altona, where he intended to re-embark for England in the steam-vessel which had, during the interval of his continental tour, made a voyage to England, and was again on her way to the Elbe. At the suggestion, however, of Professor Schumacher, the astronomical professor at Copenhagen, but residing at Altona, in whose society he passed a great portion of his time, he accompanied that gentleman to Bremen, in order to make the acquaintance of the venerable Dr. Olbers, who, since his retirement from an extensive medical practice, had entirely devoted his time to the pursuit of his favourite science astronomy; as well as to be introduced to Professor Gauss, of Gottingen, who happened to be at that time carrying on his geodetical operations for the admeasurement of the kingdom of Hanover.

Davy expressed a great desire to see the telescope with which Dr. Olbers had discovered the two planets, Pallas and Vesta, and which to his great surprise turned out to be a very ordinary instrument. His personal intercourse with these two celebrated philosophers appeared to afford him the highest satisfaction; and he spent two days most agreeably in their society.

In his "Salmonia," he gives us some account of his adventures as an angler during this short excursion to Norway and Sweden. "All the Norwegian rivers," says he, "that I tried (and they were in the southern parts) contained salmon. I fished in the Glommen, one of the largest rivers in Europe; in the Mandals, which appeared to me the best fitted for taking salmon; and in the Arendal; but, though I saw salmon rise in these rivers, I never took a fish larger than a sea-trout; of these I always caught many--and even in the _fiords_, or small inland salt-water bays; but, I think, never any one more than a pound in weight. It is true that I was in Norway in the beginning of July, in exceedingly bright weather, and when there was no night; for even at twelve o'clock the sky was so bright, that I read the smallest print in the columns of a newspaper. I was in Sweden later--in August: I fished in the magnificent Gotha, below that grand fall, Trollhetta, which to see is worth a voyage from England; but I never raised there any fish worth taking. I caught, in this noble stream, a little trout about as long as my hand; and the only fish I got to eat at Trollhetta was bream."

He again embarked, on the 14th of August, on board the Comet steam-vessel, which had ascended the Weser as high as her draught of water would allow, and reached England, after a very boisterous passage, on the 17th of the same month; indeed, the vessel left the mouth of the Weser with a contrary wind, and the pilot was unwilling to put to sea, but Davy insisted on proceeding without delay. During the whole passage he suffered extremely from sea-sickness, and in a letter written to Professor Schumacher, shortly after landing, he remarks that "the sea is a glorious dominion, but a wretched habitation."

* * * * *

On the 9th of June 1825, Sir Humphry read before the Royal Society his third and most elaborate paper upon Copper sheathing, entitled "Farther Researches on the Preservation of Metals by Electro-chemical Means."

In this memoir, he states it to be his belief, that there is nothing in the poisonous nature of the copper to prevent the adhesion of weeds and testaceous animals; for he observes, that they will readily adhere to the poisonous salts of lead which commonly form upon the metal protecting the fore-part of the keel; and even upon copper, provided it be in such a state of chemical combination as to be insoluble. It is then, in his opinion, the _solution_ of the metal--the _wear_ of its surface, by keeping it smooth, which prevents the adhesion of foreign matter. Whenever the copper is unequally worn, deposits will, without doubt, rest in the rough parts, or depressions in the metal, and afford a soil or bed in which sea-weeds can fix their roots, and to which zoophytes and shell-fish can adhere; but there is another cause of foulness on the protected sheathing, arising from the deposit of earthy matter upon the copper, in consequence of its electro-negative condition.

In relation to this subject, Davy has offered some observations upon the effects produced by partial formations of rust, which appear to me to be exceedingly interesting and important.

When copper has been applied to the bottom of a ship for a certain time, he says, a green coating, or rust, consisting of oxide, sub-muriate, and carbonate of copper, forms upon it; not equally throughout, but partially, and which, it is evident, must produce a _secondary_, partial, and unequal action, since those substances are negative with respect to metallic copper, and will consequently, by producing with it a Voltaic circuit, occasion a more rapid corrosion of those parts still exposed to sea-water: from this cause, sheets are often found perforated with holes in one part, after having been used for five or six years; while in other parts they are comparatively sound.[95] In like manner, the heads of the mixed metal nails, consisting of copper alloyed by a small quantity of tin, which are in common use in the Navy, give rise to oxides that are negative with respect to the copper, so that the latter is often worn into deep and irregular cavities in their vicinity.

[95] The rusting of a common piece of iron, if carefully
inspected, furnishes a beautiful illustration of this secondary
action. The oxide, at first a mere speck, and formed perhaps by a
globule of water, becomes negative with respect to the contiguous
surface, and by thus forming a Voltaic circuit, exalts its
oxidability, and the rust consequently extends in a circle.

A series of very interesting experiments, fully detailed in this memoir, which were instituted for the purpose of ascertaining the extent of the diminution of electrical action in instances of imperfect or irregular conducting surfaces, led him to the general conclusion, that a very small quantity of the imperfect or fluid conductor was sufficient to transmit the electrical power, or to complete the chain. This induced him to try whether copper, if nailed upon wood, and protected merely by zinc or iron on its _under_ surface, or on that next the wood, might not be defended from corrosion: a question of great practical moment with regard to the arrangement of protectors. For this purpose, he covered a piece of wood with small sheets of copper, a nail of zinc of about 1/100 part of the surface having been previously driven into the wood: the copper surface remained perfectly bright in sea-water for many weeks; and when the result was examined, it was found that the zinc had only suffered partial corrosion; that the wood was moist, and that, on the interior of the copper there was a considerable portion of revived zinc, so that the negative electricity, by its operation, provided materials for its future and constant excitement. In several trials of the same kind, iron was used with similar results; and in all these experiments there appeared to be this peculiarity in the appearance of the copper, that unless the protecting metal below was in a large mass, there were no depositions of calcareous or magnesian earths upon the metal; it was clean and bright, but never coated. The copper in these experiments was nailed sometimes upon paper, sometimes upon the mere wood, and sometimes upon linen; and the communication was partially interrupted between the external and internal surfaces by cement; but even one side or junction of a sheet seemed to allow sufficient communication between the moisture on the under surface and the sea-water without, to produce the electrical effect of preservation. This last experiment of Davy is of greater importance than may at first appear, in showing what a small proportion of conducting fluid will complete a circuit, and in thus explaining phenomena, as I shall presently show, which might not otherwise be suspected to have an electrical origin.

These results upon perfect and imperfect conductors led him to another enquiry, important as it relates to the practical application of the principle, namely, as to the extent and nature of the contact or relation between the copper and the preserving metal. He was unable to produce any protecting action of zinc or iron upon copper through the thinnest stratum of air, or the finest leaf of mica, or of dry paper; but the action of the metals did not seem to be much impaired by the ordinary coating of oxide or rust; nor was it destroyed when the finest bibulous, or _silver-paper_, as it is commonly called, was between them, being moistened with sea-water. He made an experiment with different folds of this paper. Pieces of copper were covered with one, two, three, four, five, and six folds; and over them were placed pieces of zinc, which were fastened closely to them by thread; each piece of copper, thus protected, was exposed in a vessel of sea-water, so that the folds of paper were all moist.

It was found in the case in which a single leaf of paper was between the zinc and the copper, there was no corrosion of the copper; in the case in which there were two leaves, there was a very slight effect; with three, the corrosion was distinct; and it increased, till with six folds the protecting power appeared to be lost; and in the case of the single leaf, the result differed only from that produced by immediate contact, in there not being any deposition of earthy matter. Other experiments likewise proved that there was no absolute contact of the metals through the moist paper; for, although a thin plate of mica, as before stated, entirely destroyed the protecting effect of zinc, yet when a hole was made in it, so as to admit a very thin layer of moisture between the zinc and copper, the corrosion of the latter, though not prevented, was considerably diminished.

The experimental part of this paper concludes with an account of various trials to determine the electro-chemical powers of metals in menstrua out of the contact, or to a certain extent removed from the contact of air; in order, if possible, to diminish the rapid waste of the protecting metals. In the progress of these experiments he exhibits, in a most beautiful manner, the singular effect of different proportions of a fixed alkali, when mixed with sea-water, in rendering the iron, in its Voltaic connection with copper, more or less negative.

He terminates the paper with some observations of a practical nature, relative to the best modes of rendering iron applicable to the purposes of protection; but, as these have been already embodied in the investigation, it is not necessary to notice them farther in this place.

That I may give to the history of this subject all the perspicuity which it can derive from the connexion of its several parts, I shall now, in defiance of chronological order, proceed to consider his last Bakerian Lecture, "On the Relations of Electrical Changes," which was read before the Royal Society, on the 8th of June 1826. In which, after referring to his former papers on the chemical agencies of electricity, and the general laws of decomposition which were developed in them, he enters into some historical details respecting the origin and progress of electro-chemical science; being induced so to do, from a knowledge of the very erroneous statements which had been published upon the subject abroad, and repeated in this country. At the conclusion of this lecture, in reverting to the subject of Voltaic protection, he says: "A great variety of experiments, made in different parts of the world, have proved the full efficacy of the electro-chemical means of preserving metals, particularly the copper sheathing of ships; but a hope I had once indulged, that the peculiar electrical state would prevent the adhesion of weeds or insects, has not been realized; protected ships have often indeed returned, after long voyages, perfectly bright,[96] and cleaner than unprotected ships; yet this is not always the case; and though the _whole_ of the copper may be preserved from chemical solution in steam-vessels (from the rapidity of their motion) by these means,--yet they must be adopted in common ships only so as to preserve a portion,--so applied, as to suffer a certain solution of the copper;[97] and an absolute remedy for adhesions is to be sought for by other more refined means of protection, and which appear to be indicated by these researches.

[96] The Carnbrea Castle, a large vessel, of upwards of six
hundred and fifty tons, was furnished with four protectors,
two on the stern, and two on the bow, equal together to about
1-104th of the surface of copper. She had been protected more
than twelve months, and had made the voyage to Calcutta and back.
She came into the river perfectly bright; and, when examined in
the dry-dock, was found entirely free from any adhesion, and
offered a beautiful and almost polished surface; and there seemed
to be no greater wear of copper than could be accounted for from
mechanical causes.

[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.

These are important facts, and should dispose those who may preside over judicial enquiries, to pause before they infer the inferiority of copper sheeting from the rapidity of its decay.[99]--I have now concluded a review of those admirable researches which led Sir Humphry Davy to suggest and mature a plan for arresting the corrosion of the copper sheathing of vessels by Voltaic action. Mr. Babbage has said that he was authorised in stating, that "this was regarded by Laplace as the greatest of Davy's discoveries." I do not think, however, that it should be considered in the light of a separate performance: we do injustice to the philosopher by regarding it as an independent and isolated discovery; for it was the result of a long series of enquiries, which commenced by establishing the laws of electro-chemistry,--which led him to the decomposition of the alkalies and earths,--suggested to his unwearied genius a succession of novel researches, in a new field of enquiry,--and concluded, as we have seen, in producing the most striking results by means of the greatest simplicity. Not once during the progress of this enquiry had he any occasion to retrace his steps for the purpose of correction: justly has he observed in his last Bakerian Lecture, that, notwithstanding the various novel views which have been brought forward 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 communication; and which, after the lapse of twenty years, has continued, as it was in the beginning, the guide and foundation of all his researches.

[99] This observation was suggested by an examination of a late
judgment of the Court of Common Pleas, in the case of Jones
_v._ Bright and others, on showing cause against rule for a new
trial. This was an action brought by the Plaintiff against the
Defendants for selling him copper, for the purpose of sheathing
the ship Isabella, which, from the rapidity of its corrosion,
was inferred to have an inherent defect in its composition. In
this case it was held, that with respect to _warranty_, there is
a very wide difference as it applies to articles which are not
the subject of manufacture, and those which are the produce of
manufacture and of human industry. In the one case, it may be
that no prudence, no care, could have guarded against a secret
defect; in the other, by using due care, and providing proper
materials, any defect in the manufacture may be guarded against.
"In the case of the bowsprit, the man did not make the timber
which composed the bowsprit; he merely cut it out, and fitted
it to meet the purpose, and could therefore by no means have
guarded against the rottenness in the centre of that bowsprit:
but if a man makes copper, he may guard against inherent defects
in that copper, by taking care that the copper contains a proper
proportion of pure copper; and also by taking care that it is so
well manufactured, that it does not drink in a greater quantity
of oxygen than ought to be admitted into it, and that that
oxygen, which of necessity gets in, (for some will,) shall be
so distributed, that it shall not operate, as in the opinion of
an intelligent witness the oxygen in this case did operate, by
forming itself in patches, and thereby rendering it soft, and
rendering the copper incapable of resisting the influence of
salt-water--that he can guard against." With all due deference
to the learned Judge, suppose it be shown that no human wisdom
can guard against those circumstances by which a portion of the
copper surface may be rendered more highly electro-positive,
what becomes of the judgment? That the decay of copper sheathing
is effected by extrinsic causes, and does not necessarily
depend upon an inherent defect in the metal, may be proved in
numerous ways. If it were owing to the quality of the copper,
why should five, ten, or twenty sheets out of a hundred, made
from the same charge of metal in a furnace and manufactured under
precisely similar circumstances, be affected, and the remainder
be perfectly sound? Why, again, should sheets, made from several
distinct charges, placed on a particular vessel, be acted upon,
while the same copper on other bottoms is not more than usually
dissolved? Did any inherent defect exist in the metal, it surely
must have equally affected the whole batch.

It is possible that, in some cases, in consequence of the sheets
not having been properly cleansed before they are rolled, a
portion of the oxide may be pressed into them by the rollers. In
such a case, a Voltaic effect might be produced, and portions of
the metallic surface rendered more electro-positive.

The President and Council of the Royal Society appear to have been swayed by this consideration, when they adjudged to him "A Royal Medal,[100] for his Bakerian Lecture on the relations of electrical changes, considered as the last link, in order of time, of the splendid chain of discoveries in chemical electricity, which have been continued for so many years of his valuable life."

[100] In the year 1825, His Majesty George IV. communicated to
the Royal Society, through Mr. Peel, his intention to found two
gold medals, of the value of fifty guineas each, to be awarded
annually by the Council of the Royal Society, in such a manner
as shall, by the excitement of competition among men of science,
seem best calculated to promote the object for which the Royal
Society was instituted.

Thus had Davy now received from the Royal Society all the honours they were capable of conferring upon him. In the year 1805, they adjudged to him the medal on Sir Godfrey Copley's donation for his various communications published in the Philosophical Transactions; in 1817, they awarded him the Rumford medals for his papers on combustion and flame; and in 1827, upon the grounds just stated, the President and Council expressed their unabated admiration by conferring upon him the only medal which remained for his acceptance--that which had been recently founded by their patron, his late Majesty.

Having thus disposed of the speculative part of his admirable enquiry, it will be interesting to pause in our narrative, in order to take a philosophical review of the progress of Voltaic discovery, in its relations to this particular object. It is a subject well calculated to afford a valuable lesson to the experimentalist, and at the same time to furnish illustrations, more striking even than that of the safety-lamp, of the necessity of that complicated species of machinery, without which the human mind is frequently unable to grapple with the simplicities of truth. It is true, that the fact of a galvanic effect being excited by the contact of two dissimilar metals was noticed in the earliest stages of the enquiry, but it is equally evident that the phenomena which attended it, and the laws by which it was directed, required for their discovery and elucidation the assistance of the Voltaic battery. In reference to Davy, it may be here repeated, that the power of obtaining simple results, through complicated means, was one of the most distinguishing features of his genius.

It has been stated, that the Alarm frigate, the first coppered ship in our Navy, displayed very striking evidence of the effect of Voltaic action, in the rapid corrosion of its iron.[101] As early as 1783, after copper sheathing had become general, the Government issued orders that all the bolts under the line of fluitation should, in future, be of copper; but at that period, it was not possible that any idea could be entertained as to the true nature of the operation by which the iron was thus rapidly corroded, for it was only in the year 1797 that Dr. Ash noticed, for the first time, a phenomenon which was subsequently referred to the action of a simple Voltaic circuit.

[101] Numerous are the instances of later date which might be
adduced in illustration of the same fact; and it is now generally
supposed that it may have been a frequent cause of ships
foundering at sea. By oxidation, the volume of the iron at first
increases, and then diminishes; in consequence of which the ship
leaks, or, to use a technical expression, becomes "_bolt sick_."
When the Salvador del Mundo was docked at Plymouth, in February
1815, the iron fastenings were in such a state of corrosion, that
five planks near the bilge dropped into the dock when the water
left her.

It is a very curious circumstance in the history of this subject, that, for many years after the Voltaic influence had been recognised as the agent in metallic corrosion, so far from the existence of the accompanying phenomenon of preservation being suspected, it was even supposed that the metals mutually corroded each other. At so late a date as 1813, we find Davy himself, in the letter addressed to M. Alavair, published in these memoirs,[102] dwelling upon the necessity of avoiding metallic contact, in order to prevent _corrosion_, without throwing out the most distant hint as to the simultaneous production of a converse effect.

[102] Page 13 of this Volume.

The first distinct notice of a metal being preserved from oxidation by the contact of a dissimilar metal, is at once referred to a chemical law, without a reference even to its possible connection with Voltaic action; and, striking as the fact may now appear, it never attracted much attention. M. Proust observed, that although copper vessels be so imperfectly tinned, as to leave portions of the surface uncovered, still, in cooking utensils, we shall be equally protected from the poisonous effects of the former metal; because, says he, the superior readiness with which tin is oxidized and acted upon by acids, when compared with copper, will not allow this latter metal to appropriate to itself a single atom of oxygen.[103] The same chemist observes, that if lead be associated with tin, it will be incapable of furnishing to acids any saturnine impregnation, since the latter, being more oxidable than the former, will exclusively dissolve, and thus prevent the former from being attacked.

[103] As far as the principle of Voltaic protection goes,
this may be very true; but it must be remembered that the
acid generally present upon these occasions is acetic acid,
which rises in distillation with water, so that at the boiling
temperature it will be carried beyond the sphere of Voltaic
influence, and may thus act upon the denuded copper as much as
though tin were not present.

Whether the principle of Voltaic protection be applicable or not to the purpose of preserving copper sheathing, it is evident that it will suggest numerous other expedients of high importance in the arts, while it will explain phenomena previously unintelligible. 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 also extended the principle for the preservation of steel instruments by guards of zinc: razors and lancets may be thus defended with perfect success. In the construction of monuments which are to transmit to posterity the record of important events, the artist will be careful in avoiding the contact of different metals: it is thus that the Etruscan inscriptions, engraved upon pure lead, are preserved to the present day; while medals of mixed metals of a much more recent date are corroded.

Numerous are the facts daily presented before us, which receive from this principle a satisfactory explanation. To the philosopher, the examination of its agencies will furnish a perpetual source of instruction and amusement; and I will here enumerate a few simple instances of its effects: in the first place, for the purpose of showing that, whenever a principle or discovery involves or unfolds a law of Nature, its applications are almost inexhaustible, and that, however abstracted it may appear, it is sooner or later employed for the common purposes of life; and in the next place, in the hope of convincing the reader, that there does not exist any source of pleasure so extensive and so permanent as that derived from the stores of philosophy. The saunterer stumbles over the stone that may cross his path, and vents only his vexation at the interruption; but to the philosopher there is not a body, animate or inanimate, with which he can come in contact, that does not yield its treasures at his approach, and contribute to extend the pleasures of his existence.

I well remember some years ago, that, in passing through Deptford, my curiosity was excited by the extraordinary brilliancy of a portion of the gilded sign of an inn in that town, while its other parts had entirely lost their metallic lustre. Having obtained a ladder, I ascended to the sign, in order, if possible, to solve the problem that had so greatly interested me: the mystery immediately vanished; for an iron nail appeared in the centre of the spot, which had protected the copper leaf for several inches around it. Any person may easily satisfy himself of the efficacy of such protection, in his rambles through the metropolis, by noticing the gilded, or rather coppered, sugar-loaves[104] so commonly suspended over the shops of grocers, when he will frequently perceive that the parts into which the iron supports have entered, unless the latter have been painted, shine preeminently brilliant. If a still more familiar example of the effect of a simple Voltaic circuit be required, it is afforded by the iron palisadoes, where the iron is constantly corroded at its point of contact with the lead by which it is cemented into the stone. These examples are not only interesting from their simplicity, but from their demonstrating the small quantity of a conducting fluid which is sufficient to transmit the electrical power, or to complete a simple circuit: a fact which, it will be remembered, the experiments of Davy had before established.[105]

[104] There is an excellent example at this time in the London
Road leading to the Elephant-and-Castle.

[105] Page 248 of this Volume.

As our knowledge advances, these principles will no doubt derive other illustrations, and be found capable of more extensive application; for as yet we are but in the infancy of the enquiry. I have lately been engaged in a series of experiments, the results of which, I confidently anticipate, will lead to some new facts connected with the changes produced on the negative metal of a Voltaic circuit; an account of which I hope shortly to submit to the Royal Society. I shall on this occasion merely notice one result, which appears to me to admit of an immediate application to one of the most important circumstances of life--the purity of water contained in leaden cisterns.

My attention has for several years been directed to the state of the water with which the metropolis is supplied; and upon having been lately requested to propose a remedy for preventing the action of a spring in the neighbourhood of London upon lead, which it had been found to corrode in a very rapid manner, I suggested the expediency of protecting the pipes and cisterns with surfaces of iron; but before such a plan was put in execution, I proposed to try its efficacy in the laboratory:--the first result was very startling; for, instead of preventing, as I had anticipated, I found that it greatly increased, the solution of the lead. After various experiments, I arrived at the conclusion, that lead, when rendered negative by iron, and placed in contact with weak saline solutions,--such, for instance, as common spring water,--was dissolved; in consequence of the decomposition of the salts and the transference of their elements according to the general law, the acid passing to the iron, and the alkali to the lead; and so powerfully is this latter body acted upon by an alkali, that, if a slip of it be immersed in a solution of potash or soda, its crystalline texture is so rapidly developed, that its surface exhibits an appearance similar to that presented by tin-plate, and which is designated by the term _moirée_.

I apprehend that most of the anomalous cases of the solution of lead in common water, which have for so many years embarrassed the chemist, may thus receive an explanation. An eminent physician lately informed me, that some time since he was called upon to attend a family who had evidently suffered from the effects of saturnine poison, and that he well remembers there was an iron pump in the cistern that supplied the water. Upon showing the results of my experiment to a no less eminent chemist, he was immediately reminded of a circumstance which occurred at Islington, where the water was found to corrode the lead in which it was received: in this vessel there was an iron bar; and the fact would not have attracted his notice, nor have been impressed upon his recollection, but from the unusual state of corrosion in which it appeared.

I shall conclude these observations by an account of "the change which some musket balls, taken out of Shrapnell's shells, had undergone," by Mr. Faraday, and which is published in the 16th volume of the Quarterly Journal of Science, for the year 1823. This history is not only interesting on account of the high chemical character of its author, but satisfactory as being in direct opposition to previously established facts; and cannot therefore have received any bias from preconceived theory.

"Mr. Marsh of Woolwich gave me some musket balls, which had been taken out of Shrapnell's shells. The shells had lain in the bottoms of ships, and probably had sea-water amongst them. When the bullets are put in, the aperture is merely closed by a common cork. These bullets were variously acted upon: some were affected only superficially, others more deeply, and some were entirely changed. The substance produced is hard and brittle; it splits on the ball, and presents an appearance like some hard varieties of hæmatite; its colour is brown, becoming, when heated, red; it fuses on platinum foil into a yellow flaky substance like litharge. Powdered and boiled in water, no muriatic acid or lead was found in solution. It dissolved in nitric acid without leaving any residuum, and the solution gave very faint indications only of muriatic acid. It is a _protoxide of lead_, perhaps formed, in some way, by the galvanic action of the iron shell and the leaden ball, assisted probably by the sea-water. It would be very interesting to know the state of the shells in which a change like this has taken place to any extent. _It might have been expected, that as long as any iron remained, the lead would have been preserved in the metallic state._"

In one experiment, I found that a piece of lead protected by iron underwent solution in water containing nitrate of potash, while it resisted the action of very dilute nitric acid: upon this point, however, farther enquiry is necessary; for I subsequently failed in producing the same effect, owing, no doubt, to having employed too strong an acid.

Let us return from this digression to the subject of Sir Humphry Davy's Protectors. It only remains for me to relate the results which followed the practical application of the Voltaic principles which his various experiments had developed.

In the month of May 1824, directions were issued by the Lords of the Admiralty to protect, in future, the copper sheathing of all his Majesty's ships which might be taken into dock, upon the plan proposed by Sir Humphry Davy.

The protectors were bars of iron six inches wide at their base, three inches in thickness in their centre, and, in outward form, the segment of an extended circle. They were usually placed on each side of the ship in a horizontal position, viz. in midships about three feet under water--on the keel in a line with these--and the remainder in the fore and afterparts of the ship (about three feet under the line of fluitation), as far forward and abaft as the curvatures of their respective bodies would allow of their lying flat upon the surface of the copper.

As it is difficult by verbal description alone to convey a sufficiently distinct idea of this subject to persons unacquainted with naval architecture, I have introduced a sketch, exhibiting the _general_ position of the _Protectors_, although they are necessarily exaggerated in size, or they would have appeared as mere specks upon the drawing.

On several ships, some of the protectors, in the stem and the stern, were placed _vertically_; in which case they were fastened to the stems and stern-posts; and in this manner they were found to act more powerfully in preserving the copper, than when they were all placed horizontally. The ends of the protectors were rounded, in order to prevent any great resistance to the water, and they were fastened to the bottoms of the ships with copper bolts, the iron being counter-sunk to receive their heads, and the holes were then filled with carbonate of lime, or Parker's cement. To bring about the best possible contact of all the copper sheets, their edges, which lap over each other, where the nails are driven to fasten them to the ships, were rubbed bright, first with sand-paper, and finally with glass-paper.

Shortly after the ships thus protected were sent to sea, it was evident to all on board, from their dull sailing, that the bottoms had become very foul; and on being examined in dry docks, it was found that the copper was completely covered with sea-weed, shell-fish of various kinds, and myriads of small marine insects. Upon their removal, however, it was found, on weighing the sheets, that the copper had suffered little or no loss; thus proving that, although its practical application had failed from unforeseen circumstances, the principle of protection was true, and had fully justified the expectation of its success.

The copper near the protectors was much more foul than that at a greater distance from them; and there was, moreover, a considerable deposit of carbonate of lime, and of carbonate and hydrate of magnesia, in their vicinity.

Sir Humphry Davy immediately suggested, as a remedy for this evil, that the bottoms should be scraped, and the copper washed with a small quantity of acidulous water; and he also proposed that the protectors should in future be placed under, instead of over, the copper sheathing. This plan was immediately adopted. Discs of cast-iron three and a half inches in diameter, and one-fourth of an inch in thickness, were let into the plank of the bottom of the Glasgow, of fifty guns, on the starboard side only--the larboard side having been left without any protection. These discs were in the proportion of one to every four sheets of copper, and over them were placed pieces of brown paper, and over the paper thin sheet-lead, so that the latter metal was in contact with the copper sheathing. A similar experiment was also tried on the Zebra, of eighteen guns, substituting, however, discs of zinc[106] for those of iron.

[106] It would appear that Davy latterly preferred zinc to
iron, as the protecting metal. In a letter, dated October
1826, addressed to a ship-owner, who had made some enquiries
of him upon the subject, he says--"The rust of iron, if a ship
is becalmed, seems to promote the adhesion of weeds; I should
therefore always prefer pieces of zinc, which may be very much
smaller, and which, in the cases I have heard of their being
used, have had the best effect."

The bottom of the Glasgow was examined twelve months afterwards, when the discs of iron were found oxidated throughout, presenting in their appearance the characters of plumbago. The copper on the starboard side was preserved, but covered with weeds and shell-fish. The sheets on the larboard had undergone the usual waste, but were clean.

The Zebra was docked four years after the experiment had commenced, when the zinc protectors were perfect, and it did not appear that they had exerted any influence in preserving the copper, as it had wasted equally on both sides. It may be presumed in this case that the Voltaic circuit had by some fault in the arrangement been interrupted.

The apparent conversion of iron into a substance resembling plumbago, by the action of sea-water, has been frequently noticed. The protectors thus changed[107] were, to a considerable depth from the surface, so soft as to be easily cut by a knife; but after being exposed for some time to the action of the atmosphere, they became harder, and even brittle. A portion of this soft substance having been wrapped in paper for the purpose of examination, and placed in the pocket of a shipwright, gave rise to a very curious and unexpected result: at first, the artist, like Futitorious with his chestnuts, thought he perceived a genial warmth; but the effect was shortly less equivocal; the substance became hot, and presently passed into a state of absolute ignition. Various theories have been suggested for its explanation: Mr. Daniell has advanced an opinion which supposes the formation of silicon, and thus accounts for the spontaneous ignition by the action of air.

[107] In the Annals of Philosophy (vol. v.) may be found a paper
by Dr. Henry, on the conversion of cast iron pipes into plumbago.
This change appears to have been effected by the action of water
containing muriate of soda, and the muriates of lime and of
magnesia. Cast iron contains a considerable portion of carbon;
the change is therefore readily explained on the supposition of
the removal of the principal metallic part by these salts. The
muriates of lime and magnesia have been observed by Dr. Henry to
discharge writing ink from the labels of bottles, to which they
had been accidentally applied; and the same ingenious chemist has
been baffled in his attempts to restore the legibility of ink
upon paper which had been exposed to sea-water. The texture of
the paper was not injured, but the iron basis of the ink, as well
as the gallic acid, was entirely removed.

The disadvantages which arise from the foulness of ships' bottoms, particularly when on foreign stations, where there are no dry docks to receive them, are so serious, that the Government was obliged, in July 1825, to order the discontinuance of the protectors on all sea-going ships; but directed that they should still be used upon all those that were laid up in our ports. When, however, an examination of the latter took place, they were found to be much more foul than those which had been in motion at sea: shell-fish of various kinds had adhered to them so closely, that it was even necessary to use percussion to remove them, which not only indented the copper, but in many instances actually fractured it.

Under all these discouraging circumstances, the unwelcome conviction was forced upon the agents of Government, that the plan was incapable of successful application, and it was accordingly altogether abandoned in September 1828.

Such were the results of the experiments carried on in the ports of England, for the protection of copper sheathing, from the success of which Sir H. Davy justly expected honours, fame, and reward. That his disappointment was great, may be readily imagined, and it is supposed to have had a marked influence upon his future character. It is much to be regretted that his vexation should have been heightened by the unjust and bitter attacks made upon him by the periodical press, and by those subalterns in science, who, unable to appreciate the beauty of the principle he had so ably developed, saw only in its details an object for sarcasm, and in its failure an opportunity for censure; while those whose stations should have implied superior knowledge, in the pride and arrogance of assumed contempt, sought a refuge from the humiliation of ignorance.

That Davy was severely hurt by these attacks, is a fact well known to his friends. In a letter to Mr. Children he says: "A mind of much sensibility might be disgusted, and one might be induced to say, Why should I labour for public objects, merely to meet abuse?--I am irritated by them more than I ought to be; but I am getting wiser every day--recollecting Galileo, and the times when philosophers and public benefactors were burnt for their services." In another letter he alludes to the sycophancy of a chemical journal, which, after the grossest abuse, suddenly turned round, and disgusted him with its adulation. "I never shake hands," says he, "with chimney sweepers, even when in their May-day clothes, and when they call me '_Your honour_.'"