[Note 1\12: _Hist. Nat._ lib. xxxvi. c. 25.]

But the doctrines of magnetism, like those of heat, have an

additional claim on our notice from the manner in which they are

exemplified in the globe of the earth. The subject of _terrestrial

magnetism_ forms a very important addition to the general facts of

magnetic attraction and repulsion. The property of the magnet by

which it directs its poles exactly or nearly north and south, when

once discovered, was of immense importance to the mariner. It does

not {218} appear easy to trace with certainty the period of this

discovery. Passing over certain legends of the Chinese, as at any

rate not bearing upon the progress of European science,[2\12] the

earliest notice of this property appears to be contained in the Poem

of Guyot de Provence, who describes the needle as being magnetized,

and then placed in or on a straw, (floating on water, as I presume:)

Puis se torne la pointe toute

Contre l'estoile sans doute;

that is, it turns towards the pole-star. This account would make the

knowledge of this property in Europe anterior to 1200. It was

afterwards found[3\12] that the needle does not point exactly

towards the north. Gilbert was aware of this deviation, which he

calls the _variation_, and also, that it is different in different

places.[4\12] He maintained on theoretical principles also,[5\12]

that at the same place the variation is constant; probably in his

time there were not any recorded observations by which the truth of

this assertion could be tested; it was afterwards found to be false.

The alteration of the variation in proceeding from one place to

another was, it will be recollected, one of the circumstances which

most alarmed the companions of Columbus in 1492. Gilbert says,[6\12]

"Other learned men have, in long navigations, observed the

differences of magnetic variations, as Thomas Hariot, Robert Hues,

Edward Wright, Abraham Kendall, all Englishmen: others have invented

magnetic instruments and convenient modes of observation, such as

are requisite for those who take long voyages, as William Borough in

his Book concerning the variation of the compass, William Barlo in

his supplement, William Norman in his _New Attractive_. This is that

Robert Norman (a good seaman and an ingenious artificer,) who first

discovered the _dip_ of magnetic iron." This important discovery was

made[7\12] in 1576. From the time when the difference of the

variation of the compass in different places became known, it was

important to mariners to register the variation in all parts of the

world. Halley was appointed to the command of a ship in the Royal

Navy by the Government of William and Mary, with orders "to seek by

observation the discovery of the rule for the variation of the

compass." He published Magnetic Charts, which {219} have been since

corrected and improved by various persons. The most recent are those

of Mr. Yates in 1817, and of M. Hansteen. The dip, as well as the

variation, was found to be different in different places. M.

Humboldt, in the course of his travels, collected many such

observations. And both the observations of variation and of dip

seemed to indicate that the earth, as to its effect on the magnetic

needle, may, approximately at least, be considered as a magnet, the

poles of which are not far removed from the earth's poles of

rotation. Thus we have a _magnetic equator_, in which the needle has

no dip, and which does not deviate far from the earth's equator;

although, from the best observations, it appears to be by no means a

regular circle. And the phenomena, both of the dip and of the

variation, in high northern latitudes, appear to indicate the

existence of a pole below the surface of the earth to the north of

Hudson's Bay. In his second remarkable expedition into those

regions, Captain Ross is supposed to have reached the place of this

pole; the dipping-needle there pointing vertically downwards, and

the variation-compass turning towards this point in the adjacent

regions. We shall hereafter have to consider the more complete and

connected views which have been taken of terrestrial magnetism.

[Note 2\12: _Enc. Met._ art. _Magnetism_, p. 736.]

[Note 3\12: Before 1269. _Enc. Met._ p. 737.]

[Note 4\12: _De Magnete_, lib. iv. c. 1.]

[Note 5\12: c. 3.]

[Note 6\12: Lib. i. c. 1.]

[Note 7\12: _Enc. Met._ p. 738.]

In 1633, Gellibrand discovered that the variation is not constant,

as Gilbert imagined, but that at London it had diminished from

eleven degrees east in 1580, to four degrees in 1633. Since that

time the variation has become more and more westerly; it is now

about twenty-five degrees west, and the needle is supposed to have

begun to travel eastward again.

The next important fact which appeared with respect to terrestrial

magnetism was, that the position of the needle is subject to a small

_diurnal_ variation: this was discovered in 1722, by Graham, a

philosophical instrument-maker, of London. The daily variation was

established by one thousand observations of Graham, and confirmed by

four thousand more made by Canton, and is now considered to be out

of dispute. It appeared also, by Canton's researches, that the

diurnal variation undergoes an annual inequality, being nearly a

quarter of a degree in June and July, and only half that quantity in

December and January.

Having thus noticed the principal facts which belong to terrestrial

magnetism, we must return to the consideration of those phenomena

which gradually led to a consistent magnetic theory. Gilbert

observed that both smelted iron and hammered iron have the magnetic

virtue, {220} though in a weaker degree than the magnet

itself,[8\12] and he asserted distinctly that the magnet is merely

an ore of iron, (lib. i. c. 16, Quod magnes et vena ferri idem

sunt.) He also noted the increased energy which magnets acquire by

being _armed_; that is, fitted with a cap of polished iron at each

pole.[9\12] But we do not find till a later period any notice of the

distinction which exists between the magnetical properties of soft

iron and of hard steel;--the latter being susceptible of being

formed into _artificial magnets_, with permanent poles; while soft

iron is only _passively magnetic_, receiving a temporary polarity

from the action of a magnet near it, but losing this property when

the magnet is removed. About the middle of the last century, various

methods were devised of making artificial magnets, which exceeded in

power all magnetic bodies previously known.

[Note 8\12: Lib. i. c. 9-13.]

[Note 9\12: Lib. ii. c. 17.]

The remaining experimental researches had so close an historical

connexion with the theory, that they will be best considered along

with it, and to that, therefore, we now proceed.

CHAPTER II.

PROGRESS OF MAGNETIC THEORY.

THEORY OF MAGNETIC ACTION.--The assumption of a fluid, as a mode of

explaining the phenomena, was far less obvious in magnetic than in

electric cases, yet it was soon arrived at. After the usual

philosophy of the middle ages, the "forms" of Aquinas, the "efflux"

of Cusanus, the "vapors" of Costæus, and the like, which are

recorded by Gilbert,[10\12] we have his own theory, which he also

expresses by ascribing the effects to a "formal efficiency;"--a

"_form_ of primary globes; the proper entity and existence of their

homogeneous parts, which we may call a primary and radical and

astral _form_;"--of which forms there is one in the sun, one in the

moon, one in the earth, the latter being the magnetic virtue.

[Note 10\12: Gilb. lib. ii. c. 3, 4]

Without attempting to analyse the precise import of these expressions,

we may proceed to Descartes's explanation of magnetic phenomena. The

mode in which he presents this subject[11\12] is, perhaps, the {221}

most persuasive of his physical attempts. If a magnet be placed among

iron filings, these arrange themselves in curved lines, which proceed

from one pole of the magnet to the other. It was not difficult to

conceive these to be the traces of currents of ethereal matter which

circulate through the magnet, and which are thus rendered sensible

even to the eye. When phenomena could not be explained by means of one

vortex, several were introduced. Three Memoirs on Magnetism, written

on such principles, had the prize adjudged[12\12] by the French

Academy of Sciences in 1746.

[Note 11\12: _Prin. Phil._ pars c. iv. 146.]

[Note 12\12: Coulomb, 1789, p. 482.]

But the Cartesian philosophy gradually declined; and it was not

difficult to show that the _magnetic curves_, as well as other

phenomena, would, in fact, result from the attraction and repulsion

of two poles. The analogy of magnetism with electricity was so

strong and clear, that similar theories were naturally proposed for

the two sets of facts; the distinction of bodies into conductors and

electrics in the one case, corresponding to the distinction of soft

and hard steel, in their relations to magnetism. Æpinus published a

theory of magnetism and electricity at the same time (1759); and the

former theory, like the latter, explained the phenomena of the

opposite poles as results of the excess and defect of a magnetic

"fluid," which was dislodged and accumulated in the ends of the

body, by the repulsion of its own particles, and by the attraction

of iron or steel, as in the case of induced electricity. The Æpinian

theory of magnetism, as of electricity, was recast by Coulomb, and

presented in a new shape, with two fluids instead of one. But before

this theory was reduced to calculation, it was obviously desirable,

in the first place, to determine the law of force.

In magnetic, as in electric action, the determination of the law of

attraction of the particles was attended at first with some

difficulty, because the action which a finite magnet exerts is a

compound result of the attractions and repulsions of many points.

Newton had imagined the attractive force of magnetism to be

inversely as the cube of the distance; but Mayer in 1760, and

Lambert a few years later, asserted the law to be, in this as in

other forces, the inverse square. Coulomb has the merit of having

first clearly confirmed this law, by the use of his

torsion-balance.[13\12] He established, at the same time, other very

important facts, for instance, "that the directive magnetic force,

which the earth exerts upon a needle, is a constant quantity,

parallel {222} to the magnetic meridian, and passing through the

same point of the needle whatever be its position." This was the

more important, because it was necessary, in the first place, to

allow for the effect of the terrestrial force, before the mutual

action of the magnets could be extricated from the phenomena.[14\12]

Coulomb then proceeded to correct the theory of magnetism.

[Note 13\12: _Mem. A. P._ 1784, 2d Mem. p. 593.]

[Note 14\12: p. 603.]

Coulomb's reform of the Æpinian theory, in the case of magnetism, as

in that of electricity, substituted two fluids (an _austral_ and a

_boreal_ fluid,) for the single fluid; and in this way removed the

necessity under which Æpinus found himself, of supposing all the

particles of iron and steel and other magnetic bodies to have a

peculiar repulsion for each other, exactly equal to their attraction

for the magnetic fluid. But in the case of magnetism, another

modification was necessary. It was impossible to suppose here, as in

the electrical phenomena, that one of the fluids was accumulated on

one extremity of a body, and the other fluid on the other extremity;

for though this might appear, at first sight, to be the case in a

magnetic needle, it was found that when the needle was cut into two

halves, the half in which the austral fluid had seemed to

predominate, acquired immediately a boreal pole opposite to its

austral pole, and a similar effect followed in the other half. The

same is true, into however many parts the magnetic body be cut. The

way in which Coulomb modified the theory so as to reconcile it with

such facts, is simple and satisfactory. He supposes[15\12] the

magnetic body to be made up of "molecules or integral parts," or, as

they were afterwards called by M. Poisson, "magnetic elements." In

each of these elements, (which are extremely minute,) the fluids can

be separated, so that each element has an austral and a boreal pole;

but the austral pole of an element which is adjacent to the boreal

pole of the next, neutralizes, or nearly neutralizes, its effect; so

that the sensible magnetism appears only towards the extremities of

the body, as it would do if the fluids could permeate the body

freely. We shall have exactly the same result, as to sensible

magnetic force, on the one supposition and on the other, as Coulomb

showed.[16\12]

[Note 15\12: _Mem. A. P._ 1789, p. 488.]

[Note 16\12: _Mem. A. P._ p. 492.]

The theory, thus freed from manifest incongruities, was to be

reduced to calculation, and compared with experiment; this was done

in Coulomb's Seventh Memoir.[17\12] The difficulties of calculation

in this, as in the electric problem, could not be entirely

surmounted by the analysis of Coulomb; but by various artifices, he

obtained theoretically the {223} relative amount of magnetism at

several points of a needle,[18\12] and the proposition that the

directive force of the earth on similar needles saturated with

magnetism, was as the cube of their dimensions; conclusions which

agreed with experiment.

[Note 17\12: _A. P._ 1789.]

[Note 18\12: p. 485.]

The agreement thus obtained was sufficient to give a great

probability to the theory; but an improvement of the methods of

calculation and a repetition of experiments, was, in this as in

other cases, desirable, as a confirmation of the labors of the

original theorist. These requisites, in the course of time, were

supplied. The researches of Laplace and Legendre on the figure of

the earth had (as we have already stated,) introduced some very

peculiar analytical artifices, applicable to the attractions of

spheroids; and these methods were employed by M. Biot in 1811, to

show that on an elliptical spheroid, the thickness of the fluid in

the direction of the radius would be as the distance from the

centre.[19\12] But the subject was taken up in a more complete

manner in 1824 by M. Poisson, who obtained general expressions for

the attractions or repulsions of a body of any form whatever,

magnetized by influence, upon a given point; and in the case of

spherical bodies was able completely to solve the equations which

determine these forces.[20\12]

[Note 19\12: _Bull. des Sc._ No. li.]

[Note 20\12: _A. P._ for 1821 and 2, published 1826.]

Previously to these theoretical investigations, Mr. Barlow had made

a series of experiments on the effect of an iron sphere upon a

compass needle; and had obtained empirical formulæ for the amount of

the deviation of the needle, according to its dependence upon the

position and magnitude of the sphere. He afterwards deduced the same

formulæ from a theory which was, in fact, identical with that of

Coulomb, but which he considered as different, in that it supposed

the magnetic fluids to be entirely collected at the surface of the

sphere. He had indeed found, by experiment, that the surface was the

only part in which there was any sensible magnetism; and that a thin

shell of iron would produce the same effect as a solid ball of the

same diameter.

But this was, in fact, a most complete verification of Coulomb's

theory. For though that theory did not suppose the magnetism to be

collected solely at the surface, as Mr. Barlow found it, it followed

from the theory, that the _sensible_ magnetic intensity assumed the

same distribution (namely, a surface distribution,) as if the fluids

could permeate the whole body, instead of the "magnetic elements"

only. Coulomb, indeed, had not expressly noticed the result, that

the sensible {224} magnetism would be confined to the surface of

bodies; but he had found that, in a long needle, the magnetic fluid

might be supposed to be concentrated very near the extremities, just

as it is in a long electric body. The theoretical confirmation of

this rule among the other consequences of the theory,--that the

sensible magnetism would be collected at the surface,--was one of

the results of Poisson's analysis. For it appeared that if the sum

of the electric elements of the body was equal to the whole body,

there would be no difference between the action of a solid sphere

and very thin shell.

We may, then, consider the Coulombian theory to be fully established

and verified, as a representation of the laws of magnetical

phenomena. We may add, as a remarkable and valuable example of an

ulterior step in the course of sciences, the application of the laws

of the distribution of magnetism to the purposes of navigation. It

had been found that the mass of iron which exists in a ship produces

a deviation in the direction of the compass-needle, which was termed

"local attraction," and which rendered the compass an erroneous

guide. Mr. Barlow proposed to correct this by a plate of iron placed

near the compass; the plate being of comparatively small mass, but,

in consequence of its expanded form, and its proximity to the

needle, of equivalent effect to the disturbing cause.

[2nd Ed.] [This proposed arrangement was not successful, because as

the ship turns into different positions, it may be considered as

revolving round a vertical axis; and as this does not coincide with

the magnetic axis, the relative magnetic position of the disturbing

parts of the ship, and of the correcting plate, will be altered, so

that they will not continue to counteract each other. In high

magnetic latitudes the correcting plate was used with success.

But when iron ships became common, a correction of the effect of the

iron upon the ship's compass in the general case became necessary.

Mr. Airy devised the means of making this correction. By placing a

magnet and a mass of iron in certain positions relative to the

compass, the effect of the rest of the iron in the ship is

completely counteracted in all positions.[21\12]]

[Note 21\12: See _Phil. Trans._ 1836.]

But we have still to trace the progress of the theory of terrestrial

magnetism.

_Theory of Terrestrial Magnetism._--Gilbert had begun a plausible

course of speculation on this point. "We must reject," he

says,[22\12] "in {225} the first place, that vulgar opinion of

recent writers concerning magnetic mountains, or a certain magnetic

rock, or an imaginary pole at a certain distance from the pole of

the earth." For, he adds, "we learn by experience, that there is no

such fixed pole or term in the earth for the variation." Gilbert

describes the whole earth as a magnetic globe, and attributes the

variation to the irregular form of its protuberances, the solid

parts only being magnetic. It was not easy to confirm or refute this

opinion, but other hypotheses were tried by various writers; for

instance, Halley had imagined, from the forms of the lines of equal

variation, that there must be four magnetic poles; but Euler[23\12]

showed that the "Halleian lines" would, for the most part, result

from the supposition of two magnetic poles, and assigned their

position so as to represent pretty well the known state of the

variation all over the world in 1744. But the variation was not the

only phenomenon which required to be taken into account; the dip at

different places, and also the intensity of the force, were to be

considered. We have already mentioned M. de Humboldt's collection of

observations of the dip. These were examined by M. Biot, with the

view of reducing them to the action of two poles in the supposed

terrestrial magnetic axis. Having, at first, made the distance of

these poles from the centre of the earth indefinite, he found that

his formulæ agreed more and more nearly with the observations, as

the poles were brought nearer; and that fact and theory coincided

tolerably well when both poles were at the centre. In 1809,[24\12]

Krafft simplified this result, by showing that, on this supposition,

the tangent of the dip was twice the tangent of the latitude of the

place as measured from the magnetic equator. But M. Hansteen, who

has devoted to the subject of terrestrial magnetism a great amount

of labor and skill, has shown that, taking together all the

observations which we possess, we are compelled to suppose four

magnetic poles; two near the north pole, and two near the south

pole, of the terrestrial globe; and that these poles, no two of

which are exactly opposite each other, are all in motion, with

different velocities, some moving to the east and some to the west.

This curious collection of facts awaits the hand of future

theorists, when the ripeness of time shall invite them to the task.

[Note 22\12: Lib. iv. c. 1. _De Variatione._]

[Note 23\12: _Ac. Berlin_, 1757.]

[Note 24\12: _Enc. Met._ p. 742.]

[2nd Ed.] [I had thus written in the first edition. The theorist who

was needed to reduce this accumulation of facts to their laws, {226}

had already laid his powerful hand upon them; namely, M. Gauss, a

mathematician not inferior to any of the great men who completed the

theory of gravitation. And institutions had been established for

extending the collection of the facts pertaining to it, on a scale

which elevates Magnetism into a companionship with Astronomy. M.

Hansteen's _Magnetismus der Erde_ was published in 1819. His

conclusions respecting the position of the four magnetic "poles"

excited so much interest in his own country, that the Norwegian

_Storthing_, or parliament, by a unanimous vote, provided funds for

a magnetic expedition which he was to conduct along the north of

Europe and Asia; and this they did at the very time when they

refused to make a grant to the king for building a palace at

Christiania. The expedition was made in 1828-30, and verified

Hansteen's anticipations as to the existence of a region of magnetic

convergence in Siberia, which he considered as indicating a "pole"

to the north of that country. M. Erman also travelled round the

earth at the same time, making magnetic observations.

About the same time another magnetical phenomenon attracted

attention. Besides the general motion of the magnetic poles, and the

diurnal movements of the needle, it was found that small and

irregular disturbances take place in its position, which M. de

Humboldt termed _magnetic storms_. And that which excited a strong

interest on this subject was the discovery that these magnetic

storms, seen only by philosophers who watch the needle with

microscopic exactness, rage simultaneously over large tracts of the

surface of our globe. This was detected about 1825 by a comparison

of the observations of M. Arago at Paris with simultaneous

observations of M. Kupffer at Kasan in Russia, distant more than 47

degrees of longitude.

At the instance of M. de Humboldt, the Imperial Academy of Russia

adopted with zeal the prosecution of this inquiry, and formed a chain

of magnetic stations across the whole of the Russian empire. Magnetic

observations were established at Petersburg and at Kasan, and

corresponding observations were made at Moscow, at Nicolaieff in the

Crimea, and Barnaoul and Nertchinsk in Siberia, at Sitka in Russian

America, and even at Pekin. To these magnetic stations the Russian

government afterwards added, Catharineburg in Russia Proper,

Helsingfors in Finland, Teflis in Georgia. A comparison of the results

obtained at four of these stations made by MM. de Humboldt and Dove,

in the year 1830, showed that the magnetic disturbances were

simultaneous, and were for the most parallel in their progress. {227}

Important steps in the prosecution of this subject were soon after

made by M. Gauss, the great mathematician of Göttingen. He contrived

instruments and modes of observation far more perfect than any

before employed, and organized a system of comparative observations

throughout Europe. In 1835, stations for this purpose were

established at Altona, Augsburg, Berlin, Breda, Breslau, Copenhagen,

Dublin, Freiberg, Göttingen, Greenwich, Hanover, Leipsic, Marburg,

Milan, Munich, Petersburg, Stockholm, and Upsala. At these places,

six times in the year, observations were taken simultaneously, at

intervals of five minutes for 24 hours. The _Results of the Magnetic

Association_ (Resultaten des Magnetischen Vereins) were published by

MM. Gauss and Weber, beginning in 1836.

British physicists did not at first take any leading part in these

plans. But in 1836, Baron Humboldt, who by his long labors and

important discoveries in this subject might be considered as

peculiarly entitled to urge its claims, addressed a letter to the

Duke of Sussex, then President of the Royal Society, asking for the

co-operation of this country in so large and hopeful a scheme for

the promotion of science. The Royal Society willingly entertained

this appeal; and the progress of the cause was still further

promoted when it was zealously taken up by the British Association

for the Advancement of Science, assembled at Newcastle in 1838. The

Association there expressed its strong interest in the German system

of magnetic observations; and at the instigation of this body, and

of the Royal Society, four complete magnetical observatories were

established by the British government, at Toronto, St. Helena, the

Cape of Good Hope, and Van Diemen's Land. The munificence of the

Directors of the East India Company founded and furnished an equal

number at Simla (in the Himalayah), Madras, Bombay, and Sincapore.

Sir Thomas Brisbane added another at his own expense at Kelso, in

Scotland. Besides this, the government sent out a naval expedition

to make discoveries (magnetic among others), in the Antarctic

regions, under the command of Sir James Ross. Other states lent

their assistance also, and founded or reorganized their magnetic

observatories. Besides those already mentioned, one was established

by the French government at Algiers; one by the Belgian, at

Brussels; two by Austria, at Prague and Milan; one by Prussia, at

Breslau; one by Bavaria, at Munich; one by Spain, at Cadiz; there

are two in the United States, at Philadelphia and Cambridge; one at

Cairo, founded by the Pasha of Egypt; and in India, one at

Trevandrum, established by the Rajah of Travancore; and one by {228}

the King of Oude, at Lucknow. At all these distant stations the same

plan was followed out, by observations strictly simultaneous, made

according to the same methods, with the same instrumental means.

Such a scheme, combining world-wide extent with the singleness of

action of an individual mind, is hitherto without parallel.

At first, the British stations were established for three years

only; but it was thought advisable to extend this period three years

longer, to end in 1845. And when the termination of that period

arrived, a discussion was held among the magneticians themselves,

whether it was better to continue the observations still, or to

examine and compare the vast mass of observations already collected,

so as to see to what results and improvements of methods they

pointed. This question was argued at the meeting of the British

Association at Cambridge in that year; and the conference ended in

the magneticians requesting to have the observations continued, at

some of the observatories for an indefinite period, at others, till

the year 1848. In the mean time the Antarctic expedition had brought

back a rich store of observations, fitted to disclose the magnetic

condition of those regions which it had explored. These were

_discussed_, and their results exhibited, in the _Philosophical

Transactions_ for 1843, by Col. Sabine, who had himself at various

periods, made magnetic observations in the Arctic regions, and in

several remote parts of the globe, and had always been a zealous

laborer in this fruitful field. The general mass of the observations

was placed under the management of Professor Lloyd, of Dublin, who

has enriched the science of magnetism with several valuable

instruments and methods, and who, along with Col. Sabine, made a

magnetic survey of the British Isles in 1835 and 1836.

I do not dwell upon magnetic surveys of various countries made by

many excellent observers; as MM. Quetelet, Forbes, Fox, Bache and

others.

The facts observed at each station were, the _intensity_ of the

magnetic force; the _declination_ of the needle from the meridian,

sometimes called the _variation_; and its _inclination_ to the

horizon, _the dip_;--or at least, some elements equivalent to these.

The values of these elements at any given time, if known, can be

expressed by charts of the earth's surface, on which are drawn the

_isodynamic_, _isogonal_, and _isoclinal_ curves. The second of

these kinds of charts contain the "Halleian lines" spoken of in a

previous page. Moreover the magnetic elements at each place are to

be observed in such a {229} manner as to determine both their

_periodical_ variations (the changes which occur in the period of a

day, and of a year), the _secular_ changes, as the gradual increase

or diminution of the declination at the same place for many years;

and the _irregular_ fluctuations which, as we have said, are

simultaneous over a large part, or the whole, of the earth's surface.

When these Facts have been ascertained over the whole extent of the

earth's surface, we shall still have to inquire what is the Cause of

the changes in the forces which these phenomena disclose. But as a

basis for all speculation on that subject, we must know the law of the

phenomena, and of the forces which immediately produce them. I have

already said that Euler tried to account for the Halleian lines by

means of _two_ magnetic "poles," but that M. Hansteen conceived it

necessary to assume _four_. But an entirely new light has been thrown

upon this subject by the beautiful investigations of Gauss, in his

_Theory of Terrestrial Magnetism_, published in 1839. He remarks that

the term "poles," as used by his predecessors, involves an assumption

arbitrary, and, as it is now found, false; namely, that certain

definite points, two, four, or more, acting according to the laws of

ordinary magnetical poles, will explain the phenomena. He starts from

a more comprehensive assumption, that magnetism is distributed

throughout the mass of the earth in an unknown manner. On this

assumption he obtains a function _V_, by the differentials of which

the elements of the magnetic force at any point will be expressed.

This function _V_ is well known in physical astronomy, and is obtained

by summing all the elements of magnetic force in each particle, each

multiplied by the reciprocal of its distance; or as we may express it,

by taking the sum of each element and its proximity jointly. Hence it

has been proposed[25\12] to term this function the "_integral

proximity_" of the attracting mass.[26\12] By using the most refined

{230} mathematical artifices for deducing the values of _V_ and its

differentials in converging series, he is able to derive the

coefficients of these series from the observed magnetic elements at

certain places, and hence, to calculate them for all places. The

comparison of the calculation with the observed results is, of course,

the test of the truth of the theory.

[Note 25\12: _Quart. Rev._ No. 131, p. 283.]

[Note 26\12: The function V is of constant occurrence in

investigations respecting attractions. It is introduced by Laplace

in his investigations respecting the attractions of spheroids, _Méc.

Cél._ Livr. III. Art. 4. Mr. Green and Professor Mac Cullagh have

proposed to term this function the _Potential_ of the system; but

this term (though suggested, I suppose, by analogy with the

substantive _Exponential_), does not appear convenient in its form.

On the other hand, the term _Integral Proximity_ does not indicate

that which gives the function its peculiar claim to distinction;

namely, that its differentials express the power or attraction of

the system. Perhaps _Integral Potentiality_, or _Integral

Attractivity_, would be a term combining the recommendations of both

the others.]

The degree of convergence of the series depends upon the unknown

distribution of magnetism within the earth. "If we could venture to

assume," says M. Gauss, "that the members have a sensible influence

only as far as the fourth order, complete observations from eight

points would be sufficient, theoretically considered, for the

determination of the coefficients." And under certain limitations,

making this assumption, as the best we can do at present, M. Gauss

obtains from eight places, 24 coefficients (each supplying three

elements), and hence calculates the magnetic elements (intensity,

variation and dip) at 91 places in all parts of the earth. He finds

his calculations approach the observed values with a degree of

exactness which appears to be quite convincing as to the general

truth of his results; especially taking into account how entirely

unlimited is his original hypothesis.

It is one of the most curious results of this investigation that

according to the most simple meaning which we can give to the term

"pole" the earth has only _two_ magnetic poles; that is, two points

where the direction of the magnetic force is vertical. And thus the

_isogonal curves_ may be looked upon as _deformations_ of the curves

deduced by Euler from the supposition of two poles, the deformation

arising from this, that the earth does not contain a single definite

magnet, but irregularly diffused magnetical elements, which still

have collectively a distinct resemblance to a single magnet. And

instead of Hansteen's Siberian pole, we have a Siberian region in

which the needles converge; but if the apparent convergence be

pursued it nowhere comes to a point; and the like is the case in the

Antarctic region. When the 24 Gaussian elements at any time are

known the magnetic condition of the globe is known, just as the

mechanical condition of the solar system is known, when we know the

elements of the orbits of the satellites and planets and the mass of

each. And the comparison of this magnetic condition of the globe at

distant periods of time cannot fail to supply materials for future

researches and speculations with regard to the agencies by which the

condition of the earth is determined. The condition of which we here

speak must necessarily be its _mechanico-chemical_ condition, being

expressed, as it will be, in terms of the mechanico-chemical

sciences. The {231} investigations I have been describing belong to

the mechanical side of the subject: but when philosophers have to

consider the causes of the secular changes which are found to occur

in this mechanical condition, they cannot fail to be driven to

electrical, that is, chemical agencies and laws.

I can only allude to Gauss's investigations respecting the _Absolute

Measure_ of the Earth's Magnetic Force. To determine the ratio of the

magnetic force of the earth to that of a known magnet, Poisson

proposed to observe the time of vibration of a second magnet. The

method of Gauss, now universally adopted, consists in observing the

position of equilibrium of the second magnet when deflected by the

first.

The manner in which the business of magnetic observation has been

taken up by the governments of our time makes this by far the

greatest scientific undertaking which the world has ever seen. The

result will be that we shall obtain in a few years a knowledge of

the magnetic constitution of the earth which otherwise it might have

required centuries to accumulate. The secular magnetic changes must

still require a long time to reduce to their laws of phenomena,

except observation be anticipated or assisted by some happy

discovery as to the cause of these changes. But besides the special

gain to magnetic science by this great plan of joint action among

the nations of the earth, there is thereby a beginning made in the

recognition and execution of the duty of forwarding science in

general by national exertions. For at most of the magnetic

observatories, meteorological observations are also carried on; and

such observations, being far more extensive, systematic, and

permanent than those which have usually been made, can hardly fail

to produce important additions to science. But at any rate they do

for science that which nations can do, and individuals cannot; and

they seek for scientific truths in a manner suitable to the respect

now professed for science and to the progress which its methods have

made. Nor are we to overlook the effect of such observations as

means of training men in the pursuit of science. "There is amongst

us," says one of the magnetic observers, "a growing recognition of

the importance, both for science and for practical life, of forming

exact observers of nature. Hitherto astronomy alone has afforded a

very partial opportunity for the formation of fine observers, of

which few could avail themselves. Experience has shown that magnetic

observations may serve as excellent training schools in this

respect."[27\12]] {232}

[Note 27\12: _Letter_ of W. Weber. _Brit. Assoc. Rep._ 1845, p. 17.]

The various other circumstances which terrestrial magnetism

exhibits,--the diurnal and annual changes of the position of the

compass-needle;--the larger secular change which affects it in the

course of years;--the difference of intensity at different places,

and other facts, have naturally occupied philosophers with the

attempt to determine, both the laws of the phenomena and their

causes. But these attempts necessarily depend, not upon laws of

statical magnetism, such as they have been explained above; but upon

the laws by which the production and intensity of magnetism in

different cases are regulated;--laws which belong to a different

province, and are related to a different set of principles. Thus,

for example, we have not attempted to explain the discovery of the

laws by which heat influences magnetism; and therefore we cannot now

give an account of those theories of the facts relating to

terrestrial magnetism, which depend upon the influence of

temperature. The conditions of excitation of magnetism are best

studied by comparing this force with other cases where the same

effects are produced by very different apparent agencies; such as

galvanic and thermo-electricity. To the history of these we shall

presently proceed.

_Conclusion._--The hypothesis of magnetic fluids, as physical

realities, was never widely or strongly embraced, as that of

electric fluids was. For though the hypothesis accounted, to a

remarkable degree of exactness, for large classes of the phenomena,

the presence of a material fluid was not indicated by facts of a

different kind, such as the spark, the discharge from points, the

shock, and its mechanical effects. Thus the belief of a peculiar

magnetic fluid or fluids was not forced upon men's minds; and the

doctrine above stated was probably entertained by most of its

adherents, chiefly as a means of expressing the laws of phenomena in

their elementary form.

One other observation occurs here. We have seen that the supposition

of a fluid moveable from one part of bodies to another, and capable

of accumulation in different parts of the surface, appeared at first

to be as distinctly authorized by magnetic as by electric phenomena;

and yet that it afterwards appeared, by calculation, that this must

be considered as a derivative result; no real transfer of fluid

taking place except within the limits of the insensible particles of

the body. Without attempting to found a formula of philosophizing on

this circumstance, we may observe, that this occurrence, like the

disproof of heat as a material fluid, shows the possibility of an

hypothesis which shall very exactly satisfy many phenomena, and yet

be incomplete: it {233} shows, too, the necessity of bringing facts

of all kinds to bear on the hypothesis; thus, in this case it was

requisite to take into account the facts of junction and separation

of magnetic bodies, as well as their attractions and repulsions.

If we have seen reason to doubt the doctrine of electric fluids as

physical realities, we cannot help pronouncing upon the magnetic

fluids as having still more insecure claims to a material existence,

even on the grounds just stated. But we may add considerations still

more decisive; for at a further stage of discovery, as we shall see,

magnetic and electric action were found to be connected in the

closest manner, so as to lead to the persuasion of their being

different effects of one common cause. After those discoveries, no

philosopher would dream of assuming electric fluids and magnetic

fluids as two distinct material agents. Yet even now the nature of

the dependence of magnetism upon any other cause is extremely

difficult to conceive. But till we have noticed some of the

discoveries to which we have alluded, we cannot even speculate about

that dependence. We now, therefore, proceed to sketch the history of

these discoveries.

{{235}}

BOOK XIII.

_MECHANICO-CHEMICAL SCIENCES._

(CONTINUED.)

HISTORY OF GALVANISM,

OR

VOLTAIC ELECTRICITY.

Percusssæ gelido trepidant sub pectore fibræ,

Et nova desuetis subrepens vita medullis

Miscetur morti: tunc omnis palpitat artus

Tenduntur nervi; nec se tellure cadaver

Paullatim per membra levat; terrâque repulsum est

Erectumque simul.

LUCAN. vi. 752.

The form which lay before inert and dead,

Sudden a piercing thrill of change o'erspread;

Returning life gleams in the stony face,

The fibres quiver and the sinews brace,

Move the stiff limbs;--nor did the body rise

With tempered strength which genial life supplies,

But upright starting, its full stature held,

As though the earth the supine corse repelled.

{{237}}

CHAPTER I.

DISCOVERY OF VOLTAIC ELECTRICITY.

WE have given the name of _mechanico-chemical_ to the class of

sciences now under our consideration; for these sciences are

concerned with cases in which mechanical effects, that is,

attractions and repulsions, are produced; while the conditions under

which these effects occur, depend, as we shall hereafter see, on

chemical relations. In that branch of these sciences which we have

just treated of, Magnetism, the mechanical phenomena were obvious,

but their connexion with chemical causes was by no means apparent,

and, indeed, has not yet come under our notice.

The subject to which we now proceed, Galvanism, belongs to the same

group, but, at first sight, exhibits only the other, the chemical,

portion of the features of the class; for the connexion of galvanic

phenomena with chemical action was soon made out, but the mechanical

effects which accompany them were not examined till the examination

was required by a new train of discovery. It is to be observed, that

I do not include in the class of mechanical effects the convulsive

motions in the limbs of animals which are occasioned by galvanic

action; for these movements are produced, not by attraction and

repulsion, but by muscular irritability; and though they indicate

the existence of a peculiar agency, cannot be used to measure its

intensity and law.

The various examples of the class of agents which we here

consider,--magnetism, electricity, galvanism, electro-magnetism,

thermo-electricity,--differ from each other principally in the

circumstances by which they are called into action; and these

differences are in reality of a chemical nature, and will have to be

considered when we come to treat of the inductive steps by which the

general principles of chemical theory are established. In the

present part of our task, therefore, we must take for granted the

chemical conditions on which the excitation of these various kinds

of action depends, and trace the history of the discovery of their

mechanical laws only. This rule will much abridge the account we

have here to give of the progress of discovery in the provinces to

which I have just referred. {238}

The first step in this career of discovery was that made by Galvani,

Professor of Anatomy at Bologna. In 1790, electricity, as an

experimental science, was nearly stationary. The impulse given to

its progress by the splendid phenomena of the Leyden phial had

almost died away; Coulomb was employed in systematizing the theory

of the electric fluid, as shown by its statical effects; but in all

the other parts of the subject, no great principle or new result had

for some time been detected. The first announcement of Galvani's

discovery in 1791 excited great notice, for it was given forth as a

manifestation of electricity under a new and remarkable character;

namely, as residing in the muscles of animals.[1\13] The limbs of a

dissected frog were observed to move, when touched with pieces of

two different metals; the agent which produced these motions was

conceived to be identified with electricity, and was termed _animal

electricity_; and Galvani's experiments were repeated, with various

modifications, in all parts of Europe, exciting much curiosity, and

giving rise to many speculations.

[Note 1\13: _De Viribus Electricis in Motu Musculari_, Comm. Bonon.

t. vii. 1792.]

It is our business to determine the character of each great

discovery which appears in the progress of science. Men are fond of

repeating that such discoveries are most commonly the result of

accident; and we have seen reason to reject this opinion, since that

preparation of thought by which the accident produces discovery is

the most important of the conditions on which the successful event

depends. Such accidents are like a spark which discharges a gun

already loaded and pointed. In the case of Galvani, indeed, the

discovery may, with more propriety than usual, be said to have been

casual; but in the form in which it was first noted, it exhibited no

important novelty. His frog was lying on a table near the conductor

of an electrical machine, and the convulsions appeared only when a

spark was taken from the machine. If Galvani had been as good a

physicist as he was an anatomist, he would probably have seen that

the movements so occasioned proved only that the muscles or nerves,

or the two together, formed a very sensitive indicator of electrical

action. It was when he produced such motions by contact of metals

alone, that he obtained an important and fundamental fact in science.

The analysis of this fact into its real and essential conditions was

the work of Alexander Volta, another Italian professor. Volta,

indeed, possessed that knowledge of the subject of electricity which

made a hint like that of Galvani the basis of a new science. Galvani

appears {239} never to have acquired much general knowledge of

electricity: Volta, on the other hand, had labored at this branch of

knowledge from the age of eighteen, through a period of nearly

thirty years; and had invented an _electrophorus_ and an _electrical

condenser_, which showed great experimental skill. When he turned

his attention to the experiments made by Galvani, he observed that

the author of them had been far more surprised than he needed to be,

at those results in which an electrical spark was produced; and that

it was only in the cases in which no such apparatus was employed,

that the observations could justly be considered as indicating a new

law, or a new kind of electricity.[2\13] He soon satisfied

himself[3\13] (about 1794) that the essential conditions of this

kind of action depended on the metals; that it is brought into play

most decidedly when two different metals touch each other, and are

connected by any moist body;--and that the parts of animals which

had been used discharged the office both of such moist bodies, and

of very sensitive electrometers. The _animal_ electricity of Galvani

might, he observed, be with more propriety called _metallic_

electricity.

[Note 2\13: _Phil. Trans._ 1793, p. 21.]

[Note 3\13: See Fischer, viii. 625.]

The recognition of this agency as a peculiar kind of _electricity_,

arose in part perhaps, at first, from the confusion made by Galvani

between the cases in which his electrical machine was, and those in

which it was not employed. But the identity was confirmed by its

being found that the known difference of electrical conductors and

non-conductors regulated the conduction of the new influence. The

more exact determination of the new facts to those of electricity

was a succeeding step of the progress of the subject.

The term "animal electricity" has been superseded by others, of

which _galvanism_ is perhaps the most familiar. I think it will

appear from what has been said, that Volta's office in this

discovery is of a much higher and more philosophical kind than that

of Galvani; and it would, on this account, be more fitting to employ

the term _voltaic electricity_; which, indeed, is very commonly

used, especially by our most recent and comprehensive writers.

Volta more fully still established his claim as the main originator

of this science by his next step. When some of those who repeated

the experiments of Galvani had expressed a wish that there was some

method of multiplying the effect of _this_ electricity, such as the

Leyden phial supplies for common electricity, they probably thought

their wishes far from a realization. But the _voltaic pile_, which

Volta {240} described in the _Philosophical Transactions_ for 1800,

completely satisfies this aspiration; and was, in fact, a more

important step in the history of electricity than the Leyden jar had

been. It has since undergone various modifications, of which the

most important was that introduced by **Cruickshanks, who[4\13]

substituted a trough for a pile. But in all cases the principle of

the instrument was the same;--a continued repetition of the triple

combination of two metals and a fluid in contact, so as to form a

circuit which returns into itself.

[Note 4\13: Fischer, viii. p. 683.]

Such an instrument is capable of causing effects of great intensity;

as seen both in the production of light and heat, and in chemical

changes. But the discovery with which we are here concerned, is not

the details and consequences of the effects, (which belong to

chemistry,) but the analysis of the conditions under which such

effects take place; and this we may consider as completed by Volta

at the epoch of which we speak.

CHAPTER II.

RECEPTION AND CONFIRMATION OF THE DISCOVERY OF VOLTAIC ELECTRICITY.

GALVANI'S experiments excited a great interest all over Europe, in

consequence partly of a circumstance which, as we have seen, was

unessential, the muscular contractions and various sensations which

they occasioned. Galvani himself had not only considered the animal

element of the circuit as the origin of the electricity, but had

framed a theory,[5\13] in which he compared the muscles to charged

jars, and the nerves to the discharging wires; and a controversy

was, for some time, carried on, in Italy, between the adherents of

Galvani and those of Volta.[6\13]

[Note 5\13: Ib. viii. 613.]

[Note 6\13: Ib. viii. 619.]

The galvanic experiments, and especially those which appeared to

have a physiological bearing, were verified and extended by a number

of the most active philosophers of Europe, and especially William

von Humboldt. A commission of the Institute of France, appointed in

1797, repeated many of the known experiments, but does not seem to

have decided any disputed points. The researches of this {241}

commission referred rather to the discoveries of Galvani than to

those of Volta: the latter were, indeed, hardly known in France till

the conquest of Italy by Bonaparte, in 1801. France was, at the

period of these discoveries, separated from all other countries by

war, and especially from England,[7\13] where Volta's Memoirs were

published.

[Note 7\13: _Biog. Univ._, art. _Volta_, (by Biot.)]

The political revolutions of Italy affected, in very different

manners, the two discoverers of whom we speak. Galvani refused to

take an oath of allegiance to the Cisalpine republic, which the

French conqueror established; he was consequently stripped of all

his offices; and deprived, by the calamities of the times, of most

of his relations, he sank into poverty, melancholy, and debility. At

last his scientific reputation induced the republican rulers to

decree his restoration to his professorial chair; but his claims

were recognised too late, and he died without profiting by this

intended favor, in 1798.

Volta, on the other hand, was called to Paris by Bonaparte as a man

of science, and invested with honors, emoluments, and titles. The

conqueror himself, indeed, was strongly interested by this train of

research.[8\13] He himself founded valuable prizes, expressly with a

view to promote its prosecution. At this period, there was something

in this subject peculiarly attractive to his Italian mind; for the

first glimpses of discoveries of great promise have always excited

an enthusiastic activity of speculation in the philosophers of

Italy, though generally accompanied with a want of precise thought.

It is narrated[9\13] of Bonaparte, that after seeing the

decomposition of the salts by means of the voltaic pile, he turned

to Corvisart, his physician, and said, "Here, doctor, is the image

of life; the vertebral column is the pile, the liver is the

negative, the bladder the positive, pole." The importance of voltaic

researches is not less than it was estimated by Bonaparte; but the

results to which it was to lead were of a kind altogether different

from those which thus suggested themselves to his mind. The

connexion of mechanical and chemical action was the first great

point to be dealt with; and for this purpose the laws of the

mechanical action of voltaic electricity were to be studied.

[Note 8\13: Becquerel, _Traité d'Electr._ t. i. p. 107.]

[Note 9\13: Ib. t. i. p. 108.]

It will readily be supposed that the voltaic researches, thus begun,

opened a number of interesting topics of examination and discussion.

These, however, it does not belong to our place to dwell upon at

present; since they formed parts of the theory of the subject, which

{242} was not completed till light had been thrown upon it from

other quarters. The identity of galvanism with electricity, for

instance, was at first, as we have intimated, rather conjectured

than proved. It was denied by Dr. Fowler, in 1793; was supposed to

be confirmed by Dr. Wells two years later; but was, still later,

questioned by Davy. The nature of the operation of the pile was

variously conceived. Volta himself had obtained a view of it which

succeeding researches confirmed, when he asserted,[10\13] in 1800,

that it resembled an electric battery feebly charged and constantly

renewing its charge. In pursuance of this view, the common

electrical action was, at a later period (for instance by Ampère, in

1820), called _electrical tension_, while the voltaic action was

called the _electrical current_, or _electromotive action_. The

different effects produced, by increasing the size and the number of

the plates in the voltaic trough, were also very remarkable. The

power of producing heat was found to depend on the size of the

plates; the power of producing chemical changes, on the other hand,

was augmented by the number of plates of which the battery

consisted. The former effect was referred to the increased

_quantity_, the latter to the _intensity_, of the electric fluid. We

mention these distinctions at present, rather for the purpose of

explaining the language in which the results of the succeeding

investigations are narrated, than with the intention of representing

the hypotheses and measures which they imply, as clearly

established, at the period of which we speak. For that purpose new

discoveries were requisite, which we have soon to relate.

[Note 10\13: _Phil. Trans._ p. 403.]

CHAPTER III.

DISCOVERY OF THE LAWS OF THE MUTUAL ATTRACTION AND REPULSION OF

VOLTAIC CURRENTS.--AMPÈRE.

IN order to show the place of voltaic electricity among the

mechanico-chemical sciences, we must speak of its mechanical laws as

separate from the laws of electro-magnetic action; although, in

fact, it was only in consequence of the forces which conducting

voltaic wires exert upon magnets, that those forces were detected

which they exert upon each {243} other. This latter discovery was

made by M. Ampère; and the extraordinary rapidity and sagacity with

which he caught the suggestion of such forces, from the

electro-magnetic experiments of M. Oersted, (of which we shall speak

in the next chapter,) well entitle him to be considered as a great

and independent discoverer. As he truly says,[11\13] "it by no means

followed, that because a conducting wire exerted a force on a

magnet, two conducting wires must exert a force on each other; for

two pieces of soft iron, both of which affect a magnet, do not

affect each other." But immediately on the promulgation of Oersted's

experiments, in 1820, Ampère leapt forwards to a general theory of

the facts, of which theory the mutual attraction and repulsion of

conducting voltaic wires was a fundamental supposition. The

supposition was immediately verified by direct trial; and the laws

of this attraction and repulsion were soon determined, with great

experimental ingenuity, and a very remarkable command of the

resources of analysis. But the experimental and analytical

investigation of the mutual action of voltaic or electrical

currents, was so mixed up with the examination of the laws of

electro-magnetism, which had given occasion to the investigation,

that we must not treat the two provinces of research as separate.

The mention in this place, premature as it might appear, of the

labors of Ampère, arises inevitably from his being the author of a

beautiful and comprehensive generalization, which not only included

the phenomena exhibited by the new combinations of Oersted, but also

disclosed forces which existed in arrangements already familiar,

although they had never been detected till the theory pointed out

how they were to be looked for.

[Note 11\13: _Théorie des Phénom. Electrodynamiques_, p. 113.]

CHAPTER IV.

DISCOVERY OF ELECTRO-MAGNETIC ACTION.--OERSTED.

THE impulse which the discovery of galvanism, in 1791, and that of

the voltaic pile, in 1800, had given to the study of electricity as

a mechanical science, had nearly died away in 1820. It was in that

year that M. Oersted, of Copenhagen, announced that the conducting

{244} wire of a voltaic circuit, acts upon a magnetic needle; and

thus recalled into activity that endeavor to connect magnetism with

electricity, which, though apparently on many accounts so hopeful,

had hitherto been attended with no success. Oersted found that the

needle has a tendency to place itself _at right angles_ to the

wire;--a kind of action altogether different from any which had been

suspected.

This observation was of vast importance; and the analysis of its

conditions and consequences employed the best philosophers in Europe

immediately on its promulgation. It is impossible, without great

injustice, to refuse great merit to Oersted as the author of the

discovery. We have already said that men appear generally inclined

to believe remarkable discoveries to be accidental, and the

discovery of Oersted has been spoken of as a casual insulated

experiment.[12\13] Yet Oersted had been looking for such an

_accident_ probably more carefully and perseveringly than any other

person in Europe. In 1807, he had published[13\13] a work, in which

he professed that his purpose was "to ascertain whether electricity,

in its most latent state, had any effect on the magnet." And he, as

I know from his own declaration, considered his discovery as the

natural sequel and confirmation of his early researches; as, indeed,

it fell in readily and immediately with speculations on these

subjects then very prevalent in Germany. It was an accident like

that by which a man guesses a riddle on which his mind has long been

employed.

[Note 12\13: See _Schelling ueber Faraday's Entdeckung_, p. 27.]

[Note 13\13: Ampère, p. 69.]

Besides the confirmation of Oersted's observations by many

experimenters, great additions were made to his facts: of these, one

of the most important was due to Ampère. Since the earth is in fact

magnetic, the voltaic wire ought to be affected by terrestrial

magnetism alone, and ought to tend to assume a position depending on

the position of the compass-needle. At first, the attempts to

produce this effect failed, but soon, with a more delicate

apparatus, the result was found to agree with the anticipation.

It is impossible here to dwell on any of the subsequent researches,

except so far as they are essential to our great object, the progress

towards a general theory of the subject. I proceed, therefore,

immediately to the attempts made towards this object. {245}

CHAPTER V.

DISCOVERY OF THE LAWS OF ELECTRO-MAGNETIC ACTION.

ON attempting to analyse the electro-magnetic phenomena observed by

Oersted and others into their simplest forms, they appeared, at

least at first sight, to be different from any mechanical actions

which had yet been observed. It seemed as if the conducting wire

exerted on the pole of the magnet a force which was not attractive

or repulsive, but _transverse_;--not tending to draw the point acted

on nearer, or to push it further off, in the line which reached from

the acting point, but urging it to move at right angles to this

line. The forces appeared to be such as Kepler had dreamt of in the

infancy of mechanical conceptions; rather than such as those of

which Newton had established the existence in the solar system, and

such as he, and all his successors, had supposed to be the only

kinds of force which exist in nature. The north pole of the needle

moved as if it were impelled by a vortex revolving round the wire in

one direction, while the south pole seemed to be driven by an

opposite vortex. The case seemed novel, and almost paradoxical.

It was soon established by experiments, made in a great variety of

forms, that the mechanical action was really of this transverse

kind. And a curious result was obtained, which a little while before

would have been considered as altogether incredible;--that this

force would cause a constant and rapid revolution of either of the

bodies about the other;--of the conducting wire about the magnet, or

of the magnet about the conducting wire. This was effected by Mr.

Faraday in 1821.

The laws which regulated the intensity of this force, with reference

to the distance and position of the bodies, now naturally came to be

examined. MM. Biot and Savart in France, and Mr. Barlow in England,

instituted such measures; and satisfied themselves that the

elementary force followed the law of magnitude of all known

elementary forces, in being inversely as the square of the distance;

although, in its direction, it was so entirely different from other

forces. But the investigation of the _laws of phenomena_ of the

subject was too closely connected with the choice of a mechanical

theory, to be established {246} previously and independently, as had

been done in astronomy. The experiments gave complex results, and

the analysis of these into their elementary actions was almost an

indispensable step in order to disentangle their laws. We must,

therefore, state the progress of this analysis.

CHAPTER VI.

THEORY OF ELECTRODYNAMICAL ACTION.

AMPÈRE'S THEORY.--Nothing can show in a more striking manner the

advanced condition of physical speculation in 1820, than the

reduction of the strange and complex phenomena of electromagnetism

to a simple and general theory as soon as they were published.

Instead of a gradual establishment of laws of phenomena, and of

theories more and more perfect, occupying ages, as in the case of

astronomy, or generations, as in the instances of magnetism and

electricity, a few months sufficed for the whole process of

generalization; and the experiments made at Copenhagen were

announced at Paris and London, almost at the same time with the

skilful analysis and comprehensive inductions of Ampère.

Yet we should err if we should suppose, from the celerity with which

the task was executed, that it was an easy one. There were required

in the author of such a theory, not only those clear conceptions of

the relations of space and force, which are the first conditions of

all sound theory, and a full possession of the experiments; but also

a masterly command of the mathematical arms by which alone the

victory could be gained, and a sagacious selection of proper

experiments which might decide the fate of the proposed hypothesis.

It is true, that the nature of the requisite hypothesis was not

difficult to see in a certain vague and limited way. The

conducting-wire and the magnetic needle had a tendency to arrange

themselves at right angles to one another. This might be represented

by supposing the wire to be made up of transverse magnetic needles,

or by supposing the needle to be made up of transverse

conducting-wires; for it was easy to conceive forces which should

bring corresponding elements, either magnetic or voltaic, into

parallel positions; and then the {247} general phenomena above

stated would be accounted for. And the choice between the two modes

of conception, appeared at first sight a matter of indifference. The

majority of philosophers at first adopted, or at least employed, the

former method, as Oersted in Germany, Berzelius in Sweden, Wollaston

in England.

Ampère adopted the other view, according to which the magnet is made

up of conducting-wires in a transverse position. But he did for his

hypothesis what no one did or could do for the other: he showed that

it was the only one which would account, without additional and

arbitrary suppositions, for the facts of _continued_ motion in

electromagnetic cases. And he further elevated his theory to a

higher rank of generality, by showing that it explained,--not only

the action of a conducting-wire upon a magnet, but also two other

classes of facts, already spoken of in this history,--the action of

magnets upon each other,--and the action of conducting-wires upon

each other.

The deduction of such particular cases from the theory, required, as

may easily be imagined, some complex calculations: but the deduction

being satisfactory, it will be seen that Ampère's theory conformed

to that description which we have repeatedly had to point out as the

usual character of a true and stable theory; namely, that besides

accounting for the class of phenomena which suggested it, it

supplies an unforeseen explanation of other known facts. For the

mutual action of magnets, which was supposed to be already reduced

to a satisfactory theoretical form by Coulomb, was not contemplated

by Ampère in the formation of his hypothesis; and the mutual action

of voltaic currents, though tried only in consequence of the

suggestion of the theory, was clearly a fact distinct from

electromagnetic action; yet all these facts flowed alike from the

theory. And thus Ampère brought into view a class of forces for

which the term "electromagnetic" was too limited, and which he

designated[14\13] by the appropriate term _electrodynamic_;

distinguishing them by this expression, as the forces of an electric

_current_, from the _statical_ effects of electricity which we had

formerly to treat of. This term has passed into common use among

scientific writers, and remains the record and stamp of the success

of the Amperian induction.

[Note 14\13: _Ann. de Chim._, tom. xx. p. 60 (1822).]

The first promulgation of Ampère's views was by a communication to

the French Academy of Sciences, September the 18th, 1820; Oersted's

discoveries having reached Paris only in the preceding July. {248}

At almost every meeting of the Academy during the remainder of that

year and the beginning of the following one, he had new

developements or new confirmations of his theory to announce. The

most hypothetical part of his theory,--the proposition that magnets

might be considered in their effects as identical with spiral

voltaic wires,--he asserted from the very first. The mutual

attraction and repulsion of voltaic wires,--the laws of this

action,--the deduction of the observed facts from it by

calculation,--the determination, by new experiments, of the constant

quantities which entered into his formulæ,--followed in rapid

succession. The theory must be briefly stated. It had already been

seen that parallel voltaic currents attracted each other; when,

instead of being parallel, they were situate in any directions, they

still exerted attractive and repulsive forces depending on the

distance, and on the directions of each element of both currents.

Add to this doctrine the hypothetical constitution of magnets,

namely, that a voltaic current runs round the axis of each particle,

and we have the means of calculating a vast variety of results which

may be compared with experiment. But the laws of the elementary

forces required further fixation. What _functions_ are the forces of

the distance and the directions of the elements?

To extract from experiment an answer to this inquiry was far from

easy, for the elementary forces were mathematically connected with

the observed facts, by a double mathematical integration;--a long,

and, while the constant coefficients remained undefined, hardly a

possible operation. Ampère made some trials in this way, but his

happier genius suggested to him a better path. It occurred to him,

that if his integrals, without being specially found, could be shown

to vanish upon the whole, under certain conditions of the problem,

this circumstance would correspond to arrangements of his apparatus

in which a state of equilibrium was preserved, however the form of

some of the parts might be changed. He found two such cases, which

were of great importance to the theory. The first of these cases

proved that the force exerted by any element of the voltaic wire

might be resolved into other forces by a theorem resembling the

well-known proposition of the parallelogram of forces. This was

proved by showing that the action of a straight wire is the same

with that of another wire which joins the same extremities, but is

bent and contorted in any way whatever. But it still remained

necessary to determine two fundamental quantities; one which

expressed the _power_ of the distance according to which the force

varied; the other, the {249} degree in which the force is affected

by the _obliquity_ of the elements. One of the general causes of

equilibrium, of which we have spoken, gave a relation between these

two quantities;[15\13] and as the power was naturally, and, as it

afterwards appeared, rightly conjectured to be the inverse square,

the other quantity also was determined; and the general problem of

electrodynamical action was fully solved.

[Note 15\13: Communication to the Acad. Sc., June 10, 1822. See

Ampère, _Recueil_, p. 292.]

If Ampère had not been an accomplished analyst, he would not have

been able to discover the condition on which the nullity of the

integral in this case depended.[16\13] And throughout his labors, we

find reason to admire, both his mathematical skill, and his

steadiness of thought; although these excellences are by no means

accompanied throughout with corresponding clearness and elegance of

exposition in his writings.

[Note 16\13: _Recueil_, p. 314.]

_Reception of Ampère's Theory._--Clear mathematical conceptions, and

some familiarity with mathematical operations, were needed by

readers also, in order to appreciate the evidence of the theory;

and, therefore, we need not feel any surprise if it was, on its

publication and establishment, hailed with far less enthusiasm than

so remarkable a triumph of generalizing power might appear to

deserve. For some time, indeed, the greater portion of the public

were naturally held in suspense by the opposing weight of rival

names. The Amperian theory did not make its way without contention

and competition. The electro-magnetic experiments, from their first

appearance, gave a clear promise of some new and wide

generalization; and held out a prize of honor and fame to him who

should be first in giving the right interpretation of the riddle. In

France, the emulation for such reputation is perhaps more vigilant

and anxious than it is elsewhere; and we see, on this as on other

occasions, the scientific host of Paris springing upon a new subject

with an impetuosity which, in a short time, runs into controversies

for priority or for victory. In this case, M. Biot, as well as

Ampère, endeavored to reduce the electro-magnetic phenomena to

general laws. The discussion between him and Ampère turned on some

points which are curious. M. Biot was disposed to consider as an

elementary action, the force which an element of a voltaic wire

exerts upon a magnetic particle, and which is, as we have seen, at

right angles to their mutual distance; and he conceived that {250}

the equal reaction which necessarily accompanies this action acts

oppositely to the action, not in the same line, but in a parallel

line, at the other extremity of the distance; thus forming a

primitive _couple_, to use a technical expression borrowed from

mechanics. To this Ampère objected,[17\13] that the _direct_

opposition of all elementary action and reaction was a universal and

necessary mechanical law. He showed too that such a couple as had

been assumed, would follow as a _derivative_ result from his theory.

And in comparing his own theory with that in which the voltaic wire

is assimilated to a collection of transverse magnets, he was also

able to prove that no such assemblage of forces acting to and from

fixed points, as the forces of magnets do act, could produce a

continued motion like that discovered by Faraday. This, indeed, was

only the well-known demonstration of the impossibility of a

perpetual motion. If, instead of a collection of magnets, the

adverse theorists had spoken of a magnetic _current_, they might

probably interpret their expressions so as to explain the facts;

that is, if they considered every element of such a current as a

magnet, and consequently, every point of it as being a north and a

south point at the same instant. But to introduce such a conception

of a magnetic current was to abandon all the laws of magnetic action

hitherto established; and consequently to lose all that gave the

hypothesis its value. The idea of an electric current, on the other

hand, was so far from being a new and hazardous assumption, that it

had already been forced upon philosophers from the time of Volta;

and in this current, the relation of _preceding_ and _succeeding_,

which necessarily existed between the extremities of any element,

introduced that relative polarity on which the success of the

explanations of the facts depended. And thus in this controversy,

the theory of Ampère has a great and undeniable superiority over the

rival hypotheses.

[Note 17\13: Ampère, _Théorie_, p. 154.]

CHAPTER VII.

CONSEQUENCES OF THE ELECTRODYNAMIC THEORY.

IT is not necessary to state the various applications which were

soon made of the electro-magnetic discoveries. But we may notice one

{251} of the most important,--the _Galvanometer_, an instrument

which, by enabling the philosopher to detect and to measure

extremely minute electrodynamic actions, gave an impulse to the

subject similar to that which it received from the invention of the

Leyden Phial, or the Voltaic Pile. The strength of the voltaic

current was measured, in this instrument, by the deflection produced

in a compass-needle; and its sensibility was multiplied by making

the wire pass repeatedly above and below the needle. Schweigger, of

Halle, was one of the first devisers of this apparatus.

The substitution of electro-magnets, that is, of spiral tubes

composed of voltaic wires, for common magnets, gave rise to a

variety of curious apparatus and speculations, some of which I shall

hereafter mention.

[2nd Ed.] [When a voltaic apparatus is in action, there may be

conceived to be a current of electricity running through its various

elements, as stated in the text. The force of this current in

various parts of the circuit has been made the subject of

mathematical investigation by M. Ohm.[18\13] The problem is in every

respect similar to that of the flow of heat through a body, and

taken generally, leads to complex calculations of the same kind. But

Dr. Ohm, by limiting the problem in the first place by conditions

which the usual nature and form of voltaic apparatus suggest, has

been able to give great simplicity to his reasonings. These

conditions are, the linear form of the conductors (wires) and the

steadiness of the electric state. For this part of the problem Dr.

Ohm's reasonings are as simple and as demonstrative as the

elementary propositions of Mechanics. The formulæ for the electric

force of a voltaic current to which he is led have been

experimentally verified by others, especially Fechner,[19\13]

Gauss,[20\13] Lenz, Jacobi, Poggendorf, and Pouillet.

[Note 18\13: _Die Galvanische Kette Mathematisch bearbeitet von Dr.

G. S. Ohm_, Berlin, 1827.]

[Note 19\13: _**Mass-bestimmungen über die Galvanische Kette._

Leipzig, 1831.]

[Note 20\13: _Results of the Magnetic Association._]

Among ourselves, Mr. Wheatstone has confirmed and applied the views

of M. Ohm, in a Memoir[21\13] _On New Instruments and Processes for

determining the Constants of a Voltaic Circuit_. He there remarks

that the clear ideas of electromotive forces and resistances,

substituted by Ohm for the vague notions of quantity and intensity

which have long been prevalent, give satisfactory explanations of

the most important difficulties, and express the laws of a vast

number of phenomena {252} in formulæ of remarkable simplicity and

generality. In this Memoir, Professor Wheatstone describes an

instrument which he terms _Rheostat_, because it brings to a common

standard the voltaic currents which are compared by it. He

generalizes the language of the subject by employing the term

_rheomotor_ for any apparatus which originates an electric current

(whether voltaic or thermoelectric, &c.) and _rheometer_ for any

instrument to measure the force of such a current. It appears that

the idea of constructing an instrument of the nature of the Rheostat

had occurred also to Prof. Jacobi, of St Petersburg.]

[Note 21\13: _Phil. Trans._ 1843. Pt. 11.]

The galvanometer led to the discovery of another class of cases in

which the electrodynamical action was called into play, namely,

those in which a circuit, composed of two metals only, became

electro-magnetic by _heating_ one part of it. This discovery of

_thermo-electricity_ was made by Professor Seebeck of Berlin, in

1822, and prosecuted by various persons; especially by Prof.

Cumming[22\13] of Cambridge, who, early in 1823, extended the

examination of this property to most of the metals, and determined

their thermo-electric order. But as these investigations exhibited

no new mechanical effects of electromotive forces, they do not now

further concern us; and we pass on, at present, to a case in which

such forces act in a manner different from any of those already

described.

[Note 22\13: _Camb. Trans._ vol. ii. p. 62. _On the Development of

Electro-Magnetism by Heat._]

DISCOVERY OF DIAMAGNETISM.

[2nd Ed.] [By the discoveries just related, a cylindrical spiral of

wire through which an electric current is passing is identified with

a magnet; and the effect of such a spiral is increased by placing in

it a core of soft iron. By the use of such a combination under the

influence of a voltaic battery, magnets are constructed far more

powerful than those which depend upon the permanent magnetism of

iron. The electro-magnet employed by Dr. Faraday in some of his

experiments would sustain a hundred-weight at either end.

By the use of such magnets Dr. Faraday discovered that, besides

iron, nickel and cobalt, which possess magnetism in a high degree,

many bodies are magnetic in a slight degree. And he made the further

very important discovery, that of those substances which are not

magnetic, many, perhaps all, possess an opposite property, in virtue

of which he terms them _diamagnetic_. The opposition is of this

{253} kind;--that magnetic bodies in the form of bars or needles,

if free to move, arrange themselves in the _axial_ line joining the

poles; diamagnetic bodies under the same circumstances arrange

themselves in an _equatorial_ position, perpendicular to the axial

line. And this tendency he conceives to be the result of one more

general; that whereas magnetic bodies are attracted to the poles of

a magnet, diamagnetic bodies are repelled from the poles. The list

of diamagnetic bodies includes all kinds of substances; not only

metals, as antimony, bismuth, gold, silver, lead, tin, zinc, but

many crystals, glass, phosphorus, sulphur, sugar, gum, wood, ivory;

and even flesh and fruit.

It appears that M. le Bailli had shown, in 1829, that both bismuth

and antimony and bismuth repelled the magnetic needle; and as Dr.

Faraday remarks, it is astonishing that such an experiment should

have remained so long without further results. M. Becquerel in 1827

observed, and quoted Coulomb as having also observed, that a needle

of wood under certain conditions pointed across the magnetic curves;

and also stated that he had found a needle of wood place itself

parallel to the wires of a galvanometer. This he referred to a

magnetism transverse to the length. But he does not refer the

phenomena to elementary repulsive action, nor show that they are

common to an immense class of bodies, nor distinguish this

diamagnetic from the magnetic class, as Faraday has taught us to do.

I do not dwell upon the peculiar phenomena of copper which, in the

same series of researches, are traced by Dr. Faraday to the combined

effect of its diamagnetic character, and the electric currents

excited in it by the electro-magnet; nor to the optical phenomena

manifested by certain transparent diamagnetic substances under

electric action; as already stated in Book ix.[23\13]]

[Note 23\13: See the _Twentieth Series of Experimental Researches in

Electricity_, read to the Royal Society, Dec. 18, 1845.]

CHAPTER VIII.

DISCOVERY OF THE LAWS OF MAGNETO-ELECTRIC INDUCTION.--FARADAY.

IT was clearly established by Ampère, as we have seen, that magnetic

action is a peculiar form of electromotive actions, and that, in

{254} this kind of agency, action and reaction are equal and

opposite. It appeared to follow almost irresistibly from these

considerations, that magnetism might be made to produce electricity,

as electricity could be made to imitate all the effects of

magnetism. Yet for a long time the attempts to obtain such a result

were fruitless. Faraday, in 1825, endeavored to make the

conducting-wire of the voltaic circuit excite electricity in a

neighboring wire by induction, as the conductor charged with common

electricity would have done, but he obtained no such effect. If this

attempt had succeeded, the magnet, which, for all such purposes, is

an assemblage of voltaic circuits, might also have been made to

excite electricity. About the same time, an experiment was made in

France by M. Arago, which really involved the effect thus sought;

though this effect was not extricated from the complex phenomenon,

till Faraday began his splendid career of discovery on this subject

in 1832. Arago's observation was, that the rapid revolution of a

conducting-plate in the neighborhood of a magnet, gave rise to a

force acting on the magnet. In England, Messrs. Barlow and Christie,

Herschel and Babbage, repeated and tried to analyse this experiment;

but referring the forces only to conditions of space and time, and

overlooking the real cause, the electrical currents produced by the

motion, these philosophers were altogether unsuccessful in their

labors. In 1831, Faraday again sought for electro-dynamical

induction, and after some futile trials, at last found it in a form

different from that in which he had looked for it. It was then seen,

that at the precise time of making or breaking the contact which

closed the galvanic circuit, a momentary effect was induced in a

neighboring wire, but disappeared instantly.[24\13] Once in

possession of this fact, Mr. Faraday ran rapidly up the ladder of

discovery, to the general point of view.--Instead of suddenly making

or breaking the contact of the inducing circuit, a similar effect

was produced by removing the inducible wire nearer to or further

from the circuit;[25\13]--the effects were increased by the

proximity of soft iron;[26\13]--when the soft iron was affected by

an ordinary magnet instead of the voltaic wire, the same effect

still recurred;[27\13]--and thus it appeared, that by making and

breaking magnetic contact, a momentary electric current was

produced. It was produced also by moving the magnet;[28\13]--or by

moving the wire with reference to the magnet.[29\13] Finally, it was

found that the earth might supply the place of a magnet {255} in

this as in other experiments;[30\13] and the mere motion of a wire,

under proper circumstances, produced in it, it appeared, a momentary

electric current.[31\13] These facts were curiously confirmed by the

results in special cases. They explained Arago's experiments: for

the momentary effect became permanent by the revolution of the

plate. And without using the magnet, a revolving plate became an

electrical machine;[32\13]--a revolving globe exhibited

electro-magnetic action,[33\13] the circuit being complete in the

globe itself without the addition of any wire;--and a mere motion of

the wire of a galvanometer produced an electro-dynamic effect upon

its needle.[34\13]

[Note 24\13: _Phil. Trans._ 1832, p. 127, First Series, Art. 10.]

[Note 25\13: Art. 18.]

[Note 26\13: Art. 28.]

[Note 27\13: Art. 37.]

[Note 28\13: Art. 39.]

[Note 29\13: Art. 53.]

[Note 30\13: Second Series, _Phil. Trans._ p. 163.]

[Note 31\13: Art. 141.]

[Note 32\13: Art. 150.]

[Note 33\13: Art. 164.]

[Note 34\13: Art. 171.]

But the question occurs, What is the general law which determines

the direction of electric currents thus produced by the joint

effects of motion and magnetism? Nothing but a peculiar steadiness

and clearness in his conceptions of space, could have enabled Mr.

Faraday to detect the law of this phenomenon. For the question

required that he should determine the mutual relations in space

which connect the magnetic poles, the position of the wire, the

direction of the wire's motion, and the electrical current produced

in it. This was no easy problem; indeed, the mere relation of the

magnetic to the electric forces, the one set being perpendicular to

the other, is of itself sufficient to perplex the mind; as we have

seen in the history of the electrodynamical discoveries. But Mr.

Faraday appears to have seized at once the law of the phenomena.

"The relation," he says,[35\13] "which holds between the magnetic

pole, the moving wire or metal, and the direction of the current

evolved, is very simple (so it seemed to him) although rather

difficult to express." He represents it by referring position and

motion to the "magnetic curves," which go from a magnetic pole to

the opposite pole. The current in the wire sets one way or the

other, according to the direction in which the motion of the wire

cuts these curves. And thus he was enabled, at the end of his Second

Series of _Researches_ (December, 1831), to give, in general terms,

the law of nature to which may be referred the extraordinary number

of new and curious experiments which he has stated;[36\13]--namely,

that if a wire move so as to cut a magnetic curve, a power is called

into action which tends to urge a magnetic current through the wire;

and that if a mass move so that its parts do not move in the same

direction across the magnetic curves, {256} and with the same angular

velocity, electrical currents are called into play in the mass.

[Note 35\13: First Series, Art. 114.]

[Note 36\13: Art. 256-264.]

This rule, thus simple from its generality, though inevitably

complex in every special case, may be looked upon as supplying the

first demand of philosophy, _the law of the phenomena_; and

accordingly Dr. Faraday has, in all his subsequent researches on

magneto-electric induction, applied this law to his experiments; and

has thereby unravelled an immense amount of apparent inconsistency

and confusion, for those who have followed him in his mode of

conceiving the subject.

But yet other philosophers have regarded these phenomena in other

points of view, and have stated the laws of the phenomena in a

manner different from Faraday's, although for the most part

equivalent to his. And these attempts to express, in the most simple

and general form, the law of the phenomena of magneto-electrical

induction, have naturally been combined with the expression of other

laws of electrical and magnetical phenomena. Further, these

endeavors to connect and generalize the Facts have naturally been

clothed in the garb of various Theories:--the _laws of phenomena_

have been expressed in terms of the supposed _causes of the

phenomena_; as fluids, attractions and repulsions, particles with

currents running through them or round them, physical lines of

force, and the like. Such views, and the conflict of them, are the

natural and hopeful prognostics of a theory which shall harmonize

their discords and include all that each contains of Truth. The

fermentation at present is perhaps too great to allow us to see

clearly the truth which lies at the bottom. But a few of the leading

points of recent discussions on these subjects will be noticed in

the Additions to this volume.

CHAPTER IX.

TRANSITION TO CHEMICAL SCIENCE.

THE preceding train of generalization may justly appear extensive,

and of itself well worthy of admiration. Yet we are to consider all

that has there been established as only one-half of the science to

which it belongs,--one limb of the colossal form of Chemistry. We

{257} have ascertained, we will suppose, the laws of Electric

Polarity; but we have then to ask, What is the relation of this

Polarity to Chemical Composition? This was the great problem which,

constantly present to the minds of electro-chemical inquirers, drew

them on, with the promise of some deep and comprehensive insight

into the mechanism of nature. Long tasks of research, though only

subsidiary to this, were cheerfully undertaken. Thus Faraday[37\13]

describes himself as compelled to set about satisfying himself of

the identity of common, animal, and voltaic electricity, as "the

decision of a doubtful point which interfered with the extension of

his views, and destroyed the strictness of reasoning." Having

established this identity, he proceeded with his grand undertaking

of electro-chemical research.

[Note 37\13: Dec. 1832. _Researches_, 266.]

The connexion of electrical currents with chemical action, though

kept out of sight in the account we have hitherto given, was never

forgotten by the experimenters; for, in fact, the modes in which

electrical currents were excited, were chemical actions;--the action

of acids and metals on each other in the voltaic trough, or in some

other form. The dependence of the electrical effect on these

chemical actions, and still more, the chemical actions produced by

the agency of the poles of the circuit, had been carefully studied;

and we must now relate with what success.

But in what terms shall we present this narration? We have spoken of

chemical actions,--but what kind of actions are these?

_Decomposition_; the _resolution_ of compounds into their

ingredients; the separation of _acids_ from _bases_; the reduction

of bodies to _simple elements_. These names open to us a new drama;

they are words which belong to a different set of relations of

things, a different train of scientific inductions, a different

system of generalizations, from any with which we have hitherto been

concerned. We must learn to understand these phrases, before we can

advance in our history of human knowledge.

And how are we to learn the meaning of this collection of words? In

what other language shall it be explained? In what terms shall we

define these new expressions? To this we are compelled to reply,

that we cannot translate these terms into any ordinary

language;--that we cannot define them in any terms already familiar

to us. Here, as in all other branches of knowledge, the meaning of

words is to be sought in the progress of thought; the history of

science is our {258} dictionary; the steps of scientific induction

are our definitions. It is only by going back through the successful

researches of men respecting the composition and elements of bodies,

that we can learn in what sense such terms must be understood, so as

to convey real knowledge. In order that they may have a meaning for

us, we must inquire what meaning they had in the minds of the

authors of our discoveries.

And thus we cannot advance a step, till we have brought up our

history of Chemistry to the level of our history of

Electricity;--till we have studied the progress of the analytical,

as well as the mechanical sciences. We are compelled to pause and

look backwards here; just as happened in the history of astronomy,

when we arrived at the brink of the great mechanical inductions of

Newton, and found that we must trace the history of Mechanics,

before we could proceed to mechanical Astronomy. The terms "force,

attraction, inertia, momentum," sent us back into preceding

centuries then, just as the terms "composition" and "element" send

us back now.

Nor is it to a small extent that we have thus to double back upon

our past advance. Next to Astronomy, Chemistry is one of the most

ancient of sciences;--the field of the earliest attempts of man to

command and understand nature. It has held men for centuries by a

kind of fascination; and innumerable and endless are the various

labors, the failures and successes, the speculations and

conclusions, the strange pretences and fantastical dreams, of those

who have pursued it. To exhibit all these, or give any account of

them, would be impossible; and for our design, it would not be

pertinent. To extract from the mass that which is to our purpose, is

difficult; but the attempt must be made. We must endeavor to analyse

the history of Chemistry, so far as it has tended towards the

establishment of general principles. We shall thus obtain a sight of

generalizations of a new kind, and shall prepare ourselves for

others of a higher order.

{{259}}

BOOK XIV.

_THE ANALYTICAL SCIENCE._

HISTORY OF CHEMISTRY.

. . . . . . . Soon had his crew

Opened into the hill a spacious wound,

And digged out ribs of gold . . . .

Anon out of the earth a fabric huge

Rose like an exhalation, with the sound

Of dulcet symphonies and voices sweet,

Built like a temple.

MILTON. _Paradise Lost_, i.

{{261}}

CHAPTER I.

IMPROVEMENT OF THE NOTION OF CHEMICAL ANALYSIS, AND RECOGNITION OF

IT AS THE SPAGIRIC ART.

THE doctrine of "the four elements" is one of the oldest monuments

of man's speculative nature; goes back, perhaps, to times anterior

to Greek philosophy; and as the doctrine of Aristotle and Galen,

reigned for fifteen hundred years over the Gentile, Christian, and

Mohammedan world. In medicine, taught as the doctrine of the four

"elementary qualities," of which the human body and all other

substances are compounded, it had a very powerful and extensive

influence upon medical practice. But this doctrine never led to any

attempt actually to analyse bodies into their supposed elements: for

composition was inferred from the resemblance of the qualities, not

from the separate exhibition of the ingredients; the supposed

analysis was, in short, a decomposition of the body into adjectives,

not into substances.

This doctrine, therefore, may be considered as a negative state,

antecedent to the very beginning of chemistry; and some progress

beyond this mere negation was made, as soon as men began to endeavor

to compound and decompound substances by the use of fire or mixture,

however erroneous might be the opinions and expectations which they

combined with their attempts. Alchemy is a step in chemistry, so far

as it implies the recognition of the work of the cupel and the

retort, as the produce of analysis and synthesis. How perplexed and

perverted were the forms in which this recognition was clothed,--how

mixed up with mythical follies and extravagancies, we have already

seen; and the share which Alchemy had in the formation of any

sounder knowledge, is not such as to justify any further notice of

that pursuit.

The result of the attempts to analyse bodies by heat, mixture, and

the like processes, was the doctrine that the first principles of

things are _three_, not four; namely, _salt_, _sulphur_, and

_mercury_; and that, of these three, all things are compounded. In

reality, the doctrine, as thus stated, contained no truth which was

of any value; for, though the chemist could extract from most bodies

portions which he called salt, {262} and sulphur, and mercury, these

names were given, rather to save the hypothesis, than because the

substances were really those usually so called: and thus the

supposed analyses proved nothing, as Boyle justly urged against

them.[1\14]

[Note 1\14: Shaw's Boyle. _Skeptical Chymist_, pp. 312, 313. &c.]

The only real advance in chemical theory, therefore, which we can

ascribe to the school of _the three principles_, as compared with

those who held the ancient dogma of the four elements, is, the

acknowledgment of the changes produced by the chemist's operations,

as being changes which were to be accounted for by the union and

separation of substantial elements, or, as they were sometimes

called, of _hypostatical principles_. The workmen of this school

acquired, no doubt, a considerable acquaintance with the results of

the kinds of processes which they pursued; they applied their

knowledge to the preparation of new medicines; and some of them, as

Paracelsus and Van Helmont, attained, in this way, to great fame and

distinction: but their merits, as regards theoretical chemistry,

consist only in a truer conception of the problem, and of the mode

of attempting its solution, than their predecessors had entertained.

This step is well marked by a word which, about the time of which we

speak, was introduced to denote the chemist's employment. It was

called the _Spagiric art_, (often misspelt _Spagyric_,) from two

Greek words, (σπάω, ἀγείρω,) which mean to _separate_ parts, and to

_unite_ them. These two processes, or in more modern language,

_analysis_ and _synthesis_, constitute the whole business of the

chemist. We are not making a fanciful arrangement, therefore, when

we mark the recognition of this object as a step in the progress of

chemistry. I now proceed to consider the manner in which the

conditions of this analysis and synthesis were further developed.

CHAPTER II.

DOCTRINE OF ACID AND ALKALI.--SYLVIUS.

AMONG the results of mixture observed by chemists, were many

instances in which two ingredients, each in itself pungent or

destructive, being put together, became mild and inoperative; each

{263} counteracting and neutralizing the activity of the other. The

notion of such opposition and neutrality is applicable to a very

wide range of chemical processes. The person who appears first to

have steadily seized and generally applied this notion is Francis de

la Boé Sylvius; who was born in 1614, and practised medicine at

Amsterdam, with a success and reputation which gave great currency

to his opinions on that art.[2\14] His chemical theories were

propounded as subordinate to his medical doctrines; and from being

thus presented under a most important practical aspect, excited far

more attention than mere theoretical opinions on the composition of

bodies could have done. Sylvius is spoken of by historians of

science, as the founder of the _iatro-chemical_ sect among

physicians; that is, the sect which considers the disorders in the

human frame as the effects of chemical relations of the fluids, and

applies to them modes of cure founded upon this doctrine. We have

here to speak, not of his physiological, but of his chemical views.

[Note 2\14: Sprengel. _Geschichte der Arzneykunde_, vol. iv.

Thomson's _History of Chemistry_ in the corresponding part is

translated from Sprengel.]

The distinction of _acid_ and _alkaline_ bodies (_acidum_,

_lixivum_) was familiar before the time of Sylvius; but he framed a

system, by considering them both as eminently acrid and yet

opposite, and by applying this notion to the human frame. Thus[3\14]

the lymph contains an acid, the bile an alkaline salt. These two

opposite acrid substances, when they are brought together,

_neutralize_ each other (_infringunt_), and are changed into an

intermediate and milder substance.

[Note 3\14: _De Methodo Medendi_, Amst. 1679. Lib. ii. cap. 28,

sects. 8 and 53.]

The progress of this doctrine, as a physiological one, is an

important part of the history of medical science in the seventeenth

century; but with that we are not here concerned. But as a chemical

doctrine, this notion of the opposition of acid and alkali, and of

its very general applicability, struck deep root, and has not been

eradicated up to our own time. Boyle, indeed, whose disposition led

him to suspect all generalities, expressed doubts with regard to

this view;[4\14] and argued that the supposition of acid and

alkaline parts in all bodies was precarious, their offices

arbitrary, and the notion of them unsettled. Indeed it was not

difficult to show, that there was no one certain criterion to which

all supposed acids conformed. Yet the general conception of such a

combination as that of acid and alkali was supposed to {264} be,

served so well to express many chemical facts, that it kept its

ground. It is found, for instance, in Lemery's _Chemistry_, which

was one of those in most general use before the introduction of the

phlogistic theory. In this work (which was translated into English

by Keill, in 1698) we find alkalies defined by their effervescing

with acids.[5\14] They were distinguished as the _mineral_ alkali

(soda), the _vegetable_ alkali (potassa), and the _volatile_ alkali

(ammonia). Again, in Macquer's _Chemistry_, which was long the

text-book in Europe during the reign of phlogiston, we find acids

and alkalies, and their union, in which they rob each other of their

characteristic properties, and form neutral salts, stated among the

leading principles of the science.[6\14]

[Note 4\14: Shaw's _Boyle_, iii. p. 432.]

[Note 5\14: Lemery, p. 25.]

[Note 6\14: Macquer, p. 19.]

In truth, the mutual relation of acids to alkalies was the most

essential part of the knowledge which chemists possessed concerning

them. The importance of this relation arose from its being the first

distinct form in which the notion of chemical attraction or affinity

appeared. For the acrid or caustic character of acids and alkalies

is, in fact, a tendency to alter the bodies they touch, and thus to

alter themselves; and the neutral character of the compounds **is

the absence of any such proclivity to change. Acids and alkalies

have a strong disposition to unite. They combine, often with

vehemence, and produce neutral salts; they exhibit, in short, a

prominent example of the chemical attraction, or affinity, by which

two ingredients are formed into a compound. The relation of _acid_

and _base_ in a salt is, to this day, one of the main grounds of all

theoretical reasonings.

The more distinct development of the notion of such chemical

attraction, gradually made its way among the chemists of the latter

part of the seventeenth and the beginning of the eighteenth century,

as we may see in the writings of Boyle, Newton, and their followers.

Beecher speaks of this attraction as a _magnetism_; but I do not

know that any writer in particular, can be pointed out as the person

who firmly established the general notion of _chemical attraction_.

But this idea of chemical attraction became both more clear and more

extensively applicable, when it assumed the form of the doctrine of

_elective_ attractions, in which shape we must now speak of it. {265}

CHAPTER III.

DOCTRINE OF ELECTIVE ATTRACTIONS. GEOFFROY. BERGMAN.

THOUGH the chemical combinations of bodies had already been referred

to attraction, in a vague and general manner, it was impossible to

explain the changes that take place, without supposing the

attraction to be greater or less, according to the nature of the

body. Yet it was some time before the necessity of such a

supposition was clearly seen. In the history of the French Academy

for 1718 (published 1719), the writer of the introductory notice

(probably Fontenelle) says, "That a body which is united to another,

for example, a solvent which has penetrated a metal, should quit it

to go and unite itself with another which we present to it, is a

thing of which the possibility had never been guessed by the most

subtle philosophers, and of which the explanation even now is not

easy." The doctrine had, in fact, been stated by Stahl, but the

assertion just quoted shows, at least, that it was not familiar. The

principle, however, is very clearly stated[7\14] in a memoir in the

same volume, by Geoffroy, a French physician of great talents and

varied knowledge, "We observe in chemistry," he says, "certain

relations amongst different bodies, which cause them to unite. These

relations have their _degrees_ and their _laws_. We observe their

different degrees in this;--that among different matters jumbled

together, which have a certain disposition to unite, we find that

one of these substances always unites constantly with a certain

other, preferably to all the rest." He then states that those which

unite by preference, have "plus de rapport," or, according to a

phrase afterwards used, more _affinity_. "And I have satisfied

myself," he adds, "that we may deduce, from these observations, the

following proposition, which is very extensively true, though I

cannot enunciate it as universal, not having been able to examine

all the possible combinations, to assure myself that I should find

no exception." The proposition which he states in this admirable

spirit of philosophical caution, is this: "In all cases where two

substances, {266} which have any disposition to combine, are united;

if there approaches them a third, which has more affinity with one

of the two, this one unites with the third and lets go the other."

He then states these affinities in the form of a Table; placing a

substance at the head of each column, and other substances in

succession below it, according to the order of their affinities for

the substance which stands at the head. He allows that the

separation is not always complete (an imperfection which he ascribes

to the glutinosity of fluids and other causes), but, with such

exceptions, he defends very resolutely and successfully his Table,

and the notions which it implies.

[Note 7\14: _Mém. Acad. Par._ 1718, p. 202.]

The value of such a tabulation was immense at the time, and is even

still very great; it enabled the chemist to trace beforehand the

results of any operation; since, when the ingredients were given, he

could see which were the strongest of the affinities brought into

play, and, consequently, what compounds would be formed. Geoffroy

himself gave several good examples of this use of his table. It was

speedily adopted into works on chemistry. For instance,

Macquer[8\14] places it at the end of his book; "taking it," as he

says, "to be of great use at the end of an elementary tract, as it

collects into one point of view, the most essential and fundamental

doctrines which are dispersed through the work."

[Note 8\14: Pref., p. 13.]

The doctrine of _Elective Attraction_, as thus promulgated,

contained so large a mass of truth, that it was never seriously

shaken, though it required further development and correction. In

particular the celebrated work of Torbern Bergman, professor at

Upsala, _On Elective Attractions_, published in 1775, introduced

into it material improvements. Bergman observed, that not only the

order of attractions, but the sum of those attractions which had to

form the new compounds, must be taken account of, in order to judge

of the result. Thus,[9\14] if we have a combination of two elements,

_P_, _s_, (potassa and vitriolic acid), and another combination,

_L_, _m_, (lime and muriatic acid,) though _s_ has a greater

affinity for _P_ than for _L_, yet the sum of the attractions of _P_

to _m_, and of _L_ to _s_, is greater than that of the original

compounds, and therefore if the two combinations are brought

together, the new compounds, _P_, _m_, and _L_, _s_, are formed.

[Note 9\14: _Elect. Attract._, p. 19.]

The Table of Elective Attractions, modified by Bergman in pursuance

of these views, and corrected according to the advanced knowledge of

the time, became still more important than before. The next step

{267} was to take into account the quantities of the elements which

combined; but this leads us into a new train of investigation, which

was, indeed, a natural sequel to the researches of Geoffroy and

Bergman.

In 1803, however, a chemist of great eminence, Berthollet, published

a work (_Essai de Statique Chimique_), the tendency of which

appeared to be to throw the subject back into the condition in which

it had been before Geoffroy. For Berthollet maintained that the

rules of chemical combination were not definite, and dependent on

the nature of the substances alone, but indefinite, depending on the

quantity present, and other circumstances. Proust answered him, and

as Berzelius says,[10\14] "Berthollet defended himself with an

acuteness which makes the reader hesitate in his judgment; but the

great mass of facts finally decided the point in favor of Proust."

Before, however, we trace the result of these researches, we must

consider Chemistry as extending her inquiries to combustion as well

as mixture, to airs as well as fluids and solids, and to weight as

well as quality. These three steps we shall now briefly treat of.

[Note 10\14: _Chem._ t. iii. p. 23.]

CHAPTER IV.

DOCTRINE OF ACIDIFICATION AND COMBUSTION.--PHLOGISTIC THEORY.