Chapter XIII (1)

The Liquefaction of Chlorine Gas first effected by Mr. Faraday,
and witnessed by the Author.--Sir H. Davy continues the
investigation.--His paper on the application of Liquefiable
Gases as mechanical agents.--Other probable uses of these
bodies.--He proposes several methods to prevent the fumes which
arise from Smelting-furnaces.--Importance of the subject. His
Letters to Mr. Vivian.--The Government solicit the advice of
the Royal Society on the subject of protecting the Copper
Sheathing of Ships from the action of sea-water.--Sir H.
Davy charges himself with this enquiry.--He proposes a plan
of protection founded on Voltaic principles.--His numerous
experiments.--He embarks on board the Comet steam-vessel
bound to Heligoland, in order to try his plan on a vessel
in motion.--He arrives at Mandal, lands, and fishes in the
lakes.--The Protectors washed away.--He teaches the inhabitants
of Christiansand to crimp fish--He remains a few days at
Arendal.--A Norwegian dinner.--The Protectors are examined and
weighed.--Results of the experiment.--The steam-vessel proceeds
up the Glommen.--He visits the great waterfall--Passes into
Sweden.--Has an interview with the Crown Prince of Denmark,
and afterwards with Prince Christian at Copenhagen.--He
visits Professor Oersted.--He proceeds to Bremen to see Dr.
Olbers.--Returns to England.--His third paper read before the
Royal Society.--Voltaic influence of patches of rust.--A small
quantity of fluid sufficient to complete the circuit.--He
receives from the Royal Society the Royal Medal.--The Progress
of Voltaic discovery reviewed.--The principle is of extensive
application.--The Author's researches into the cause of the
solution of Lead in spring water.--An account of the numerous
trials of Protectors.--Failure of the plan.--Report of the
French on the state of the protected frigate La Constance.--Dr.
Revere's new plan of Protection.

Every incident, however trifling, if it relates to a great scientific discovery, merits the attention of the historian. As it accidentally occurred to me, and to me alone, to witness the original experiment by which Mr. Faraday first condensed chlorine gas into a liquid, I shall here state the circumstances under which its liquefaction was effected.

I had been invited to dine with Sir Humphry Davy, on Wednesday the 5th of March 1823, for the purpose of meeting the Reverend Uriah Tonkin, the heir of his early friend and benefactor of that name.[80] On quitting my house for that purpose, I perceived that I had time to spare, and I accordingly called in my way at the Royal Institution. Upon descending into the laboratory, I found Mr. Faraday engaged in experiments on chlorine and its hydrate in closed tubes. It appeared to me that the tube in which he was operating upon this substance contained some oily matter, and I rallied him upon the carelessness of employing soiled vessels. Mr. Faraday, upon inspecting the tube, acknowledged the justness of my remark, and expressed his surprise at the circumstance. In consequence of which, he immediately proceeded to file off the sealed end; when, to our great astonishment, the contents suddenly exploded, and the oily matter vanished!

[80] Sir Humphry had expressed to me, on the preceding Thursday,
at the Royal Society, his wish to purchase the old house in
Penzance, which, as the reader will remember, was the early scene
of his chemical operations; and, at his request, I conversed with
Mr. Tonkin upon the subject; but it immediately appeared that
the interest which the Corporation of Penzance possessed in the
estate presented an insurmountable obstacle to the accomplishment
of his object.

Mr. Faraday was completely at a loss to explain the occurrence, and proceeded to repeat the experiment with a view to its elucidation. I was unable, however, to remain and witness the result.

Upon mentioning the circumstance to Sir Humphry Davy after dinner, he appeared much surprised; and after a few moments of apparent abstraction, he said, "I shall enquire about this experiment to-morrow."

Early on the next morning, I received from Mr. Faraday the following laconic note:

DEAR SIR,

The _oil_ you noticed yesterday turns out to be liquid chlorine.

Yours faithfully,

M. FARADAY.

It is well known that, before the year 1810, the solid substance obtained by exposing chlorine, as usually procured, to a low temperature, was considered as the gas itself reduced into that form: Sir Humphry Davy, however, corrected this error, and first showed it to be a hydrate, the pure gas not being condensable even at a temperature of-40° Fahrenheit.

Mr. Faraday had taken advantage of the cold season to procure crystals of this hydrate, and was proceeding in its analysis,[81] when Sir Humphry Davy suggested to him the expediency of observing what would happen if it were heated in a close vessel; but this suggestion was made in consequence of the inspection of results already obtained by Mr. Faraday, and which must have led him to the experiment in question, had he never communicated with Sir Humphry Davy upon the subject. This avowal is honestly due to Mr. Faraday.

[81] The results are contained in a short paper in the Quarterly
Journal of Science, vol. xv.

On exposing the hydrate, in a tube hermetically sealed, to a temperature of 100°, the substance fused, the tube became filled with a bright yellow atmosphere, and, on examination, was found to contain two fluid substances: the one, about three-fourths of the whole, was of a faint yellow colour, having very much the appearance of water; the remaining fourth was a heavy, bright yellow fluid, lying at the bottom of the former, without any apparent tendency to mix with it.

By operating on the hydrate in a bent tube hermetically sealed, Mr. Faraday found it easy, after decomposing it by a heat of 100°, to distil the yellow fluid to one end of the tube, and thus to separate it from the remaining portion. If the tube were now cut in the middle, the parts flew asunder, as if with an explosion, the whole of the yellow portion disappeared, and there was a powerful atmosphere of chlorine produced; the pale portion, on the contrary, remained, and when examined, proved to be a weak solution of chlorine in water, with a little muriatic acid, probably from the impurity of the hydrate used. When that end of the tube in which the yellow fluid lay was broken under a jar of water, there was an immediate production of chlorine gas.

After several conjectures as to the nature of the changes thus produced, Mr. Faraday arrived at its true explanation; viz. that the chlorine had been entirely separated from the water by the heat, and condensed into a dry fluid by the mere pressure of its own abundant vapour. He subsequently confirmed these views by condensing chlorine in a long tube, by mechanical pressure, applied by means of a condensing syringe, and which farther enabled him to ascertain that the degree of pressure necessary for this effect was about that of four atmospheres.

To Mr. Faraday's paper upon this subject, published in the Philosophical Transactions for the year 1823, Sir Humphry Davy thought proper to add a "Note on the condensation of muriatic acid gas into the liquid form."

The circumstances under which this was effected are briefly these. On the morning (Thursday, March 6th,) after Mr. Faraday had condensed chlorine, Sir Humphry Davy had no sooner witnessed the result, than he called for a strong glass tube, and, having placed in it a quantity of muriate of ammonia and sulphuric acid, and then sealed the end, he caused them to act upon each other, and thus condensed the muriatic acid, which was evolved, into a liquid. The condensation of carbonic acid gas, nitrous oxide gas, and several others, were in succession treated with similar success; but, as I regard the discovery as strictly belonging to Mr. Faraday, I shall confine myself to the relation of those experiments and deductions which, with equal justice, I must assign to Sir Humphry Davy.

He observes, "that the generation of elastic substances in close vessels, either with or without heat, offers much more powerful means of approximating their molecules than those dependent upon the application of cold, whether natural or artificial: for, as gases diminish only about 1/450 in volume for every--degree of Fahrenheit's scale, beginning at ordinary temperatures, a very slight condensation only can be produced by the most powerful freezing mixtures, not half as much as would result from the application of a strong flame to one part of a glass tube, the other part being of ordinary temperature: and when attempts are made to condense gases into liquids by sudden mechanical compression, the heat, instantly generated, presents a formidable obstacle to the success of the experiment; whereas, in the compression resulting from their slow generation in close vessels, if the process be conducted with common precautions, there is no source of difficulty or danger; and it may be easily assisted by artificial cold in cases when gases approach near to that point of compression and temperature at which they become vapours."

On the 17th of April 1823, he communicated to the Royal Society a paper "On the application of Liquids formed by the condensation of Gases as mechanical agents."

He states that doubts may, for various philosophical reasons, exist as to the economical results to be obtained by employing the steam of water under great pressures, and at very elevated temperatures; but that no doubts can arise with respect to the use of such liquids as require for their existence even a compression equal to that of the weight of thirty or forty atmospheres; and where common temperatures, or slight elevations of them, are sufficient to produce an immense elastic force; and when the principal question to be discussed is, whether the effect of mechanical motion is to be most easily produced by an increase or diminution of heat by artificial means.

With the assistance of Mr. Faraday, he made several experiments on the differences between the increase of elastic force in gases under high and low pressures, by similar increments of temperature. In an experiment made with carbonic acid, its force was found to be nearly equal to that of air compressed to one-twentieth at 12° Fah. and of air compressed to one-thirty-sixth at 32 degrees, making an increase equal to the weight of thirteen atmospheres by an increase of twenty of temperature!

In applying, however, the condensed gases as mechanical agents, Davy admits that there will be some difficulty; "the materials of the apparatus must be as strong and as perfectly joined as those used by Mr. Perkins in his high-pressure steam-engine: but the small differences of temperature to produce an elastic force equal to the pressure of many atmospheres, will render the risk of explosion extremely small;" and he adds, "that if future experiments should realize the views here developed, the mere difference of temperature between sunshine and shade, and air and water, or the effects of evaporation from a moist surface, will be sufficient to produce results, which have hitherto been obtained only by a great expenditure of fuel."

If this be true, who can say that future generations shall not perform their voyages in _gas_-vessels, across the Atlantic Ocean, with no other fuel than that which a common taper may supply? I fear, however, that in this scientific reverie, Davy merely looked at the difference of the sensible temperatures, and entirely neglected, in his calculation, the quantity of heat rendered latent during the change of the liquid into the gaseous state; and which, perhaps, is far more considerable in the application of these fluids than in that of water; but even in this latter case, the great expenditure of heat in working the steam-engine, is in the portion rendered latent, and which cannot, by any contrivance, be brought again into operation, after it has performed its duty. That a philosopher who had, during the whole progress of his researches, directed such unremitting attention to the subject of Heat, should have wholly overlooked an objection arising out of one of its most familiar phenomena, is scarcely less extraordinary than his having, on another occasion,[82] advanced to a conclusion in direct opposition to the very principle of Electricity, which his own discoveries had established.

[82] I here allude to an anecdote related by Mr. Babbage, in
his "Reflections on the Decline of Science in England;" a work,
by the by, which strongly reminds me of a practical bull. A
gentleman, anxious to escape the tax on armorial bearings,
wrote a long letter to the Commissioners, stating I do not know
how many reasons to show that he could never have used them;
and, after all, sealed the letter with his own coat of arms!
Had Mr. Babbage hoped to convince the reader that Science was
actually on the decline in this country, he should never have
written a work which gives the lie to the title-page. Now for
the anecdote.--"Meeting Dr. Wollaston one morning in the shop
of a bookseller, I proposed this question: If two volumes of
hydrogen and one of oxygen are mixed together in a vessel, and
if by mechanical pressure they can be so condensed as to become
of the same specific gravity of water, will the gases, under
these circumstances, unite and form water? 'What do you think
they will do?' said Dr. W. I replied, that I should rather expect
they would unite. 'I see no reason to suppose it,' said he. I
then enquired whether he thought the experiment worth making.
He answered, that he did not, for that he should think it would
certainly _not_ succeed.

"A few days after, I proposed the same question to Sir Humphry
Davy. He at once said, 'They will become water of course:' and
on my enquiring whether he thought the experiment worth making,
he observed that it was a good experiment, but one which it was
hardly necessary to make, as it must succeed.

"These were off-hand answers, which it might perhaps be hardly
fair to have recorded, had they been of persons of less eminent
talent; and it adds to the curiosity of the circumstance to
mention, that I believe Dr. Wollaston's reason for supposing no
union would take place, arose from the nature of the electrical
relations of the two gases remaining unchanged: an objection
which did not weigh with the philosopher whose discoveries had
given birth to it."

Davy succeeded in liquefying gases by a method which, at first view, appears very paradoxical--_by the application of heat!_ The method consists in placing them in one leg of a bent sealed tube, confined by mercury, and applying heat to ether, or alcohol, or water, in the other end. In this manner, by the pressure of the vapour of ether, he liquefied prussic gas and sulphurous acid gas; which gases, on being reproduced, occasioned cold.

There can be little doubt, he thinks, that these general facts of the condensation of the gases will have many practical applications. They offer, for instance, easy methods of impregnating liquids with carbonic acid and other gases, without mechanical pressure. They afford means of producing great diminutions of temperature, by the rapidity with which large quantities of liquids may be rendered aëriform; and as compression occasions similar effects to cold, in preventing the formation of elastic substances, there is great reason to believe that it may be successfully employed for the preservation of animal and vegetable substances for the purposes of food.

Davy might also have added, that the same general views will explain natural and other phenomena not previously understood. They certainly afford a plausible explanation of the nature of _blowers_ in coal-mines; and they may lead to more satisfactory views on other subjects of geology. They assign a limit to the expansive force of gas under increasing pressure, and account for effects connected with the _blasting_ of rocks, which would otherwise appear anomalous.[83]

[83] In the year 1812, Mr. Babbage attempted to ascertain whether
pressure would prevent decomposition: for this purpose, a hole
about thirty inches deep, and two inches in diameter, was bored
downward into a limestone rock, into which was then poured a
quantity of strong muriatic acid, and a conical wooden plug, that
had been previously soaked in tallow, was immediately driven
hard into the mouth of the hole. It was expected either that the
decomposition would be prevented, or that the gas developed would
split the rock by its expansive force: but nothing happened.
Now, it is most probable that a part of the carbonic acid had
condensed into a liquid, and thus prevented that developement of
power which Mr. Babbage had expected would have torn the rock
asunder.

It may be stated, greatly to the honour of Davy, that there never occurred any question of scientific interest or difficulty in which he did not cheerfully offer his advice and assistance. Few Presidents of the Royal Society have ever exerted their influence and talents with so much unaffected zeal for the promotion of scientific objects, and for the welfare of scientific men. In the year 1821, the Great Hafod copper-works, in the neighbourhood of Swansea, were indicted for a nuisance, in consequence of the alleged destructive effects of the fumes which arose during the smelting of the ores. When we learn that the amount of wages paid by the proprietors of the works in this district exceeds 50,000_l._, per annum; that twelve thousand persons, at least, derive their support from the smelting establishments; that a sum of not less than 200,000_l._ sterling is annually circulated in Glamorganshire and the adjoining county, in consequence of their existence; that they pay to the collieries no less than from 100,000_l._ to 110,000_l._ per annum for coal; that one hundred and fifty vessels are employed in the conveyance of ore, and, supposing each upon an average to be manned by five seamen, that they give occupation to seven hundred and fifty mariners, a more serious calamity can scarcely be imagined than the stoppage of such works: we may therefore readily believe, that Davy entered most ardently into the consideration of some plan by which the fumes might be prevented, and the alleged nuisance abated.

Through the kind attention of my friend Mr. Vivian, I am enabled to insert the following letters.

TO JOHN HENRY VIVIAN, ESQ.

London, Jan. 9, 1822.

MY DEAR SIR,

As you expressed a wish that I should commit to writing those
opinions which I mentioned in conversation, when I had the
pleasure of visiting you at Marino, after inspecting your
furnaces and witnessing your experiments on the smoke arising
from them, I lose no time in complying with your desire.

It is evident that the copper ore cannot be properly calcined
without a copious admission of air into the furnaces, which
must cause the sulphurous acid gas formed in the calcination
to be mixed with very large quantities of other elastic
fluids, which presents great mechanical, as well as chemical
difficulties to its condensation or decomposition.

To persons acquainted with chemistry, a number of modes of
effecting these objects are known. Of condensation, for
instance, by water, by the formation of sulphuric acid, by
alkaline lixivia, by alkaline earths, &c. Of decomposition,
by hydrogen, by charcoal, by hydro-carbonous substances, and
by metals; but to most of these methods there are serious and
insurmountable objections, depending upon the diluted state of
the acid gas, and the expenses required.

To form sulphuric acid, or to decompose by charcoal or
hydrogen, or to condense by alkaline lixivia, or by alkaline
earths, from the nature of the works, and of the operations for
which they were intended, I conceive impracticable except at an
expense that could not be borne; and the only processes which
remain to be discussed are those by hydro-carbonous substances,
and by the action of water.

There can be no doubt that the gas may be decomposed by the
action of heated hydro-carbonous gases from the distillation
of coal; but for this purpose there must be a new construction
of the furnaces, and more than double, probably triple,
the quantity of fuel would be required, supposing even the
Swansea coal to contain the common average of bitumen; and
this method must be infinitely more expensive, and liable to
many more objections, than the one you have so ingeniously
employed--absorption by water.

As water costs nothing, and as a supply is entirely in
your power, the application of it offers comparatively few
difficulties; and it has the great advantage of freeing the
smoke from fluoric and arsenious compounds, which would not be
perfectly effected by any other method.

The experiments of MM. Phillips and Faraday prove, that your
shower baths have already entirely destroyed all the fluoric
and arsenious fumes of the smoke, and by a _certain_ quantity
of water, the smoke may undoubtedly be entirely freed from
sulphurous acid gas.

This, _your own_ plan, is the one that I strongly recommend to
you to proceed with, and, if necessary, to extend.

Perhaps you may find an additional shower bath near the colder
part of the flue useful. I have no idea that steam passed into
the hot part of the flue can be of the least service; but if
passed out with the smoke through the stack, it may tend to
convert such residual portion of sulphurous acid gas, exposed
to fresh air, into sulphuric acid. Could you not likewise
try a stream of _cold_ water passing along the bottom of the
horizontal flue?[84]

I do not think the advantages of your improvements can be
fairly appreciated, till the effects of your smoke are
determined by actual experiments and fair trials.

Yours, &c.

H. DAVY.

[84] For the purpose of acting by its cooling power in condensing
vapour, which would carry down sulphurous acid with it. It would
likewise assist by direct absorption. H. D.

TO THE SAME.

London, May 12, 1823.

MY DEAR SIR,

I return you my thanks for the copies you were so good as
to send me of your work on the modes you have adopted for
rendering copper smoke innoxious, &c. I have read it with very
great pleasure, and I am sure there can be but one feeling, and
that of strong admiration, at the exertions you have made, and
the resources you have displayed, in subduing the principal
evils of one of our most important national manufactures. I
trust you will have no more trouble on this subject, and that
it will only occur to you in an agreeable form, with the high
approbation as well as grateful feelings of your neighbours;
and that your example will be followed.

A Committee of the Royal Society has been formed for
investigating the causes of the decay of copper sheeting in
the Navy, as I mentioned to you. The Navy Board has sent us a
number of specimens of copper in different stages of decay. We
have our first meeting to examine them on Thursday, and I shall
have much pleasure in communicating to you our results. I wish
I could do it in person.

I am going into Hampshire on Sunday next to fish near
Fordingbridge for a week, and to try the Avon and its tributary
streams.

I was going to give you an account of some experiments which
Mr. Faraday has made by my directions in generating gases in
close vessels as liquids, but I find I have not time. I have
already found an application of this discovery, which I hope
will supersede _steam_, as a difference of a few degrees of
temperature gives the elastic force of many atmospheres.

Hoping to see you soon, I am, with best respects to Mrs.
Vivian, and love to the charming little Bessy,

My dear Sir, yours sincerely obliged,

H. DAVY.

* * * * *

I proceed now to relate the history of an elaborate experimental enquiry, instituted for the purpose of ascertaining the chemical nature and causes of the well-known corrosive action of sea-water upon metallic copper; in order, if possible, to obviate that serious evil in naval economy--the rapid decay of the copper sheathing on the bottoms of our ships. An investigation which Sir Humphry Davy commenced in the year 1823, and prosecuted with his characteristic zeal and happy talent during a considerable period; when, at length, paradoxical as it may appear, the truth of his theory was completely established by the failure of his remedy!

From the several original documents which have been placed at my disposal, and from the valuable communications and kind assistance of my friend Mr. Knowles, I trust I shall be enabled to offer to the scientific reader a more complete and circumstantial history of this admirable enquiry than has been hitherto presented to the public.

The results he produced are equally interesting and important, whether we contemplate them biographically, as indicative of the peculiar genius by which they were obtained; or, scientifically, in their connexion with the electro-chemical theory, to the farther developement and illustration of which they have so powerfully contributed; or, economically, as the probable means by which the hand of Time may be averted, an increased durability imparted to rapidly perishable works of art, and monuments of human genius transmitted to posterity, in all their freshness, through a long succession of ages.

It is probable that, in the earliest period of naval architecture, some expedient[85] was practised, in order to protect ships' bottoms from the ravages of marine worms.[86] The use of metallic sheathing, however, is of ancient date. The galley supposed to have belonged to the Emperor Trajan was sheathed with sheets of lead, which were fastened with copper nails.[87] The same metal was also used in the earlier periods of our naval history;[88] and it is worthy of remark, that the circumstances which led to its disuse, were the rapid corrosion of the _rother irons_, (from the formation of a Voltaic circle,) and the accumulation of sea-weed.

[85] Mr. Knowles, in his "Inquiry into the Means which have been
taken to preserve the British Navy," observes, that the first
sheathing was probably the hides of animals covered with pitch,
or with asphaltum, which led to the use of thin boards, having,
in some cases, lime, and in others lime and hair, between them
and the bottom of the ships.

[86] The worms infesting the timber of ships are--the _Teredo_,
the _Lepisma_, and the _Pholas_. The first of these, however,
which was imported from India, is by far the most destructive;
and I am informed by Mr. Knowles, that it is more abundant at
Plymouth than on any other part of the coast where there is a
dock-yard; and although on the shores of England it is not of a
very large size, yet it is a formidable enemy to the safety of
those ships which have not a metallic sheathing to cover their
bottoms. In the East Indies, and off the coast of Africa, the
_Teredo_ is of very large size; and holes have been bored by them
in the timber of at least seven-eighths of an inch in diameter.

[87] Alberti Archeti.

[88] In the year 1670, an Act of Parliament was passed, granting
unto Sir Philip Howard and Francis Watson, Esq. the sole use of
the manufacture of milled lead for sheathing ships; and, in the
year 1691, twenty ships had been sheathed with lead, manufactured
by them, and which was fastened with copper nails.--See
_Knowles's Inquiry_.

In the year 1761, copper plates were first used as sheathing on the Alarm frigate, of thirty-two guns;[89] a second underwent this operation in 1765, a third in 1770, four in 1776, nine in 1777; and, in the course of the three following years, the whole British navy was coppered: an event which may be considered as forming an important era in the naval annals of the country.

[89] The copper sheathing was removed from this ship in 1763,
when all the iron was found to be much corroded, the pintles and
braces nearly eaten through, and the false keel lost, from the
decay of the keel staples and the bolt fastenings. Thus, in the
very first coppered ship, the Voltaic effect, produced by the
contact of copper and iron, was displayed in a very striking
manner.

The expense attending the use of copper for this purpose, in consequence of its corrosion and decay by salt-water, has always been felt as a serious objection to its use, and various suggestions have from time to time occurred, and numerous experiments been made, in the hope of obviating the evil,[90] but without any great degree of success.

[90] An experiment was tried by painting or varnishing their
inner surfaces, but the use of brown paper which has been dipped
in tar, and placed between the wood and copper, is now considered
to be the best mode. A solution of caoutchouc spread on paper
was tried on the bottom of Sir W. Curtis's yacht; but, on
examination, it was pronounced to be less efficacious than tarred
paper.

The solution of the metal, however, has been found to vary in degree at different anchorages: at Sheerness, for instance, its rapidity is very great, in consequence of the copper being subjected to the alternate action of the sea, which flows in there from the British Channel, and to the flux of water down the two great rivers, the Thames and Medway, loaded, as they necessarily must be, with the products of animal and vegetable decomposition.

In order, if possible, to obtain a remedy for this evil, the naval departments of the Government requested, in the latter part of the year 1823, the advice of the President and Council of the Royal Society, as to the best mode of manufacturing copper sheets, or of preserving them, while in use, against the corrosive effects of oxidation.

Sir H. Davy charged himself with this enquiry; the results of which he communicated to the Royal Society, in three elaborate memoirs. The first was read on the 22nd of January 1824; the second, on the 17th of June, in the same year; and the third, and concluding paper, on the 9th of June 1825.

A very general belief prevailed, that sea-water had little or no action on _pure_ copper, and that the rapid decay of that metal on certain ships was owing to its impurity. On submitting, however, various specimens of copper to the action of the sea-water, Sir H. Davy came to a conclusion, in direct opposition to such an opinion;[91] and Mr. Knowles informed me, in a late conversation upon the subject, that the attempts to purify the metal, since the Government has manufactured its own copper sheathing, has been the cause of its more rapid decay. It will however presently appear, that the relative durability of the metallic sheets must also be influenced by circumstances wholly independent of their quality, some of which are very probably, even in our present advanced state of chemical knowledge, not thoroughly understood.

[91] In two instances, the copper (from the Batavier and from
the Plymouth yacht) which had remained perfect for twenty-seven
years, was found to be alloyed. In the former one there was an
alloy of one three-hundredth part of zinc; and, in the latter,
the same proportion of tin. On the other hand, in the case of
the copper on the Tartar's bottom, which was nearly destroyed in
four years, upon being submitted to chemical examination by Mr.
Phillips, it was found to be very pure copper.

Alloys of copper have generally been found more durable than the
unmixed metal; and various patents have been taken out for the
fabrication of such compounds; but metallic sheets so composed
have been found to be too hard and brittle, and not to admit of
that flexibility which is necessary for their application to a
curved surface; the consequence of which has been, that they have
cracked upon the ship's bottom.

Sir H. Davy, on entering upon the examination of this subject, very justly considered, that to ascertain the exact nature of the chemical changes which take place in sea-water, by the agency of copper, ought to be the first step in the enquiry; for, unless the cause were thoroughly understood, how could the evil be remedied?

On keeping a polished piece of copper in contact with sea-water, the following were the effects which successively presented themselves. In the course of two or three hours, the surface of the metal exhibited a yellow tarnish, and the water in which it was immersed contracted a cloudiness, the hue of which was at first white, but gradually became green. In less than a day, a bluish-green precipitate appeared, and constantly continued to accumulate in the bottom of the vessel; at the same time, the surface of the copper corroded, appearing red in the water, and grass-green where it was in contact with air. Upon this grass-green matter carbonate of soda formed; and these changes continued until the water became much less saline. The green precipitate he ascertained to consist of an insoluble compound of copper, (which he thinks may be considered as a _hydrated sub-muriate_,) and hydrate of magnesia.[92]

[92] The Muriate of Magnesia is the most active salt in sea-water.

According to his own views of the nature of chlorine, he immediately perceived that neither soda nor magnesia could appear in sea-water by the action of a metal, unless in consequence of an absorption or transfer of oxygen, which in this case must either be derived from the atmosphere, or from the decomposition of water: his experiments determined that the former was the source which supplied it. By reasoning upon these phenomena, and applying for their explanation his electro-chemical theory, which had shown that chemical attractions may be exalted, modified, or destroyed, by changes in the electrical states of bodies, he was led to the discovery of a remedy for the corrosion of copper, by the very principle which enabled him, sixteen years before, to decompose the fixed alkalies.

When he considered that copper is but weakly positive in the electro-chemical scale, and that it can only act upon sea-water when in a positive state, it immediately occurred to him that, if it could be 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, 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 highly satisfactory and conclusive. A piece of zinc, not larger than 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 connexion 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 sea-water; the copper remained perfectly polished. The same experiment was repeated with iron, and after the lapse of a month, the copper was in both instances found as bright as when it was first introduced; whilst similar pieces of copper, undefended, underwent in the same water very 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.

There was however one point which still remained for enquiry. As the ocean may be considered in its relation to the quantity of copper in a ship, as an infinitely extended conductor, it became necessary to ascertain whether that circumstance would influence the results. For this purpose, he placed 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 sheathing of a ship. The result was perfectly satisfactory. The defended copper underwent no change; the undefended tarnished, and deposited a green powder.[93]

[93] During the course of some experiments in which I have been
lately engaged, a simple mode of exhibiting the principle of
protection occurred to me, which, I believe, has not before
been suggested; at least, I cannot find any notice of such
an experiment. As I consider it admirably calculated for
illustration, I will here describe it. Let two slips of copper
of equal size, the one protected with a piece of zinc, the other
unprotected, be plunged into two wine-glasses filled with a
solution of ammonia. In a short time, the liquor containing the
unprotected copper will assume an intensely blue colour; the
other will remain colourless for any length of time. The theory
is obvious. When metallic copper is placed in contact with an
ammoniacal solution, a protoxide of the metal is formed which
is colourless,--and will remain so, if the contact of air be
prevented; but on exposure to the atmosphere, it passes into
a state of peroxide, which is dissolved by the ammonia, and
produces an intensely blue solution. In the case of the protected
copper, the metal is incapable of attracting a single atom of
oxygen, in consequence of having been rendered negative by the
zinc, and consequently no solution can take place.

Davy having thus satisfied his own mind as to the truth of his views, communicated to Government, in January 1824, the important fact of his having discovered a remedy for the evil of which they had complained; and that the corrosion of the copper sheathing of his Majesty's ships might be prevented by rendering the copper electro-positive, by means of the contact of tin, zinc, lead, iron, or any other easily oxidable metal; and that he was prepared to carry his plan into effect.

A proposition from a philosopher of such known science, and upon a subject of such great importance to the navigation and commerce of the country, immediately obtained all the attention it deserved; and an order was made that the plan of protection should, under the superintendence of Sir H. Davy, be forthwith tried upon the bottom of a sailing cutter.

To give to his discovery farther publicity, Sir Humphry requested that three models of ships might be exhibited in the spacious hall of the Navy Office in Somerset House; the copper of one of which he proposed should be protected by bands of zinc, that of another by plates of wrought iron soldered on the sheathing, while the third should have its copper exposed without any protection whatever.

These models were floated in sea-water for several months; and the experiment fully confirmed the results he had previously obtained in his laboratory. The models were from time to time examined by persons of the highest scientific character, as well as by others of great naval celebrity; and so alluring was the theory, and so conclusive the experiments, that, instead of waiting the result of the slow but more certain ordeal to which the plan had been submitted, it was immediately put into extensive practice, both in the Government service and on the bottoms of ships belonging to private individuals.

To those the least acquainted with the principles of Voltaic action, it was only necessary to state the proposition, in order to command their assent to its truth. The utility of the plan therefore was never questioned, but the claims of Davy to the originality of the invention were doomed to meet with immediate opposition.[94]

[94] Amongst other counter-claims, there appeared, in a weekly
publication entitled "The Mechanic's Magazine," a statement in
favour of a person of the name of Wyatt, founded on the following
advertisement in "The World" newspaper of April 16, 1791. "By
the King's Patent, tinned copper sheets and pipes manufactured
and sold by Charles Wyatt of Birmingham. These sheets, amongst
other advantages, are particularly recommended for sheathing of
ships, as they possess all the good properties of copper, with
others obviously superior." It is unnecessary to observe that,
except their object, there is nothing in common in the inventions
of Davy and Wyatt. The superiority claimed by Wyatt consisted
merely in coating the copper with some substance less corrosive
by sea-water than that metal: an idea borrowed from the common
practice of tinning copper vessels.

The correctness of the principle having been established, it became, in the next place, necessary to determine the most eligible metal to be used for protection; the proportion which it must bear to the surface of the copper-sheathing below the waterline; the form least likely to offer resistance to the sea, and to impede the sailing of the vessel; and lastly, its most convenient position on the ship's bottom. To ascertain these several points, Lord Melville and the Lords of the Admiralty desired the Commissioners of the Navy Board, and of the Dock-yards, to afford Sir Humphry every assistance and facility for prosecuting the necessary experiments; and he accordingly made many very extensive trials, not only on copper sheets which were immersed in the sea, but also on the bottoms of a considerable number of boats which had been coppered for that purpose, and exposed to the flow of the tide in Portsmouth harbour; upon which occasions he varied the nature as well as the proportions of the protecting metal. The results were communicated to the Royal Society, and they constituted the materials for his second memoir on the subject.

"When the metallic protector was from 1/20 to 1/110 parts of its surface, there was no corrosion nor decay of the copper; with smaller quantities, such as from 1/200 to 1/400, the copper underwent a loss of weight, which was greater in proportion as the protector was smaller; and, as a proof of the universality of the principle, it was found that even 1/1000 part of cast iron saved a certain proportion of the copper.

"The sheeting of boats and ships, protected by the contact of zinc, or 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. Fortunately, in the course of these experiments, it was proved that cast iron, the substance which is cheapest and most easily procured, is likewise most fitted for the protection of the copper. It lasts longer than malleable iron, or zinc; and the plumbaginous substance which is left by the action of sea-water upon it, retains the original form of the iron, and does not impede the electrical action of the remaining metal."

In the earlier stage of the investigation, it had been suggested by Mr. Knowles, and several other persons, that by rendering the copper innoxious, it was probable sea-weeds might adhere to the sheets; but this objection he answered by stating, that negative electricity could not be supposed favourable to animal and vegetable life; and as it occasioned the deposition of magnesia, a substance exceedingly noxious to land vegetables, upon the copper surface, he entertained no difficulty upon that subject: in this, however, he was fatally mistaken. He found, after a trial of several weeks, that the metallic surface became coated with carbonate of lime and magnesia, and that, under such circumstances, weeds adhered to the coatings, and marine insects collected upon them; but at the same time he observed, that when the proportion of cast iron, or zinc, was below 1/150, the electrical power of the copper being less negative, no such deposition occurred; and that although the surface had undergone a slight degree of solution, it remained perfectly clean: a fact which he considered of great importance, as it pointed out the _limits of protection_; and makes the application of a _very small_ quantity of the oxidable metal more advantageous, in fact, than that of a larger one.

During the course of these experiments, many singular facts occurred to him, which tended to confirm his views of electro-chemical action. Amongst the various details which remained for his investigation, the relations between the surface of the protector, and that of the copper sheathing, under the different circumstances of temperature, saltness of the sea, and rapidity of the ship's motion, presented themselves as objects of great importance; and an opportunity occurred which enabled him to pursue them by actual observation and experiment.

In the month of June 1824, a steam-vessel, H.M. ship the Comet, was, at the express request of the King of Denmark, ordered to proceed to Heligoland, for the purpose of fixing with precision, by means of numerous chronometers, the longitude of that island, in order to connect the Danish with the British survey; and the Board of Longitude having recommended that the voyage should be extended as far as the Naze of Norway, for the purpose of ascertaining also the longitude of that important point, Sir H. Davy thought that this vessel would afford him the means of performing his desired experiments upon protected and unprotected copper sheets, when under the influence of rapid motion; and upon application to the Board of Admiralty, he obtained the entire disposal of the vessel after the required observations had been completed, as long as the season would allow her going to sea; and, that every facility might be afforded him, a skilful carpenter was put on board, to prepare whatever might be necessary for the prosecution of the enquiry.

For the following account of his adventures upon this occasion I am indebted to Dr. Tiarks, who, in his character of astronomical observer, superintended the expedition.

In the first instance, Davy directed to be constructed a number of oblong, rectangular, thin plates of copper, the surface of which should exceed that of a square foot: in the centre of these plates was fastened a slip of copper, by means of which other pieces of copper, which had small plates of iron of various dimensions attached to them, were fixed to the plate, by merely sliding them into the groove thus prepared for their reception. The plates were all carefully weighed previously to the experiment, and the pieces of iron were considered as representing the various proportions of iron and copper surfaces within whose limits Sir H. Davy had been led, by former experiments, to expect that the best proportion would be found. These plates were afterwards slipped into wooden frames, and nailed to the ship's side, over a piece of thick canvass, for the purpose of intercepting every possible communication between them and the copper sheathing.

It was proposed that, after each trip, these plates should be accurately weighed, in order to ascertain the loss which they severally might sustain from the corrosive action of the sea, while thus protected by different proportions of iron surface; and, to ensure every possible accuracy, he carried with him the excellent balance, constructed by Ramsden, which is in possession of the Royal Society.

Sir H. Davy, accompanied by Lord Clifton, embarked at Greenwich on the 30th of June, and the vessel arrived at Heligoland on the 2nd of July. Here, as they remained not more than one day, the plates were not examined, although the Master expressed strong doubts as to their safety. The vessel then proceeded, by order of Sir Humphry, to Norway, a country which he was, for several reasons, very desirous of visiting, especially for the sake of determining a doubtful point in ornithology, upon which he subsequently corresponded with Professor Rheinhard, of Copenhagen.

The difference of longitude, also, between that country and Greenwich, not having been accurately ascertained, offered perhaps an additional reason for thus deviating from a course which, it must be confessed, was at variance with the original plan of the expedition.

After a severe gale of wind on the 4th of July, the vessel arrived, on the day following, at Rleve, near Mandal, and afterwards proceeded to this latter place, at which Davy remained for several days, during which interval the vessel made a tour to the Naze, and took in coal.

On the arrival of the vessel in the port, the plates were immediately examined; but, to the great disappointment of Sir Humphry, it was discovered that every one of the protectors had been washed away, and that most of the plates had sustained considerable injury.

With the country around Mandal he was much pleased; for, although it is far from being fertile, the scenery is rendered exceedingly striking and beautiful by the numerous lakes which wash the feet of high and sometimes perpendicular mountains, at that time clothed with the rich verdure of their summer herbage.

Sir Humphry made several excursions into the interior of the country, and derived much amusement from angling in the lakes; and had it not been from his own inspection of the roads, and the information which he collected respecting them, together with an indisposition of his fellow-traveller, Lord Clifton, he would have made an extensive land journey through the country; but, under the existing circumstances, he determined to return to England through Denmark and Germany. He therefore at once resolved to take the steam-boat with him as far as Sweden, where the excellent roads would enable him, without inconvenience, to reach Gottenburg, and thence to continue his route through Denmark to Germany. The vessel proceeded accordingly to Christiansand, the chief town of a country of the same name.

Having been provided with some spare plates and protectors, he fixed them to the ship's side at Mandal, as he was informed that the voyage could be entirely performed within the rocks, with which the whole coast of Norway is so plentifully studded; but a short traverse through an open part of the sea, not far from Mandal, again defeated his object. The protectors were washed away, and no result was obtained.

At Christiansand he remained a few days, in order to try some new plates, which were constructed there under his own inspection. Upon this occasion he made an excursion to the falls of the Torjedahl, distant about six miles from the town. The river abounds with salmon, which were easily caught in their descent from the falls, by an apparatus contrived for that purpose. Sir Humphry amused himself by teaching the inhabitants the operation of _crimping_, and he declared the flavour of the fish to be superior to any salmon he had ever tasted.

It was at Christiansand that he became acquainted with the Norwegian race of ponies, so well adapted for mountainous countries; and which, at his recommendation, were afterwards introduced into England by Mr. Knight, of Downton Castle.

From Christiansand the vessel proceeded on her route eastward to Arendal, where she arrived on the 12th, after a passage of only a few hours. The route lay entirely within the rocks,--and so narrow were the passages, that the vessel could frequently not pass the rocks on either side without touching them.

At Arendal, which is the chief place of a remarkable mining district, Sir Humphry was well received by the Messrs. Dedehamys, two brothers, and the leading merchants of the place, with whom he made several excursions to the neighbouring mines. He was also invited by them to meet at their beautiful country seats the most respectable inhabitants of the town.

In the house of Mr. Dedehamy, Davy was introduced into Norwegian society, and, for the first time, had an opportunity of witnessing the customs and manners of the country.

A short time before dinner, the guests were summoned to partake of pickled fish, anchovies, and smoked salmon, with rum, brandy, and wine, which were placed on small tables in the drawing-room in which the company assembled. This custom of taking salt provisions, together with spirits, just before dinner, is very general in the North, and is considered as the best means of preparing the stomach, and of provoking an appetite for the approaching meal.

The very numerous party, which, with the exception of the hostess and her daughter, consisted entirely of men, were then ushered into two large rooms, one not being sufficiently spacious to accommodate them, and each person took his seat promiscuously. At the beginning of the dinner, large basins filled with sugar were carried round by the host's daughter, followed by a servant, from which each gentleman took a large handful. Sir Humphry, surprised at so singular a ceremony, enquired its meaning; when the host very good-humouredly answered, that in Norway they thought, if the wine was good it could not be spoiled by sugar,--and if bad, that it would be improved by it. Davy immediately followed the example of the company, and helped himself to the sugar.