Researches on diamagnetism and magne-crystallic action including the question of diamagnetic polarity

DIAMAGNETISM

AND

MAGNE-CEYSTALLIC ACTION

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jf

THE ROYAL INSTITUTION MAGNET.

RESEAKCHES ON

DIAMAGNETISM AND
MAGNE-CRYSTALLIC ACTION

INCLUDING THE QUESTION OF
DIAMAGNETIC POLARITY

BY
JOHN TYNDALL, D. C. L., LL. D., R E. S.

NEW YORK
D. APPLETON AND COMPANY * HlX

1888 *W

*

Authorized Edition.

TO

PROFESSOK WILIIELM WEBEE,

OP GOTTINGEN,

WITH THE DEEP RESPECT OF ITS AUTHOR,
THIS BOOK IS DEDICATED.

PEE FACE

TO

THE NEW EDITION.

BEGUN in Marburg, continued in Berlin, and ended in the
quiet laboratory of the Royal Institution, the researches
here presented to the reader cover the first six years of
my experimental work. It was difficult work, and the
discipline it involved was of high value to me as a pre-
paration for labours more difficult still. The forces to be
investigated were so weak, and their action was so com-
plex, that in dealing with them the extreme of delicacy
had to be combined with the maximum of power. Hence,
indeed, the divergences and discussions which, for several
years, the questions here considered provoked among emi-
nent scientific men. At the time referred to, the subject
was one of universal interest ; which, in view of its theo-
retic significance, is sure, in due time, to reappear.

The first investigation of the series, conducted in com-
panionship with my friend Professor Knoblauch, treats of
the deportment of crystals, and of other bodies possessing
a definite structure, in the magnetic field. Pliicker had
discovered that deportment, and had deduced from it the
existence of new forces and new laws, having an important
bearing not only on the phenomena of magnetism, but
on those of light. Faraday followed Plucker and verified

Vlll PREFACE.

him, adding, moreover, another to the list of forces already
assumed. These forces were alleged to possess an indivi-
duality wholly distinct from magnetism and diamagnetism.
Special experiments, indeed, were executed by Faraday, to
prove that neither attraction nor repulsion had anything to
do, and, as a consequence, that polarity could have nothing
to do, with the phenomena.

This conclusion landed him in serious difficulty, and
his musings on the insoluble enigma thereby created are
profoundly interesting. He visualises the crystalline parti-
cles, and the power which makes them cohere in regular
order. He looks at his magnet in relation to these par-
ticles and to this power ; and he concludes that it is
impossible to conceive of the results otherwise than as
being due to the interaction of the magnetic force and
the forces which built the crystal. This was his way of
looking at the problem. To him, as he reflects upon it,
the magne-crystallic force appears ' to be very strange and
striking in its character. It is not polar, for there is no
attraction or repulsion. What then is the nature of the
force which turns the crystal round, and makes it affect
a magnet ? I do not remember,' he continues, ' such a
case of force as the present one, where a body is brought
into position only, without attraction or repulsion.' After
advancing what he considers to be 'a very striking series
of proofs that neither attraction nor repulsion governs'
the conduct of crystals in the magnetic field, he winds up
with the emphatic inference that this new force 'is dis-
tinct in its character from the magnetic and diamagnetic
forms of force.'

So thought, and so reasoned, this incomparable experi-
menter. His views were assuredly strange, but they
brought into play the driving-force of his emotions. Here,
as in many other cases, the very strangeness of Faraday's
conclusions constituted a stimulus which urged him into
regions where the art and instinct of the experimenter

PEEFACE. IX

were supreme, and from which he was sure to return en-
riched with the spoils of discovery.

In the researches here thrown together the experiments
of Pliicker on crystals are carefully repeated and greatly
multiplied in number. Standing as a mathematician in
his own department, in the first rank, and fortunate, be-
yond many, in the discovery of facts, his conclusions from
his experiments were, at the beginning, precipitate. His
first striking generalization, indeed, was corrected by him-
self; but his second statement of the law of magne-crystallic
action was as faulty as the first. Pasteur truly describes
the art of experiment as beset with difficulty and danger.
Pliicker, when he passed suddenly from mathematics to
physics, was not sufficiently aware of this. He did not
give himself sufficient time to vary his combinations, and
check his results, before publishing his conclusions. Still,
he must, I think, be credited with a large measure of that
experimental instinct, which, in Faraday, rose to the dignity
of a new sense, enabling him to see in each fact extensions
and applications beyond the discernment of ordinary men.

Pliicker concluded that the magnetic deportment of a
crystal, and its optical deportment, went hand in hand —
that from either of them the other could be inferred. He
announced the important law that negative crystals, when
suspended in the magnetic field with their optic axes
horizontal, took up, on the development of the magnetic
force, a definite position — always setting the optic axes at
right angles to the direction of the magnetic force; while
positive crystals, under the same influence, set their axes
from pole to pole. In the latter case the axes were said to
be attracted, in the former case, repelled. This was the
second generalization, which embodied Pliicker's correction
of his first. Let us consider it for a moment. It is well
known that in crystals one constituent can often be sub-
stituted for another, without change of external form or
internal structure. Isomorphous crystals are thus rendered

X PEEFACB.

possible. We can replace a diamagnetic atom by a mag-
netic one, without disturbing the molecular architecture,
or the optical phenomena dependent on it. Carbonate
of lime, carbonate of lime and iron, and pure carbonate
of iron, are cases in point. They are all of the same
rhomboidal form ; they have the same cleavages which,
if followed sufficiently far, would show them to possess
the same molecular structure. This identity of structure
makes them alike in optical character. They are all three
' negative ' crystals. But the atomic change from calcium
to iron, which does not affect the optical deportment,
completely reverses the magnetic deportment. This single
instance suffices to invalidate Pliicker's second magnetic
classification ; while it also disposes of the proposition, so
often repeated, that magne-crystallic action is indepen-
dent of the magnetism or diamagnetism of the mass of the
crystal. A host of other exceptions and considerations are,
however, adduced.

But a still more fundamental question than that of
magne-crystallic action stirred the scientific mind at the
period here referred to. The character of the diamagnetic
force itself was a subject of doubt and discussion. Was
it a polar force, like magnetism, or an unpolar force, like
gravity ? Diamagnetic repulsion obviously augmented with
the strength of the operating magnet. With feeble magnets
it was hardly sensible ; with strong ones, especially when
the more powerful diamagnetic substances like bismuth
and antimony were operated on, the repulsion was very
sensible indeed. Was this enhancement of the action with
the rise of magnetic power due to the magnet alone ?
Was there no response on the part of the diamagnetic
body, like the separation of magnetic fluids in the theory
of Poisson, or the arrangement of molecular currents in
the theory of Ampdre ? This portion of the question was
answered by Eeich, E. Becquerel, and myself, in different
ways, but with the same result. It was proved that it was

PEEFACE. XI

not the mere matter of the diamagnetic body (to which
permanence of quantity must be ascribed) that was re-
pelled, but something which, as in the case of magnetism,
rose and fell, within wide limits, in exact proportion to
the rise and fall of the magnetic power.

The question of diamagnetic polarity, round which the
discussion was warmest and most prolonged, comes here
into view. After the discovery of diamagnetism, Faraday
had thrown out the idea that its phenomena might be ex-
plained by assuming in diamagnetic bodies a polarity the
reverse of that of magnetic bodies. But he soon abandoned
this hypothesis, and never afterwards became reconciled to
it. Here, I doubt not, he was swayed, in part, by the
results of experiments which he had undertaken in repeti-
tion of a series by Professor W. Weber ; and, in part, by
the sheer unthinkability of either the theory of magnetic
fluids, or the theory of molecular currents, as then held,
when applied to the fundamental phenomenon of diamag-
netic repulsion. It was as a refuge from this difficulty
that Professor Weber propounded and developed a theory
by which he avoided the contradictions involved in the
application to diamagnetism of the theory of Ampere. In
iron, according to the latter, the act of magnetization con-
sists in rendering pre-existent currents wholly or partially
parallel to a common plane ; attraction being due to the
fact that the directions of these currents are the same as
those of the influencing magnet. In bismuth, according
to Weber's theory, the molecular currents are not pre-
existent, but induced ; and, in accordance with Faraday's
law, are opposed in direction to the currents which excite
them. Hence the repulsion of the bismuth. Ordinary
induced currents cease, in a moment, because of the
resistance of the conductors through which they pass.
Weber, therefore, provides his induced molecular currents
with channels of no resistance in which, once started, they
can permanently circulate. As justly remarked in a letter

Xll PREFACE.

from Professor Weber to myself, this hypothesis of non-
resisting circuits is also included in the theory of Ampere.
Nobody, of course, who accepts unreservedly this theory,
as applied to iron and steel, will find any difficulty in the
conception that these channels of perfect conductivity sur-
round atoms which, in their aggregate form, constitute the
most powerful insulators. Shell-lac, sulphur, and glass, for
example, which are all diamagnetic, must be assemblages
of such atoms with their circuits. As a speculation, Weber's
theory is beautiful and consistent, and if it affords repose
and satisfaction to his powerful mind, it is sure to do the
same to the minds of others.

But, being a matter of fact, the question of diamagnetic
polarity lies apart from these theoretic considerations. The
knowledge that a magnet has two poles does not require to
be prefaced by a general theory of magnetism. The essence
of magnetic polarity consists in the simultaneous and in-
separable existence, or development, of two hostile powers
which, in action, always resolve themselves into mechanical
couples. Here, it may be said in passing, the key of all
Faraday's difficulties — the solution of all the mechanical
paradoxes which so perplexed him — is to be found. The
facts of magnetic polarity can be mastered and made sure
of by anybody possessing a bar magnet and a magnetic
needle, or even two magnetic needles. And passing from
steel magnets to bars of iron in helices through which
electric currents flow, the polarity of the iron is as much a
matter of experimental certainty as the polarity of the
magnetized steel. The question to be decided was : Do
diamagnetic bodies, under magnetic influence, show this
doubleness of action ? To put the case strongly, iron is
repelled by a magnet, as well as attracted; is bismuth
attracted by a magnet, as well as repelled ? That it is so
is abundantly proved in the following pages. Faraday,
over and over again, observed this attraction ; but it came
to him in the disguise of magne-crystallic action, in which,

PREFACE. Xlll

according to his view, neither attraction nor repulsion had
any share.

The subject of diamagnetic polarity was first definitely
approached by me in the investigation described in the
' Third Memoir ' of this series but I had not, at the time,
the apparatus and material needed to carry the enquiry
out. Thanks to the Council of the Royal Society, this
want was soon supplied ; and I faced the investigation
recorded in the l Fourth Memoir,' with the resolution to
leave no stone unturned in the effort to arrive at the truth.
The deportment of diamagnetic bodies was subjected to an
exhaustive comparison with that of magnetic bodies, and
the antithesis between them, when acted on by all possible
combinations of electro-magnets and electric currents, was
proved to be absolute and complete. Under the same
conditions of excitement the repulsion of the one class of
bodies had its complement in the attraction of the other ;
the north and south magnetism of the one class had its
complement in the south and north magnetism of the
other. When the end of an excited iron bar was repelled
by a magnetic pole the end of a bismuth bar, under the
same influence, was attracted by the same pole; every
deflection, moreover, produced by the combined action of
magnets and helices, in the one case, had its exact com-
plement in an opposite deflection in the other. No rea-
sonable doubt, therefore, could rest upon the mind that the
diamagnetic force possessed precisely the same claim to
the title of a polar force as the magnetic.

This conclusion is further illustrated and enforced by
the experiments recorded in the ' Fifth Memoir.' These
experiments were executed with a most delicate apparatus,
expressly devised for me by Professor Weber, of Gottingen,
and constructed by Leyser, of Leipzig, with consummate
accuracy and skill. With it the various objections which
had been urged against Weber's own results were en-
tirely removed. The severest conditions laid down by the

XIV PREFACE.

opponents of diamagnetic polarity were accepted and ful-
filled. Conductors and insulators— liquids and solids —
were subjected to this new test, and by it also diamagnetic
polarity was shown to rest upon as safe a basis as the old
and long-recognized magnetic polarity itself.

The argument was rounded off by the application of
the doctrine of polarity to magne-crystallic phenomena.
This subject is formally approached towards the end of the
* Fourth Memoir,' where certain objections which had been
urged by Matteucci are examined and removed. In the
' Sixth Memoir ' the application is carried on. By com-
bining with the doctrine of polarity, the differential attrac-
tion and repulsion, first observed in the case of bismuth
by Faraday, and extended to other crystals, and to com-
pressed substances, in the ' Second Memoir ' by myself,
all difficulties are caused to disappear ; the cases cited by
Faraday to prove that neither attraction nor repulsion was
involved in these phenomena being shown to be simple
mechanical consequences of the contemporaneous action of
both attraction and repulsion.

I have aimed at rendering this volume small and
handy, by omitting various topics which were introduced
in the first edition.

J. TYNDALL.

HIND HEAD,

April, 1838.

CONTENTS.

FIRST MEMOIR

FAOX

THE MAGNETO-OPTIC PROPERTIES OF CRYSTALS AND THE RE-
LATION OF MAGNETISM AND DIAMAGNETISM TO MOLECULAR
ARRANGEMENT . 1

SECOND MEMOIR.
ON DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION . . 47

THIRD MEMOIR.

ON THE POLARITY OF BISMUTH, INCLUDING AN EXAMINATION
OF THE MAGNETIC FIELD 94

FOURTH MEMOIR.

ON THE NATURE OF THE FORCE BY WHICH BODIES ARE RE-
PELLED FROM THE POLES OF A MAGNET .... Ill

FIFTH MEMOIR.

FURTHER RESEARCHES ON THE POLARITY OF THE DIAMAGNETIC
FORCE 193

XVI CONTENTS.

SIXTH MEMOIR.

PAGE

ON THE RELATION OF DIAMAGNETIC POLARITY TO MAGNE-

CEYSTALLIC ACTION 225

1. LETTER PROM PROFESSOR W. WEBER . . . 243

2. FARADAY ON MEDIA 250

8. ON THE EXISTENCE OP A MAGNETIC MEDIUM IN

SPACE .... .... 256

4. FARADAY'S LETTER TO MATTEUCCI .... 263

5. CHANGE OF FORM BY MAGNETISATION . . . 268

6. THE POLYMAGNET 274

7. STEEL MOULDS FOR COMPRESSION .... 281

INDEX. . 283

LIST OF PLATES.

FRONTISPIECE.— THE ROYAL INSTITUTION MAGNKT

PLATE I.— DEPORTMENT OF PARAMAGNETIC AND
DIAMAGNETIC BARS, NORMAL AND AB-
NORMAL, WHEN ACTED ON BY HELICES
AND MAGNETS To face p. 153

PLATE IA.-DITTO „ 157

PLATE II. — POLAR ANTITHESIS OF IRON AND BIS-
MUTH BARS „ 162

PLATE IU.— DITTO ,,164

PLATE III.— DEPORTMENT OF BISMUTH BAR ACTED

ON BY FOUR ELECTRO-MAGNETS . . „ 167

PLATE IV.— THE POLYMAGNET IN DETAIL . . „ 274

PLATE V.— DITTO ........ ,,275

PLATE VI — THE RHEOTROPE ,,276

PLATE VII. — THE POLYMAGNET COMPLETE , 277

THE ROYAL INSTITUTION ELECTRO-MAGNET.
(See Frontispiece. ,)

THE Electro-magnet represented in the Frontispiece is that
generally used by Faraday in his researches on Diamagnetism.
He employed a retort stand for suspension, covering the poles
by a square glass shade, B c, to protect the suspended body from
currents of air.

The magnet is formed from the link of a great chain-cable ;
its section is a distorted square, rounded off at the corners. The
magnet, coil inclusive, weighs 272 Ibs.

On the ends of the magnet stand two pieces of iron, p p,
which are the movable poles. They represent those most com-
monly used by Faraday. Various other poles, however, with
rounded, conical, and chisel ends, and some with perforations
to allow a beam of light to pass through them, were employed
from time to time.

Right and left of the drawing, at R and L, are shown, in plan,
the pole ends, with a little bar in its two characteristic posi-
tions, axial and equatorial, between them.

To enable suspended conductors, such as copper cubes or
spheres, to rotate in the magnetic field, with the axis of rotation
parallel to the lines of force, I had the magnet supported by the
pivot A, which permits its two arms to be placed, the one above
the other, in a horizontal position.

J.T.

FIEST MEMOIR.

TEE MAGNETO-OPTIC PEOPEETIES OF CRYSTALS
AND THE RELATION OF MAGNETISM AND DIA-
MAGNETISM TO MOLECULAR ARRANGEMENTS

IN the year 1846 our views of magnetic action received,
through the researches of Faraday, an extraordinary expan-
sion. The experiments of Brugmans, Le Baillif, Seebeck,
and Becquerel had already proved the power to be active
beyond the limits usually assigned to it ; but these ex-
periments were isolated and limited in number. Faraday
was the first to establish the broad fact, that there is no
known body indifferent to magnetic influence when the
latter is strongly developed. The nature of magnetic
action was then found to be twofold, attractive and re-
pulsive ; thus dividing bodies into two great classes, which
are respectively denominated magnetic and diamagnetic.

The representative of the former class is iron, which,
being brought before the single pole of a magnet, is
attracted; the representative of the latter class is bismuth,
which, being brought before the single pole of a magnet,
is repelled.2

If a little bar of iron be hung freely between the two
poles of a magnet, it will set its longest dimension in the

1 Published jointly with Professor Knoblauch in the Philosophical
Magazine, July 1850,

2 Faraday afterwards suggested that the general term magnetism
should include both the magnetism of iron and that of bismuth, which he
respectively designated paramagnetism and diamagnetism.

2 DIAMAGNETISM AND MAGNE-CEYSTALLIC ACTION.

line joining the poles; a little bar of bismuth, on the
contrary, will set its longest dimension at right angles to
the line joining the poles. —

The position of the iron is termed by Faraday the
axial position, that of the bismuth the equatorial posi-
tion. We shall have occasion to use these terms.

These discoveries, opening, as they did, a new field in
physical science, invited the labours of scientific men on
the Continent. Weber, CErsted, Eeich, and others have
occupied themselves with the subject. But, if we except
the illustrious discoverer himself, there is no investigator
in this branch of science whose labours have been so richly
rewarded as those of Professor Pliicker of Bonn.

In 1847 Pliicker had a magnet constructed of the
fame size and power as that described by Faraday,1 his
object being to investigate the influence of the fibrous
constitution of plants upon their magnetic deportment.
While conducting these experiments, he was induced to
try whether crystalline structure exercised an influence.
' The first experiment,' says Pliicker, * gave an immediate
and decided reply.'

Following up his investigations with crystals, he was
led to the affirmation of the following two laws : —

4 When any crystal whatever with one optic axis is
brought between the poles of a magnet, the axis is repelled
by each of the poles ; and if the crystal possess two axes,
each of these is repelled, with the same force, by the two
poles.

1 The force which causes this repulsion is independent
of the magnetism or diamagnetism of the mass of the
crystal j it decreases with the distance more slowly than
the magnetic influence exerted by the palest 2

It i?9 perhaps, worth explaining that if, on exciting the

1 Phil. Mag., vol. xxviii. p. 396.

2 PoggendorfFs Annalen, vol. Ixxii. p. 75.

LAWS OF PLUCKER. 3

magnet, the optic axis take up the axial position, it is
said to be attracted ; if the equatorial, it is said to be
repelled.

The first experiment of Pliicker, which led to the
affirmation of these laws, was made with tourmaline. A
plate of the crystal which had been prepared for the
purposes of polarisation, twelve millimetres long, nine
wide, and three thick, was suspended by a silk fibre
between the poles of an electro-magnet. On sending a
current round the latter, the plate, which was magnetic,
set itself as an ordinary magnetic substance would do, with
its longest dimension from pole to pole. The optic axis of
the crystal, thus suspended, was vertical.

On hanging the crystal, however, with its optic axis
horizontal, when the magnet was excited, the plate stood
no longer as a magnetic substance, but as a diamagnetic ;
its longest dimension being at right angles to the line
joining the poles. The optic axis of the crystal was found
to coincide with its length, and the peculiar deportment
was considered as a proof that the optic axis was repelled.

This law was further established by experiments with
Iceland spar, quartz, zircon, beryl, &c., and, as above
stated, included crystals of all kinds, both optic positive
and negative. It has, however, lately undergone consider-
able modification at the hands of Pliicker himself. In
a letter to Faraday, which appears at page 450, vol. xxxiv.
of the ' Philosophical Magazine,' he expresses himself as
follows : —

' The first and general law I deduced from my last
experiments is the following : — " There will be either
repulsion or attraction of the optic axes by the poles of a
magnet, according to the crystalline structure of the crystal.
If the crystal is a negative one, there will be repulsion ;
if it is a positive one, there will be attraction" ' l
1 Phil. Mag., vol. xxxiv. p. 450.

4 DIAMAGNETISM AND MAGNE-CRFSTALLIC ACTION.

This law applies to crystals possessing two optic axes,
each of the said axes being attracted or repelled according
as the crystal is positive or negative. It will simplify the
subject if we regard the line bisecting the acute angle
enclosed by the two axes as the resultant of attraction or
repulsion ; for the sake of convenience, we shall call this
the middle line. In positive crystals, therefore, the
middle line, according to the above law, must stand axial ,
in negative crystals, equatorial. It is also evident that
the plane passing through the optic axes must, in the one
class of crystals, stand from pole to pole, in the other class
at right angles to the line joining the poles.

In explaining this new modification of the law,
Pliicker lays particular emphasis upon the fact that the
attraction or repulsion is the result of an independent force,
connected in no way with the magnetism or diamagnet-
ism of the mass of the crystal ; and this view is shared
by Faraday, who, in expressing his concurrence with
Pliicker, denominates the force in question an ' optic axis
force.' l

The experiments described in our first paper upon this
subject2 furnish, we conceive, sufficient ground of dissent
from these views. In the case of five crystals of pure
carbonate of lime (Iceland spar), we found the law of
Pliicker strictly verified, all five crystals being dia-
magnetic ; on replacing, however, a portion of the carbonate
of lime by carbonate of iron, nature herself being the
chemist in this case, the crystal was no longer diamagnetic,
but magnetic ; in every other respect it was physically
unchanged ; its optical properties remained precisely as
before, the crystal of carbonate of lime and the crystal of
carbonate of lime and iron being both negative. In the

1 Phil. Trans., 1849, p. 32.

2 Phil. Mag., vol. xxxvi. p. 178. A short preliminary notice printed
further on.

LAWS EXAMINED. 6

one case, however, the optic axis was attracted ; in the
other the said axis was repelled, the attraction being
evidently caused by the passage of the crystal from the dia-
magnetic into the magnetic state.

We have examined other crystals of the same form as
Iceland spar, both magnetic and diamagnetic. In all cases
the former act in a manner precisely similar to the
carbonate of lime and iron already described, while the
latter behave as the pure carbonate of lime. The following
are examples : —

Nitrate of Soda. — This crystal is of the same form
as carbonate of lime, and, like it, diamagnetic. Its
deportment is in every respect the same. A rhombus
cloven from the crystal and suspended horizontally between
the poles sets its longer diagonal axial. Suspending the
full crystal between the poles, with its optic axis horizontal,
on exciting the magnet this axis sets itself equatorial.

Breunnerite. — This is a crystal composed principally
of carbonate of lime and carbonate of magnesia, but con-
taining a sufficient quantity of the carbonate of iron to
render it magnetic. Suspended in the magnetic field, the
optic axis sets from pole to pole.

Dolomite. — In this crystal a portion of the lime is
replaced by protoxide of iron and protoxide of manganese,
which ingredients render it magnetic. The optic axis sets
from pole to pole.

Carbonate of Iron. — In the cases just cited, the substi-
tution of iron for calcium was partial ; in the case now
before us the substitution is complete. This crystal
differs in nothing, save in the energy of its action, from
the magnetic crystals already described. If a full
crystal be hung between the poles, with its optic axis
horizontal, on sending a current round the magnet the
axis sets strongly in the line joining the poles, vibrates
through it quickly for a time, and finally comes to rest

6 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

there. If a thin rhombus be cloven from the crystal and
suspended from one of its obtuse angles with its parallel
faces vertical, it will set itself exactly equatorial. In this
case it is easy to see that the horizontal projection of the
optic axis, which passes through the obtuse angle of the
crystal, stands axial. Hung from its acute angle, the
rhombus takes up an oblique position, making a constant
angle with the line joining the poles. To this position, if
forcibly removed from it, it will invariably return. The
position may be either right or left of the axial line ;
but the angle of obliquity is always the same, being the
angle which the optic axis makes with the face of the
rhombus. Hung from the obtuse angle the obliquity is
nothing — from the acute angle it is a maximum ; the
rhombus is capable of all degrees of obliquity between
these extremes, the optic axis setting in all cases from
pole to pole.

Oxide of Iron. — The above phenomena are exhibited
even in a more striking manner by this crystal. So strong
is the directive power that a rhombus, suspended from one
of its obtuse angles, will set itself strongly equatorial,
though its length may be fifteen or twenty times its
breadth.

What is the conclusion to be drawn from these experi-
ments ? We have first of all a diamagnetic crystal of
pure carbonate of lime, which sets its optic axis equatorial.
On substituting for a portion of the lime a quantity of
protoxide of iron sufficient to render the crystal weakly
magnetic, we find the axis attracted instead of repelled.
Keplacing a still further quantity of the diamagnetic lime by
a magnetic constituent, we find the attraction stronger, the
force with which the optic axis takes up the axial position
increasing as the magnetic constituents increase. These
experiments appear to be irreconcilable with the state-
ment, that the position of the optic axis is independent

LAWS EXAMINED. 7

of the magnetism or diamagnetism of the mass of the
crystal.

Turning now to crystals possessing two optic axes, we
find the law of Pliicker equally untenable.

Dichroite. — This crystal, as is well known, receives its
name from its ability to transmit light of two different
colours. The specimen examined by us is a cube. In the
direction of the ' crystallographic' axis, which coincides
with the ' 'middle line, the light transmitted is yellowish ;
through the other four sides of the cube it is a deep blue.
Suspended with the middle line horizontal, whatever be
the position of that line before closing the circuit, the
instant the magnetic force is developed it turns with sur-
prising energy into the axial position and becomes fixed
there. According to the law, however, the middle line
should stand equatorial, for the crystal is negative.1

Sulphate of Baryta (Heavy spar). — The form of this
crystal is a prism whose base is a rhombus, the four sides
being perpendicular to the base. It cleaves parallel to the
sides and base. Suspended between the poles, with the
axis of the prism vertical, on exciting the magnet, though
the crystal is diamagnetic, the long diagonal sets itself
axial. It agrees thus far with the carbonate of lime.
Suspended from the acute angle formed by two sides of
the prism, on closing the circuit the axis sets parallel
to the line joining the poles, and remains there as long
as the force is active. Suspending the crystal from its
obtuse angle, the axis being still horizontal, on closing the
circuit the axis sets itself equatorial. A plane perpendicular
to the rhombic base, and passing through the long diagonal,
contains the two optic axes, which are inclined to each
other at an angle of 38°. The middle line bisecting this
angle is parallel to the axis of the prism, and hence stands
axial or equatorial, according as the prism is suspended

2 ' Brewster'E list.

8 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

from its acute or its obtuse angle. The position of the
middle line is therefore a function of the point of sus-
pension, varying as it varies ; at one time supporting
the law of Pliicker, and at another time contradicting it.
Heavy spar is positive.

Sulphate of Strontia (Codestine). — This is also a
positive crystal, its form being precisely that of heavy
spar ; the only difference is this, that, in Coelestine, the
optic axes enclose an angle of 50° instead of 38°. The
corroboration and contradiction exhibited by heavy spar
are exhibited here also.

Sulphate of Zinc. — Suppose the crystalline prism to
be hung from its end, and the line which stands
equatorial when the magnet is excited carefully marked.
A plate taken from the crystal, parallel to this line and to
the axis of the prism, displays, on examination with
polarised light, the ring systems surrounding the ends
of the two optic axes. The middle line which bisects
the acute angle enclosed by these axes, is perpendicular
to the surface of the plate, and therefore stands axial.
It ought, however, to stand equatorial, for the crystal is
negative.

Sulphate of Magnesia. — Suspending the crystalline
prism from its end, and following the method applied in
the case of sulphate of zinc, we discover the ring systems
and the position of the middle line. This line stands
axial ; the crystal is nevertheless negative.

Topaz. — This being one of the crystals pronounced by
Pliicker as peculiarly suited to the illustration of his
new law, it is perhaps on that account deserving of more
than ordinary attention. In the letter to Faraday, before
alluded to, he writes : —

'The crystals most fitted to give evidence of this
law are diopside (a positive crystal), cyanite, topaz (both
negative), and others crystallising in a similar way. In

LAWS EXAMINED. 9

these crystals the line (A), bisecting the acute angles made
by the two optic axes, is neither perpendicular nor parallel
to the axis (B) of the prism. Such a prism, suspended
horizontally, will point neither axially nor equatorially,
but will take always a fixed intermediate direction. This
direction will continually change if the prism be turned
round its own axis (B). It may be proved by a simple
geometrical construction, which shows that during one
revolution of the prism round its axis (B), this axis, without
passing out of two fixed limits c and D, will go through
all intermediate positions. The directions c and D, where
the crystal returns, make, either with the line joining the
two poles, or with the line perpendicular to it, on both
sides of these lines, angles equal to the angle included by
A and B ; the first being the case if the crystal be a. positive
one, the last if a negative one. Thence it follows that if
the crystal, by any kind of horizontal suspension, should
point to the poles of a magnet, it is a positive one ; if it
should point equatorially, it is a negative one.' *

In experimenting with this crystal, we have found the
greatest care to be necessary. Its diamagnetic force is
so weak, that the slightest local impurity, contracted by
handling or otherwise, is sufficient to derange its action.
The crystals as they come from the mineralogist are unfit
for exact experiment. We have found it necessary to boil
those we have used in muriatic acid, and to scour them
afterwards with tine white sand, reduced to powder in a
mortar. These precautions taken, we have been unable
to obtain the results described by Pliicker. We have
examined five specimens of topaz from Saxony, the axial
dimension of some of them exceeding the dimension per-
pendicular thereto by one-half ; the axis, notwithstanding,
stands in all cases from pole to pole. Two specimens of
Brazilian topaz, the one of an amber colour, the other almost

1 Phil. Mag., vol. xxxir. p. 450.

10 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

as clear as distilled water, gave the same results ; the axes
of the crystals stand from pole to pole, and turning round
makes no difference. On a first examination, some of the
crystals exhibited an action similar to that described
by Pliicker ; but after boiling and scouring, these
irregularities disappeared, and the axes one and all stood
axial.

One crystal in particular caused us considerable em-
barrassment. Its action was irregular, and the irregu-
larity remained after the adoption of the methods described
to ensure purity. On examination, however, a splinter
from one of its sides was found to be attracted, a splinter
from the side opposite was found to be repelled. To the
naked eye the crystal appeared clean and clear. On
examination, however, under a powerful microscope, the
side of the crystal from which the magnetic splinter was
taken was found dotted with small black particles im-
bedded in its mass ; the other side of the crystal was
perfectly transparent. On cleaving away the impurities,
the irregularity vanished, and the crystal stood as the
others.

In the letter quoted, diopside is pronounced to
be a positive crystal. On examination with circular
polarized light, as recommended by Dove,1 we find the
crystal to be negative. The same method pronounces
topaz positive, instead of negative, as affirmed by
Pliicker. The specimens we have examined in this way
are from Brazil and Saxony. Aberdeen topaz we have not
examined, but it also is classed by Brewster among posi-
tive crystals. The obliquity of the middle line of topaz
does not exist in the specimens which have come under
our notice ; it is exactly perpendicular to the planes of
principal cleavage, and consequently exactly parallel to
the axis of the prism. This agrees with the results of
1 Poggendorffs Annalen, vol. xl. pp. 457, 482.

LAWS EXAMINED. 11

Brewster, who found the optic axes to be ' equally inclined
to the plain of cleavage.' '

In experimenting with weak diamagnetic crystals, the
greater the number of examples tested the better ; as, if local
impurity be present, it is thus more liable to detection.
Our results with heavy spar have been confirmed by ten
different crystals ; with coelestine, by five ; and with topaz,
as has been stated, by seven. The suspending fibre, in
these and similar instances, was a foot in length and ., .^ 0-
of an inch thick, or about one-eighth of the diameter of a
human hair.

Sugar. — It is well known that this crystal forms a
prism with six sides, two of which are generally very
prominent, the principal cleavage being parallel to these
two, and to the wedge-like edge which runs along the end
of the prism. The plane of the optic axes is perpendicular
to the axis of the prism, and their ends may be found by
cutting out a plate parallel to that axis, and inclined to
the principal cleavage at an angle of about 20°. Such a
plate exhibits both ring systems symmetrically, while a
plate parallel to the principal cleavage exhibits one system
only. Suspended between the excited poles, with the axis
of the prism horizontal, and the principal cleavage ver-
tical, the plane of the optic axes sets axial. According to
the law of Pliicker, it ought to stand equatorial, for the
crystal is negative.

Rock-crystal (Quartz). — This crystal has undergone
more than one examination by the learned German, its
deportment being, ' contrary to all expectation,' very weak —
a result, it may be remarked, difficult of explanation on the
hypothesis of an ' optic axis force.' Pliicker's first experi-
ments with this crystal were apparently made with great
exactitude, the crystal being reduced to a spherical shape,
and the influence of mere form thus annulled. These
1 Lardner's Encyclopaedia, Optics, p. 204.

12 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

experiments proved the optic axis to be repelled. Later
researches, however, induced the philosopher to alter his
opinion, and accordingly, in his last memoir,1 we find
quartz ranked with those crystals whose optic axes are
attracted, with the remark ' weak ' added parenthetically.
We have not been able to obtain this deportment. After
the washing and scouring process, the finest and most
transparent crystals we could procure confirmed the first
experiments of Pliicker, and therefore contradict the
new modification of his law. It is almost incredible how
slight an impurity is sufficient to disturb the action of this
crystal. A specimen with smaller crystals attached to it,
or growing through it, is suspicious and ought to be
rejected. Clear isolated crystals are alone suitable. We
must remark that a fine cube, with faces half an inch
square, suspended with the optic axis horizontal, showed
no directive action ; either one or the other of the diago-
nals set itself from pole to pole, though the axis ran
parallel to four of the faces.

As far as it has been practicable, we have ourselves
cut, cloven, and examined the optical properties of the
crystals which have passed through our hands, testing, in
every possible case, the results of others by actual experi-
ment. Most of the crystals in Brewster's list have been
gone through in this way. Iceland spar, quartz, mica,
arragonite, diopside, lepidolite, topaz, saltpetre, sugar,
sulphate of zinc, sulphate of magnesia, and others have
been examined and verified. In two cases, however, our
results differed from the list, these being sulphate of
nickel and borax. A prism of sulphate of nickel was
suspended from its end between the poles ; on exciting
the magnet it tojk up a determinate position. When it
came to rest, a line parallel to the magnetic axis was
marked thereon, and a plate taken from the crystal parallel

1 Poggendorff's Annalen, vol. Ixxviii. p. 428.

LAWS EXAMINED. 13

to this line and to the axis of the prism. Such a plate,
ground thin, exhibited in the polari scope a pair of very
beautiful ring systems. The ring systems of borax were
found in a similar manner. The middle line, therefore, in
both cases stood equatorial, and, according to the list,
would contradict the law of Pliicker, for both are there
set down as positive. A careful examination with circular
polarised light led us to the opposite conclusion. We
thought it worth while to send specimens of each to
Berlin, so as to have them examined by Professor Dove,
the author of the method by which we examined them.
The crystals have been returned to us with a note certi-
fying that they are negative, thus confirming our obser-
vations.

Yellow Ferrocyanide of Potassium. — This crystal
does not stand in the list of Brewster, and we have sought
for it in other lists in vain. In one German work on
physics we find Blutlaugensalz set down as a negative
crystal with one optic axis, but whether the red or yellow
salt is meant, the author does not explain. We have
examined the crystal ourselves, and find it positive with two
optic axes. The middle line stands perpendicular to the
principal cleavage. Suspended with this line horizontal,
on closing the circuit it sets itself equatorial. Another
exception to the law under consideration is here ex-
hibited.

Pliicker recommends the magnet as a practical
means of determining whether a crystal is positive or
negative ; this method being attended with the peculiar
advantage that it can be applied in the case of opaque
crystals, where all the ordinary methods fail. We find
accordingly, in his last memoir on this subject, that
metallic and other opaque crystals have optical properties
attributed to them. Antimony is negative with one
optic axis; bismuth and arsenic are positive with one

14 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

optic axis. The foregoing experiments demonstrate
the insecurity of the basis on which this classification
rests.

By looking back upon the results described, it will be
seen that we have drawn from each respective class of
crystals one or more examples which disobey the law of
Pliicker. Of positive crystals with one axis, we have
quartz ; of positive crystals with two axes, we have heavy
spar, ccelestine and ferrocyanide of potassium. Of nega-
tive crystals with one axis, we have carbonate of lime
and iron, and several others ; of negative crystals with
two axes, we have dichroite, sugar, sulphate of zinc,
and sulphate of magnesia. It is but just, however, to
state that, in a considerable number of cases, we have
found the law confirmed. Tourmaline, idocrase, beryl,
Iceland spar, saltpetre, arragonite, and many others, all
confirm it. Singularly enough, these are the very crystals
with which Pliicker has experimented. It is therefore
not to be wondered at, that he should be led by such a
mass of concurring evidence to pronounce his law general.
Had bis experiments embraced a sufficient number of
cases, they would doubtless have led him to the same
conclusion to which ours have conducted us.

Faraday has devoted considerable time to the in-
vestigation of this intricate subject. His most notable
experiments are those with bismuth, antimony, arsenic,
sulphate of iron, and sulphate of nickel, which experi-
ments we have carefully repeated.

Bismuth. — Crystals of bismuth we have ourselves
prepared, by melting the metal in a Hessian crucible,
placed within a larger one and surrounded by fine sand.
In this state it was allowed to cool slowly, until a thin
crust gathered on the surface. At this point the crust
was pierced, and the molten metal underneath poured
out, thus leaving the complete crystals clustering round

LAWS EXAMINED. 15

the sides and bottom. Our experiments with these
crystals corroborate, to the letter, those so minutely
described by Faraday in the Bakerian Lecture, delivered
before the Eoyal Society in 1849.1

Arsenic. — Our arsenic we procured at the druggists'.
It is well known that this metal is usually obtained by
the sublimation of its ore, the vapour being condensed in
suitable receivers, where it is deposited in a crystalline
form. There is a difference of opinion between Faraday
and Pliicker as regards this metal ; the former holding it
for diamagnetic, the latter for magnetic. Several speci-
mens, obtained from different druggists, corroborated the
view of Pliicker. They were all magnetic.

About half an ounce of the metal was introduced into
a glass tube, closed at one end and open at the other.
About five inches of the tube, near the open end, was
crammed full of copper turnings, and the open end intro-
duced through a small aperture into the strong draft of a
flue from a heated oven. The portion of the tube con-
taining the copper turnings was heated to redness, and by
degrees the oxygen within the tube was absorbed. The
arsenic at the other end was then heated and sublimed.
After some time the vapour was allowed to condense
slowly, and a metallic deposit was the consequence — the
arsenic thus obtained was diamagnetic. The deportment
of the crystal is desciibed by Faraday in the place above
referred to.

Antimony. — A difference of opinion exists with re-
gard to the action of this crystal also. Keferring to the
deportment assigned to it by Faraday, Pliicker writes,
'to my astonishment, however, antimony behaved in a
manner directly the reverse. While on the one side a
prism of bismuth, whose principal cleavage coincided with
the base of the prism, set itself axial ; and on the other

Phil. Trans., 1849, p. 1.

16 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTIOX.

side a plate of arsenic, which, on account of its magnetism,
ought to stand axial, set itself equatorial; a plate of
antimony deviated completely from this deportment, and
although the mass was strongly diamagnetic, set itself
decidedly axial?

Piiicker's results differ from those of Faraday in two
particulars : first, a plate of antimony, similar to
that described by the German philosopher, is found by
Faraday to stand equatorial instead of axial ; secondly,
the following phenomena, observed by Faraday, appear
not to have exhibited themselves in Pliicker's experi-
ments : — ' On the development of the magnetic force,
the crystal went up to its position slowly, and pointed
as with a dead set. Other crystals did the same im-
perfectly; and others again made one or perhaps two
vibrations, but all appeared as if they were moving in a
thick fluid, and were, in that respect, utterly unlike
bismuth, in the freedom and mobility with which it
vibrated. If the crystalline mass was revolving when the
magnetic force was excited, it suddenly stopped, and was
caught in a position which might, as was found by experi-
ence, be any position. The arrest was followed by a
revulsive action on the discontinuance of the electric
current.' *

In most of the specimens examined by us these phe-
nomena were also absent, and the results of Pliicker
presented themselves. Three specimens, however, behaved
exactly in the manner described by Faraday, exhibit-
ing a singular inertness when the magnetic force was
present, and a revulsion from the poles on breaking the
circuit. To ascertain, if possible, the cause of this differ-
ence, we dissolved an example of each class in muriatic
acid, precipitated the antimony with distilled water, and

1 Phil. Trans., 1849, p. 14. For an explanation see Phil. Mug , vol
xxviii. p. 460.

FAILURE OF LAWS. 17

tested the clear filtrate with ferrocyanide of potassium
The specimen which agreed with Pliicker exhibited a faint
bluish tint, characteristic of the presence of iron ; that
which corroborated Faraday showed not the slightest
trace of this metal. The iron, though thus revealing
itself, must have been present in a quantity exceedingly
minute, for the antimony was diamagnetic. Whether this
has been the cause of the difference between the two
philosophers we will not undertake to say; irregular
crystalline structure may also have had an influence.

We have here a crowd of examples of crystalline action
in the magnetic field, but as yet not a word of explanation.
Pliicker's hypothesis has evidently failed. We now turn
to the observations of Faraday, and shall endeavour to
exhibit, in the briefest manner possible, the views of this
profound investigator.

After a general description of the action of bismuth
between the poles, Faraday writes : — ' The results are, alto-
gether, very different from those produced by diamagnetic
action. They are equally distinct from those dependent
on ordinary magnetic action. They are also distinct from
those discovered and described by PI ticker, in his beautiful
researches into the relation of the optic axis to magnetic
action; for there the force is equatorial, whereas here it is
axial. So they appear to present to us a new force, or a
new form of force in the molecules of matter, which, for
convenience' sake, I will conventionally designate by a
new word, as the magne-crystallic force.'1

4 The magne-crystallic force appears to be very clearly
distinguished from either the magnetic or diamagnetic
forces, in that it causes neither approach nor recession ;
consisting not in attraction or repulsion, but in its giving
a certain determinate position to the mass under its
influence, so that a given line in relation to the mass is

1 Phil. Trans., 1849, p. 4.

18 DIAMAGNETISM AND MAGXE-CRYSTALLIC ACTION.

brought by it into a given relation with the direction of
the external magnetic power.' l

The line through the crystal which sets itself with
greatest force from pole to pole, is termed by Faraday
the magne-crystallic axis of the crystal. He proves by
experiment that bismuth has exactly the same amount of
repulsion whether this axis be parallel or transverse to
the lines of magnetic force acting on it.8

' In other experiments a vertical axis was constructed
of cocoon silk, and the body to be examined was attached
to it at right angles as radius; a prismatic crystal of
sulphate of iron, for instance, whose length was four times
its breadth, was fixed on the axis with its length as radius
and its magne-crystallic axis horizontal, and therefore as
tangent ; then, when this crystal was at rest under the
torsion force of the silken axis, an electro-magnetic pole
was so placed that the a.xial line of magnetic force should
be, when exerted, oblique to both the length and the
magne-crystallic axis of the crystal ; and the consequence
was, that, when the electric current circulated round the
magnet, the crystal actually receded from the magnet
under the influence of the force, which tended to place
the magne-crystallic axis and the magnetic axis parallel.
Employing a crystal or plate of bismuth, that body could
be made to approach the magnetic pole under the influence
of the magne-crystallic force ; and this force is so strong
as to counteract either the tendency of the magnetic body
to approach, or of the diamagnetic body to retreat, when
it is exerted in the contrary direction.' Hence Faraday
concludes that it is neither attraction nor repulsion which
causes the set or determines the final position of a magne-
crystallic body.3

1 Phil. Trans., 1849, p. 22.

* Faraday afterwards corrected this.

' Phil. Mag., vol. xxxiv. p. 77.

FARADAY'S HYPOTHESIS OF NEW FORCES. i«

'As made manifest by the phenomena, the magne-
crystallic force is a force acting at a distance, for the
crystal is moved by the magnet at a distance, and the
crystal can also move the magnet at a distance.' Fara-
day obtained the latter result by converting a steel
bodkin into a magnet, and suspending it freely in the
neighbourhood of the crystal. The tendency of the needle
was always to place itself parallel to the magne-crystallic
axis.

Crystals of bismuth lost their power of pointing at the
moment the metal began to fuse into drops over a spirit-
lamp or in an oil-bath. ' Crystals of antimony lost their
magne-crystallic power below a dull red heat, and just as
they were softening so as to take the impression of the
copper loop in which they were hung.' Iceland spar and
tourmaline, on the contrary, on being raised to the highest
temperature which a spirit-lamp could give, underwent
no diminution of force ; they pointed equally well as
before.

Faraday finally divides the forces belonging to crystals
into two classes — inherent and induced. An example
of the former is the force by which a crystal modifies a
ray of light which passes through its mass ; the second is
developed exclusively by magnetic power. To this latter,
as distinct from the other, Faraday has given the name
'magneto-crystallic. To account for crystalline action
in the magnetic field, we have, therefore, the existence
of three new forces assumed : — the optic axis force,
the magne-crystallic force, and the magneto-crystallic
force.

With regard to the experimental portion of Faraday's
labours on this subject, we have only to express our ad-
miration of the perfect exactitude with which the results
are given. It appears to us, however, a matter of exceed-
ing difficulty to obtain a clear notion of any such force

20 DJAMAGNETISM AND MAGXE-CKYSTALLIC ACTION.

as he has described; that is to say, a force proceeding
from the pole of a magnet, and capable of producing such
motions in the magnetic field, and yet neither attractive
nor repulsive.

That a crystal of bismuth should approach the mag-
netic pole, and that a crystal of sulphate of iron should
recede therefrom, appears, at first sight, anomalous, but
certainly not more so than other phenomena connected
with one of Faraday's most celebrated discoveries, and
explained in a beautiful and satisfactory manner by him-
self.

If we hang a penny from its edge in the magnetic
field, and so arrange the suspending thread that the coin,
before the magnetic power is developed, shall make an
angle of 45°, or thereabouts, with the line joining the poles ;
then, on closing the circuit, and sending a current round
the mag-net, the coin will suddenly turn, as if it made an
effort to set itself from pole to pole ; and if its position
beforehand be nearly axial, this effort will be sufficient to
set it exactly so ; the penny thus behaving, to all appear-
ance, as if it were attracted by the poles.

The real cause of this, however, is repulsion. During
the development of magnetic power, an electric current is
aroused in the copper coin, which circulates round the
coin in a direction opposite to that of the current which
passes from the battery round the coils of the magnet.
The effect of this induced current is to create a polar
axis in the copper ; and when the direction of the
current is considered, it is easy to see that the north
end of this axis must face the north pole of the magnet,
arid will consequently be repelled. On looking therefore
at the penny, apparently attracted as above described,
we must, if w« would conceive rightly of the matter,
withdraw our attention from the coin itself, and fix it
on a line passing through its centre, and at right angles

HYPOTHESIS EXAMINED. 21

to its flat surface ; this is the polar axis of the penny,
the repulsion of which causes the apparent attraction.

We do not mean to say that any such action as that
here described takes place with a bismuth crystal in the
magnetic field. The case is cited merely to show that the
* approach' of the bismuth crystal, noticed by Faraday,
maybe really due to repulsion', and the 'recession' of
the sulphate of iron really due to attraction.

Our meaning will perhaps unfold itself more clearly as
we proceed. If we take a slice of apple, about the same
size as the penny, but somewhat thicker, and pierce it
through with short bits of iron wire, in a direction per-
pendicular to its flat surface, such a disc, suspended in the
magnetic field, will, on the evolution of the magnetic
force, recede from the poles and set its horizontal diameter
strongly equatorial; not by repulsion, but by the at-
traction of the iron wires passing through it. If, instead
of iron, we use bismuth wire, the disc, on exciting the
magnet, will turn into the axial position ; not by attrac-
tion, but by the repulsion of the bismuth wires passing
through it.

If we suppose the slice of apple to be replaced by a
little cake made of a mixture of flour and iron filings, the
bits of wire running through this will assert their pre-
dominance as before ; for though the whole is strongly
magnetic, the superior energy of action along the wire will
determine the position of the mass. If the bismuth wire,
instead of piercing the apple, pierce a little cake made of
flour and bismuth filings, the cake will stand between the
poles as the apple stood ; for though the whole is dia-
magnetic, the stronger action along the wire will be the
ruling agency as regards position.

Is it not possible to conceive an arrangement among
the molecules of a magnetic or diamagnetic crystal, capable
of producing a visible result similar to that here described ?

i>2 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

If, for example, in a magnetic or diamagnetic mass, two
directions exist, in one of which the contact of the particles
is closer than in the other, may we not fairly conclude
that the strongest exhibition of force will be in the former
line, which therefore will signalise itself between the
poles, in a manner similar to the bismuth or iron wire ?
If analogic proof be of any value, we have it here of
the very strongest description. For example : — bismuth
is a brittle metal, and can readily be reduced to a tine
powder in a mortar. Let a teaspoonful of the powdered
metal be wetted with gum-water, kneaded into a paste,
and made into a little roll, say an inch long and a quarter
of an inch across. Hung between the excited poles, it
will set itself like a little bar of bismuth — equatorial.
Place the roll, protected by bits of pasteboard, within
the jaws of a vice, squeeze it flat, and suspend the plate
thus formed between the poles. On exciting the magnet
the plate will turn, with the energy of a magnetic sub-
stance, into the axial position, though its length may be
ten times its breadth.

Pound a piece of carbonate of iron into fine powder,
and form it into a roll in the manner described. Hung
between the excited poles, it will set as an ordinary
magnetic substance — axial. Squeeze it in the vice and
suspend it edgeways, its position will be immediately
reversed. On the development of the magnetic force, the
plate thus formed will recoil from the poles, as if violently
repelled, and take up the equatorial position.

We have here ' approach ' and * recession,' but the
cause is evident. The line of closest contact is perpen-
dicular in each case to the surface of the plate — a conse-
quence of the pressure which the particles have undergone
in this direction ; and this perpendicular sets axial or
equatorial according as the plate is magnetic or diamag-
netic. We ha're here a ' directive force,' but it is attraction

HYPOTHESIS EXAMINED. 23

or repulsion modified. May not that which has been here
effected by artificial means occur naturally? Must it
not actually occur in most instances ? for, where perfect
homogeneity of mass does not exist, there will always be
a preference shown by the forces for some particular direc-
tion. This election of a certain line is therefore the
rule and not the exception. It will assist both the
reader and us if we give this line a name ; we therefore
propose to call it the line of elective polarity.* In
magnetic bodies this line will set axial, in diamagnetic
equatorial.

' The relation of the magne-crystallic force,' says
Faraday, 'to the magnetic field is axial and not equa-
torial.' This he considers to be proved by the follow-
ing considerations : — Suppose a crystal of bismuth so
suspended that it sets with its maximum degree of force,
then if the point of suspension be moved 90° in the
axial plane, so that the line which in the last case stood
horizontal and axial, may now hang vertical, then the
action is a minimum : now, contends Faraday, if the
force were equatorial this change in the axial plane
ought not to have affected it ; that is to say, if the force
act at right angles to the axial plane, it is all the
same which point of the plane is chosen as the point of
suspension.

This seems a fair conclusion ; but the other is just as
fair — that, if the force be axial, a change of the point of
suspension in the equatorial plane cannot disturb it. In
sulphate of nickel, Faraday finds the line of maximum
force to be parallel to the axis of the prism. Whatever,
therefore, be the point of suspension in the plane perpen-
dicular to the axis, the action ought to be the same. On
examining this crystal it will probably be found that two

1 The principal axis of magnetic induction. — J. T. 1870.

24 DIAMAGXETISM AND MAGNE-CRYSTALLIC ACTION.

opposite corners of the parallelepiped are a little flattened.
Let the prism be hung with its axis horizontal and this
flattening vertical, and after the evolution of the mag-
netic force let the oscillations of the prism be counted.
Move the point of suspension 90° in the equatorial plane,
so that the flattening shall be horizontal, and again count
the oscillations. The numbers expressing the oscilla-
tions in the two cases will be very different. The former
will be a maximum, the latter a minimiwn. But if the
force be axial this is impossible, therefore the force is not
axial.

Whatever be the degree of conclusiveness which at-
taches itself to the reasoning of Faraday drawn from
bismuth ; precisely the same degree attaches to the reason-
ing drawn from sulphate of nickel. The conclusions are
equal and opposite, and hence destroy each other. It will
probably be found that the reasoning in both cases is
entirely correct ; that the force is neither axial nor equa-
torial, in the sense in which these terms are used.

A number of thin plates, each about half an inch
square, were cut from almond kernels, with an ivory blade,
parallel to the cleft which divides the kernel into two
lobes. These were laid one upon the other, with strong
gum between them, until a cube was obtained. A few
minutes in the sunshine sufficed to render the cube dry
enough for experiment. Hung between the poles, with
the line perpendicular to the layers horizontal, on ex-
citing the magnet this line turned and set itself parallel
to the magnetic resultant passing through the mass. The
action here was a maximum,. Turning the cube round
90° in the axial plane, there was scarcely any directive
action. If the word ' crystal ' be substituted for ' cube '
in the description of this deportment, every syllable of it
is applicable to the case of bismuth ; and if the deport-
ment of the crystal warrant the conclusion that the force

HYPOTHESIS EXAMINED. 25

is axial, the deportment of the cube warrants the same
conclusion. Is the force axial in the case of the cube ?
Is the position of the line perpendicular to its layers due
to the ' tendency ' of that line to set itself parallel to the
magnetic resultant ? The kernel is strongly diamagnetic,
and the position of the perpendicular is evidently a
secondary result, brought about by the repulsion of the
layers. Is it not then possible, that the approach of
the magne-crystallic axis, in bismuth, to the magnetic
resultant, is really due to the repulsion of the planes of
cleavage ?

But here the experiment with the silken axis meets us ;
which showed that, so far from attraction being t?he cause
of action in a magnetic crystal, there was actual recession ;
and so far from repulsion being the cause in a diamagnetic
crystal, there was actual approach. This objection it is
our duty to answer.

A model was constructed of powdered carbonate of
iron, about 0*3 of an inch long and 0-1 in thickness, and,
by attention to compression, it was arranged that the line
of elective polarity through the model was perpendicular
to its length. Hanging a weight from one end of a fibre
of cocoon silk a vertical axis was obtained ; a bit of card
was then slit and fitted on to the axis, so that when the
model was laid on one side, the card stood like a little
horizontal table in the middle of the magnetic field. The
length of the model extended from the central axis to
the edge of the card, so that when the mass swung round,
its line of elective polarity was tangent to the circle
described.

When the model was made to stand between the flat-
faced poles obliquely, the moment the magnet was excited
it moved, tending to set its length equatorial and its line
of elective polarity parallel to the lines of magnetic force.
In this experiment the model of carbonate of iron, though

20 DIAMAGNETiSM AND MAGNE-CKYSTALLIC ACTION.

a magnetic body and strongly attracted by such a magnet
as that used, actually receded from the magnetic pole.

If, instead of the model of carbonate of iron, we sub-
stitute a crystal of sulphate of iron, we have the experiment
instituted by Faraday to prove the absence of attraction or
repulsion. The dimensions are his dimensions, the arrange-
ment is his arrangement, and the deportment is the exact
deportment which he has observed. We have copied his
very words, these words being perfectly descriptive of the
action of the model. If, then, the experiment be <a
striking proof that the effect is not due to attraction or re-
pulsion ' in the one case, it must also be such in the other
case ; but the great experimenter will, we imagine, hardly
push his principles so far. He will, we doubt not, be ready
to admit, that it is more probable that a line of elective
polarity exists in the crystal, than that a magne-crystallic
axis exists in the model.1

By a similar proceeding, using bismuth powder instead
of carbonate of iron, the action of Faraday's plate of
bismuth may be exactly imitated. The objection to the
conclusion, that the approach of the magne-crystallic axis,
in bismuth, to the magnetic resultant, is due to the re-
pulsion of the planes of cleavage, is thus, we conceive,
fairly met.

Let us look a little further into the nature of this
magne-crystallic force, which, as is stated, is neither
attraction nor repulsion, but gives position only. The
magne-crystallic axis, says Faraday, tends to place it-
self parallel to the magnetic resultant passing through the
crystal ; and in the case of a bismuth plate, the recession
from the pole and the taking up of the equatorial position
is not due to repulsion, but to the endeavour of the

1 The term magne-crystallic axis may with propriety be retained,
even should our views prove correct ; but then it must be regarded as a
subdenomination of the line of elective polarity.

FAILURE OF HYPOTHESIS. 27

bismuth to establish the parallelism before mentioned.
Leaving attraction and repulsion out of the question, we
find it extremely difficult to affix a definite meaning to
the words ' tends ' and 'endeavour.' * The force is due,' says
Faraday, ' to that power of the particles which makes
them cohere in regular order, and gives the mass its crystal-
line aggregation, which we call at times the attraction of
aggregation, and so often speak of as acting at insensible
distances.' We are not sure that we fully grasp the mean-
ing of the philosopher in the present instance ; for the
difficulty of supposing that what is here called the attrac-
tion of aggregation, considered apart from magnetic
attraction or repulsion, can possibly cause the rotation
of the entire mass round an axis, and the taking up
of a fixed position by the mass, with regard to sur-
rounding objects, appears to us insurmountable. We
have endeavoured to illustrate the matter, to our own
minds, by the action of a piece of leather brought near
a red-hot coal. The leather will curl, and motion will be
caused, without the intervention of either attraction or
repulsion, in the present sense of these terms ; but this
motion exhibits itself in an alteration of shape, which is
not at all the case with the crystal. Even if the direct
attraction or repulsion of the poles be rejected, we do not
see how the expressed relation between the magne-crystal-
lic axis and the direction of the magnetic resultant is
possible, without including the idea of lateral attraction
between these lines, and consequently of the mass associated
with the former. In the case of flat poles, the magnetic
resultant lies in a straight line from pole to pole across the
magnetic field. Let us suppose, at any given moment,
this line and the magne-crystallic axis of a properly
suspended crystal to cross each other at an oblique angle ;
let the crystal be forgotten for a moment, and the atten-
tion fixed on those two lines. Let us suppose the former

28 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

line fixed, and the latter free to rotate, the point of inter-
section being regarded as a kind of pivot round which
it can turn. On the evolution of the magnetic force, the
magne-crystallic axis will turn and set itself alongside the
magnetic resultant. The matter may be rendered very
clear by taking a pair of scissors, partly open, in the hand,
holding one side fast, and then closing them. The two
lines close in a manner exactly similar ; and all that is
required to make the illustration perfect, is to suppose
this power of closing suddenly developed in the scissors
themselves. How should we name a power resident in the
scissors and capable of thus drawing the blades together ?
It may be called a * tendency,' or an * endeavour,' but the
word attraction seems to be as suitable as either.

The symmetry of crystalline arrangement is annihilated
by reducing the mass to powder. ' That force among the
particles which makes them cohere in regular order ' is
here ineffective. The magne-crystallic force, in short, is
reduced to nothing, but we have the same results. If,
then, the principle of elective polarity, the mere modifica-
tion of magnetism or diamagnetism by mechanical arrange-
ment, be sufficient to explain the entire series of crystalline
phenomena in the magnetic field, why assume the existence
of this new force, the very conception of which is attended
with so many difficulties ? l

1 ' Perhaps,' says Mr. Faraday, in a short note referring to ' the
strange and striking character ' of these forces, ' these points may find
their explication hereafter in the action of contiguous particles.'

MOLECULAR ARRANGEMENT. 29

APPLICATION OF THE PRINCIPLE OF ELECTIVE
POLARITY TO CRYSTALS.

We shall now endeavour to apply the general principle
of elective polarity to the case of crystals. This principle
may be briefly enunciated as follows : —

If the arrangement of the component molecules of
any crystal be such as to present different degrees of
proximity in different directions, then the line of closest
proximity, other circumstances being equal, will be that
chosen by the respective forces for the exhibition of their
greatest energy. If the mass be magnetic, this line ivill
set axial ; if diamagnetic, equatorial.

From this point of view, the deportment of the two
classes of crystals, represented by Iceland spar and car-
bonate of iron, presents no difficulty. This crystalline
form is the same ; and as to the arrangement of the
molecules, what is true of one will be true of the other.
Supposing, then, the line of closest proximity to coincide
with the optic axis ; this line, according to the principle
expressed, will stand axial or equatorial, according as the
mass is magnetic or diamagnetic, which is precisely what
the experiments with these crystals exhibit.

Analogy, as we have seen, justifies the assumption here
made. It will, however, be of interest to inquire, whether
any discoverable circumstance connected with crystalline
structure exists, upon which the difference of proximity
depends; and, knowing which, we can pronounce with
tolerable certainty, as to the position which the crystal
will take up in the magnetic field.

The following experiments will perhaps suggest a reply.

If a prism of sulphate of magnesia be suspended between
the poles with its axis horizontal, on exciting the magnet
the axis will take up the equatorial position. This is not
entirely due to the form of the crystal ; for even when its

30 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

axial dimension is shortest, the axis will assert the equatorial
position ; thus behaving like a magnetic body, setting its
longest dimension from pole to pole.

Suspended from its end with its axis vertical, the prism
will take up a determinate oblique position. When the
crystal has come to rest, let that line through the mass
which stands exactly equatorial be carefully marked. Lay
a knife-edge along this line, and press it in the direction
of the axis. The crystal will split before the pressure,
disclosing shining surfaces of cleavage. This is the only
cleavage the crystal possesses, and it stands equatorial.

Sulphate of zinc is of the same form as sulphate of
magnesia, and its cleavage is discoverable by a process
exactly similar to that just described. Both crystals set
their planes of cleavage equatorial. Both are diamagnetic.

Let us now examine a magnetic crystal of similar form.
Sulphate of nickel is, perhaps, as good an example as we
can choose. Suspended in the magnetic field with its axis
horizontal, on exciting the magnet the axis will set itself
from pole to pole ; and this position will be persisted in,
even when the axial dimension is shortest. Suspended
from its end, the crystalline prism will take up an oblique
position with considerable energy. When the crystal thus
suspended has come to rest, mark the line along its end
which stands axial. Let a knife-edge be laid on this line,
.and pressed in a direction parallel to the axis of the prism.
The crystal will yield before the edge, and discover a per-
fectly clean plane of cleavage.

These facts are suggestive. The crystals here experi-
mented with are of the same outward form ; eacli has
but one cleavage ; and the position of this cleavage, with
regard to the form of the crystal, is the same in all. The
magnetic force, however, at once discovers a difference of
action. The cleavages of the diamagnetic specimens
stand equatorial ; of the magnetic, axial.

INFLUENCE OP CLEAVAGE. 31

A cube cut from a prism of scapolite, the axis of the
prism being perpendicular to two of the parallel faces of
the cube, suspended in the magnetic field, sets itself with
the axis of the prism from pole to pole.

A cube of beryl, of the same dimensions, with the axis
of the prism from which it was taken also perpendicular
to two of the faces, suspended as in the former case, sets
itself with the axis equatorial. Both these crystals are
magnetic.

The former experiments showed a dissimilarity of
action between magnetic and diamagnetic crystals. In
the present instances both are magnetic, but still there is
a difference ; the axis of the one prism stands axial, the
axis of the other equatorial. With regard to the explana-
tion of this, the following fact is significant. Scapolite
cleaves parallel to its axis, while beryl cleaves perpen-
dicular to its axis ; the cleavages in both cases, therefore,
stand axial, thus agreeing with sulphate of nickel. The
cleavages hence appear to take up a determinate position,
regardless of outward form, and they seem to exercise a
ruling power over the deportment of the crystal.

A cube of saltpetre, suspended with the crystallographic
axis horizontal, sets itself between the poles with this axis
equatorial.

A cube of topaz, suspended with the crystallographic
axis horizontal, sets itself with this axis from pole to pole.

We have here a kind of complementary case to the
former. Both these crystals are diamagnetic. Saltpetre
cleaves parallel to its axis ; topaz perpendicular to its
axis. The planes of cleavage, therefore, stand in both
cases equatorial, thus agreeing with sulphate of zinc and
sulphate of magnesia.1

1 Topaz possesses other cleavages, but for the sake of simplicity we
have not introduced them ; more especially as they do not appear to
vitiate the action of the one introduced, which is by far the most
cnmplele.

32 DTAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

"Where do these facts point ? A moment's speculation
will perhaps be allowed us here. May we not suppose
these crystals to be composed of layers indefinitely thin,
laid side by side, within the range of cohesion, which holds
them together, but yet not in absolute contact ? This
seems to be no strained idea ; for expansion and contrac-
tion by heat and cold compel us to assume that the
particles of matter in general do not touch each other ;
that there are unfilled spaces between them. In such
crystals as we have described, these spaces may be con-
sidered as alternating with the plates which compose the
crystal. From this point of view it seems very natural
that the magnetic laminae should set themselves axial, and
the diamagnetic equatorial.1

- We have a very fine description of sand-paper here.
The sand or emery on the surface is magnetic, while the
paper itself is comparatively indifferent. By cutting a
number of strips of this paper, an inch long and a quarter
of an inch wide, and gumming them together so as to
form a parallelepiped, we obtain a model of magnetic
crystals which cleave parallel to their axis ; the layers of
sand representing the magnetic crystalline plates, and the
paper the intermediate space between two plates. For
such a model one position only is possible between the
poles, the axial. If, however, the parallelepiped be built
up of squares, equal in area to the cross section of the
model just described, by laying square upon square until
the pile reaches the height of an inch, we obtain a model
of those magnetic crystals which cleave perpendicular to

1 In these speculations we have made use of the commonly received
notion of matter. Faraday, for reasons derived from electric con-
ductibility, and from certain anomalies with regard to the combina-
tions of potassium and other bodies, considers this notion erroneous.
Nothing, however, could be easier than to translate the above into a
language agreeing with the views of Faraday. The interval of space
between the laminas would then become intervals of mealier force, and
the result of our reasoning would be the same as before.

DEPORTMENT OP MODELS. 83

their axes. Such a model, although its length be four
times its thickness, and the whole strongly magnetic, will,
on closing the circuit, recede from the poles as if repelled,
and take up the equatorial position with great energy.
The deportment of the first model is that of scapolite ;
of the second, that of beryl. By using a thin layer of
bismuth paste instead of the magnetic sand, the deport-
ment of saltpetre and topaz will be accurately imitated.

Our fundamental idea is, that crystals of one cleavage
are made up of plates indefinitely thin, separated by
spaces indefinitely narrow. If, however, we suppose two
cleavages existing at right angles to each other, then we
must relinquish the notion of plates and substitute that of
little parallel bars ; for the plates are divided into such by
the second cleavage. If we further suppose these bars to
be intersected by a cleavage at right angles to their length?
then the component crystals will be little cubes, as in the
case of rock-salt and other crystals. By thus increasing the
cleavages, the original plates may be subdivided indefi-
nitely, the shape of the little component crystal bearing
special relation to the position of the planes. It is an
inference which follows immediately from our way of
viewing the subject, that if the crystal have several planes
of cleavage, but all parallel to the same straight line, this
line, in the case of magnetic crystals, will stand axial ; in
the case of diamagnetic, equatorial. It also follows, that in
the so-called regular crystals, in rock-salt, for instance, the
cleavages annul each other, and consequently no directive
power will be exhibited, which is actually the case.

Everything which tends to destroy the cleavages tends
also to destroy the directive power ; and here the tem-
perature experiments of Faraday receive at once their
solution. Crystals of bismuth and antimony lose their
directive power just as they melt., for at this particular
instant the cleavages disappear. Iceland spar and tour-

34 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

maline, on the contrary, retain their directive power, for
in their case the cleavages are unaffected. The deport-
ment of rock crystal, whose weakness of action appears
to have taken both Faraday and Pliicker by surprise
— as here the optic axis force, without assigning any
reason, has thought proper to absent itself almost totally
— follows at once from the homogeneous nature of its
mass ; it is almost like glass, which possesses no directive
power ; its cleavages are merely traces of cleavage. If,
instead of possessing planes of cleavage, a crystal be com-
posed of a bundle of fibres, the forces may be expected to
act with greater energy along the fibre than across it.
Anything, in short, that affects the mechanical arrange-
ment of the particles will affect, in a corresponding degree,
the line of elective polarity. There are crystals which are
both fibrous and have planes of cleavage, the latter often
perpendicular to the fibre ; in this case two opposing
arrangements are present, and it is difficult to pronounce
beforehand which would predominate.1

The same difficulty extends to crystals possessing
several planes of cleavage, oblique to each other, and
having no common direction. In many cases, however,
the principle may be successfully applied. We shall
content ourselves in making use of it to explain the
deportment of that class of crystals, of which, as to form,
Iceland spar is the type.

For the sake of simplicity, we will commence our
demonstration with an exceedingly thin rhombus cloven
from this crystal. Looking down upon the flat surface of
such a rhombus, what have we before us ? It is cleavable
parallel to the four sides. Hence our answer must be, t an
indefinite number of smaller rhombuses held symmetri-
cally together by the force of cohesion.' Let us confine

1 It is probable that the primitive plates themselves have different
arrangements of the molecules along and across them.

APPLICATION TO CRYSTALS. So

our attention, for a moment, to two rows of these rhom-
buses; the one ranged along the greater diagonal, the
other along the less. A moment's consideration will
suffice to show, that whatever be the number of small
rhombuses supposed to stand upon the long diagonal,
precisely the same number must fit along the short one ;
but in the latter case they cure closer together. The
matter may be rendered very plain by drawing a lozenge
on paper, with opposite acute angles of 77°, being those
of Iceland spar. Draw two lines, a little apart, parallel
to opposite sides of the lozenge, and nearly through its
centre ; and two others, the same distance apart, parallel
to the other two sides of the figure. The original rhombus
is thus divided into four smaller ones ; two of which stand
upon the long diagonal, and two upon the short one, each
of the four being separated from its neighbour by an
interval which may be considered to represent the interval
of cleavage in the crystal. The two which stand upon the
long diagonal, L, have their acute angles opposite; the
two which stand upon the short diagonal, s, have their
obtuse angles opposite. The distance between the two
former, across the interval of cleavage, is to the distance
between the two latter, as L is to s, or as the cosine of
38° SO' to its sine, or as 4 : 3. We may conceive the size
of these rhombuses to decrease till they become molecular ;
the above ratio will then appear in the form of a differ-
ential quotient, but its value will be unaltered. Here, then,
we have along the greater diagonal a row of magnetic or
diamagnetic molecules, the distance between every two
being represented by the number 4 ; and along the short
diagonal a row of molecules, the distance between every
two being represented by the number 3. In the magnetic
field, therefore, the short diagonal will be the line of
elective polarity ; and in magnetic crystals will stand
axial, in diamagnetic equatorial, which is precisely the

86 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

case exhibited by experiment. Thus the apparent
anomaly of carbonate of lime setting its long diagonal
axial, and carbonate of iron its short diagonal axial, seems
to be fully explained; the position of the former line
being due, not to any endeavour on its part to stand
parallel with the magnetic resultant, but being the sim-
ple consequence of the repulsion of the short diagonal.

There is no difficulty in extending the reasoning used
above to the case of full crystals. If this be done, it will
be seen that the line of closest proximity coincides with
the optic axis, which axis, in the magnetic field, will
signalise itself accordingly. A remarkable coincidence
exists between this view and that expressed by Mitscher-
lich in his beautiful investigation on the expansion of
crystals by heat.1 ' If,' says this gifted philosopher, ' we
imagine the repulsive force of the particles increased by
the accession of heat, then we must conclude that the line
of greatest expansion will be that in which the atoms lie
most closely together.' This line of greatest expansion
Mitscherlich found, in the case of Iceland spar, to co-
incide with the optic axis. The same conclusion has thus
been arrived at by two modes of reasoning, as different as
can well be conceived.

If, then, speculation and experiment concur in pro-
nouncing the line of closest proximity among the particles
to be that in which the magnetic and diamagnetic forces
will exhibit themselves with peculiar energy, thus deter-
mining the position of the crystalline mass between the
poles, we are furnished with a valuable means of ascer-
taining the relative values of this proximity in different
directions through the mass. An order of contact might,
perhaps, by this means be established, of great interest
in a mineralogical point of view. In the case of a right
rhombic prism, for example, the long diagonal of the
1 Poggcndorff's Amutlen, vol. x. p. 138.

MAGNE-CRYSTALLIC ACTION FURTHER IMITATED. 37

base may denote an order of contact very different from
that denoted by the short one ; and the line at right
angles to the diagonals, that is, the axis of the prism, a
contact very different from both. We can compare these
lines two at a time. By hanging the short diagonal
vertical in the magnetic field, its rotatory power is
annulled, and we can compare the long diagonal and the
axis. By hanging the long diagonal vertical, we can
compare the short diagonal and the axis. By hanging
the axis vertical, we can compare the two diagonals.
From this point of view the deportment of heavy spar and
coelestine, so utterly irreconcilable with the assumption of
an optic axis force, presents no difficulty. If we suppose
. the proximity along the axis of the prism to be inter-
mediate between the proximities along the two diagonals,
the action of both crystals follows as a necessary conse-
quence. Suspended from one angle, the axis must stand
from pole to pole ; from the other angle, it must stand
equatorial.

A ball of dough, made from bismuth powder, was
placed between two bits of glass and pressed to the thick-
ness of a quarter of an inch. It was then set edgeways
between the plates and pressed again, but not so strongly
as in the former case. A model of heavy spar was cut from
the mass, so that the shorter diagonal of its rhombic base
coincided with the line of greatest compression, the axis of
the model with the direction of less compression, and the
longer diagonal of the base with that direction in which
no pressure had been exerted. When this model was
dried and suspended in the magnetic field, there was no
recognisable difference between its deportment and that
of heavy spar.

When a crystal cleaves symmetrically in several planes,
all parallel to the same straight line, and, at the same
time, in a direction perpendicular to this line, then the

38 DIAMAGNETISM AND MAGNE-CKYSTALLIC ACTION.

latter cleavage, if it be more eminent than the former,
may be expected to predominate ; but when the cleavages
are oblique to each other, the united action of several
minor cleavages may be such as to overcome the principal
one, or so to modify it that its action is not at all the same
as that of a cleavage of the same value unintersected by
others. A complex action among the particles of the
crystal itself may contribute to this result, and possibly in
some cases modify even the influence of proximity. If we
hang a magnetic body between the poles, it always shows
a preference for edges and corners, and will spring to a
point much more readily than to a surface. Diamag-
netic bodies, on the contrary, will recede from edges and
corners. A similar action among the crystalline par-
ticles may possibly bring about the modification we have
hinted at.

During this investigation a great number of crystals
have passed through our hands, but it is useless to cumber
the reader with a recital of them. The number of natural
crystals have amounted to nearly one hundred ; while
through the accustomed kindness of Professor Bunsen, the
entire collection of artificial crystals, which his laboratory
contains, has been placed at our disposal.1

We now pass over to a brief examination of the
basis on which the second law rests — the affirmation,
namely, that ' the magnetic attraction decreases in a
quicker ratio than the repulsion of the optic axis.'
The ingenuity of this hypothesis, and its apparent
sufficiency to account for the phenomena observed by
Pliicker, are evident. It will be seen, however, that this
repulsion arises from quite another cause — a source of
error which has run undetected through the entire series
of this philosopher's inquiries.

1 We gladly make use of this opportunity to express our obligation
to Dr. Debus, the able assistant in the chemical laboratory.

ACTION BETWEEN POINTED POLES. 39

The following experiment is a type of those which led
Pliicker to the above conclusion. A tourmaline crystal
36 millimeters long and 4 millimeters wide was suspended
between a pair of pointed movable poles, so that it could
barely swing between them. It set its length axial. On
removing the poles to a distance and again exciting the
magnet the crystal set equatorial. The same occurred,
if the poles were allowed to remain as in the former
case, when the crystal was raised above them or sunk
beneath them. Thus, as the crystal was withdrawn
from the immediate neighbourhood of the poles it
turned gradually round and finally set itself equa-
torial.*

A similar action was observed with staurolite, beryl,
idocrase, smaragd, and other crystals.

We have repeated these experiments in the manner
described, and obtained the same results. A prism of
tourmaline three-quarters of an inch long and a quarter of
an inch across was hung between a pair of poles with
conical points, an inch apart. On exciting the magnet
the crystal set axial. When the poles were withdrawn
to a distance, on the evolution of the force the crystal
set equatorial. An exceedingly weak current was here
used ; a single Bunsen's cell being found more than suffi-
cient to produce the result.

According to the theory under consideration, the tour
maline, in the first instance, stood from pole to pole
because the magnetism was strong enough to overcome
the repulsion of the optic axis. This repulsion, decreas-
ing more slowly than the magnetic attraction, necessarily
triumphed when the poles were removed to a sufficient
distance. Between a pair of flat poles, however, this same
crystal could never take up the axial position. On bring-

1 Poggendorff's Ann-alen, vol. Ixxii. p. 31.

40 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

ing the faces within half an inch of each other, and
exciting the magnet by a battery of thirty-two cells, the
crystal vibrated between the faces without touching either.
The same occurred when one cell, six cells, twelve cells,
and twenty cells, respectively, were employed.

If the attraction increases, as stated, more quickly
than the hypothetic repulsion, how can the impotence
of attraction in the case before us be accounted for ? We
have here a powerful current, and poles only half an inch
apart; power and proximity work together, but their
united influence is insufficient to pull the crystal into the
axial line. The cause of the phenomenon must it seems be
sought, not in optic repulsion, but in the manner in which
the magnetic force is applied. The crystal is strongly
magnetic, and the pointed poles exercise a concentrated
local action. The mass at both ends of the crystal, when
in the neighbourhood of the points, is powerfully attracted,
while the action on the central parts, on account of their
greater distance, is comparatively weak. Between the
flat poles, on the contrary, the crystal finds itself, as it
were, totally immersed in the magnetic influence ; its
entire mass is equally affected, and the whole of its
directive power developed. The similarity of action
between the flat poles and the points, withdrawn to a
distance, is evident. In the latter case, the force, radiat-
ing from the points, has time to diffuse itself, and fastens
almost uniformly upon the entire mass of the crystal, thus
calling forth, as in the former case, its directive energy ;
and the equatorial position is the consequence. The dis-
position of the lines of force, in the case of points, is
readily observed by means of iron filings, strewn on paper
and brought over the poles. When the latter are near
each other, on exciting the magnet, the filings are gathered
in and stretch in a rigid line from point to point ; accord-
ing as the poles are withdrawn, the magnetic curves take

PLUCKER VERIFIED BY FARADAY. 41

a wider range, and at length attain a breadth sufficient to
encompass the entire mass of the crystal.1

As the local attraction of the mass in the case of
magnetic crystals deranges the directive power and over-
comes it, so will the local repulsion of the mass in
diamagnetic crystals. A prism of heavy spar, whose length
was twice its breadth, hung from its acute angle, stood
between the flat poles axial, between the points equatorial.
On making its length and breadth alike, the axis of the
prism stood from pole to pole, whether the conical points
or flat faces were used. Shortening the axial direction a
little more, and suspending the crystal from its obtuse
angle, the axis between the flat poles stood equatorial, and,
consequently, the longest dimension of the crystal, axial ;
between the points, owing to the repulsion of the extreme
ends, the length stood equatorial. Similar experiments
were made with ccelestine and topaz ; but all with the
same general result.

' I had the advantage,' says Faraday, ' of verifying
Pliicker's results under his own personal tuition, in respect
of tourmaline, staurolite, red ferrocyanide of potassium,
and Iceland spar. Since then, and in reference to the
present inquiry, I have carefully examined calcareous spar,
as being that one of the bodies which was at the same
time free from magnetic action, and so simple in its
crystalline relations as to possess but one optic axis.

' When a small rhomboid about 0'3 of an inch in its
greatest dimension was suspended with its optic axis
horizontal between the pointed poles of the electro-magnet,
approximated as closely as they can be to allow free motion,
the rhomboid set in the equatorial direction, and the optic
axis coincided with the magnetic axis ; but if the poles be
separated to the distance of a half or three-quarters of an

1 Faraday has already pointed out 'the great value of a mag-
netic field of uniform force.' — Phil. Trans., 1849, p. 4.

42 DIAMAGNETISM AND MAGNE-CEYSTALLIC ACTION.

inch, the rhomboid turned through 90° and set with the
optic axis in the equatorial direction, and the greatest
length axial. In the first instance the diamagnetic force
overcame the optic axis force ; in the second the optic
axis force was the stronger of the two.'

The foregoing considerations will, we believe, render it
very clear that the introduction of this optic axis force
is altogether unnecessary ; the case being simply one of
local repulsion. Faraday himself found that the crystal
between the flat poles could never set its optic axis from
pole to pole ; between the points alone was the turning
round of the crystal possible. We have made the experi-
ment. A fine large crystal of Iceland spar, suspended
between the near points, set its optic axis from point to
point ; between the distant points the axis stood equatorial.
The crystal was then removed from the magnetic field,
placed in an agate mortar and pounded to powder. The
powder was dissolved in muriatic acid. From the solution
it was precipitated by carbonate of ammonia. The
.precipitate thus obtained, as is well known, is exactly of
the same chemical constitution as the crystal. This
precipitate was mixed with gum water and squeezed in one
direction. From the mass thus squeezed a model of
Iceland spar was made, the line of greatest compression
through the model coinciding with that which represented
the optic axis. This model imitated, in every respect,
the deportment observed by Faraday. Between the near
points the optic axis stood from point to point, between
the distant points equatorial. It cannot, however, be
imagined that the optic axis force survived the pounding,
dissolving, and precipitating. Further, this optic axis
force is a sword which cuts two ways ; if it be assumed
repulsive, then the deportment of the compound carbonate
of lime and iron is unexplainable ; if attractive, it fails in
the case of Iceland spar.

SOLUTION OF ACTION BETWEEN POINTS. 43

It is a remarkable fact, that all those crystals which
exhibit this phenomenon of turning round, cleave either
perpendicular to their axes or oblique to them, furnishing
a resultant which acts in the direction of the perpendicular.
Beryl is an example of the former ; the crystal just examined,
Iceland spar, is an example of the latter. This is exactly
what must have been expected. In the case of a magnetic
crystal, cleavable parallel to its length alone, there is no
reason present why the axial line should ever be forsaken.
But if the cleavages be transverse, or oblique, so as to
furnish a line of elective polarity in the transverse direction,
two diverse causes come into operation. By virtue of its
magnetism, the crystal seeks to set its length axial, as
a bit of iron or nickel would do ; but in virtue of its
molecular structure, it seeks to place a line at right angles
to its length axial. For the reasons before adduced, if the
near points be used, the former is triumphant ; if the
points be distant, the latter predominates.

We noticed in a former paper a description of gutta-
percha of a fibrous texture, which, on being suspended
between the poles, was found to accept magnetic induc-
tion with peculiar facility along the fibre. A piece was
cut from this substance, of exactly the same size as the
tourmaline crystal, described at the commencement of this
section. The fibre was transverse to the length of the
piece. Suspended in the magnetic field, the gutta-percha
exhibited all the phenomena of the crystal.

One of the sand-paper models before described is still
more characteristic as regards this turning round on the
removal of the poles to a distance. We allude to that
whose magnetic layers of emery are perpendicular to its
length. The deportment of this model, if we except its
greater energy, is not to be distinguished from that of a
prism of beryl. Between the near points both model and
crystal set axial, between the distant points equatorial,

44 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

while between the flat poles the deportment, as before
described, is exactly the same. The magnetic laminae of
beryl occupy the same position, with regard to its axis, as
the magnetic laminae of the model, with regard to its axis.
There is no difference in construction, save in the superior
workmanship of nature, and there is no difference at all as
regards deportment. Surely these considerations suggest
a common origin for the phenomena exhibited by both.

We have the same action in the case of the compressed
dough, formed respectively from the powdered carbonate
of iron and powdered bismuth. A plate of the former,
three-quarters of an inch square and one- tenth of an inch
in thickness, stands between the conical poles, brought
within an inch of each other, exactly axial ; between the
same poles, two inches apart, it stands equatorial. A plate
of compressed bismuth dough stands, between the near
points, equatorial, between the distant points, axial.

Any hypothesis which solves these experiments must
embrace crystalline action also ; for the results are not to
be distinguished from each other. But in the above cases
an optic action is out of the question. With the similarity
of structure between beryl and the sand-paper model, above
described — with the complete identity of action which
they exhibit, before us, is it necessary, in explanation of
that action, to assume the existence of a force which,
in the case of the crystal, is all but inconceivable,
and in the case of the model is not to be thought of ? In
his able strictures on the theory of M. Becquerel,1 Pliicker
himself affirms, that we have no example of a force
which is not associated with ponderable matter. If this
be the case as regards the optic axis force, if the attrac-
tion and repulsion attributed to it be actually exerted
on the mass of the crystal, how is it to be distinguished
from magnetism or diamagnetism ? The assumption of
1 Poggendorfi's Annalen, vol. Ixxvii. p. 578.

NEW FORCES NON-EXISTENT, 45

Faraday appears to be the only refuge here : the denial
of attraction and repulsion altogether.

In the first section of this memoir it has been proved,
by the production of numerous exceptions, that the law
of Pliicker, as newly revised, is untenable. It has also
there been shown, that the experiments upon which
Faraday grounds his hypothesis of a purely directive force,
are referable to quite another cause. In the second
section an attempt has been made to connect this cause
with crystalline structure, and to prove its sufficiency
to produce the particular phenomea a exhibited by crystals.
In the third section we find the principle entering into
the most complicated instances of these phenomena, and
reducing them to cases of extreme simplicity. The choice,
therefore, rests between the assumption of three new forces
which seem but lamely to execute their mission, and that
simple modification of existing forces, to which we have
given the name elective polarity, and which seems suf-
ficiently embracing to account for all.

It appears then to be sufficiently establi hed, that from
the deportment of crystalline bodies in the magnetic field,
no direct connection between light and magnetism can be
inferred. A rich possession, as regards physical discovery,
seems to be thus snatched away from us ; but the re-
sult will be compensatory. That a certain relation exists,
with respect to the path chosen by both forces through
transparent bodies, must be evident to any one who care-
fully considers the experiments described in this memoir.
The further examination of this deeply interesting subject
we defer to another occasion.

Nature acts by general laws, to which the terms great
and small are unknown ; and it cannot be doubted that
the modifications of magnetic force, exhibited by bits
of copperas and sugar in the magnetic field, display them-

46 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

selves on a large scale in the crust of the earth itself.
A lump of stratified grit exhibits elective polarity. It
is magnetic, but will set its planes of stratification from
pole to pole, though it should be twice as long in the
direction at right angles to these planes. A new factor
appears thus to enter our speculations as to the position
of the magnetic poles of our planet — the influence of
stratification and plutonic disturbance upon the magnetic
and electric forces.

MAEBUEG : May, 1850.

Note, 1870. — I wish to direct attention here to a paper
written by Pliicker, and translated by myself, for the new
series of ' Scientific Memoirs,' published by Taylor and Francis
(1853). In this paper Pliicker approached much more closely
than he had previously done to the views expressed in the
foregoing memoir. But his paper, which had been written in
December, 1849, remained unprinted till 1852. — J. T.

SECOND MEMOIR.

ON DIAMAGNETISM AND MAGNE-CRYSTALLIC
A CTION.

[This investigation was conducted by me in the laboratory of
Professor Magnus, of Berlin, during the spring of 1851, and
it was communicated to the British Association at its meeting
at Ipswich the same year. It was also published in the
'Philosophical Magazine' for September, 1851.— J. T. 1870.]

§ 1. On Diamagnetism.

FIVE years ago Faraday established the existence of the
force called diamagnetism, and from that time to the
present some of the first minds in Germany, France, and
England have been devoted to the investigation of this
subject. One of the most important aspects of the inquiry
is the relation which subsists between magnetism and
diamagnetism. Are the laws which govern both forces
identical? Will the mathematical expression of the
attraction in the one case be converted into the expression
of the repulsion in the other by a change of sign from
positive to negative ?

The conclusions arrived at by Pliicker in this field
of inquiry are exceedingly remarkable and deserving of at-
tention. His first paper, ' On the relation of Magnetism
and Diamagnetism,' is dated from Bonn, September 8,
1847, and will be found in Poggendorff s Annalen and in
Taylor's ' Scientific Memoirs.' He sets out with the ques-
tion, ' Is it possible, by mixing a magnetic substance with

48 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

a diamagnetic, so to balance the opposing forces that
an indifferent body will be the result ? ' This question he
answers in the negative. 'The experiments,' he writes,
' which I am about to describe, render it necessary that
every thought of the kind should be abandoned.'

One of these experiments will serve as a type of
the whole, and will show the foundation on which the
negative reply rests. A piece of cherry-tree bark, 15
millims. long and 7 millims. wide, was suspended freely
between the two movable poles of an electro-magnet ; on
bringing the points of the poles so near each other that
the bark had barely room to swing between them, it
set itself, like a diamagnetic substance, with its length
perpendicular to the line which united the two poles.
On removing the poles to a distance, or on raising the
bark to a certain height above them, it turned round and
set its length parallel to the line joining the poles.
As usual, I shall call the former position the equatorial,
and the latter position the axial. Thus when the poles
were near, diamagnetism was predominant, and caused the
mass to set equatorial; when the poles were distant,
magnetism, according to the notion of Pliicker, was pre-
dominant, and caused the mass to set axial. From this
he concludes, ' That in the cherry-tree bark two distinct
forces are perpetually active ; and that one of them,
the magnetic, decreases more slowly with the distance
than the other, the diamagnetic. '

In a later memoir ! this predominance of the dia-
magnetic force at a short distance is affirmed to be
due to the more general law, that when a magnet ope-
rates upon a substance made up of magnetic and dia-
magnetic constituents, if the power of the magnet be
increased, the diamagnetism of the substance increases
in a much quicker ratio than the magnetism ; so that,
1 Poggendorff's Annalen, vol. Ixxv. p. 413.

BERLIN INVESTIGATION. 49

without altering the distance between it and the magnet,
the same substance might at one time be attracted, and
at another time repelled, by merely varying the strength
of the exciting current.

This assertion is supported by a number of experi-
ments, in which a watch-glass containing mercury was
suspended from one end of a balance. The watch-glass
was magnetic, the mercury was diamagnetic. When the
glass was suspended at a height of 3*5 millims. above the
pole of the magnet, and the latter was excited by a bat-
tery of four cells, an attraction of one milligramme was
observed ; when the magnet was excited by eight cells,
the attraction passed over into a repulsion of the same
amount.

It is to be regretted that Pliicker, instead of giving
us the actual strength of the exciting current, has men-
tioned merely the number of cells employed. From this
we can get no definite notion as to the amount of mag-
netic force evolved in the respective cases. It depends of
course upon the nature of the circuit whether the current
increases with the number of cells or not. If the exterior
resistance be small, an advance from four to eight cells
will make very little difference ; if the outer resistance be
a vanishing quantity, one cell is as good as a million.1

During an investigation on the magneto-optic pro-
perties of crystals,2 which I had the pleasure of conducting
in connection with my friend Professor Knoblauch, I had
repeated opportunities of observing phenomena exactly
similar to those observed with the cherry-tree bark ;
but a close study of the subject convinced me that the
explanation of these phenomena by no means necessi-
tated the hypothesis of two forces acting in the manner

1 The usual arrangement of the cells is here assumed ; that is, where
the negative component of one cell is connected with the positive com-
ponent of the next.

8 Phil. Mag., July 1850.

60 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

described. Experiment further convinced me, that a
more delicate apparatus than the balance used by Pliicker
would be better suited to the measurement of such feeble
manifestations of force.

An exact acquaintance with electro-magnetic attrac-
tions appeared to be a necessary discipline for the success-
ful investigation of diamagnetic phenomena ; and pur-
suing this idea, an inquiry was commenced last November
into the action of an electro-magnet upon masses of soft
iron. I was finally led to devote my entire attention to
the attraction of soft iron spheres, and the results obtained
were so remarkable as to induce me to devote a special
memoir to them alone.1

In this investigation it was proved, that a ball of soft
iron, separated by a small fixed distance from the pole of an
electro-magnet, was attracted with a force exactly propor-
tional to the square of the exciting current.2 Now this
attraction is in each case the produce of two factors, one
of which represents the magnetism of the magnet, and the
other the magnetism of the ball. For example, if the
magnetism of the magnet at any given moment be re-
presented by the number 4, and that of the ball by 3, the
attraction, which is a consequence of their reciprocal
action, is represented by the product 12. If we now sup-
pose the magnetism of the magnet to be doubled by a
current of double strength, the ball will have its magnet-
ism also doubled, and the attraction resulting will be
expressed by 8 x 6, or 48. Thus we see that the doub-
ling of the power of the magnet causes four times the
attraction ; and that while the attraction increases as the
square of the current, the magnetism of the ball increases
in the simple ratio of the current itself.

1 Phil. Mag., April 1851. Poggendorff's Annalen, May 1851.

2 This had been already proved by Lenz and Jacobi, but the employ-
ment of the iron spheres renders the result particularly sharp and
exact.

TORSION BALANCE CONSTRUCTED. 61

The way to a comparison of magnetism and dia-
magnetism is thus cleared. We know the law according to
which the magnetism of an iron ball increases, and we
have simply to ascertain whether the diamagnetism of a
bismuth ball follows the same law. For the investigation
of this question I constructed the following apparatus.

In two opposite sides of a square wooden box were
sawn two circular holes about four inches in diameter.
The holes were diagonally opposite to each other, and
through each a helix of copper wire was introduced and
wedged fast. Each helix contained a core of soft iron,
which was pushed so far forward that a line parallel to the
sides of the box through which the helices entered, and
bisecting the other two sides, was a quarter of an inch
distant from the interior end of each core. The distance
between the two interior ends was six inches, and in this
space a little beam of light wood was suspended. At the
ends of the beam two spoon-shaped hollows were worked
out, in which a pair of small balls could be conveniently
laid. The beam rested in a paper loop, which was at-
tached to one end of a fine silver wire. The wire passed
upward through a glass tube nearly three feet in length,
and was connected at the top with a torsion head. The
tube was made fast in a stout plate of glass, which was
laid upon the box like a lid, thus protecting the beam
from currents of air. A floor of Bristol board was fixed a
little below the level of the axes of the cores, the ' board '
being so cut as to fit close to the helices : the two corners
of the floor adjacent to the respective cores and diagonally
opposite to each other, bore each a graduated quadrant.
When the instrument was to be used, two balls of the
substance to be experimented with were placed upon the
spoon-shaped hollows of the beam and exactly balanced.
The balance was established by pushing the beam a little
in the required direction through the paper loop in which

52 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

it loosely rested ; and to accomplish this with greater ease,
two square pieces were sawn out of the sides of the
box, and two others were exactly fitted into the spaces
thus opened ; these pieces could be taken out at pleasure,
and the hand introduced without raising the lid. The
torsion-head was arranged so that when the beam bearing
the balls came to rest, a thin glass fibre attached to the
beam pointed to zero on the graduated quadrant under-
neath, while the index of the head pointed also to the
zero of the graduated circle above. A current was sent
through the helices so as to cause the two magnetic poles
which operated on the diamagnetic balls to be of opposite
polarities. The balls were repelled when the current flowed.
Preserving the current constant, the index above was
turned in a direction opposed to the repulsion until the
beam stood again at zero, The torsion necessary to effect
this is evidently the expression of the repulsive force
exerted at this particular distance.

Fig. 1 represents the appearance of the beam and
helices when looked down upon through the glass lid.
Fig. 2 represents the beam and balls attached to the sus-
pending wire.

When the glass index pointed to zero, an interval of
about -j^th of an inch usually separated the nearest sur-
faces of the diamagnetic balls from the core ends. The
intensity of the current was measured by a tangent gal-
vanometer, and it was varied by means of a rheostat.
Always before commencing a series of experiments, the
little beam was tested. With very strong currents it was
found to be slightly diamagnetic ; but so feeble, that its
action, even supposing it not to follow the same law of
increase as the ball (which, however, it certainly does),
could cause no measurable disturbance.

I neglected no precaution to secure the perfect purity
of the substances examined. The entire investigation was

PURIFICATION OF BISMUTH. 63

conducted in the private cabinet of Professor Magnus in
Berlin ; and at the same time Dr. Schneider happened to
be engaged in the professor's laboratory in determining

the atomic weight of bismuth. He was kind enough
to give me a portion of this substance, prepared in the
following way : — The metal of commerce was dissolved in
nitric acid and precipitated with distilled water ; whatever
iron was present remained in the solution. The preci-
pitate was filtered, washed for six days successively, and
afterwards reduced by means of black flux. The metal
thus obtained was again melted in a Hessian crucible, and
saltpetre was gradually added, the mass at the same time
being briskly stirred. Every remaining trace of foreign
ingredient was thus oxidised and rose to the surface, from
which it was carefully skimmed. The metal thus purified
was cast into a bullet-mould, the interior surface of which
was coated by a thin layer of oil ; the outer surface of
each bullet was carefully scraped away with glass, the ball
was then scoured with sea-sand, and finally boiled in
hydrochloric acid. The bismuth balls thus purified were

54 DIAMAGNETISM AND MAGNE-CKYSTALLIC ACTION.

placed upon the hollows of the beam, Fig. 2, and their
repulsions by currents of various strengths determined in
the manner indicated. The series of repulsions thus ob-
tained are exactly analogous to the series of attractions
in the experiments with the balls of iron. Now the
square roots of the attractions give a series of numbers
exactly proportional to the currents employed ; and the
question to be decided is, — ' Will the square roots of the
repulsions give a similar series, or will they not ? '

Calling the angle which the needle of the tangent
compass, under the influence of the current, makes with
the magnetic meridian a, then if the repulsion of the
bismuth ball follow the same law as the attraction of the
iron one, we shall have the equation

v*T = n tan a,

where T represents the torsion necessary to bring the
beam back to zero, and n is a constant depending on the
nature of the experiment. The following tables will show
the fulfilment or non-fulfilment of this equation : —

Table I. — Bismuth spheres, 8 millims. diameter.
n=ll-7.

a

tan o

T

-v/T

n tan o

10°

0-176

5

2-23

2-06

20

0-364

16-3

4-04

4-25

30

0-577

42-3

6-50

674

35

0-700

64

8

8-19

40

0-839

100

10

9-81

45

1-000

136

11-66

11-7

50

1-192

195

13-96

13-95

A second series was made with a pair of spheres of the
bismuth of commerce with the same result.

Sulphur is also a diamagnetic substance, but a much

REPULSIONS MEASURED.

55

weaker one than bismuth. The next series of experiments
were made with two balls of this substance.

Table II. — Sulphur spheres, 8 millimx. diameter.

a

tan a

T

A/T

n tan a

20° 0

0-364

1-2

MO

1-20

30 45

0-595

30

1-73

1-96

41 20

0-880

8-0

2-83

2-5)0

54 0

1-376

21-0

4-58

4-54

A pair of sulphur balls were next taken of nearly twice
the diameter of the preceding.

Table III. — Sulphur spheres, 13'4 millims. diameter.
n=6-7.

a

tan o

T

SI

n tan a

20° 0

0-364

6-2

2-45

2-44

30 45

0-595

is-o

3-87

3-93

41 20

0-880

34 5

5-90

5-89

54 0

1-376

890

9-43

9-22

The sulphur from which these balls were made was the
material of commerce. After the experiments one of the
balls was placed in a clean porcelain crucible and brought
over the flame of a spirit-lamp ; the sulphur melted,
ignited, and disappeared in sulphurous acid vapour. A
portion of solid substance remained in the crucible un-
volatilised. This was dissolved in hydrochloric acid, and
ferrocyanide of potassium was added ; the solution turned
immediately blue ; iron was present. The other ball was
submitted to a similar examination, and with the same
result ; both balls contained a slight admixture of iron.

In this case, therefore, the two opposing forces, magnet-

r,6 DIAMAGNETISM AND MAGNE-CRYSTALL1C ACTION.

ism and diamagnetism, were actually present, but we find
the equation ^/T=n tan a fulfilled notwithstanding. Did
one of the forces increase with the ascending magnetic
power more quickly than the other, this result would be
impossible.

Flowers of sulphur were next tried, but found to con-
tain a considerable quantity of iron. I have to thank
Professor Magnus for a portion of a native crystal of the
substance obtained in Sicily, which upon trial was found
to be perfectly pure. From this two small pellets were
formed and laid upon the torsion -balance : they gave the
following results: —

Table IV. — Spheres of Native Sulphur.

a

tan o

T

VT

n tan a

20°

0-364

0-9

0-95

0-96

30

0-577

2-5

1-58

1-53

40

0-839

5-0

2-24

2-22

45

1-000

7-0

2-64

2-(>5

50

1-192

10-0

3-16

3-16

The next substance chosen was calcareous spar. The
corners of the crystalline rhomb were first filed away, and
the mass thus rendered tolerably round; it was then
placed between two pieces of soft sandstone, in each of
which a hollow, like the cavity of a bullet-mould, had
been worked out. By turning the stones, one right and
the other left, and adding a little water, and a little
patience, the crystal was at length reduced to a spheri-
cal form. The ball was then washed, and its surface care-
fully cleansed in dilute hydrochloric acid. The first pair
of balls were from the neighbourhood of Clitheroe in
Lancashire.

REPULSIONS MEASURED.

57

Table V. — Spheres of Calcareous Spar, 9 '2 miUlms. diameter.
w=3-7.

a

tan a

T

*'T

n tan a

20°

0-364

1-8

l*8ft

1-34

25

0-466

30

1-73

1-72

30

0-577

4-5

2-12

2-13

35

0-700

7-0

2-64

2-59

40

0-839

9-7

3-11

3-10

45

1-000

14-0

3-74

3-70

The spar from which these balls were taken was not
quite transparent; to ascertain whether its dullness was
due to the presence of iron, a crystal which weighed about
3 grammes was dissolved in hydrochloric acid ; the solu-
tion was exposed in a flat basin to the air, and the iron, if
present, suffered to oxidise ; ferrocyanide of potassium
was added, but not the slightest tinge indicative of iron
was perceptible.

Experiments were next made with a pair of spheres of
calcareous spar from Andreasberg in the Harz Mountains.

Table VI. — Spheres of Calcareous Spar, 10'8 millims. diameter.

a

tan a

T

VT

n tan a

20° 0

0-364

2-8

1-68

1-82

25 0

0-466

5-0

2-21

2-33

30 0

9-577

8-0

2-83

2-83

35 0

0-700

11-2

3-35

37,0

37 30

0-767

14-5

3-81

3-83

57 0

1-540

60-0

7-75

7-70

The spar from which these balls were taken was per-
fectly transparent. After the experiment, they were
partially dissolved in hydrochloric acid, and the solution
tested as in the former case for iron. No trace of irom
was present.

58 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

The conclusion to be drawn from all these experiments,
and from many others which I forbear citing, is, that the
law of increase for a diamagnetic body is exactly the same
as for a magnetic one. I had proceeded further with
this investigation than the point now attained, when I
learned that a memoir on dia magnetism by M. Edmond
Becquerel had appeared in the May number of the
Annales de Chimie et de Physique.1 In this memoir the
views of the Bonn philosopher are also controverted, and a
number of experiments are adduced to prove the identity
of the laws which regulate magnetic attraction and dia-
magnetic repulsion. The argument employed by M.
Becquerel is the same in principle as that furnished by the
foregoing experiments. He proves that the repulsion of
bars of bismuth, sulphur and wax, increases as the square
of the exciting current, and that the attraction of a little
bar of iron follows the same law. We have both been
guided in our inquiries by the same fundamental thought,
though our modes of carrying out the thought are
different.

1 In fact M. Edmond Becquerel had proved, in the year 1850, that
diamagnetic repulsion followed the law of squares. My experiments
on this subject, though different in form, are to be regarded as mere
verifications of his. See Annales de Chimie et de Physique, vol. xxviii.
p. 301. In the very able memoir referred to in the text, he amply illus-
trates the law of attraction and repulsion ; and there also he repeats
the theoretic conclusion already adverted to, which in his own words is
this : —

' Cette hypothese consiste a supposer qu'il n'y a pas deux genres
d'actions differentes produites sur les corps par les aimants, actions
magnetiques et actions diamagnetiques, mais bien un seul genre d'ac-
tion, une aimantation par influence, et que la repulsion exercee sur les
substances qui s'eloignent des poles des aimants est due a ce que les
corps sont entoures par uu milieu plus magnetique qu'elles.'

'Je n'ai presente,' he adds, 'cette explication du diamagnetisme
que pour lier entre eux, d'une maniere plus simple, je crois, qu'on ne
1'avait fait jusqu'ici, les effets du diamagn6tisme sur les differents
corps soumis a son action.' — Annales de Chimie et de Physique, vol.
xxxii. p. 112.

DIAMAGNETIC INDUCTION. 59

I have observed many phenomena, which, without
due consideration, would lead us directly to Pluck er's
conclusions ; and a few of which may be here described.
The bismuth balls were placed upon the beam, and one
core was excited ; on the top of the ball opposite that core,
a particle of iron, not the twentieth part of a common pin-
head in size, was fixed. A current of 10° circulated in
the helix, and the beam came to rest at the distance of 4°
from the zero of the lower graduation. The current was
then permitted to increase gradually. The magnetism of
the iron particle and the diamagnetism of the bismuth
rose of course along with it, but the latter triumphed ;
the beam was repelled, and finally came to rest against a
stop which was placed 9° distant.

The particle of iron was removed, and a small crystal
of carbonate of iron was put in its place ; a current of
15° circulated in the helix, and the beam came to rest
at about 3° distant from zero. The current was raised
gradually, but before it had reached 300,1 diamagnetism
conquered, and the beam receded to the stop as before.

Thinking that this apparent triumph of diamagnetism
might be due to the fact that the crystal of carbonate
of iron had become saturated with magnetism, and that it
no longer followed the law of increase true for a larger
piece of the substance, I tested the cr}7stal with currents
up to 49°; the attractions were exactly proportional to
the squares of the exciting currents.

Thinking also that a certain reciprocal action between
the bismuth and the crystal, when both were placed
together in the magnetic field, might so modify the latter
as to produce the observed result, I removed the crystal,
and placed a cube of the zinc of commerce upon the
opposite end of the beam. The zinc was slightly mag-

1 Currents of 10°, of 15°, of 30°, &c., signify currents which pro-
duced these respective deflections of the tangent-compass needle.

GO DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

netic. Bismuth and zinc were thus separated by an
interval of 6 inches ; both cores were excited by a current
of 10°, and the beam, after some oscillations, came to rest
at 4° distant from zero. The current was now gradually
raised, but when it reached 35° of the graduated quadrant,
the beam receded and was held firmly against the stop.
When the circuit was broken it left the stop, and, after
some oscillations, came to rest at zero.

These experiments seem fully to bear out the notion
of Pliicker. In each case we waited till both forces were
in equilibrium ; and it might be thought that if the
forces followed the same law, the beam ought not to
move. Let us, however, clear the experiment of all
mystery. When the beam was in equilibrium with a
current of 10°, let us ask what forces were opposed to the
repulsion of the bismuth ? There was, first of all, the
attraction of the zinc ; but besides this, there was a
torsion of 4° ; for the position of equilibrium for the beam
with the un excited magnet was at zero. Let its suppose
the magnetism of the zinc at the distance of 4, and with
the current 10°, to be equal to 8 of torsion; this, added
to the 4 already present, will give the force opposed to
the bismuth ; the repulsion of the latter is therefore equal
to 12. Let us now conceive the current raised from 10°
to 35°, that is quadrupled.1 Supposing the magnetism of
the zinc to be increased in proportion to the strength of
the current, its attraction will now be 32 ; this, added to
4 of torsion, which remains constant, makes 36, which is
therefore the force exerted against the bismuth by a
current of 35° under the present circumstances. But the
repulsion of the bismuth being also quadrupled, it is now
48. This, opposed to a force of 36, necessarily conquers,
and the beam is repelled.

We thus see that, although the magnetic force on one
* The tangent of 35° being four times the tangent of 10°.

DIFFERENTIAL REPULSIONS. 01

side, and the diamagnetic on the other side, follow pre-
cisely the same law, the introduction of the small constant
4° entirely destroys the balance of action, so that to all
appearance diamagnetism. increases in a much quicker
ratio than magnetism. Such a constant has probably
crept into the experiments of Pliicker ; an inadvertency
not to be wondered at, when we remember that the force
was new at the time, and our knowledge of the precautions
necessary for its accurate investigation very imperfect.

§ 2. On Magne-crystallio action.

Pliicker has discovered that, when a crystal of pure
carbonate of lime is suspended in the magnetic field with
its optic axis horizontal, the said axis always sets itself
equatorial. He attributed this action of the spar to a
repulsion of the optic axis by the magnet, which is inde-
pendent of the magnetism or diamagnetism of the mass
of the crystal. It was the product of a new force, which
Faraday has named 'the optic axis force.'

In the memoirs published by Knoblauch and myself,
this view is controverted, and it is there proved that the
action of the crystal, so far from being independent of the
magnetism or diamagnetism of its mass, is totally changed
by the substitution of a magnetic constituent for a dia-
magnetic. Our experiments led us to the conclusion, that
the position of the crystal of carbonate of lime was due to
the superior repulsion of the mass of the crystal in the
direction of the optic axis. This view, though supported
by the strongest presumptive facts, has remained up to
the present time without direct proof; if, however, a
difference of repulsion, such as that we have supposed,
actually exists, it may be expected to manifest itself upon
the torsion-balance.

But the entire repulsion of calcareous spar is so feeble,
that to discover a differential action of this kind requires

62 DIAMAGXET1SM AND MAGNE-CRTSTALLIC ACT10X.

great nicety of experiment. I returned to this subject
three different times ; twice I failed, and despaired of
being able to establish a difference with the apparatus at
my command. But the thought clung to me, and after
an interval of some weeks, I resolved to try again.1

The spheres of calcareous spar were placed upon th
beam, and the latter was exactly balancedr The in
above was so placed, that when the beam came to resi
the attached glass fibre exactly coincided with a fine black
line drawn upon the Bristol board underneath. T\vo dot
were placed upon the glass cover, about the fiftieth of ai
inch asunder, and the fibre was observed through the in
terval between them. The beam was about four inche
below the cover, and parallax was thus avoided. On ex
citing both cores the balls receded, the index of the torsion
head was softly turned against the recession, till the fibr
was brought once more into exact coincidence with th
fine black line, and the torsion necessary to effect thi
was read off upon the graduated circle above.

The repulsion of the spheres was measured in fou
different directions : —

1 . The optic axes were parallel to the axes of the iroi
cores.

2. The spheres were turned through an arc of 90°, so
that the optic axes were at right angles to the cores.

1 ' The torsion balance was placed before a window through which
the sun shone in the forenoon. In experimenting with spheres o
bismuth, I was often perplexed and baffled by the contradictory result
obtained ai different hours of the same day. With spheres of cal
careous spar, where the diamagnetic action was weaker, the dis-
crepancies were still more striking. Once while gazing puzzled at the
clear ball of spar resting on the torsion balance, my attention was
drawn to the bright spot of sunlight formed by the convergence of the
rays which traversed the spar, and the thought immediately occurred
to me that this little " fire-place " might create currents of air strong
enough to produce the observed anomalies. The shutting out of the
light entirely removed the cause of the disturbance ; which how^vei
was mainly due to the heating of the glass lid of the balance.' — Phil
Mag. vol. iii. p. 128.

DIFFERENTIAL REPULSIONS. 63

3. The spheres were turned 90° in the same direction,
so that the other ends of the axes faced the cores.

4. The spheres were turned 90° further, so that their
axes were again at right angles to the cores, but with the
opposite surface to that in (2) facing the latter.

The following are the respective repulsions : —

Repulsion.

1st position 28'5

2nd position 26-5

3rd position 27'0

4th position 24-5

[Mean of repulsions along optic axis . 27-8
„ „ across „ . 25-5

Or as JOO: 91-7]

Each of the helices surrounding the cores was composed
of two insulated wires ; the four ends of these could be
so combined that the current could pass through both at
the same time, as if they were a single wire, or it could
be caused to traverse one wire after the other. The first
arrangement was advantageous when a small exterior
resistance was an object to be secured, the second when
the force of the battery was such as to render exterior
resistance to a certain extent a matter of indifference. In
the foregoing experiments the first of these arrangements
was adopted. Before commencing, I had taken fresh acid
and freshly amalgamated zinc cylinders, so that the bat-
tery was in good condition. The second arrangement
was then adopted, that is to say, the current was allowed
to traverse one wire after the other, and the following
repulsions were observed ; the numbers refer to the po-
sitions already indicated.

Repulsion.
1st position ....... 57

2nd position 51

3rd position 53

4th position 48

[Mean of repulsions along optic axis . . 55
M „ across „ . . 49*5

Or as 100 : 90]

64 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

These experiments furnish the direct proof that cal-
careous spar is repelled most strongly in the direction of
the optic axis. That Faraday has not succeeded in es-
tablishing a difference here is explained by reference to
his mode of experiment. He observed the distance to
which the spar was repelled, and found this the same for
all positions of the crystal. The magnetic force at this
distance is too weak to show a difference. In the above
experiments, on the contrary, the crystal was forced back
into a portion of the magnetic field where the excitement
was intense, and here for the first time the difference rises
to a measurable quantity.

Carbonate of iron is a crystal of the same form as cal-
careous spar, the iron filling up, so to speak, the exact
space vacated by the calcium. This crystal is strongly
magnetic ; suspended in the magnetic field, that line
which in calcareous spar sets equatorial, sets here axial,
but with an energy far surpassing the spar ; a greater
differential action may therefore be anticipated.

A pair of spheres were formed from the crystal, but
their attraction was so strong, that to separate them from
the magnet would strain the wire beyond its limits ot
elasticity; one sphere only could therefore be used, the
other being used as a balance-weight merely. The
core opposite to the latter was removed, and the current
sent round that helix only which surrounded the former.
A piece of Bristol board was placed against the end of the
core, and the torsion-head was so turned that when the
index above pointed to zero, the little sphere was on the
verge of contact. The magnet was then excited and the
sphere attracted. The index was then turned in a direction
opposed to the attraction until the ball gave way ; the
torsion necessary to effect this expresses the attraction.
The crystal was first placed so that its axis was parallel to
that of the magnet, and afterwards so that it was perpen-

ATTRACTIONS MEASURED.

dicular to the same. The following tables exhibit the
results in both cases respectively : —

Fable VII. — Carbonate of Iron. Axis of Crystal parallel to
axis of Magnet. n=25-5.

a

tan a

T

-v/T

n tan a

15

0-268

43

6.56

6-57

20

0-364

80

8-94

8-91

25

0-466

129

11-36

11-42

30

0-577

200

14-14

14-14

Table VIII. — Carbonate of Iron. Axis oj Crystal
perpendicular to axis of Magnet. n=20'7.

a

tan a

T

A/T

n tan a

15

0-268

30-5

5-52

5-55

20

0-364

56-0

7-48

7-53

25

0-466

92-5

9-62

9-64

30

0-577

142-5

11-44

11-44

We learn from these experiments that the law accord-
ing to which the attraction of carbonate of iron increases,
is exactly the same as that according to which the repul-
sion of the calcareous spar increases, and that the respective
forces manifest themselves in both cases with the greatest
energy in the direction of the optic axis, the attraction
along the optic axis being to that across the same axis, in
all four cases, as 100 I 71 nearly.

Let us observe for an instant the perfect antithesis
which exists between carbonate of lime and carbonate of
iron. The former is a diamagnetic crystal. Suspended
before the single pole of a magnet, the entire mass is re-
pelled, but the mass in one direction is repelled with
peculiar force, and this direction, when the crystal is
suspended in the magnetic field, recedes as far as possible

66 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION

from the poles, and finally sets equatorial. The crystal of
carbonate of iron is, on the contrary, strongly magnetic ;
suspended before a single pole the entire mass is attracted,
but in one direction the mass is attracted with peculiar
energy, and this direction, when the crystal is suspended
in the magnetic field, will approach the poles and finally
set axial.

Sulphate of iron in the magnetic field displays a direc-
tive action considerably inferior to that of carbonate of
iron. Some large crystals were obtained from a chemical
manufactory, and from these I cut two clean cubes. Each
was suspended by a cocoon fibre in the magnetic field, and
the line which stood axial was marked upon it. The white
powder which collects by efflorescence around these crys-
tals was washed away, and two transparent cubes remained
These were laid upon the torsion-balance, and instead of
the Bristol board used in the last experiment, two plates
of glass were placed against the core ends ; the adhesion
of the cubes, which in delicate experiments of this nature
sometimes enters as a disturbing element, was thus re-
duced to a minimum. As in the case of carbonate of iron,
one core only was excited. The cube opposite to this core
was first so placed that the line which stood axial in the
magnetic field was parallel to the axis of the core ; pre-
serving this line horizontal, the three remaining faces
were presented successively to the core, and the attraction
measured in each particular case; the attractions were
as follows : —

Cube of Sulphate of Iron, edges 10 millims.

Attraction

1st position 43-0

2nd position 36-3

3rd position 4OO

4th position ...... 34.6

[Mean of attraction along axis . . . 41*5
„ „ across „ . . . iio'4

Or as 100 : 85 nearly.]

DIFFERENTIAL ATTRACTIONS. 67

From an article translated from PoggendorfFs Annalen,
and published in the June number of the ' Philosophical
Magazine,' it will be seen that Professor Pliicker has ex-
perimented with a cube of sulphate of iron, and has
arrived at results which he adduces against the theory of
magne-crystallic action advanced by Knoblauch and my-
self. He rightly concluded that if the position of the
crystal, suspended between two poles, were due to the
superior attraction exerted in a certain direction, this
peculiarity ought to exhibit itself in the attraction of the
entire mass of the crystal by the single pole of a magnet.
He brings this conclusion to the test of experiment, sus-
pends the crystal from one end of a balance, weighs the
attraction in different directions, but finds no such differ-
ence as that implied by the conclusion. This result, I
believe, is entirely due to the imperfection of his appara-
tus ; I have tried a very fine balance with even worse
success than Pliicker. Although the torsion-balance
furnishes a means of experiment immeasurably ficer, still,
even with it, great delicacy of manipulation and a consider-
able exercise of patience are necessary to insure invariable
success.

Faraday has discovered, that if a bismuth crystal be
suspended in the magnetic field, it will set itself so that a
line perpendicular to the plane of most eminent cleavage
will be axial; this line he calls the magne-crystallic axis
of the crystal. In the memoir by Knoblauch and myself
before alluded to, the position of the magne-crystallic axis
is affirmed to be a secondary result, depending on the fact
that the mass in the direction of the planes of cleavage is
most strongly repelled. The general fact of superior re-
pulsion in the direction of the cleavages has been already
demonstrated by Faraday.

Our torsion-balance furnishes us with a quantitative
confirmation of Faraday's result. Two cubes of bis-

68 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

muth were prepared, in each of which the plane of most
eminent cleavage formed two of the opposite sides Sus-
pended by a fibre of cocoon-silk in the magnetic field, the
line perpendicular to the cleavage turned into the axial
position, or what amounts to the same as far as the eye is
concerned, the cleavage itself receded from the poles and
stood equatorial. These cubes were placed one on each
end of the torsion-balance ; first, so that the plane of most
eminent cleavage was parallel to the axes of the cores,
and afterwards perpendicular to these axes. The respec-
tive repulsions are stated in the following tables.

Tattle IX. — Cubes of Bismuth, edges 6 millims. Plane of most
eminent cleavage parallel to axes of cores.

a

T

20

11-7

30

34-8

40

78

45

111

50

153

Table X. — The same cubes. Plane of most eminent cleavage
perpendicular to axes of cores.

a

T

20

8

30

23

40

53

45

76-5

50

110

A comparison of these two tables shows us that the re-
pulsion of the cubes, when the plane of most eminent
cleavage was parallel to the magnetic axis, is to the repul-
sion when the said plane was perpendicular thereto, in the
ratio nearly of 100 : 71.

COMPRESSED POWDERS.

09

What is it, then, which causes this superior manifes-
tation of force in a certain direction ? To this question
experiment returns the following reply : — ' If the arrange-
ment of the component particles of any body be such as
to present different degrees of proximity in different
directions, then the line of closest proximity, other circum-
stances being equal, will be that of strongest attraction in
magnetic bodies and of strongest repulsion in diamagnetic
bodies.'

The torsion-balance enables us to test this theory. A
quantity of bismuth was ground to dust in an agate
mortar, gum-water was added, and the mass was kneaded
to a stiff paste. This was placed between two glasses and
pressed together; from the mass when dried two cubes
were taken, the line of compression being perpendicular
to two of the faces of each cube and parallel to the other
four. Suspended by a silk fibre in the magnetic field,
upon closing the circuit the line of compression turned
strongly into the equatorial position, exactly as the plane
of most eminent cleavage in the case of the crystal. The
cubes were placed one upon each end of the torsion-
balance ; first with the line of compression parallel to the
cores, and secondly with the same line perpendicular to
the cores. The following are the repulsions exhibited in
both cases respectively.

Table XI. — Cubes of powdered Bismuth, edges 7 millims. Line
of compression parallel to axes of cores.

a.

tan a

T

VT

8-3 x tan o

30

0-577

22

4-69

4-78

40

0-839

46

6-78

6-96

45

1-000

67

8-19

8-30

50

1-192

98

9-89

9-89

From this table we see that the law of increase for the

70 DIAMAGNET1SM AND MAGNE-CRYSTALLIC ACTION.

artificial cube is the same as that for diamagnetic sub-
stances generally.

Table XII. — The same cubes. Line of compression
perpendicular to cores.

a

T

30

13

40

31

45

46

50

67

A comparison of the two tables shows us that the line
which stands equatorial in the magnetic field is most
strongly repelled upon the torsion-balance, exactly as in
the case of the crystal; the repulsion in the direction of
this line and in a direction perpendicular to the same
being in the ratio of 100 .' 66 nearly. Similar experi-
ments were made with cubes of powdered carbonate of
iron. The line of compression set axial in the magnetic
field, and on the torsion-balance the attraction along this
line was a maximum.

[Summary. — Differential attractions and repulsions of
magnetic and diamagnetic bodies : —

Along axis Across axis

Carbonate of iron (attraction) , . 100 . . .71

Carbonate of lime (repulsion) . . 100 . . .90

Sulphate of iron (attraction) . . 100 . . .85

Bismuth (repulsion) < 100 . . .71

Compressed bismuth .

Along line of
pressure

100 .

Across line of
pressure

. 66

In all cases in magnetic bodies the line of strongest
attraction sets from pole to pole, while in diamagnetic
bodies the line of strongest repulsion sets equatorial.]

At the last meeting of the British Association, an ob-

THOMSON'S ARGUMENT. 71

jection, which will probably suggest itself to all who study
the subject as profoundly as he has done, was urged, viva
voce, against this mode of experiment by Sir William
Thomson. ' You have,' he said, * reduced the mass to
powder, but you have not thereby destroyed the crystalline
property ; your powder is a collection of smaller crystals,
and the pressing of the mass together gives rise to a pre-
dominance of axes in a certain direction ; so that the re-
pulsion and attraction of the line of compression which
you refer to the mere closeness of aggregation is, after all,
a product of crystalline action.'

1 know that this objection, which was specially direc-
ted against the experiment made with powdered bismuth
and carbonate of lime, floats in the minds of many both
in Germany and England, and I am therefore anxious to
give it a full and fair reply. I might urge, that in the
case of the bismuth powder at least, the tendency of com-
pression would be to place the little component crystals in
such a position, that a deportment precisely the reverse of
that actually observed might be anticipated. If we pound
the crystal to the finest dust, the particles of this dust,
to render Thomson's hypothesis intelligible, must have a
certain predominant shape, otherwise there is no reason to
suppose that pressure will always cause the axes of the
little crystals to take up the same predominant direction.
Now what shape is most likely here ? The crystal cleaves
in one direction more easily than in any other ; is it not
then probable that the powder will be chiefly composed of
minute scales, whose opposite flat surfaces are the surfaces
of principal cleavage? And what is the most probable
effect of compression ? Will it not be to place these little
scales with their flat surfaces perpendicular to the line in
which the pressure is exerted ? In the crystal, the line
perpendicular to the principal cleavage sets axial, and
hence it might be expected that the line of compression in

72 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

the model would set axial also ; it does not, however, — it
sets equatorial.

This, however, though a strong presumptive argument,
is not yet convincing ; and it is no easy matter to find one
that shall be so. Bismuth powder will remain crystalline,
and carbonate of lime is never free from suspicion. I
thought I had found an unexceptionable substance in
chalk, inasmuch as Ehrenberg has proved it to be a
mere collection of microscopic shells; but Professor Ehren-
berg himself informs me, that even these shells, which
require a high magnifying power to render them visible,
are in their turn composed of infinitesimal crystals of cal-
careous spar. In this dilemma one way remains open to
us : we will allow the objection to stand, and follow it out
to its inevitable consequences ; if these are opposed to fact,
the objection necessarily falls.

Let us suppose the bismuth powder to be rearranged,
so that the perfect crystal from which it was obtained is
restored. In this case the axes of all the little component
crystals are parallel, they work all together, and hence their
action must be greater than if only a majority of them
were parallel. In a bismuth crystal, therefore, the differ-
ence of action in the line of the magne-crystallic axis,
and in a line perpendicular thereto, must be a maximum.
It must, for example, be greater than any difference which
the model of bismuth powder can exhibit ; for a portion of
the force attributed to the axes must in this case be an-
nulled by the confused grouping of the little component
crystals. In the words of Professor Thomson, it is merely
a balance of action brought about by predominance, which
can make itself manifest here. Hence, if we measure the
repulsion of the crystal in a direction parallel to the prin-
cipal cleavage, and in a direction perpendicular to it, and
also measure the repulsion of the model in the line of
compression and in a line perpendicular to it, the ratio of

REPLY TO ARGUMENT. 73

the two former repulsions, that is, of the first to the second,
must be greater than the ratio of the. two latter, that is,
of the third to the fourth,

Turning to Tables IX. and X., we see that the ratio of
the repulsion of the crystal in the direction of principal
cleavage to the repulsion in a direction perpendicular to

15

the same is expressed by the fraction — = I -36. Turning

to Tables XI. and XII., we find that the ratio of the repulsion

of the model in the line of compression to the repulsion in a

3
line perpendicular to it is expressed by the fraction - = l-5.

fi

In the latter case, therefore, we have the greatest differ-
ential effect ; which result, were the repulsion due to the
mere predominance of axes, as urged by Thomson, would
be tantamount to the conclusion that a part is greater
than the whole. This result has been entirely unsought.
The models were constructed with the view of establish-
ing the general fact, that the repulsion in the line of
compression is greatest. That this has fallen out in the
manner described is a pure accident. I have no doubt
whatever that models might be made in which this dif-
ference of action would be double that exhibited by the
crystal.

The case, however, is not yet free from suspicion ; the
gum-water with which it is necessary to bind the powder
may possibly exert some secret influence. When isinglass
or jelly is compressed, we know that it exhibits optical
phenomena similar to those exhibited by crystals ; and the
. squeezing of the metallic dough may induce a kind of
crystalline structure on the part of the gum sufficient to
produce the phenomena observed.

An experiment to which I was conducted by the follow-
ing accident will set this doubt, and I believe all other
doubts regarding the influence of compression, completely

74 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTIOX,

at rest. Having repeated occasion to refer to the deport-
ment of crystals in the magnetic field, so as to be able to
compare this deportment with the attraction or repulsion
of the entire mass upon the torsion-balance, through the
kindness of Professor Magnus, the great electro-magnet of
the University of Berlin1 was placed in the room where I
experimented. One morning a cube of bismuth was sus-
pended between the movable poles, and not knowing the
peculiarities of the instrument, I chanced to bring the
poles too near each other. On closing the circuit, the
principal cleavage of the crystal receded to the equator.
Scarcely however was this attained, when the poles were
observed moving towards each other, and before I had
time to break the circuit, they had rushed together and
caught the crystal between them. The pressure exerted
squeezed the tube to about three-fourths of its former thick-
ness, and it immediately occurred to me that the theory of
proximity, if it were true, ought to tell here. The pressure
brought the particles of the crystal in the line of compres-
sion more closely together, and hence a modification, if
not an entire subversion of the previous action, was to be
expected.

Having liberated the crystal, I boiled it in hydrochloric
acid, so as to remove any impurity it might have contracted
by contact with the iron. It was again suspended between
the poles, and completely verified the foregoing anticipa-
tion. The line of compression, that is, the magne-crystallic
axis of the crystal, which formerly set from pole to pole,
now set strongly equatorial. I then brought the poles
intentionally near each other, and allowed them to close
once more upon the already compressed cube ; its original
deportment was thereby restored. This I repeated several
times with several different crystals, and with the same

1 A notion of the power of this magnet may be derived from
the fact, that the copper helices alone which surrounded the pillars of
soft iron weighed 243 pounds.

COMPRESSED CRYSTALS.

75

unvarying result ; the line of compression always stood
equatorial, and it was a matter of perfect indifference
whether this line was the magne-crystallic axis or not.
The experiment was then repeated with a common vice. I
rubbed the letters from two copper coins with sandstone,
and polished the surfaces ; between the plates thus obtained
various pieces of bismuth were forcibly squeezed ; in this
way plates were procured about as thick as a shilling, and
from half an inch to an inch in length. Although the
diamagnetism of the substance tended strongly to cause
such a plate, suspended from its edge between the poles,
to take up the equatorial position, although the force
attributed to the magne-crystallic axis worked in each case
in unison with the diamagnetism of the mass, every plate
set nevertheless with its length from pole to pole, and its
magne-crystallic axis equatorial.

This superior repulsion of the line of compression mani-
fests itself upon the torsion-balance also. The cubes of bis-
muth crystal already made use of were squeezed in a vice
to about four-fifths of their former thickness ; the line of
compression in each case being perpendicular to the prin-
cipal cleavage, and consequently parallel to the magne-
crystallic axis. From the masses thus deformed, two new
cubes were taken ; these laid upon the torsion-balance
in the positions indicated in the tables, gave the following
results :—

Table XIII. — Bismuth Crystals, compressed cubes. Plane of
most eminent cleavage parallel to axf.s of magnets.

a

T

20

7-8

30

21

40

47

45

67

50

101

76 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

Table XIV. — The same cubes. Plane of most eminent cleavage
perpendicular to axes of magnets.

a

T

20

9

SO

25-5

40

57-3

45

79

50

113

Looking back to Tables IX. and X., we see that the
line which was there repelled most strongly is here repelled
most feebly, and vice versa, the change being due to com-
pression. The ratio there is 100 I 71 ; here it is 100 : 112
nearly.

I have been careful to make similar experiments with
substances concerning whose amorphism there can be but
little doubt. A very convenient substance for showing the
influence of compression is the white wax used in candles.
The substance is diamagnetic. A little cylinder of the
wax suspended in the magnetic field set with its axis equa-
torial. It was then placed between two stout pieces of
glass and squeezed as thin as a sixpence ; suspended from
its edge, the plate thus formed set its length, which coin-
cided with the axis of the previous cylinder, axial, and its
shortest dimension equatorial.

The plate was then cut into little squares, which were
laid one upon the other and pressed together to a
compact cubical mass. Two such cubes were placed upon
the torsion-balance, and the repulsion in the line of com-
pression, and in a line perpendicular to the same, were
determined — the former was considerably the greater.

The crumb, scooped from a fresh roll, was placed between
the glass plates, and squeezed closely together ; after re-
maining in the vice for half an hour, a rectangle was taken
from the plate thus formed, and suspended from its edge in

REVERSAL OF MAGNE-CRYSTALLIC ACTION. 77

the magnetic field ; it set like a magnetic body, with its
length from pole to pole. The mass was diamagnetic, its
line of compression was repelled, and an apparent attraction
of the plate was the consequence.

Fine wheat-flour was mixed with distilled water into a
stiff paste, and the diamagnetic mass was squeezed into
thin cakes. The cakes when suspended from the edges set
always with their longest dimension from pole to pole, the
line of compression being equatorial.

Eye-flour, from which the Germans make their black
bread, was treated in the same manner and with the same
result.

I have an oblong plate of shale from the neighbourhood
of Blackburn in Lancashire, which imitates M. Pliicker's
first experiment with tourmaline with perfect exactitude.
The mass is magnetic, like the tourmaline. Suspended
from the centre of one of its edges, it sets axial ; this cor-
responds to the position of the tourmaline when the optic
axis is vertical. Suspended from the centre of the adjacent
edge, it sets even more strongly equatorial ; this corre-
sponds with the tourmaline when the optic axis is horizontal.
If the eyes be closed, and the respective positions of the
plate of shale ascertained by means of touch, and if the same
be done with Pliicker's plate of tourmaline, it will be im-
possible to distinguish the one deportment from the other.

With regard to tae experiment with the cherry-tree
bark, I have a bar of chemically pure bismuth which does
not contain a trace of magnetism, and which exhibits the
precise phenomena observed with the bark. These pheno-
mena do not therefore necessitate the hypothesis of two
conflicting forces, the one or the other of which predomi-
nates according as the poles of the magnet are more or less
distant. I have already commenced an investigation in
which the deportment of the bark and other phenomena
of an analogous nature will be more fully discussed.

78 DIAMAGNETISM AND MAGNE-CKYSTALLIC ACTION",

Every inquirer who has occupied himself experiment-
ally with electro-magnetic attractions must have been
struck with the great and speedy diminution of the force
by which soft iron is attracted, when the distance is aug-
mented, in the immediate neighbourhood of the poles. In
experiments with spheres of soft iron, I have usually found
that a distance of y-J-jj th of an inch between the sphere and
the magnet is sufficient to reduce the force with which the
former is attracted to -j^th of the attraction exerted when
the sphere is in contact. To any one acquainted with this
fact, and aware, at the same time, of the comparative
sluggishness with which a bismuth ball moves in obedience
to the repulsive force even when close to the poles, a law
the exact reverse of that affirmed by Pliicker must appear
exceedingly probable.

The bismuth balls were placed upon the torsion balance ;
on the top of one of them a particle of iron filing was
fixed, and with this compound mass the space opposite to
a core excited by a current of 50° was sounded. The beam
was brought by gentle pushing into various positions, some-
times close to the magnet, sometimes distant. The position
of equilibrium for the beam when the core was un excited
was always zero. When the beam was pushed to a distance
of 4° (about y^ths of an inch) from the core end, on excit-
ing the magnet it receded still further and rested against
a stop at 9° distant. When the current was interrupted
the beam left the stop and approached the core ; but if,
before it had attained the third or fourth degree, the
circuit was closed, the beam was driven back and rested
against the stop as before.

Preserving the current constant at 50°, the index of
the torsion-head was turned gently against the repulsion,
and in this way the ball was caused slowly to approach the
magnet. The repulsion continued until the glass fibre of
the beam pointed to 2° ; here an attractive force suddenly

ANOMALIES EXAMINED. 70

manifested itself, the ball passed speedily on to contact
with the core end, to separate it from which a torsion of
50° was requisite.

The circuit was broken and the beam allowed to come
to rest at zero, a space of about y¥th of an inch inter-
vening between the ball and the end of the magnet ; on
closing the circuit the beam was attracted. The current
was once more interrupted, and the torsion-head so ar-
ranged, that the beam came to rest at 3° distant ; on
establishing the current again the beam was repelled.
Between 0° and 3° there was a position of unstable equili-
brium for the beam ; from this place to the end of the
magnet attraction was triumphant, beyond this place repul-
sion prevailed.

Here we see, that on approaching the pole, the attraction
of the magnetic particle mounts much more speedily than
the repulsion of the diamagnetic ball ; a result the reverse of
that arrived at by the learned Professor, but most certainly
coincident with what everybody who has closely studied
electro-magnetic attractions would expect. Shall we there-
fore conclude that 'magnetism ' increases more quickly than
1 diamagnetism ? ' The experiment by no means justifies
so wide a generalisation. If magnetism be limited to the
attraction of soft iron, then the above conclusion would be
correct ; but it is not so limited. Pliicker calls the attrac-
tion of .his watch-glass magnetism, the attraction of a salt
of iron bears the same name, and it so happens that the
attraction of a salt of iron on approaching the poles in-
creases incomparably more slowly than the attraction of
iron itself. The proof of this remarkable fact I will now
proceed to furnish.

From one end of a very fine balance a sphere of soft
iron, £th of an inch in diameter was suspended. Under-
neath, and about £th of an inch distant from the ball when
the balance stood horizontal, was the flat end of a straight

80 DIAMAGNETISM AND MAGNE-CKYSTALLIC ACTION.

electro-magnet. On sending a current of 30° through the
surrounding helix, the ball was attracted, and the force
necessary to effect a separation was measured : it amounted
to 90 grammes. A plate of thin window-glass was then
placed upon the end of the magnet, and the ball allowed
to rest upon it. The weight necessary to effect a separa-
tion, when the magnet was excited by the same current,
amounted to 1 gramme. Here an interval of about yg-th
of an inch was sufficient to reduce the attractive force to
o^th of that exerted in the case of contact.

A sphere of sulphate of iron, of somewhat greater
diameter than the iron ball, was laid upon one end of the
torsion-balance; the adjacent core was excited by a cur-
rent of 30°, and the force necessary to effect a separa-
tion of the core from the sphere was determined : it
amounted to 20° of torsion. The plate of glass used in the
last experiment was placed against the core end, and the
force necessary to effect a separation from it, with a cur-
rent of 30°, was also determined. The difference, which
in the case of the soft iron amounted to -|-§-ths of the
primitive attraction, was here scarcely appreciable. At a
distance of ygth of an inch the sphere of sulphate of iron
was almost as strongly attracted as when in immediate
contact.

Similar experiments were made with a pellet of car-
bonate of iron, and with the same result. At a distance
of ^-th of an inch the attraction was two-thirds of that
exerted in the ease of contact. An interval of yoVoth °f
an inch is more than sufficient to effect a proportionate
diminution in the case of soft iron.

A salt of iron in the immediate neighbourhood of the
poles behaves like iron itself at a considerable distance,
and the deportment of bismuth is exactly similar. A
slight change of position will make no great difference of
attraction in the one case or of repulsion in the other.

SUMMARY OF RESULTS. 81

To make the antithesis between magnetism and diamag-
netism perfect, we require a yet undiscovered metal,
which shall bear the same relation to bismuth, antimony,
sulphur, &c., which iron does to a salt of iron. Whether
nature has such a metal in store for the enterprising-
physicist, is a problem on which I will hazard no con-
jecture.

PRINCIPAL RESULTS OF THE FOREGOING INVESTIGATION.

1 . The repulsion of a diamagnetic substance placed
at a fixed distance from the pole of a magnet is governed
by the so/me law as the attraction of a magnetic sub-
stance.

2. The entire mass of a magnetic substance is most
strongly attracted when the attracting force acts parallel
to that line which sets axial when the substance is sus-
pended in the magnetic field ; and the entire mass of a
diamagnetic substance is most strongly repelled when
the repulsion acts parallel to the lime which sets equa-
torial in the magnetic field.

3. The superior attraction and repulsion of the mass
in a particular direction is due to the fact, that in this
direction the material particles are ranged more closely
together than in other directions ; the force exerted being
attractive or repulsive according as the particles are
magnetic or diamagnetic. This is a law applicable to
matter in general, the phenomena exhibited bv crystals

the magnetic field being particular manifestations of
the same.

BEBLLN : June, 1851.

82 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

ADDITIONS AND REMARKS, 1870.
Poisson's prediction of Magne-crystallic action.

In March 1851, Professor, now Sir William Thomson,
drew attention to an exceedingly remarkable instance of
theoretic foresight on the part of Poisson, with reference
to the possibility of magne-crystallic action.

' Poisson,' says Sir William, ' in his mathematical
theory of magnetic induction, founded on the hypothesis
of magnetic fluids, " moving within the infinitely small
magnetic elements," of which he assumes magnetisable
matter to be constituted, does not overlook the possibility
of those magnetic elements being non-spherical and sym-
metrically arranged in crystalline matter, and he remarks
that a finite spherical portion of such a substance would,
when in the neighbourhood of a magnet, act differently
according to the different positions into which it might
be turned with its centre fixed. But " such a circum-
stance not having yet been observed," he excludes the
consideration of the structure which would lead to it
from his researches, and confines himself in his theory of
magnetic induction to the case of matter consisting either
of spherical magnetic elements or of non-symmetrically
disposed elements of any forms. Now, however, when
a recent discovery of Pliickor's has established the very
circumstance, the observation of which was wanting to
induce Poisson to enter upon a full treatment of the sub-
ject, the importance of working out a magnetical theory of
magnetic induction is obvious.'

Sir William Thomson then proceeds to make the
necessary ' extension of Poisson's mathematical theory of
magnetic induction ' ; and he publishes the following
striking quotation : —

* La forme des elemens pourra aussi influer sur cette

TOISSONS PREDICTION. 83

intensite ; et cette influence aura cela de particulier,
qu'elle ne sera pas la meme en des sens differens. Suppo-
sons, par exemple, que les elemens magnetiques sont des
ellipsoi'des dont les axes ont la meme direction dans toute
1'etendue d'un meme corps, et que ce corps est line sphere
aimante"e par influence, dans laquelle la force coercitive
est nulle ; les attractions ou repulsions qu'elle exercera an
dehors seront differentes dans le sens des axes de ces
elemens et dans tout autre sens ; en sorte que si 1'on fait
tourner cette sphere sur elle-meme, son action sur un
meme point changera, en general, en grandeur et en
direction. Mais si les elemens magnetiques sont des
spheres de diametres egaux ou inegaux, ou bien s'ils
ecartent de la forme spherique, mais qu'ils soient disposes
sans aucune regularite dans 1'interieur d'un corps aimante
par influence, leur forme n'influerait plus sur les resultats,
qui dependront seulement de la somme de leurs volumes,
comparee au volume entier de ce corps, et qui seront alors
les memes en tout sens. Ce dernier cas est celui du fer
forge, et sans doute aussi des autres corps non cristallises
dans lesquels on a observe le magnetisme. Mais il serait
curieux de chercher si le premier cas n'aurait pas lieu
lorsque ces substances sont cristallisees ; on pourrait
s'assurer par 1'experience soit en approchant un cristal
d'une aiguille aimantee, librement suspendue, soit en
faisant osciller de petites aiguilles taillees dans des cristaux
en toute sorte de sens, et soumises a Faction d'un tres-fort
aimant.' (Mem. de 1'Institut, 1821-22. Paris, 1826.)

Subsequent to the foregoing inquiries, I had a power-
ful and delicate torsion-balance constructed for me by Mr.
Becker, and in the autumn of 1855, 1 examined with it the
differential attractions and repulsions of large additional
number of crystals and compressed substances.

Dichroite was one of the crystals then examined. It
was magnetic. The form was a cube with two pairs of

84 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

faces parallel to the crystallographic axis, and one pair
perpendicular to it. The crystal was found to possess
three magnetic axes of unequal values. Measured twice in
each case by the torsion-balance the attraction of the mass
along the three axes respectively was —

Least axis Middle axis Greatest axis

222 293 300

225 288 300

Mean . . 223-5 290-5 300

When the crystal \vas suspended from its centre of
gravity with the least and greatest axes horizontal, the
rapidity of its vibration was greater than when the inter-
mediate axis was pitted against either of the two others.
Depending as it did upon the differential induction, the
rate of vibration ought of course to be highest where the
difference is greatest.

Various other crystals possessing three magnetic axes
were examined at the time here referred to. The deport-
ment when suspended from their centres of gravity in the
magnetic field was always in harmony with the differential
attractions and repulsions of the mass as measured by
the torsion -balance. Numerous compressed substances were
also examined, and their deportment on the torsion-
balance compared with their deportment in the magnetic
field. As far as the experiments extended the harmony
observed in the case of crystals was exhibited here also.

It would give me great pleasure to go again over the
ground traversed in the preceding papers. The experi-
ments, I think, are secure ; but I should like to review the
molecular theory of the whole subject, and examine still
further the remarkable variations of magnetic capacity
produced by mechanical strains and pressures. In 1855 a
great number of experiments were made on compressed
powders, but I was deflected from the subject immediately
afterwards, and from 1856 to the present time I have

MUTUAL INDUCTION OF PARTICLES. 85

been unable to bestow any attention on the subject of
diamagnetism. A rich reward is probably here in store
for the young investigator.

In the foregoing pages, the mutual inductive action of
the particles of carbonate of iron is referred to. Their
shape ought also to be taken into account. From a long
list of experiments I will take one which bears upon this
point.

Pure white wax is strongly diamagnetic. When
squeezed between clean plates it always sets the line of
compression equatorial in the magnetic field.

A crystal of pure carbonate of iron was pounded to an
extremely fine powder in a mortar. The finger and thumb
were dipped into the mixture, and the powder adhering to
them was in great part brushed away by mutual friction.
The minute residue was mixed with a quantity of white
wax. The mass was then squeezed ; square plates were
taken from the flattened mass, and laid one upon another
to form a cube. Suspended in the magnetic field it set
the line of compression axial.

When the smallness of the quantity of magnetic
powder here employed and its extremely sparse diffusion
in the mass of the wax are taken into consideration, it can
hardly be supposed that the setting of the line of compres-
sion axial was due to the mutual induction of the particles.
It is, perhaps, more probable that the pressure brought
the axes of the minute crystals composing the dust into
partial parallelism with the line of compression. This
would be the natural result of the shape of the particles.
The longest dimension would tend to set perpendicular to
the direction of pressure, and this, in the particular case
before us, would bring the direction of maximum magne-
tisation parallel to the same line. The surmise of Sir
William Thomson may, in this case, be justified.

But though this action may occur in the case of

SO D1AMAGNET1SM AND MAGNE-CRYSTALLIC ACTION.

carbonate of iron, it fails in its application to compressed
bismuth crystals. There is nothing in the structure of
the crystal to warrant the notion that the effect of
compression is merely to re-arrange the particles. By
mechanical pressure a new magnetic capacity is here
superinduced.

Three other cubes were formed of the wax in the
manner above described, the wax being kneaded in the
three respective cases with increasing quantities of the
carbonate of iron. The mixture was then compressed,
and it was found that the adherence of the line of com-
pression to the line joining the poles became stronger as
the quantity of the carbonate of iron dust was increased.

But now a curious effect is to be mentioned which
needs further examination. A quantity of very fine oxide
of iron was mixed with the powder of the carbonate, and
the smallest pinch of the mixture was kneaded into a
lump of wax. Cubes were formed of the substance in the
usual manner. But while the pure carbonate always
caused the line of compression to set axial ; the admixture
of the oxide entirely changed this deportment, and caused
the direction of pressure to set equatorial.

Three other cubes were formed containing gradually
increasing quantities of the oxide. In all cases the line
of compression set equatorial.

A class of results of which this is a type was forced on
my attention by the anomalous behaviour of the carbonate
of iron in certain cases. The line of compression some-
times sets axial, sometimes equatorial ; the discrepancies
being finally traced to the oxide which adhered here and
there as a crust to the pure crystal. A great number of
different powders were thus examined ; and indeed, iron
itself was reduced to powder in various ways. The greatest
difficulty in these experiments arose from the fact that in
strongly magnetic substances the slightest elongation of

HEAT AND MAGNETISM IN ROCK-CRYSTAL. 87

the particle was sufficient to determine its position. The
coercive force of all magnetic powders was also a source of
confusion and difficulty.

At the time here referred to I also tried various ex-
periments with a view of connecting calorific conduction
with magnetic induction. Heat and magnetism do not
seem to be operated upon equally by molecular arrange-
ment. By a beautiful and simple mode of experiment, de
Senarmont has shown that crystals conduct heat differently
in different directions, and one of the best examples of this
difference is furnished by rock-crystal. Coating a plate
of the substance with wax, and passing through the plate
a heated wire, the heat communicated to the crystal
melts the wax into an oval, the longest axis of which is
parallel to the axis of the crystal.1 As regards heat the
differential action is specially striking, but hardly any
crystal is more inactive than quartz in the magnetic
field. Hence the state of the ether, or of the molecules,
which produces great differences as regards calorific con-
duction, may produce no sensible difference as regards mag-
netic induction. Sulphate of baryta has, according to de
Senarmont, sensibly the same calorific conductivity in all
directions ; but it has three unequal axes of magnetic
induction ; two parallel to the two diagonals of the base,
and an intermediate one parallel to the axis of the prism.

The ratio of the two axes of the ellipse in rock-crystal is
as 131 : 100 ; while in calcite, which is far more energetic in
the magnetic field, the ratio is only as 111 : 100. In cal-
cite, moreover, the direction of greatest calorific conduction
is also that of highest diamagnetic induction, while in
selenite the case is reversed. In transparent tourmaline the
direction of minimum calorific conduction is parallel to the
axis ; this, at all events in coloured magnetic crystals, is the

1 Annales de Chimie et de Physique, vol. xxi. p. 457, also vol. xxii.
Heat as a Mode of Motion., 3rd edition, page 202.

88 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

direction of maximum magnetic induction. De Senarmont
says, * It is remarkable to observe that quartz, the optical
constants of which differ little among themselves, com-
pared with those of calc-spar, possesses on the contrary
conductibilities which differ far more than those of the
spar.' * The magnetic deportment of quartz is more
analogous to its optical than to its calorific deportment.
A similar remark applies to selenite. As soon as I can
command the necessary time, I shall examine whether
there is any general relation here.

1 Annales de Cliimie et de Physique, vol. xxviii. p. 279.

POLARITY OF THE DIAMAGNETIC FORCE.

Introduction, 1870.

SOON after the discovery of diamagnetism, Professor Reich,
of Freiburg, made the following very important ex-
periment. Placing a ball of bismuth on a torsion-balance
which had been previously employed in determinations of
the density of the earth, he found that ' magnet bars, on
being brought up in a horizontal direction to the case near
the ball, produced a very distinct repulsion, both when
the north and the south pole were brought near. But
when several similar bars were brought near, half with
their north and the other half with their south poles, there
was no effect perceptible, or merely a slight one arising
from the inequality of the magnets employed.'1 Prof. W.
Weber2 immediately saw the bearing of this result on
the character of diamagnetism. * From this single experi-
ment,' he says, ' it might be concluded with the greatest
probability that the origin of the diamagnetic force is not
to be sought for in the never-changing metallic particles
of the bismuth, but in an imponderable constituent moving
between them, which on the approach of the pole of a
magnet is displaced and distributed differently according
to the character of this pole.' He then inquires into the
nature of this imponderable constituent, and into its bear-
ing on the view first enunciated by Faraday, that dia-

1 Poggendorff's Annalen, vol.' Ixxiii. p. CO ; Phil. Mag. vol. xxxiv.
p. 127.

'Poggendorff's Annalen, January 7, 1818; Taylor's Scientific
Memoirs, vol. v. p. 477.

90 DIAMAGNETISM AND MAGNE-CEYSTALLIC ACTION.

magnetism might be explained by assuming the existence
of a polarity the reverse of that of magnetism. He
subjects the view to an experimental test, and shows that
a bar of bismuth which at a certain distance had no sensible
action on a magnetic needle, did exert an action on
the same needle when placed between the poles of a power-
ful magnet.1 'Between the two poles of the horseshoe
magnet,' writes Weber, ' a very perceptible and measurable
effect is exhibited, viz., a deflection of the needle, owing
to one pole being repelled and the other attracted.' He
found that when the poles of the influencing magnet were
reversed, the deflection produced by the bismuth was
reversed also ; and that when a piece of iron was substituted
for the bismuth, the deflection produced by the magnetic
metal was opposite to that produced by the diamagnetic
one. Hence he concluded that Faraday's hypothesis was
proved. To render the proof more complete, Weber made
an exceedingly skilful arrangement to show that induced
currents were excited by the diamagnetisation of bismuth
as well as by the magnetisation of iron. The proof of
diamagnetic polarity appeared, therefore, to be complete.
Faraday, however, again took up the subject. Kef er-
ring to his hypothesis of diamagnetic polarity, he says
the view was ' received so favourably by Pliicker, Eeich,
and others, but above all by W. Weber, that I had great
hope it would be confirmed ; and though certain ex-
periments of my own did not increase that hope, still
my desire and expectation were in that direction.' 'It
appeared to me,' he continues, ' that many of the results
which have been supposed to indicate a polar condition,
were only consequences of the law that diamagnetic bodies
tend to go from stronger to weaker places of magnetic

1 The action of the magnetic poles upon the suspended needle was
neutralised'by a second magnet, the needle being thus rendered suffi-
ciently sensitive to respond to the action of the bismuth.

INTRODUCTION, 1870. 91

action.' In a paper of great experimental power, he
demonstrates that the induced currents ascribed by Weber
to the diamagnetisation of bismuth were probably due to a
totally different cause ; and with regard to Weber's experi-
ment with the bar of bismuth placed between the poles of
a magnet, Faraday says, * I have repeated this experiment
most anxiously and carefully, but have never obtained the
slightest trace of action with the bismuth. I have obtained
action with the iron ; but in those cases the action
was far less than if the iron were applied outside, between
the horseshoe magnet and the needle, or to the needle
alone, the magnets being entirely away. On using a
garnet, or a weak diamagnetic substance of any kind, I
cannot find that the arrangement is at all comparable,
for readiness of indication or delicacy, with the use of
a common or an astatic needle, and therefore I do not un-
derstand how it could become a test of the polarity of
bismuth when these fail to show it.'

' Finally,' he continues, ' I am obliged to say that I
can find no experimental evidence to support the hypo-
thetical view of diamagnetic polarity, either in my own
experiments, or in the repetition of those of Weber, Reich,
or others. I do not say that such a polarity does
not exist, and I should think it possible that Weber,
by far more delicate apparatus than mine, had obtained
a trace of it, were it not that then also he would have cer-
tainly met with the far more powerful effects produced by
copper, gold, silver, and the better conducting diamag-
netics.'

In a very exhaustive and beautiful memoir translated
by myself from PoggendorfFs Annalen, vol. Ixxxvii., p.
145,1 Professor Weber returns to the subject of dia-

1 Scientific Memoirs, published by Taylor & Francis, New Series,
vol. i. p. 163.

92 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

magnetism, and considers four possible assumptions to
account for the origin of the diamagnetic effects : —

1 . The internal cause of such effects may be referred
to the existence of two magnetic fluids which are more or
less independent of the ponderable matter which carries
them.

2. They may be due to the existence of two magnetic
fluids, which are only capable of moving in con-
nexion with their ponderable carriers (rotatory molecular
magnets).

3. They may be due to the existence of permanent
molecular currents formed by the electric fluids, and which
rotate with the molecules.

4. They may be due to the existence of electric fluids,
which can be thrown into molecular currents.

Weber decides in favour of the fourth hypothesis. He
supposes that by the act of magnetisation molectilar
currents are generated in diamagnetic bodies ; which
currents, like those of Faraday, have a direction op-
posed to that of their generators. But Faraday's currents
are of vanishing duration, being immediately extinguished
by the resistance of the conductors through which they
move. Diamagnetism, however, would require per-
manent molecular currents to account for it. Weber
secures this permanence by supposing that the induced
molecular currents move in channels of no resistance *
round the molecules. This assumption enables him to
link all the phenomena of diamagnetism together in a
satisfactory manner. While recognising the extreme
beauty of the hypothesis, I should hesitate to express a
belief in its truth.

Weber also again applied his wonderful experimental
skill to the subject of currents induced by the act of

1 This, indeed, is involved in Ampere's theory of molecular currents.
Bee Letter of Prof. Weber further on.

INTRODUCTION, 1870. 93

diamagnetisation ; and in my opinion, fairly met all the
requirements of the case ; but neither his labours nor those
of Poggendorff and Pliicker produced conviction in the
mind of Faraday. The notion of a distinct diamagnetic
polarity was also opposed by others. Prof, von Feilitzsch,
for example, contended, on theoretic grounds, and backed
his contention by definite experiments, that the magnetic
excitement of bismuth and of iron were one and the same.
This was also the view of M. Becquerel. Matteucci sub-
sequently entered the field as an ardent opponent of
diamagnetic polarity.

The investigations recorded in the Third, Fourth,
Fifth, and Sixth Memoirs, but mainly in the three last,
are directed to the complete clearing up of this subject.

94 DIAMAGNET1SM AND MAGNE-CRYSTALLIC ACTION.

THIED MEMOIR.

ON THE POLARITY OF BISMUTH, INCLUDING AN
EXAMINATION OF THE MAGNETIC FIELD.1

THE polarity of bismuth is a subject on which philo-
sophers have differed and on which they continue to
differ. On the one side we have Weber, Poggendorff,
and Pliicker, each affirming that he has established this
polarity ; on the other side we have Faraday, not affirm-
ing the opposite, but appealing to an investigation which
is certainly calculated to modify whatever conviction the re-
sults of the above-named experimenters might have created.
It will probably have occurred to those occupied experi-
mentally with diamagnetic action that, whenever the
simple mode of permitting the body experimented with
to rotate round an axis passing through its own centre of
gravity, can be applied, it is preferable in point of delicacy
to all others. A crystal of calcareous spar, for example,
when suspended from a fine fibre between the poles,
readily exhibits its directive action, even in a field of
weak power ; while to establish that peculiar repulsion of
the mass which is the cause of the directive action, even
with high power and with the finest torsion-balance, is
a matter of considerable difficulty. In the knowledge of
this and in the fact of my having a piece of bismuth,
whose peculiar structure suggested the possibility of sub-
mitting the question of diamagnetic polarity to a new
test, the present brief inquiry originated.
1 Phil. Mag., Nov. 185\.

POLARITY. OF BISMUTH : FIRST GROPINGS. 95

In December 1847 a paper on 'Diamagnetic Polarity'
was read before the Academy of Sciences in Berlin by
Professor Poggendorff, the result arrived at by the writer
being, that a bismuth bar, suspended horizontally and
occupying the equatorial position between two excited
magnetic poles, was transversely magnetic — that side of
the bar which faced the north pole possessing north
polarity, and that side which faced the south pole
possessing soutli polarity; the excitation being thus the
opposite of that of iron, and in harmony with the original
conjecture of Faraday.

The method adopted by Poggendorff was as fol-
lows:— The bismuth bar was suspended within a helix of
copper wire, the coils of which were perpendicular to
the axis of the bar. The helix was placed between the
opposite poles of a magnet, so that the axis of the helix
was perpendicular to the line joining the poles. The
bismuth took up the usual equatorial position, its length
thus coinciding with the axis of the helix. On sending
an electric current through the latter the bar was weakly
deflected in a certain direction, and on reversing the
current, a feeble deflection in the opposite direction was
observed. The deflection was such as must follow from
the supposition, that the north pole of the magnet had
excited a north pole in the bismuth, and the south pole of
the magnet a south pole.

It will be at once seen that a considerable mechanical
disadvantage was connected with the fact that the distance
from pole to pole of the transverse magnet was very short,
being merely the diameter of the bar. If a piece of
bismuth, instead of setting equatorial, could be caused to
set axial, a mechanical couple of far greater power would
be presented to the action of the surrounding current.
Now it is well known that bismuth sets in the magnetic
field with the plane of most eminent cleavage equatorial :

96 DIAMAGNETISM AND MAGNE-CKYSTALLIC ACTION.

hence, if a bar of bismuth could be obtained with the said
plane of cleavage perpendicular to its length, the directive
power of such a bar might be sufficient to overcome the
tendency of its ends to proceed from stronger to weaker
places of magnetic action and to set the bar axial. After
repeated trials of melting and cooling in the laboratory of
Professor Magnus in Berlin, I succeeded in obtaining a
plate of this metal in which the plane of most eminent
cleavage was perpendicular to the flat surface of the plate,
and perfectly parallel to itself throughout. From this
plate a little cylinder, an inch long and 0-2 of an inch
in diameter, was cut, which being suspended horizontally
between the excited poles, turned strongly into the axial
position, thus behaving to all appearance as a bar of iron.

About 100 feet of copper wire overspun with silk were
wound into a helix so that the cylinder was able to swing
freely within it. Through a little gap in the side of the
helix a fine silk fibre descended, to which the bar was
attached ; to prevent the action of the bar from being dis-
turbed by casual contact with the little fibrous ends pro-
truding from the silk, a coating of thin paper was gummed
to the interior.

The helix was placed between the flat poles of an
electro-magnet, so that the direction of its coils was from
pole to pole. It being first ascertained that the bar
moved without impediment, and that it hung perfectly
horizontal, the magnet was excited by two of Bunsen's
cells ; the bar was immediately pulled into the axial line,
being in this position parallel to the surrounding coils.
A current from a battery of six cells was sent through the
helix, so that the direction of the current, in the upper
half of the helix, was from the south pole to the north
pole of the magnet. The cylinder, which an instant
before was motionless, was deflected, forming at the limit
of its swing an angle ot 70° with its former position ; the

POLARITY OF BISMUTH. 97

final position of equilibrium for the bar was at an angle
of 35°, or thereabouts, with the axial line.

Looking from the south pole towards the north pole of
the magnet, or in the direction of the current as it passed
over the bar, that end of the bar which faced the south
pole swung to the left.

The current through the helix being interrupted and
the bar brought once more to rest in the axial position
(which of course is greatly facilitated by the proper open-
ing and closing of the circuit), a current was sent through
in the opposite direction, that is from the north pole to
the south ; the end of the bar, which in the former experi-
ment was deflected to the left, was now deflected an equal
quantity to the right. I have repeated this experiment a
great number of times and on many different days with
the same result.

In this case the direction of the current by which the
magnet was excited was constant, that passing through
the helix which surrounded the bismuth cylinder being
variable. The same phenomena are exhibited if we pre-
serve the latter constant and reverse the former.

A polar action seems undoubtedly to be indicated here ;
but if a polarity be inferred, it must be assumed that the
north pole of the magnet excites a south pole in the
bismuth, and the south pole of the magnet a north pole
in the bismuth ; for by reference to the direction of the
current and the concomitant deflection, it will be seen
that the deportment of the bismuth is exactly the same as
that which a magnetised needle freely suspended between
the poles must exhibit under the same circumstances.

The bar of bismuth was then removed, and a little bar
of magnetic shale vas suspended in its stead ; it set axial.
On sending a current through the surrounding helix, it was
deflected in the same manner as the bismuth. The piece
of shale was then removed and a little bar of iron was sus-

98 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

pended within the helix ; the residual magnetism which
remained in the cores after the cessation of the exciting
current was sufficient to set the bar axial ; a very feeble
current was sent through the helix and the deflection
observed — it was exactly the same as that of the bismuth
and the shale.

These results being different from those obtained by M.
Poggendorff, I repeated his experiment with all possible
care. A bar of ordinary bismuth, an inch in length and
about 0'2 of an inch in diameter, was suspended within the
helix ; on exciting the magnet, it receded to the equator,
and became finally steady there. The axis of the bar thus
coincided with the axis of the helix. A current being sent
through the latter, the bar was distinctly deflected. Sup-
posing an observer to stand before the magnet, with the
north pole to his right and the south pole to his left, then
when a current passed through the upper half of the coil
from the north to the south pole, that end of the bismuth
which was turned towards the observer was deflected
towards the north pole ; and on reversing the current, the
same end was deflected towards the south pole. This
seems entirely to agree with the former experiment. When
the bar hung equatorially between the excited poles, on
the supposition of polarity the opposite ends of all its
horizontal diameters were oppositely polarised. Fixing
our attention on one of these diameters, and supposing
that end which faced the north pole of the magnet to be
gifted with south polarity, and the end which faced the
south pole endowed with north polarity, we see that the
deportment to be inferred from this assumption is the same
as that actually exhibited ; for the deflection of a polarised
diameter in the same sense as a magnetic needle, is equi-
valent to the motion of the end of the bar observed in the
experiment.

The following test, however, appears to be more refined

POLARITY OF BISMUTH: FIRST GROPINGS. 99

than any heretofore applied. Hitherto we have supposed
the helix so placed between the poles that the direction of
its coils was parallel to the line which united them ; let us
now suppose it turned 90° round, so that the axis of the
helix and the line joining the poles may coincide. In this
position the planes of the coils are parallel to the planes in
which, according- to the theory of Ampere, the molecular
currents of the magnet must be supposed to move ; and we
have it in our power to send a current through the helix
in the same direction as these molecular currents, or in a
direction opposed to them. Supposing the bar first experi-
mented with suspended within the coil, and occupying the
axial position between the excited poles, a current in the
helix opposed to the molecular currents of the magnet
will, according to the views of the (rerman philosophers
named at the commencement, be in the same direction as
the currents evoked in the bismuth : hence such a current
ought to exert no deflecting influence upon the bar ; its
tendency, on the contrary, must be to make the bar more
rigid in the axial position. A current, on the contrary,
whose direction is the same as that of the molecular cur-
rents in the magnet, will be opposed to those evoked in the
bismuth: and hence, under the influence of such a current,
the bar ought to be deflected.

The bar first experimented with was suspended freely
within the helix, and permitted to come to rest in the axial
position. A current was sent through the helix in the
same direction as the molecular currents of the magnet,
but not the slightest deflection of the bar was perceptible ;
when, however, the current was sent through in the oppo-
site direction, a very distinct deflection was the conse-
quence : by interrupting the current whenever the bar
reached the limit of its swing, and closing it when the
bar crossed the axial line, the action could bo increased
to such a degree as to cause the bar to make an entire

100 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

rotation round the axis of suspension. This result is diame-
trically opposed to the above conclusion [as todiamagnetic
polarity] — here again the bismuth bar behaves like a bar
of iron.

These experiments seem fully to bear out the theory
advanced by von Feilitzsch in his letter to Faraday.1
He endeavours to account for diamagnetic action on the
hypothesis that its polarity is the same as that of iron ;
'only with this difference, that in a bar of magnetic sub-
stance the intensity of the distribution over the molecules
increases from the ends to the middle, while in a bar of
diamagnetic substance it decreases from the ends to the
middle.' So far as I can see, however, the reasoning of
von Feilitzsch necessitates the assumption, that in the self-
same molecule the poles are of unequal values, that the
intensity of the one is greater than that of the other, an
assumption which will find some difficulty of access into
the speculations of most physicists. A peculiar directive
action might be readily brought about by the distribution
of magnetism assumed by von Feilitzsch ; but up to the
present time I see no way of reconciling the repulsion of
the total mass of a piece of bismuth with the idea of a
polarity similar to that of iron.

During these inquiries, an observation of Faraday
perpetually recurred to me. ' It appeared to me,' he
writes,2 ' that many of the results which had been supposed
to indicate a polar condition were only consequences of
the law that diamagnetic bodies tend to go from stronger
towards weaker places of action.' The question here arose,
whether the various actions observed might not be explained
by reference to the change effected in the magnetic field
when it is intersected by an electric current. The distribu-
tion of magnetic intensity between the poles will perhaps
be rendered most clear by means of a diagram. Let A n
1 Phil. Mag., S. 4, vol. i. p. 46. 2 Phil. Mag., S. 3, vol. xxxvii. p. 89.

EXAMINATION OF MAGNETIC FIELD.

101

represent the distance between the polar faces ; plotting
the intensity at every point in A B as an ordinate from that
point, the line which unites the ends of all these ordinates
will express the magnetic distribution. Suppose this line
to be c d e. Commencing at A, the intensity of attraction
towards this face decreases as we approach the centre d,
and at this point it is equilibrated by the equal and oppo-
site attraction towards B. Beyond d the residual attrac-
tion towards A becomes negative, that is, it is now in the
direction of d B. The point d will be a position of stable
equilibrium for a diamagnetic sphere, and of unstable

FIG. 1.

equilibrium for a magnetic sphere. But if, through the
introduction of some extraneous agency, the line of distri-
bution be shifted, pay to c'd'ef, the point will be no longer
a position of equilibrium ; the diamagnetic sphere will move
from this point to d', and the magnetic sphere will move
to the pole A.

For the purpose of investigating whether any change of
this nature takes place in the magnetic field when an elec-
tric current passes through it, I attached a small sphere of
carbonate of iron to the end of a slender beam of light
wood ; and balancing it by a little copper weight fixed to
the other end, suspended the beam horizontally from a silk

102 DIAMAGNETISM AND MAGXE-CRYSTALLIC ACTION.

fibre. Attaching the fibre to a movable point of suspension,
the little sphere could be caused to dip into the interior of
the helix as it stood between the poles, and to traverse the
magnetic field as a kind of feeler. The law of its action
being that it passes from weaker to stronger places of
force, we have in it a ready and simple means of testing
the relative force of various points of action. The point of
the beam to which the fibre was attached being cut by the
axis of the helix produced, and the sphere being also on the
same level with the axis, when the magnet was excited1 it
passed into the position occupied by the defined line in

fig. 2, thus resting against the
FIG. 2. interior of the helix a little

within its edge. On sending
a current through the helix,
which in the upper half thereof
had the direction of the arrow,
the sphere loosed from its posi-
tion, sailed gently across the
field, and came to rest in the

position of the dotted line. If, while thus sailing, the
direction of the current in the helix, or of the current by
which the magnet was excited, were reversed, the sphere
was arrested in its course and brought back to its original
position. In like manner, when the position of the sphere
between the poles was that of the dotted line, a current
sent through the helix in a direction opposed to the arrow,
caused the sphere to pass over into the position of the
defined line.

The sphere was next introduced within the opposite
edge of the helix (fig. 3). On exciting the magnet, the
beam came to rest in the position of the defined line ; on

1 One of Bunsen's cells was found sufficient; when the magnetic
power was high, the change caused by the current was not sufficient to
deflect the beam.

EXAMINATION OF MAGNETIC FIELD.

103

FIG. 3.

FIG. 4.

s

1

2

N

4

3

sending a current through the helix in the direction of the
arrow, the sphere loosed, moved towards the north pole,
and came to rest in the dotted
position. If while in this posi-
tion either the current of the
magnet or the current of the
helix were reversed, the sphere
went back ; if both were reversed
simultaneously, the sphere stood
still.

From these facts we learn, that if the magnetic field be
divided into four compartments, as
in fig. 4, the passage of an electric
current through a helix placed there-
in (the direction of the current in
the upper half of the helix being that
indicated by the arrow) will weaken

the force in the first and third quadrants, but will
strengthen it in the second and fourth. With the aid of
this simple fact we can solve every experiment made
with the bismuth bars. For instance, it was found that
when an observer stood before the magnet with a
north pole to his right and a south pole to his left, a cur-
rent passing through the upper half of the helix from
the north to the south pole deflected a bar of ordinary
bismuth, which had previously stood equatorial, so that the
end presented to the observer moved towards the north
pole. This deportment might be inferred from the con-
stitution of the magnetic field ; the bar places its ends
in quadrants 1 and 3, that is, in the positions of weakest
force.

The experiments with the other bar are capable of
an explanation just as easy. Preserving the arrangement
as in the last figure, the bismuth bar, which previously
stood axial, would be deflected by the surrounding current,

104 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

so that its two ends would occupy the quadrants 2 and 4,
that is, the positions of strongest force. Now this is
exactly what they did in the magnetic field before the
passage of any current, for the bar set axial. It was first
proved by Faraday, that the mass of a bismuth crystal
was most strongly repelled when the repulsive force acted
parallel to the planes of most eminent cleavage ; and in the
magnetic field the superior repulsion of these planes causes
them always to take up that position where the force is a
minimum. It is the equatorial setting of these planes
which causes the bar at present under consideration to
set axial. The planes of cleavage being thus the true
indicators, we see that when these set from the first to the
third quadrant, or in the line of weakest action, the ends
of the bar must necessarily occupy the second and fourth,
which is the deportment observed.

The little test-sphere can also be made available
for examining the change brought about in the mag-
netic field by the introduction of a small bar of iron,
as in the experiment of Pliicker quoted by Faraday.1
Removing the helix from the magnetic field, the little
sphere was at liberty to traverse it from wall to wall.
When the magnet was excited, the sphere passed slowly
on to the pole to which it was nearest and came to rest
against it. When forcibly brought into the centre of
the magnetic field, after a moment's apparent hesitation
it passed to one pole or the other with a certain speed ;
but when a bar of iron was brought underneath while
it was central, this speed was considerably increased.
Over the centre of the bar there was a position of unstable
equilibrium for the sphere, from which it passed right
or left, as the case might be, with greatly increased
velocity. The distribution of the force appears in this
case to have undergone a change represented by the line
1 Phil. Mag., S. 3, vol. xxxvii. p. 101.

EXAMINATION OP MAGNETIC FIELD.

105

gef in the diagram. From the centre towards the poles
the magnetic tension
steepens suddenly, the Fia 6<

quicker recession of a 3
bismuth bar towards
the equator, as ob-
served by Pliicker,
being the consequence.

Assuming the law
of action for a small
magnetic sphere to be
that it proceeds from
weaker to stronger

places of force, we find that the passage of an electric
current in the manner described so modifies the 'field'
[between flat poles], that the positions of its two diagonals
are of unequal values as regards the distribution of the
force, the position of the field intersected by the diagonal
which bisects 1 and 3, fig. 4, being weaker than the por-
tion intersected by the diagonal which bisects 2 and 4.

But here the believer in diamagnetic polarity may enter
his protest against the use which we have made of the as-
sumption. ' I grant you,' he may urge, ' that in a simple
magnetic field, consisting of the space before and around a
single pole, what you assume is correct, that a magnetic
sphere will pass from weaker to stronger places of action ;
but for a field into which several distinct poles throw their
forces, the law by no means sufficiently expresses the state
of things. If we place together two poles of equal
strengths, but of opposite qualities, close to a mass of
iron, it is an experimental fact that there is almost no
attraction ; and if they operate upon a mass of bismuth,
there is no repulsion. Why ? Do the magnetic rays,
to express the thing popularly, annul each other by
a species of interference before they reach the body; or

106 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

does the one pole induce in the body the condition upon
which the second pole acts in a sense contrary to the
first, the two poles thus exactly neutralising each other?
If the former, then I grant you that the magnetic field
is rendered weaker, nay deprived of all force if you will,
by the introduction of the second pole ; but if the
latter, then we must regard the field as possessing two
systems of forces; and it is to the peculiar inductive
property of the body, in virtue of which one system
neutralises the other, that we must attribute the Absence
of attraction or repulsion. Once grant this, however, and
the question of diamagnetic polarity, so far as you are con-
cerned, is settled in the affirmative.'

Our hypothetical ' believer ' mentions it as ' an experi-
mental fact,' that if dissimilar poles of equal strengths
operate upon a mass of bismuth there is no repulsion.
This is Keich's result — a result which I have carefully
tested and corroborated. I will now proceed to show the
grounds which the believer in diamagnetic polarity
might urge in support of his last assertion. A twelve-
pound copper helix was removed from the limb of an
electro-magnet and set upright. A magnetised sewing-
needle being suspended from one end, the other end
was caused to dip into the hollow of the spiral, and to
rest against its interior surface. When a current was sent
through the helix in a certain direction, the needle was
repelled towards the axis of the coil; the same end of
the needle, when suspended at half an inch distance from
the exterior surface of the coil, was drawn strongly up
against it. When the current was reversed, the end of the
needle was attracted to the interior surface of the coil, but
repelled from its exterior surface. If we suppose a little
mannikin swimming along in the direction of the current,
with his face towards the axis of the helix, the exterior
surface of that end towards which his left arm would point

REICH S EXPERIMENT. 107

repels the north pole of a magnetic needle, while the
interior surface of the same end attracts the north pole.
The complementary phenomena were exhibited at the other
end of the helix. Thus if we imagine two observers placed
the one within and the other without the coil, the same end
thereof would be a north pole to the one and a south pole
to the other.

If we apply these facts to the case of the helix
within the magnetic field, we see that each pole of the
magnet had two contrary poles of the helix in contact
with it ; and we moreover find that the quadrants which
we have denominated the strongest are those in which the
poles of magnet and helix were in conjunction ; while the
quadrants which we have called weakest are those in which
the poles of magnet and helix were in opposition.

' Which will you choose ? ' demands our hypothetical
friend ; ' either you must refer the weakening of a quadrant
to magnetic interference, or you must conclude, that
that induced state, whatever it be, which causes the
bismuth to be repelled by the magnet, causes it to be
attracted by the coil, the resultant being the difference of
both forces. In the same manner the strengthening of
a quadrant is accounted for by the fact, that here the
induced state which causes the bismuth to be repelled
by the magnet causes it to be repelled by the coil also,
the resultant being the sum of both forces. The matter
may be stated still more distinctly by reference to Eeich's
experiments.1 Bringing a bundle of magnet-bars to bear
upon a diamagnetic ball suspended to the end of a torsion-
balance, he found that when similar poles were presented
to the body, there was a very distinct repulsion; but
that if one half of the poles were north and the other
half south, there was no repulsion. Let us imagine
the respective halves to be brought to bear upon the
1 Phil. Mag., S. 3. vol. xxxiv. p. 127,

108 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

ball consecutively ; the first half will cause it to recede to
a certain distance; if the second and unlike half be
now brought near, the ball will approach again, and
take up its original position. The question therefore
appears to concentrate itself into the following: — Is this
" approach " due to the fact that the magnetic forces
of the two halves annul each other before they reach
the ball, or is it the result of a compensation of inductions
in the diamagnetic body itself? If a sphere of soft
iron be suspended from a thread, the north pole of a
magnet will draw it from the plumb-line; if the south
pole of an exactly equal magnet be brought close to the
said north pole, the sphere will recede to the plumb-line.
Is this recession due to a compensation of inductions in
the sphere itself, or is it not ? If the former, then, by all
parity of reasoning, we must assume a similar compen-
sation on the part of the bismuth.'

That bismuth, and diamagnetic bodies generally, suffer
induction, will, I think, appear evident from the following
considerations. The power of a magnet is practically
ascertained by the mechanical effect which it is able to
produce upon a body possessing a constant amount of
magnetism, — a hard steel needle, for instance. The
action of a magnet in pulling such a needle from the
magnetic meridian may be expressed by a weight which
acts at the end of a lever of a certain length. By easy
practical rules we can ascertain when the pull of one
magnet is twice or half the pull of another, and in such a
case we should say that the former possesses twice or half
the strength of the latter. If, however, these two magnets,
with their powers thus fixed, be brought to bear upon a
sphere of soft iron, the attraction of the one will be four
times or a quarter that of the other. The strengths of the
magnets being, however, in the ratio of 1 : 2, this attrac-
tion of 1 : 4 can only be explained by taking into account

DIAMAGNETISM AN INDUCED STATE. 109

•

the part played by the iron sphere. We are compelled to
regard the sphere as an induced magnet, whose power is
directly proportional to the inducing one. Were the
magnetism of the sphere a constant quantity, a magnet of
double power could only produce a double attraction ; but
the fact of the magnetism of the sphere varying directly
as the source of induction leads us inevitably to the law
of squares ; and conversely, the law of squares leads us to
the conclusion that the sphere has been induced.

These sound like truisms ; but if they be granted,
there is no escape from the conclusion that diamagnetic
bodies are induced ; for it has been proved by M. E.
Becquerel and myself, that the repulsion of diamagnetic
bodies follows precisely the same law as the attraction of
magnetic bodies ; the law of squares being true for both.
Now were the repulsion of bismuth the result of a force
applied to the mass alone, without induction, then, with
a constant mass, the repulsion must be necessarily propor-
tional to the strength of the magnet. But it is proportional
to the square of the strength, and hence must be the pro-
duct of induction.

In order to present magnetic phenomena intelligibly
to the mind, a material imagery has been resorted to
by philosophers. Thus we have the 'magnetic fluids'
of Poisson and the ' lines of force ' of Faraday. For
the former of these Sir \V. Thomson has recently sub-
stituted an ' imaginary magnetic matter.' The distri-
bution of this 'matter' in a mass of soft iron, when
operated on by a magnet, has attraction for its result.
We have the same necessity for an image in the case
of bismuth. If we imagine the two magnetic matters
which are distributed by induction on a piece of iron to
change places, we have a distribution which will cause
the phenomena of bismuth. Hence it is unnecessary to
assume the existence of anv new matter in the case of

110 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

*

diamagnetic bodies, their deportment being accounted for
by reference to a peculiarity of distribution. Further,
the experiments of Eeich, which prove that the matter
evoked by one pole will not be repelled by an unlike pole,
compel us to assume the existence of two kinds of matter,
and this, if I understand the term aright, is polarity.

Note added, 1870. — The foregoing slight paper could have
very little influence on the decision of so weighty a question.
In the autumn of 1854 I therefore resumed the investigation
with a desire to exhaust, if possible, the experimental portion
of it. The following memoir contains an account of the inquiry.
I had previously been examining the influence of organic struc-
ture upon the display of magnetism ; and had also been engaged
with certain laws deduced by Pliicker from his experiments
as to the diminution of magnetism and diamagnetism with the
distance. The account of these experiments precedes the real
inquiry into the relations of magnetism to diamagnetism, and
ought, perhaps, to have been published by itsel£

FOUETH MEMOIR.

ON THE NATURE OF THE FOECE BY WHICH
BODIES AEE EEPELLED FROM THE POLES
OF A MAGNET.1

Introduction.

FROM the published account of his researches it is to be
inferred, that the same heavy glass, by means of which he
first produced the rotation of the plane of polarisation
of a luminous ray, also led Faraday to the discovery of
the diamagnetic force. A square prism of the glass, 2
inches long, and 0-5 of an inch thick, was suspended with
its length horizontal between the two poles of a powerful
electro-magnet : on developing the magnetism the prism
moved round its axis of suspension, and finally set its
length at right angles to a straight line drawn from the
centre of one pole to that of the other. A prism of ordinary
magnetic matter, similarly suspended, would, as is well
known, set its longest dimension from pole to pole. To
distinguish the two positions here referred to, Faraday
introduced two new terms, which have since come into
general use: he called the direction parallel to the
line joining the poles, the axial direction, and that per-
pendicular to the said line, the equatorial direction.

The difference between this new action and ordinary
magnetic action was further manifested when a frag-
ment of the heavy glass was suspended before a single

1 Phil. Trans. 1855, p. 1 : being the Bakerinn Lecture.

112 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

electro-magnetic pole : the fragment was repelled when the
magnetism was excited. To the force which produced this
repulsion Faraday gave the name of diamagnetism.

Numerous other substances were soon added to the
heavy glass, and, among the metals, it was found that
bismuth possessed the new property in a comparatively
exalted degree. A fragment of this substance was forcibly
repelled by either of the poles of a magnet ; while a thin
bar of the substance, or a glass tube containing the bismuth
in fragments, or in powder, suspended between the two
poles of a horseshoe magnet, behaved exactly like the
heavy glass, and set its longest dimension equatorial.

These exhaustive researches, which rendered manifest
to the scientific world the existence of a pervading
natural force, glimpses of which merely had been pre-
viously obtained by Brugmans and others, were made
public at the end of 1845 ; and in 1847 Pliicker
announced his beautiful discovery of the action of a
magnet upon crystallised bodies. His first result was,
that when any crystal whatever was suspended between
the poles of a magnet, with its optic axis horizontal, a
repulsive force was exerted on the axis, in consequence of
which it receded from the poles and finally set itself at
right angles to the line joining them. Subsequent experi-
ments, however, led to the conclusion, that the axes of
optically negative crystals, only, experienced this repul-
sion, while the axes of positive crystals were attracted ; or,
in other words, set themselves from pole to pole. The
attraction and repulsion, here referred to, were ascribed
by Pliicker to the action of a force, independent of the
magnetism or diamagnetism of the mass of the crystal.1

1 ' The force which produces this repu Ision is independent of tJie magnetic
or diamagnctic condition of the mass of the crystal ; it diminishes less, as
tlie distance from the poles of the magnet increases, than the magnetic and
diamagnetic forces emanating from tlicse poles and acting upon the crystal.'

PREFATORY REMARKS. 113

Shortly after the publication of Pliicker's first me-
moir, Faraday observed the remarkable magnetic pro-
perties of crystallised bismuth ; and his researches upon
this, and other kindred points, formed the subject of the
Bakerian Lecture before the Royal Society for the year
1849.

Through the admirable lectures of Professor Bunsen on
Electro-chemistry in 1848, I was first made acquainted
with the existence of the diamagnetic force ; and in the
month of November 1849 my friend Professor Knoblauch,
then of Marburg, now of the University of Halle, sug-
gested to me the idea of repeating the experiments of
Pliicker and Faraday. He had procured the necessary
apparatus with the view of prosecuting the subject him-
self, but the pressure of other duties prevented him from
carrying out his intention. I adopted the suggestion and
entered upon the inquiry in M. Knoblauch's cabinet. Our
frequent conversations upon the subject led naturally to
our making it a joint investigation. We published our

— Prof. Pliicker in Poggendorff's Annalcn, vol. Ivii. No. 10 ; Taylor's
Scientific Memoirs, vol. v. p. 353.

The forces emanating from the poles of a magnet are thus summed
up by Pliicker : —

1st. The magnetic force in a strict sense.

2nd. The diamagnetic action discovered by Faraday^

3rd. The action exerted on the optic axes of crystals (and that pro-
ducing the rotation of the plane of polarisation which probably corre-
sponds to it). The second diminishes more with the distance than the
Jirzt, and the first more than, the third.- Taylo.v's Scientific Memoirs,
vol. v. p. 380.

The crystal (cyanite) does not point according to the magnetism of
its substance, but only in obedience to the magnetic action upon its optio
axes. — Letter to Faraday, Phil. Mag. vol. xxxiv. p. 451. The italics in
all cases are Pliicker's own.

De la Rive states the view of Pliicker to be: — 'that the axis in
its quality as axis, and independently of the very nature of the
substance of the crystal, enjoys peculiar properties, more frequently in
opposition to those possessed by the substance itself, or which at least
are altogether independent of it.' — Treatise on Electricity, vol. i.
p. 359.

114 DIAMAGNETISM AND MAGNE-CKYSTALLIC ACTION.

rssults in two papers, the first of which, containing a briet
account of some of the earliest experiments, appeared in
the ' Philosophical Magazine' for March 1850, and some
time afterwards in Poggendorffs Annalen i while the
second and principal memoir appeared in the ' Philoso-
phical Magazine' for July 1850, and in Poggendorffs
Annalen about January 1851.1 I afterwards continued
my researches in the private laboratory of Professor Mag-
nus of Berlin, who, with prompt kindness and a lively
interest in the furtherance of the inquiry, placed all
necessary apparatus at my disposal. The results of this
investigation are described in a paper published in the
4 Philosophical Magazine' for September 1851, and in
Poggendorffs Annalen, vol. Ixxxiii.

In these memoirs it was shown that the law according
to which the axes of positive crystals are attracted and
those of negative crystals repelled, was contradicted by the
deportment of numerous crystals both positive and nega-
tive. It was also proved that the force which determined
the position of the optic axes in the magnetic field was
not independent of the magnetism or diamagnetism of the
mass of the crystal ; inasmuch as two crystals, of the same
form and structure, exhibited altogether different effects,
when one of them was magnetic and the other diamag-
netic. It was shown, for example, that pure carbonate of
lime was diamagnetic, and always set its optic axis equa-
torial; but that when a portion of the calcium was
replaced by an isomorphous magnetic constituent, which
neither altered the structure nor affected the perfect
transparency of the crystal, the optic axis set itself from
pole to pole. The various complex phenomena exhibited

1 The memoirs in the ' Philosophical Magazine' were written by
myself, and the second one has, I believe, been translated into German
by Dr. Kronig ; the papers in Poggendorff's Annalen were edited by
Knoblauch.— J. T.

PEEFATORY REMARKS. 115

by crystals in the magnetic field were finally referred to
the modification of the magnetic and diamagnetic forces
by the peculiarities of molecular arrangement.

This result is in perfect conformity with all that we
know experimentally regarding the connection of matter
and force. Indeed it may be safely asserted that every
force which makes matter its vehicle of transmission must
be influenced by the manner in which the material
particles are grouped together. The phenomena of double
refraction and polarisation illustrate the influence of mo-
lecular aggregation upon light. Wertheim has shown that
the velocity of sound through wood, along the fibre, is
about five times its velocity across the fibre : De la Rive,
de Candolle, and myself have shown the influence of the
same molecular grouping upon the propagation of heat.
In the first section of the present memoir, the influence
of the molecular structure of wood upon its magnetic de
portment is described : De Senarmout has shown that the
structure of crystals endows them with different powers
of calorific conduction in different directions : Knoblauch
has proved the same to be true, with regard to the
transmission of radiant heat : Wiedemann finds the pas-
sage of frictional electricity along crystals to be affected
by structure ; and some experiments, which I have not
yet had time to follow out, seem to prove, that bismuth
may, by the approximation of its particles, be caused to
exhibit, in a greatly increased degree, those singular effects
of induction which are so strikingly exhibited by copper,
and other metals of high conducting power.

Indeed the mere a priori consideration of the subject
must render all the effects here referred to extremely
probable. Supposing the propagation of the forces to
depend upon a subtle agent, distinct from matter, it is
evident that the progress of such an agent from particle to

116 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

particle must be influenced by the manner in which these
particles are arranged. If the particles be twice as near
each other in one direction as in another, it is certain
that the agent spoken of will not pass with the same
facility in both directions. Or supposing the effects to
which we have alluded to be produced by motion of some
kind, it is just as certain that the propagation of this
motion must be affected by the manner in which the
particles which transmit it are grouped together. Whether,
therefore, we take the old hypothesis of imponderables or
the new, and more philosophic one, of modes of motion, the
result is still the same.

If this reasoning be correct, it would follow that, if
the molecular arrangement of a body be changed, such a
change will manifest itself by an alteration of deportment
towards any force operating upon the body : the action of
compressed glass upon light, which Wertheim in his recent
researches ' has so beautifully turned to account in the
estimation of pressures, is an illustration in point ; and
the inference also receives the fullest corroboration from
experiments, some of which are recorded in the papers
already alluded to, and which show that all the phe-
nomena of magne-crystallic action maybe produced by
simple mechanical agency. What the crystalline forces do
in the one case, mechanical force, under the control of
the human will, accomplishes in the other. A crystal of
carbonate of iron, for example, suspended in the magnetic
field, exhibits a certain deportment : the crystal may be
removed, pounded into the finest dust, and the particles so
put together that the mass shall exhibit the same deport-
ment as before. A bismuth crystal suspended in the mag-
netic field, with its planes of principal cleavage vertical,
will set those planes equatorial ; but when the crystalline
planes are squeezed sufficiently together by a suitable

1 Phil. Mag. October and November, 1854.

PREFATORY REMARKS. 117

mechanical force, this deportment is quite changed, the
line which formerly set equatorial now setting axial.1

Thus we find that the influence of crystallisation may
be perfectly imitated, and even overcome, by simple me-
chanical agencies. It would of course be perfectly unin-
telligible were we to speak of any direct action of the
magnetic force upon the force by which the powdered car-
bonate of iron, or the solid cube of bismuth, is com-
pressed ; such an idea, however, appears scarcely less
tenable than the notion entertained by distinguished
men who have worked at this subject ; namely, that
there is a direct action of the magnet upon the molecular
forces which built the crystal. The function of such forces,
as regards the production of the effects, is, I believe,
mediate ; the molecular forces are exerted in placing the
particles in position, and the subsequent phenomena,
whether exhibited in magne-crystallic action, in the
bifurcation and polarisation of a luminous ray, or in the
modification of any other force transmitted through the
crystal, are not due to the action of force upon force,
except through the intermediation of the particles referred
to.2

The foregoing introductory statement will, perhaps,
sufficiently indicate the present aspect of this question.
The object I proposed to myself in commencing the in-
quiry now laid before the Royal Society was to obtain, if
possible, clearer notions of the nature of the diamagnetic

1 Phil. Mag. vol. ii. Ser 4. p. 183.

2 The influence of the molecular aggregation probably manifests
itself on a grand scale in nature. The Snowdon range of mountains,
for example, is principally composed of slate rock, whose line of strike
is nearly north and south. The magnetic properties of this rock I find,
by some preliminary experiments, to be very different along the
cleavage from what they are across it, I cannot help thinking that
these vast masses, in their present position, must exert a different action
on the magnetic needle from that which would be exerted if the line
of strike were east and we?t.

118 DIAMAGNETISM AND MAGNE-CKYSTALLIC ACTION.

force than those now prevalent ; for though, in the pre-
ceding paragraphs, we have touched upon some of the most
complex phenomena of magnetism and diamagnetism, and
are able to reproduce these phenomena at will, the greatest
diversity of opinion still prevails as to the real relation-
ship of the two forces. The magnetic force, we know,
embraces both attraction and repulsion, thus exhibiting
that wonderful dual action which we are accustomed
to denote by the term polarity. Faraday was the first
who proposed the hypothesis that diamagnetic bodies,
operated on by magnetic forces, possess a polarity ' the
same in kind as, but the reverse in direction of, that
acquired by iron, nickel, and ordinary magnetic bodies
under the same circumstances.' l W. Weber sought to
confirm this hypothesis by a series of experiments, wherein
the excitement of the supposed diamagnetic polarity was
applied to the generation of induced currents — appa-
rently with perfect success. Faraday afterwards showed
and his results were confirmed by Verdet, that effects
similar to those described by the distinguished Grerman
were to be attributed, not to the excitement of diamag-
netic polarity, but to the generation of ordinary induced
currents in the metallic mass. On the question of pola-
rity Faraday's results were negative, and he therefore,
with philosophic caution, holds himself unpledged to his
early opinion. Weber, however, still retains his belief
in the reverse polarity of diamagnetic bodies, whereas
Weber's countryman von Feilitzsch, in a series of me-
moirs recently published in Poggendorff s Annalen, con-
tends that the polarity of diamagnetic bodies is precisely
the same as that of magnetic ones. In this unsettled
state of the question nothing remained for me but to
undertake a complete examination of the nature of the
diamagnetic force, and a thorough comparison of its
1 Experimental Researches, 2429, 2430.

DEPORTMENT OF WOOD. 119

phenomena with those of ordinary magnetism. This has
been attempted in the following pages : with what success
it must be left to the reader to decide.

Before entering upon the principal inquiry, I will in-
troduce one or two points which arose incidentally from the
investigation, and which appear to be worth recording.

ON THE MAGNETIC PROPERTIES OF WOOD.

No experiments have yet been made to determine the
influence of structure upon the magnetic deportment of
this substance; and even on the question whether it is
magnetic, like iron, or diamagnetic, like bismuth, differ-
ences of opinion appear to prevail. Such differences are
to be referred to the extreme feebleness of the force proper
to the wood itself, and its consequent liability to be masked
by extraneous impurity. In handling the substance in-
tended for experiment the fingers must be kept perfectly
clean, and frequent washing is absolutely necessary. After
reducing the substance to a regular shape, so as to annul
the influence of exterior form, its outer surface must be
carefully removed by glass, and the body afterwards sus-
pended by a very fine fibre between the poles of a strong
electro-magnet.

The first step in the present inquiry was to ascertain
whether the substance examined was paramagnetic l or
diamagnetic. It is well known, that, in experiments of this
kind, movable masses, or poles, of soft iron are placed upon
the ends of the electro-magnet, the distance between the
poles being varied to suit the experiment. A cube of wood

1 The effects exhibited by iron and by bismuth come properly under
the general designation of magnetic phenomena : to render their sub-
division more distinct Mr. Faraday hs*s recently introduced the word
paramagnetic to denote the old magnetic effects, of which the action of
iron is an example. Wherever the word magnetic occurs, without the
prefix, it is always the old action that is referred to.

Jl'O DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

being suspended in front of a pointed pole of this kind,
if, on exciting the magnet, the cube v,-as repelled by the
point, it was regarded as diamagnetic ; if attracted, it

was considered to be para-
FIG. i. magnetic. The force is

considerably intensified by
placing the two movable
poles as in fig. 1, and sus-
pending the cube at a on
the same level with the
points; a diamagnetic body

placed there is, on the development of the magnetic force,
forcibly driven from the line which unites the points,
while a magnetic body is forcibly drawn in between them.
Having thus observed the deportment of the mass, the
cube was next suspended between the flat ends of the
poles sketched in fig. 1. The parallel faces were about
three-quarters of an inch apart, and in each case the fibre
of the suspended wood was horizontal. The specimen first
examined was Beef-wood : suspended in the position a,
fig. 1, the mass was repelled: suspended between the flat
poles, on exciting the magnet, the cube, if in an oblique
position, turned and set its fibre equatorial. By suitably
breaking and closing the circuit the cube could be turned
180° round and held in this new position. The axial posi-
tion of the ligneous fibre was one of unstable equilibrium,
from which, if it diverged in the slightest degree right or
left, the cube turned and finally set its fibre equatorial.
The following is a statement of the results obtained with
thirty-five different kinds of wood : —

DEPORTMENT OF WOOD.

121

Talk I.

Name of wood

Deportment of
mass

Deportment of
structure

Kern arks

1. Beef -wood. .

Diamagnetic

nbre equatorial

2. Black ebony .

»

»

3. Box-wood

M

n

4. Second speci-

men

>»

>»

5. Brazil-wood

»

j»

6. Braziletto

»

M

Action decided

7. Bullet-wood

»

n

Action decided

8. Cam-wood

»

)5

9. Cocoa-wood

»

»

10. Coromandel-

wood .

•

M

Action strong

11. Green Ebony .

M

M

Action strong

12. Green-heart

M

>»

Action strong

13. Iron- wood

»

»

14. King-wood

J»

>»

Action strong

15. Locust-wood .

»

)

1C. Maple

»

>

Action decided

17. Lance- wood

»

1

Action decided

18. Olive-tree

n

»

19. Peruvian-wood.

»

»

Action strong

20. Prince's-wood .

?>

)

21. Camphor- wood.

»

)>

22. Sandal-wood .

»

»

23. Satin-wood

•

»

24. Tulip-wood

M

J>

25. Zebra-wood

»

M

26. Botany Bay

Oak

»

»

Action strong

27. Mazatlan-wood.

M

n

Action decided

28. Tamarind-

wood .

J»

»

29. Sycamore.

M

»

Action decided

30. Beech

»

j>

Action decided

31. Kuby-wood

»

32. Jacca

»

33. Oak .

j>

Action strong

34. Yew.

»

Action feeble

35. Black Oak

Paramagnetic

>

Action decided

The term ' decided ' is here used to express an action
more energetic than ordinary, but in no case does the result
lack the decision necessary to place it beyond doubt. It
must also be remarked that the term ' strong' is used in
relation to the general deportment of wood ; for, compared

122 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

with the action of many other diamagnetic bodies, the
strongest action of wood is but feeble. Simple as the prob-
lem may appear, it required considerable time and care to
obtain the results here recorded. During the first examina-
tion of the cubes eight anomalies presented themselves —
in eight cases the fibre set either oblique or axial. The
whole thirty-five specimens were carefully rescraped with
glass and tested once more ; still two remained, which,
though repelled as masses, persistently set with the fibre
axial, and oscillated round this position so steadily as to
lead to the supposition that the real deportment of the
substance was thus exhibited. I scraped these cubes ten
times successively, and washed them with all care, but the
deportment remained unchanged. The cubes, for the sake
of reference, had been stamped with diminutive numbers
by the maker of them ; and I noticed at length, that in
these two cases a trace of the figures remained ; on remov-
ing, from each, the whole surface which bore the stamp, the
cubes forsook the axial position, and set, like the others,
with the fibre equatorial.

The influence of the mere form of an impurity was
here very prettily exhibited. The cubes in question had
been stamped (probably by a steel tool) with the numbers
33 and 37, which lay in the line of the fibre ; the figures,
being dumpy little ones, caused an elongation of the
magnetic impurity along the said line, and the natural
consequence of this elongation was the deportment above
described.

Of the thirty-five specimens examined one proved to
be paramagnetic. Now, it may be asked, if the views of
molecular action stated in the foregoing pages be correct,
how is it that this paramagnetic cube sets its fibre equa-
torial ? The case is instructive. The substance (bog-oak)
had been evidently steeped in a liquid containing a small
quantity of iron in solution, whence it derived its mag-

HYPOTHESIS OF CONFLICTING FORCES. 123

netism ; but here we have no substitution of paramagnetic
molecules for diamagnetic ones, as in the cases referred to.
The extraneous magnetic constituent is practically indif-
ferent as to the direction of magnetisation, and it therefore
accommodates itself to the directive action of the wood to
which it is attached.

ON THE KOTATION OF BODIES BETWEEN POINTED
MAGNETIC POLES.

In his experiments on charcoal, wood-bark, and
other substances, Pliicker discovered some very curious
phenomena of rotation, which occurred on removing the
substance experimented on from one portion of the mag-
netic field to another. To account for these phenomena,
he assumed, that in the substances which exhibited the
rotation, two antagonistic forces were perpetually active — a
repulsive force which caused the substance to assume one
position ; and an attractive force which caused it to assume
a different position : that, of these two forces, the repulsive
diminished more quickly than the attractive, when the
distance of the body from the poles was augmented. Thus,
the former, which was predominant close to the poles, suc-
cumbed to the latter when a suitable distance was attained
— hence arose the observed rotation.

Towards the conclusion of the memoir published in
the September number of the ' Philosophical Magazine ' for
1851, 1 stated that it was my intention further to examine
this highly ingenious theory. I shall now endeavour to
fulfil the promise then made, and to state, as briefly as I
can, the real law which regulates these complex phenomena.

The masses of soft iron sketched in fig. 1 were placed
upon the ends of the electro-magnet, with their points
facing each other ; between the points the body to be
examined was suspended by a fine fibre, and could be
raised or lowered by turning a milled head. The body was

124 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

first suspended at the level of the points and its de-
portment noted, it was then slowly elevated, and the
change of position, if any, was observed. It was finally
permitted to sink below the points and its deportment
there noted also.

The following is a statement of the results ; the words
'equatorial' (E) and 'axial' (A) imply that the longest
horizontal dimension of the substance examined took up
the position denoted by each of these words respectively.
The manner in which the bars were prepared is explained
further on.

Table II.

Name of substance

Horizontal
dimensions

Deportment of
mass

Position

Between
poles

Above

Below

1. Tartaric acid . .

0-5 xO-1

Diamagnetic

E

A

A

2. A second specimen.

0-4 xO-1

„

E

A

A

3. Eed ferrocyanide

of potassium . .

0-6 xO-1

Paramagnetic

A

E

E

4. A second prism. .

0-9 xO-12

„

A

E

E

5. Citric acid . . .

0-55 x 0-25

Diamagnetic

E

A

A

6. A second specimen.

0-48 x 0-2

„

E

A

A

7. Beryl

0-45 x 0-1

Paramagnetic

A

E

E

8. Saltpetre ....

0-6 xO-3

Diamagnetic

E

A

A

9. Nitrate of soda

0-6 xO-12

„

E

A

A

10. Sulphate of iron .

0-7 xO-15

Paramagnetic

A

E

E

11. A second specimen.

0-6 xO-03

„

A

E

E

12. A third specimen .

1-0 xO-13

„

A

E

E

13. Calcareous spar. .

0-5 xO-1

Diamagnetic

E

A

A

14. A full crystal . .

—

V

E

A

A

15. Carbonate of iron .

0-5 xO-1

Paramagnetic

A

E

E

16. Carbonate of iron

powdered and

compressed . .

0-9 xO-18

„

A

E

E

17. Compressed disc .

0-8 x008

„

A

E

E

18. Bismuth ....

0-95 x 0-15

„

E

A

A

19. The same com-

pressed. . . .

0-7 xO-05

„

E

A

A

20. The same powdered

and compressed .

0-6 xO-07

Diamagnetic

E

A

A

21. Cylinder of the

same. ....

1-0 xO'15

E

A

A

22. Tourmaline . . .

2-1 xO-1

Paramagnetic

A

E

E

23. A second specimen.

1-1 xO-1

„

A

E

E

24. A third ....

0-9 xO-1

M

A

E

E

L

EOTATIOXS IN MAGNETIC FIELD.

125

Table II. — continued.

Name of substance

Horizontal
dimensions

Deportment of
mass

Position

Between
poles

Above

Below

25. Sulphate of nickel.

0-9 xO-3

Paramagnetic

A

E

E

26. A second specimen.

0-6 xO-2

»

A

E

E

27. Heavy spar . . .

0-38x0-18

Diamagnetic

E

A

A

28. A second specimen.

0-4 xO-18

»

E

A

A

29. Carbonate of tin

powdered and

compressed . .

0-34 x 0-04

M

E

A

A

30. A second specimen.

length 6 limn »idlli

m

E

A

A

31. Ammonio - phos-

phate of mag-

nesia powdered

and compressed .

0-3 xO-06

n

E

A

A

32. A second specimen.

0-5 xO-07

n

E

A

A

33. Carbonate of mag-

nesia powdered

and compressed .

0-45 x 0-04

m

E

A

A

34. Sulphate of mag-

nesia

0-32 x 0-2

»

E

A

A

35. A second specimen.

0-25xO-lo

n

E

A

A

36. Flour compressed .

0-24 x 0-04

»

E

A

A

37. Oxalate of cobalt .

0-6 xO-OS

Paramagnetic

A

E

E

These experiments might be extended indefinitely, but
we have sufficient here to enable us to deduce the law of
action. In the first place we notice, that all those substances
which set equatorial between the points and axial above
and below them, are diamagnetic ; while all those which
set axial between the points and equatorial above and
below them, are paramagnetic. When any one of the
substances here named is reduced to the spherical form,
this rotation is not observed. I possess, for example, four
spheres of calcareous spar, and when any one of them
is suspended between the points, it takes up a position
which is not changed when the sphere is raised or lowered ;
the crystallographic axis sets equatorial in all positions.
A sphere of compressed carbonate of iron, suspended
between the points, also sets that diameter along which
the pressure is exerted from pole to pole, and continues to

126 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

do so when raised or lowered. A sphere of compressed
bismuth, on the other hand, sets its line of compression
always equatorial. The position taken up by the spheres
depends solely upon the molecular structure of the sub-
stances which compose them ; but, when the mass is
elongated, another action comes into play. Such a mass
being suspended with its length horizontal, the repulsion
of its ends constitutes a mechanical couple which increases
in power with the length of the mass; and when the
body is long enough, and the local repulsion of the ends
strong enough, the couple, when it acts in opposition to
the directive tendency due to structure, is able to over-
come the latter and to determine the position of the mass.
In all the cases cited, it was so arranged that the
length of the body and its structure should act in opposi-
tion to each other. Tartaric acid and citric acid cleave
with facility in one direction, and, in the specimens used,
the planes of cleavage were perpendicular to the length of
the body. In virtue of the structure, these planes tended
to set equatorial, but the repulsion of the elongated mass
by the points prevented this, and caused the planes to set
axial. When, however, the body was raised or lowered
out of the region of local repulsion, and into a position
where the distribution of the force was more uniform, the
advantage due to length became so far diminished that
it was overcome, in turn, by the influence of structure, and
the planes of cleavage turned into the equatorial position.
In the specimen of saltpetre the shortest horizontal dimen-
sion was parallel to the axis of the crystal, which axis,
when the influence of form is destroyed, always sets
equatorial. A full crystal of calcareous spar will, when the
magnetic distribution is tolerably uniform, always set
its axis at right angles to the line joining the poles ; but
the axis is the shortest dimension of the crystal, and,
between the points, this mechanical disadvantage compels

ACTION OF HEAVY SPAR ANALYZED. 127

the influence of structure to succumb to the influence
of shape. A cube of calcareous spar, in my possession,
may be caused to set the optic axis from pole to pole
between the points, but this is evidently due to the
elongation of the mass along the diagonals ; for, when the
corner of the cube succeeds in passing the point of the
pole, the mass turns its axis with surprising energy into
the equatorial position, round which it oscillates with
great vivacity. Counting the oscillations, I found that
eighty-two were performed by the cube, when its axis was
equatorial, in the time required to perform fifty-nine,
when the axis stood from pole to pole. Heavy spar and
ccelestine are beautiful examples of directive action.
These crystals, as is well known, can be cloven into prisms
with rhombic bases : the principal cleavage is parallel
to the base of the prism, while the two subordinate cleav-
ages constitute the sides. If a short prism be suspended
in a tolerably uniform field of force, so that its rhombic
ends shall be horizontal, on exciting the magnet the short
diagonal will set equatorial, as shown in fig. 2. If the
prism be suspended with its axis and the short diagonal
horizontal, the long diagonal being therefore vertical, the
short diagonal will retain the equatorial position, while
the axis of the prism sets axial as in fig. 3. If the prism
be suspended with its long diagonal and axis horizontal,
the short diagonal being vertical, and its directive power
therefore annulled, the axis will take up the equatorial
position, as in fig. 4.

Now as the line which sets equatorial in diamagnetic
bodies is that in which the magnetic repulsion acts most
strongly,1 the crystal before us furnishes a perfect example
of a substance possessing three rectangular magnetic axes,
no two of which are equal. In the experiment cited in
Table II. page 124, the mass was so cut that the short

1 Phil. Mag., S. 4. vol. ii. p. 177.

7

128 DIAMAGNETISM AKD MAGNE-CRYSTALLIC ACTION.

diagonal of the rhombic base was perpendicular to the
length of the specimen. Carbonate of tin, and the other
powders, were compressed by placing the powder between
two clean plates of copper, and squeezing them together in

FIG. 2,

Fio. 3.

a strong vice. The line of compression in diamagnetic
bodies, as already stated, always sets equatorial, when the
field of force is uniform, or approximately so ; but, between
points, the repulsion of the ends furnishes a couple strong

ANTITHESIS OF ROTATIONS. 129

enough to overcome this directive action, causing the
longest dimension of the mass to set equatorial, and con-
sequently its line of compression axial.

The antithesis between the deportment of diamagnetic
bodies and of paramagnetic ones is thus far perfect. Be-
tween the points the former class set equatorial, the latter
axial. Eaised or lowered, the former set axial, the latter
equatorial. The simple substitution of an attractive for a
repulsive force produces this difference of effect. A sphere
of ferrocjanide of potassium, for example, always sets the
line perpendicular to the crystallographic axis from pole
to pole ; but when we take a full crystal, whose dimension
along its axis, as in one of the cases before us, is six times
the dimension at right angles to the axis, the attraction
of the ends is sufficient to overcome the directive action due
to structure, and to pull the crystal into the axial position
between the points. In a field of uniform force, or between
flat poles, the length sets equatorial, and it is, as already
insisted on, the partial attainment of such a field, at a
distance from the points, that causes the crystal to turn
from axial to equatorial when it is raised or lowered.
Beryl is a paramagnetic crystal, and when the influence
of form is annulled, it always sets a line perpendicular to
the axis of the crystal from pole to pole ; a cube of this
crystal, at present in my possession, shows this deportment
whether the poles are pointed or flat : but in the specimen
examined the dimension of the crystal along its axis was
greatest, and hence the deportment described. It is need-
less to dwell upon each particular paramagnetic body : the
same principle was observed in the preparation and choice
of all of them ; namely, that the line which, in virtue
of the internal structure of the substance, would set axial,
was transverse to the length of the body. The directive
action due to structure was thus brought into opposition
with the tendency of magnetic bodies to set their longest

130 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

dimension from pole to pole : between the points the latter
tendency was triumphant ; at a distance, on the contrary,
the influence of structure prevailed.

The substance which possesses this directive action in
the highest degree is carbonate of iron : when a lozenge,
cloven from the crystalline mass, is suspended from the
angle at which the crystallographic axis issues, there is
great difficulty in causing the plate to set axial. If the
points are near, on exciting the magnetism the whole mass
springs to one or the other of the points ; and when the
points are distant, the plate, although its length may be
twenty times its thickness, will set strongly equatorial.
An excitation by one cell is sufficient to produce this re-
sult. In the experiment cited in the table the residual
magnetism was found to answer best, as it permitted the
ends of the plate to be brought so near to the points that
the mass was pulled into the axial position. When the
magnet was more strongly excited, and the plate raised so
far above the points as to prevent its springing to either
of them, it was most interesting to watch the struggle of
the two opposing tendencies. Neither the axial nor the
equatorial position could be retained; the plate would
wrench itself spasmodically from one position into the
other, and, like a human spirit operated on by conflicting
passions, find rest nowhere.

The conditions which determine the curious effects
described in the present chapter may be briefly expressed
as follows : —

An elongated diamagnetic body being suspended in the
magnetic field, if the shortest horizontal dimension tend,
in virtue of the internal structure of the substance, to set
equatorial, it is opposed by the tendency of the longest
dimension to take up the same position. Between the
pointed poles the influence of length usually predominates;
above the points and below them the directive action due
to structure prevails.

ANTITHESIS OP KOTATIONS. 131

Hence, the rotation of a diamagnetic body, on being
raised or loivered, is always from the equatorial to the
axial position.

If the elongated mass be magnetic, and the shortest
dimension of the mass tend, in virtue of its structure, to
set from pole to pole, it is opposed by the tendency of the
longest dimension to take up the same position. Between
the points the influence of length is paramount ; above
and below the points the influence of structure prevails.

Hence, the rotation of magnetic bodies, on being
raised or lowered, is always from the axial to the equa-
toi^ial position.

The error of the explanation which referred many of
the above actions to the presence of two conflicting forces,
one of which diminished with the distance in a quicker
ratio than the other, lies in the supposition, that the
assuming of the axial position proved a body to be mag-
netic, while the assuming of the equatorial position
proved a body to be diamagnetic. This assumption was
perfectly natural in the early stages of diamagnetic
research, when the modification of magnetic force by
structure was unknown. Experience however proves
that the total mass of a magnetic body continues to be
attracted after it has assumed the equatorial position,
while the total mass of a diamagnetic body continues to
be repelled after it has taken up the axial one.

ON THE DISTRIBUTION OF THE MAGNETIC FORCE
BETWEEN TWO FLAT POLES.

In experiments where a uniform distribution of the
magnetic force is desirable, flat poles, or magnetised
surfaces, have been recommended. It has long been
known that the force proceeds with great energy from
the edges of such poles : the increase of force from the
centre to the edge has been made the subject of a special

132 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

investigation by Von Koike.1 The central portion of the
magnetic field, or space between two such magnetised
surfaces, has hitherto been regarded as almost perfectly
uniform, and indeed for all ordinary experiments the
uniformity is sufficient. But, when we examine the field
carefully, we find that the uniformity is not perfect.
Substituting, for the sake of convenience, the edge of a
pole for a point, I studied the phenomena of rotation
described in the last section, in a great number of
instances, by comparing the deportment of an elongated
body, suspended in the centre of the space between two
flat poles, with its deportment when suspended between
the top or the bottom edges. Having found that the
fibre of wood, in masses where form had no influence,
always set equatorial, I proposed to set this tendency to
contend with an elongation of the mass in a direction
at right angles to the fibre. For this purpose, thirty-one
little wooden bars were carefully prepared and examined,
the length of each bar being about twice its width, and
the fibre coinciding with the latter dimension. The bars
were suspended from an extremely fine fibre of cocoon
silk, and in the centre of the magnetic field each one
of them set its length axial, and consequently its fibre
equatorial. Between the top and bottom edges, on the
contrary, each piece set its longest dimension equatorial,
and consequently the fibre axial.

For some time I referred the axial setting of the mass,
in the centre of the field, to the directive action of the fibre,
though, knowing the extreme feebleness of this directive
action, I was surprised to find it able to accomplish what
the experiments exhibited. The thought suggested itself,
however, of suspending the bars with both the long dimen-
sion and the fibre vertical, in which position the latter
could have no directive influence. Here also, to my sur-
1 Poggerdorff's Annalen, vol. Ixxxi. p. 321.

FIELD BETWEEN FLAT POLES 133

prise, the directive action, though slightly weakened,
was the same as before : in the centre of the field the bars
took up the axial position. Bars of sulphur, wax, salt
of hartshorn, and other diamagnetic substances were next
examined : they all acted in the same manner as the
wood, and thus showed that the cause of the rotation lay,
not in the structure of the substances, but in the distri-
bution of the magnetic force around them. This distribu-
tion in fact was such, that the straight line which con-
nected the centre of one pole with that of the opposite
one was the line of weakest force. Ohm represents the
distribution of electricity upon the surfaces of conductors
by regarding the tensions as ordinates, and erecting
them from the points to which they correspond, the
steepness of the curve formed by uniting the ends of the
ordinates being the measure of the increase or diminu-
tion of tension. Taking the centre of the magnetic field
as the origin, and drawing horizontal lines axial and
equatorial, if we erect, the magnetic tensions along these
lines, we shall find a steeper curve in the equatorial than
in the axial direction. This may be proved by suspending
a bit of carbonate of iron in the centre of the magnetic
field ; on exciting the magnet, the suspended body will
move, not to the nearest portion of the flat pole, though it
may be not more than a quarter of an inch distant, but
equatorially towards the edges, though they may be two
inches distant. The little diamagnetic bars referred to
were therefore pushed into the axial position by the force
acting with superior power in an equatorial direction.

The results just described are simply due to the reces-
sion of the ends of an elongated body from places of
stronger to those of weaker force ; but it is extremely
instructive to observe how this result is modified by
structure. If, for example, a plate of bismuth, be sus-
pended between the poles with the plane of principal

134 DIAMAGNETISM AND MAGNE-C11YSTALLIC ACTION.

cleavage vertical, the plate will assert the equatorial
position from top to bottom ; and in the centre with
almost the same force as between the edges. The cause
of this lies in the structure of the bismuth. Its position
depends not so much upon the character of the magnetic
field around it, as upon the direction of the force through
it. I will not, however, anticipate matters by entering
further upon this subject at present.

COMPARATIVE VIEW OF PARAMAGNETIC AND DIAMAGNETIC
PHENOMENA.

1. State of Diamagnetic Bodies under Magnetic
Influence.

When a piece of iron is brought near a magnet, it is
attracted by the latter : this attraction is not the act of
the magnet alone, but results from the mutual action of
the magnet and the body upon which it operates. The
iron in this case is said to be magnetised by influence ;
it becomes itself a magnet, and the intensity of its mag-
netisation varies with the strength of the influencing
magnet. Poisson figured the act of magnetisation as
consisting of the decomposition of a neutral magnetic
fluid into north and south magnetism, the amount of the
decomposition being proportional to the strength of the
magnet which produces it. Ampere, discarding the
notion of magnetic fluids, figured the molecules of iron as
surrounded by currents of electricity, and conceived the
act of magnetisation to consist in setting the planes of
these molecular currents parallel to each other : the
degree of parallelism, or in other words, the intensity of
the magnetisation, depending, as in Poisson's hypothesis,
upon the strength of the influencing magnet.

The state into which the iron is here supposed to be
thrown is a state of constraint, and when the magnet is

LAW OF MAGNETIC INDUCTION. 135

removed, the substance returns to its normal condition.
Poisson's separated fluids rush together once more, and
Ampere's molecular currents return to their former
irregular positions. As our knowledge increases, we shall
probably find both hypotheses inadequate to represent the
phenomena ; the only thing certain is, that the iron, when
acted upon by the magnet, is thrown into an unusual
condition, in virtue of which it is attracted ; and that the
intensity of this condition is a function of the force which
produces it.

There are, however, bodies which, unlike iron, offer a
great resistance to the imposition of the magnetic state,
but when once they are magnetised they do not, on the
removal of the magnet, return to their neutral condition,
but retain the magnetism impressed on them. It is in
virtue of this quality that steel can be formed into com-
pass needles and permanent magnets. This power of
resistance and retention is named by Poisson coercive
force.

Let us conceive a body already magnetised, and in
which coercive force exists in a very high degree — a piece
of very hard steel for example — to be brought near a
magnet, the strength of which is not sufficient to mag-
netise the steel further. To simplify the matter, let us
fix our attention upon the south pole of the magnet, and
conceive it to act upon the north pole of the piece of
steel. Let the magnetism of the said south pole, referred
to any unit, be M, and of the north pole of the steel, M' ;
then their mutual attraction, at the unit of distance, is
expressed by the product MM'. Conceive now the
magnet to increase in power from M to nM, the steel
being still supposed hard enough to resist magnetisation
by influence ; the mutual attraction now will be

or n times the former attraction ; hence when a variable

136 DTAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

magnetic pole acts on an opposite one of constant power,
the attraction is proportional to the simple strength of the
former.

Let us now take a body whose magnetisation varies
with that of the magnet : a south pole of the strength M
induces in such a body a north pole of the strength M',
and the attraction which results from their mutual action is

MM'.

Let the strength of the influencing south pole increase
from M to nM. ; then, assuming the magnetism of the
body under influence to increase in the same ratio, the
strength of the above-mentioned north pole will become
riMf, and the attraction, expressed by the product of both,

will be

«2HM' ;

that is to say, the attraction of a body magnetised by in-
fluence, and whose magnetism varies as the strength of the
influencing magnet, is proportional to the square of the
strength of the latter.

Here then is a mark of distinction between those bodies
which have their power of exhibiting magnetic phenomena
conferred upon them by the magnet, and those whose
actions are dependent upon some constant property of the
mass : in the latter case the resultant action will be simply
proportional to the strength of the magnet, while in the
former case a different law of action will be observed.1

The examination of this point lies at the very founda-
tion of our inquiries into the nature of the diamagnetic
force. Is the repulsion of diamagnetic bodies dependent
merely on the mass considered as ordinary matter, or is it

1 This test was first pointed out in a paper on the Polarity of Bis-
muth, Phil. Mag. Nov. 1851, p. 333. I have reasons, however, to know
that the same thought occurre to Poggendorff previous to the pub-
lication of my paper. — J. T.

LAW OF D1AMAGNETIC INDUCTION. 137

due to some condition impressed upon the mass by the
influencing magnet ? This question admits of the most
complete answer either by comparing the increase of repul-
sion with the increase of power in the magnet which pro-
duces the repulsion, or by comparing the attraction of a
paramagnetic body, which we know to be thrown into an
unusual condition, with the repulsion of a diamagnetic
body, whose condition we would ascertain.

Bars of iron and bismuth, of the same dimensions,
were submitted to the action of an electro-magnet, which
was caused gradually to increase in power ; commencing
with an excitation by one cell, and proceeding up to an
excitation by ten or fifteen. The strength of the current
was in each case accurately measured by a tangent galvano-
meter. The bismuth bar was suspended between the two
flat poles, and, when the magnet was excited, it took up
the equatorial position. The iron bar, if placed directly
between the poles, would, on the excitation of the mag-
netism, infallibly spring to one of them ; hence it was
removed to a distance of 2 feet 7 inches from the centre of
the space between the poles, and in a direction at right
angles to the line which united them. The magnet being
excited, the bar, in each case, was drawn a little aside
from its position of equilibrium and then liberated, a series
of oscillations of very small amplitude followed, and the
number of oscillations accomplished in a minute was care-
fully ascertained. Tables III. and IV. contain the results
of experiments made in the manner described with bars of
iron and bismuth of the same dimensions.

138 DIAMAGNETI8M AND MAGNE-CKYSTALLIC ACTION.

TabU III.

Bar of iron, No. 1. — Length 0'8 of
an inch ; width 0'13 of an inch ;
depth 0-15 of an inch.

Strength of current Attraction

168
214
248
274
323
362
385
411

1682
2042
2532
2752
3132
3472
374*
385*

Talle IV.

Bar of Bismuth, No. 1. — Length
0-8 of an inch; width 0*13 of
an inch ; depth 0'15 of an inch.

Strength of current Eepulsion

78 782

136 1352

184 191*

226 226*

259 259»

287 291*

341 322'

377 3592

411 386*

These experiments prove that, up to a strength of
about 280, the attractive force operating upon the iron,
and the repulsive force acting upon the bismuth, are each
proportional to the square of the strength of the mag-
netising current. For higher powers, both attraction
and repulsion increase in a smaller ratio ; but it is here
sufficient to show that the diamagnetic repulsion follows
precisely the same law as the magnetic attraction. So
accurately indeed is this parallelism observed, that while
the forces at the top of the tables produce attractions and
repulsions exactly equal to the square of the strength of
the current, the same strength of 411, at the bottom of
both tables, produces in iron an attraction of 3852, and in
bismuth a repulsion of 3862. The numbers which indicate
the strength of current in the first column are the tangents
of the deflections observed in each case : neglecting the
indices, the figures in the second column express the num-
ber of oscillations accomplished in a minute, multiplied by
a constant factor to facilitate comparison ; the forces ope-
rating upon the bars being proportional to the squares of
the number of oscillations, the simple addition of the index
figure completes the expression of these forces.

In these experiments the bismuth bar set across the

METHOD OF OSCILLATION. 139

lines of magnetic force, while the bar of iron set along
them ; the former was so cut from the crystalline mass, that
the plane of principal cleavage was parallel to the length
of the bar, and in the experiments hung vertical. I
thought it interesting to examine the deportment of a bar
of bismuth which should occupy the same position, with
regard to the lines of force, as the bar of iron ; that is to
say, which should set its length axial. Such a bar is
obtained when the planes of principal cleavage are trans-
verse to the length.

Table V.

Bar of Bismuth, No. 2. — Length 0'8 of an inch ; width 0-13 of an inch;
depth 0*15 of an inch.

Set axial betnce-n the excited poles.

Strength of current Repulsion

68 67*

182 187*

218 218«

248 249»

274 273*

315 309*

364 350*

401 366"

A deportment exactly similar to that exhibited in the
foregoing cases is observed here also : up to about 280 the
repulsions are exactly proportional to the squares of the
current strengths, and from this point forward they increase
in a less ratio.

A paramagnetic substance was next examined which
set its length at right angles to the lines of magnetic force :
the substance was carbonate of iron. The native crystal-
lised mineral was reduced to powder in a mortar, and the
powder was compressed. It was suspended, like the bis-
muth, between the flat poles, with its line of compression
horizontal. When the poles were excited, the compressed
bar set the line of pressure from pole to pole, and conse-
quently its length equatorial.

140 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

Table VI.

Bar of compressed Carbonate of Iron. — Length 0-95 of an inch ; width
0-17 of an inch ; depth O23 of an inch.

Set equatorial between the excited Poles.

Strength of current Attraction

74 742

135 133*

179 ISO2

214 2182

249 248*

277 280*

341 330*

381 353*

It is needless to remark upon the perfect similarity
of deportment here exhibited to the cases previously re-
corded.

In experiments made with bars of sulphate of iron the
same law of increase was observed.

These experiments can leave little doubt upon the mind,
that if a magnetic body be attracted in virtue of its being
converted into a magnet, a diamagnetic body is repelled
in virtue of its being converted into a diamagnet. On
no other assumption can it be explained, why the repulsion
of the diamagnetic body, like the attraction of the mag-
netic one, increases in a so much quicker ratio than the
force of the magnet which produces the repulsion. But,
as this is a point of great importance, I will here introduce
corroborative evidence, derived from modes of experiment
totally different from the method already described. By
a series of measurements with the torsion-balance, in which
the attractive and repulsive forces were determined directly,
with the utmost care, the relation of the strength of the
magnet to the force acting upon the following substances
was found to be as follows :—

IDENTITY OP LAWS.

141

Table VII.

Spheres of Native Sulphur.
Strength of Eatio of

repulsions

952
15S2
224'
2642
3162

Table VIII.

Spheres of Carbonate of Lime.
Strength of Batio of

repulsions
1342
1732
2122
2642

magnet
134
172
213
259
310

3112

Table IX.

Spheres of Carbonate of Iron.

Strength of magnet

66

89
114
141

Ratio of attractions

66Z

892

1142

1412

These results confirm those of M. E. Becquerel,1 whose
experiments first showed that the repulsion of diamagne tic
bodies follows the same law as the attraction of magnetic
ones.

Bar of Sulphur. — Length 25 millims. ; weight 840 milligrms.

Squares of the Quotients of the repulsions

magnetic intensities by the magnetic intensities

36-58 0-902

27-60 0-929

26-84 0-906

16-33 0-920

The constancy of the quotient in the second column
proves that the ratio of the repulsions to the squares of the
magnetic intensities is a ratio of equality.

I will also cite a series of experiments by Mr. Joule,2
which that excellent philosopher adduces in confirmation of
the results obtained by M. E. Becquerel and myself.

Bar of Bismuth.
Strength of magnet Repulsions

1 I2

2 22
4 42

' Annales de Chimie et de Physique, 3rd series, vol. xxviii. p. 302.
8 Phil. Mag., 4th series, vol. iii. p. 32.

142 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTIOX.

Let us contrast these with the results obtained by Mr.
Joule, on permitting the magnet to act upon a hard mag-
netic needle.

Magnetic Needle. — Length 1-5 of an inch.
Strength of magnet Attraction

1 1

2 2
4 4

Here we find experiment in strict accordance with the
theoretical deduction stated at the commencement of the
present chapter. The intensity of the magnetism of the
steel needle is constant, for the steel resists magnetisation
by influence ; the consequence is that the attraction is
simply proportional to the strength of the magnet.

A consideration of the evidence thus adduced from
independent sources, and obtained by different methods,
must, I imagine, render the conclusion certain that diainag-
netic bodies, like magnetic ones, exhibit their phenomena
in virtue of a state of magnetisation induced in them by
the influencing magnet. This conclusion is in no way
invalidated by the recent researches of Pliicker, on the
law of induction in paramagnetic and diamagnetic bodies,
but, on the contrary, derives support from his experiments.
With current strengths which stand in the ratio of
1 '. 2, Pliicker finds the repulsion of bismuth to be as
1 : 3'62, which, though it falls short of the ratio of 1 '. 4,
as the law of increase according to the square of the
current would have it, suffices to show that the bismuth
was not passive, but acted the part of an induced diamag-
net in the experiments. In the case of the iron itself,
Pliicker finds a far greater divergence ; for here currents
which stand in the ratio of 1 I 2 produce attractions only
in the ratio of 1 : 2 '76.

2. Duality of Diamagnetic Excitement.
Having thus safely established the fact that diamag-

NEW TORSION BALANCE. 143

netic bodies are repelled, in virtue of a certain state into
which they are cast by the influencing magnet, the next
step of our inquiry is : — Will the state evoked by one mag-
netic pole facilitate, or prevent, the repulsion of the diamag-
netic body by a second pole of an opposite quality ? If the
force of repulsion were an action on the mass, considered
as ordinary matter, this mass, being repelled by both the
north and the south pole of a magnet, when they operate
upon it separately, ought to be repelled by the sum of the
forces of the two poles where they act upon it together.
But if the excitation of diamagnetic bodies be of a dual
nature, as is the case with the magnetic bodies, then it may
be expected that the state excited by one pole will not
facilitate, but on the contrary prevent, the repulsion of the
mass by a second opposite pole.

To solve this question the apparatus sketched in fig.
5a, Plate II. was made use of. AB and CD are two helices
of copper wire 12 inches long, of 2 inches internal, and of
5^ inches external diameter. Into them fit soft iron cores
2 inches thick : the cores are bent as in the figure, and
reduced to flat surfaces along the line e/, so that when the
two semicylindrical ends are placed together, they consti-
tute a cylinder of the same diameter as the cores within
the helices.1 In front of these poles a bar of pure bismuth
gh was suspended by cocoon silk ; by imparting a little
torsion to the fibre, the end of the bar was caused to press
gently against a plate of glass ik, which stood between it
and the magnets. By means of a current reverser the
polarity of one of the cores could be changed at pleasure ;
thus it was in the experimenter's power to excite the cores,
so that the poles PP" should be of the same quality, or of
opposite qualities.

The bar, being held in contact with the glass by a very

1 The ends of the semicylinders were turned so as to present the
apex of a truncated cone to the suspended bar of bismuth.

144 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

feeble torsion, a current was sent round the cores, so that
they presented two poles of the same name to the suspended
bismuth ; the latter was promptly repelled, and receded
to the position dotted in the figure. On interrupting
the current it returned to the glass as before. The cores
were next excited, so that two poles of opposite qualities
acted upon the bismuth ; the latter remained perfectly un-
moved.1

This experiment shows that the state, whatever it may
be, into which bismuth is cast by one pole, so far from
being favourable to the section of the opposite pole, com-
pletely neutralises the effect of the latter. A perfect analogy
is thus established between the deportment of the bismuth
and that of iron under the same circumstances ; for it is
well known that a similar neutralisation occurs in the
latter case. If the repulsion depended upon the strength
of the poles, without reference to their quality,, the repul-
sion, when the poles are of opposite names, ought to be
greater than when they are alike ; for in the former case
the poles are greatly strengthened by their mutual induc-
tive action, while, in the latter case, they are enfeebled by
the same cause. But the fact of the repulsion being depen-
dent on the quality of the pole, demonstrates that the sub-
stance is capable of assuming a condition peculiar to each
pole, or in other words, is capable of a dual excitation.2

1 A shorter bar of bismuth than that here sketched, with a light
index attached to it, makes the repulsion more evident. It may be thus
rendered visible throughout a large lecture-room.

2 Since the above was written, the opinion has been expressed to
me, that the action of the unlike poles, in the experiment before us, is
' diverted ' from the bismuth upon each other, the absence of repulsion
being due to this diversion, and not to the neutralisation of inductions
in the mass of the bismuth itself. Many, however, will be influenced
by the argument as stated in the text, who would not accept the
interpretation referred to in this note ; I therefore let the argument
stand, and hope at no distant day to return to the subject. — J. T.
May 6, 1855.

ACTION OP LIKE AND UNLIKE POLES. 145

The experiments from which these conclusions are drawn
are a manifest corroboration of those made by M. Eeich
with steel magnets.

If we suppose the flat surfaces of the two semicylinders
which constitute the ends of the cores to be in contact, and
the cores so excited that the poles P and p' are of different
qualities, the arrangement, it is evident, forms a true
electro-magnet of the horseshoe form ; and here the perti-
nency of a remark made by M. Poggendorff, with his usual
clearness of perception, becomes manifest ; namely, that
if the repulsion of diamagnetic bodies be an indifferent
one of the mass merely, there is no reason why they should
not be repelled by the centre of a magnet, as well as by its
ends.

3. Separate and joint action of a Magnet and a Voltaic
Current on Paramagnetic and Diamagnetic Bodies.

In operating upon bars of bismuth with the magnet, or
the current, or both combined, it was soon found that the
gravest mistakes might be committed if the question of
molecular structure was not attended to ; that it is not
more indefinite to speak of the volume of a gas without
giving its temperature, than to speak of the deportment
of bismuth without stating the relation of the form of the
mass to the planes of crystallisation. Cut in one direction,
a bar of bismuth will set its length parallel to an electric
current passing near it; cut in another direction, it will
set its length perpendicular to the same current. It was
necessary to study the deportment of both of these bars
separately.

A helix was formed of covered copper wire one-twen-
tieth of an inch thick : the space within the helix was
rectangular, and was 1 inch long, 0*7 inch high, and 1
inch wide : the external diameter of the helix was 3
inches. Within the rectangular space the body to be ex-

146 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

amined was suspended by a fibre which descended through
a slit in the helix. The latter was placed between the
two flat poles of an electro-magnet, and could thus be
caused to act upon the bar within it, either alone or in

FIG. 6.

combination with the magnet. The disposition will be
at once understood from fig. 6, which gives a front view of
the arrangement.

a. — Action of Magnet alone : Division of bars into
Normal and Abnormal.

A bar of soft iron suspended in the magnetic field will
set its longest dimension from pole to pole : this is the
normal deportment of paramagnetic bodies. A bar of
bismuth, whose planes of principal cleavage are through-
out parallel to its length, suspended in the magnetic field
with the said planes vertical, will set its longest dimension
at right angles to the line joining the poles : this is the
normal deportment of diamagnetic bodies. We will, there-
fore, for the sake of distinction, call the former a normal
paramayneiic bar, and the latter a normal diamagnetic
bar.

A bar of compressed carbonate of iron dust, whose

SEPAKATE ACTION OF MAGNET AND OF CURRENT. 147

shortest dimension coincides with the line of pressure,
will, when suspended in the magnetic field with the said
line horizontal, set its length equatorial. A bar of com-
pressed bismuth dust, similarly suspended, or a bar of
bismuth whose principal planes of crystallisation are
transverse to its length, will set its length axial in the
magnetic field. We will call the former of these an ab-
normal paramagnetic bar, and the latter an abnormal
diamagnetic bar.

b. — Action of Current alone on normal and
abnormal bars.

A normal paramagnetic bar was suspended in the
helix above described ; when a current was sent through
the latter, the bar set its longest horizontal dimension
parallel to the axis of the helix, and consequently per-
pendicular to the coils.

An abnormal paramagnetic bar was suspended in the
same manner ; when a current was sent through the helix,
the bar set its longest dimension perpendicular to the
axis of the helix, and consequently parallel to the coils.

A normal diamagnetic bar was delicately suspended
in the same helix ; on the passage of the current it acted
precisely as the abnormal magnetic bar ; setting its long-
est dimension perpendicular to the axis of the helix and
parallel to the coils. Skill is needed, but when a fine
fibre and sufficient power are made use of, this deportment
is obtained without difficulty.

An abnormal diamagnetic bar was suspended as
above ; on the passage of the current it acted precisely as
the normal magnetic bar : it set its length parallel to the
axis of the helix and perpendicular to the coils. Here
also, by fine manipulation, the result is obtained with
ease and certainty.

148 DIAMAGNETISM AND MAGNE-CKYSTALLIC ACTION.

c. — Action of Magnet and Current combined.
In examining this subject, eight experiments were
made with each bar ; it will be remembered that fig. 6
gives a view of the arrangement in vertical section.

1. Four experiments were made in which the magnet
was excited first, and after the suspended bar had taken
up its position of equilibrium, the deflection produced by
the passage of a current through the surrounding helix
was observed.

2. Four experiments were made in which the helix was
excited first ; and when the bar within it had taken up its
position of equilibrium, the magnetism was developed and
the consequent deflection observed.

Normal Paramagnetic Bar.

In experimenting with iron it was necessary to place
it at some distance from the magnet, otherwise the attrac-
tion of the entire mass by one or the other pole would
completely mask the action sought. Fig. 7 represents, in

FIG. 7.

plan, the disposition of things in these experiments : N
and s indicate the north and south poles of the magnet ;

JOINT ACTION OF MAGNET AND CURRENT. 149

a 6 is the bar of iron ; the helix within which the bar was
suspended is shown in outline around it ; the arrow shows
the direction of the current in the upper half of the
helix ; its direction in the under half would, of course,
be the reverse.

On exciting the magnet, the bar of iron set itself
parallel to the line joining the poles, as shown by the un-
broken line in fig. 7.

When the direction of the current in the helix was
that indicated by the arrow, the bar was deflected towards
the position dotted in the figure.

Interrupting the current in the helix, and permitting
the magnet to remain excited, the bar returned to its
former position : the current was now sent through the
helix in the direction of the arrow, fig. 8 ; the consequent
deflection was towards the dotted position.

Both the current which excited the magnet and that
which passed through the helix were now interrupted, and

Fia. 8.

the polarity of the magnet was reversed. On sending a
current through the helix in the direction of the arrow,
the bar was deflected from the position of the defined line
to that of the dotted one, fig. 9.

Interrupting the current through the helix, and per-
mitting the bar to come to rest under the influence of the

150 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

magnet alone, a current was sent through the helix in a

FIG. 9.

direction opposed to its former one : the deflection (from
full to dotted outline) was that shown in fig. 10.

The oblique position of equilibrium finally assumed by
the bar depends, of course, upon the ratio of the forces

FIG. 10.

acting upon it : in these experiments, the bar, in its final
position, enclosed an angle of about 50 degrees with the
axial line.

A series of experiments was next made, in which the

NORMAL PARAMAGNETIC BAR.

151

bar was first acted on by the current passing through the
helix, the magnet being brought to bear upon it after-
wards. On the passage of the current through the helix
in the direction shown in fig. 11, the bar set its length
parallel to the axis of the latter. On exciting the magnet

FIG. 11.

so that its polarity was that indicated by the letters N and s
in the figure, the deflection was towards the dotted position.
Interrupting the current through both magnet and
helix, and reversing the current through the latter, the bar
came to rest, as before, parallel to the axis : on exciting

FIG. 12.

le magnet, as in the last case, the deflection was that
lown in fig. 12.

Preserving the same current in the helix, and reversing

152 DIAMAGNET1SM AND MAGNE-CRYSTALLIC ACTION.

the polarity of the magnet, the deflection was that shown
in fig. 13.

FIG. 13.

Preserving the magnet-poles as in the last experiment,
and reversing the current in the helix, the deflection was
that shown in fig. 14.

FIG 14.

Thus far the results might, of course, have been pre-
dicted ; but I am anxious to go through all the phases of
this disputed question, with the view of rendering the
comparison of paramagnetism and diamagnetism com-
plete, and the inference from experiment certain.

Normal Diamagnetic Bar.

Our next step is to compare with these effects the de-
portment of a normal diamagnetic body placed under the
same conditions.

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NORMAL DIAMAGNET1C BAB. 153

With the view of increasing the force, the helix was
removed from its lateral position and placed between the
two poles, as in fig. 6, p. 146. The normal diamagnetic
bar was suspended within the helix and submitted to the
self-same mode of examination as that applied in the case
of the paramagnetic body.

The polarity first excited was that shown by the letters
s and N (south and north) in fig. 9, Plate I., and the
position of rest, when the magnet alone acted, was at right
angles to the line joining the poles, as shown in unbroken
outline; on sending a current through the helix in the
direction of the arrow, the deflection was towards the posi-
tion dotted out.

Preserving the magnetic polarity as in the last experi-
ment, the direction of the current through the helix was
reversed, and the deflection was that shown in fig. 10,
Plate I. [In all cases the motion is to be regarded as
taking place from the position shown by the full line to
that shown by the dotted line.]

Reversing the polarity of the magnet, and sending the
current through the helix in the direction of the last ex-
periment, the deflection was that shown in fig. 11.

Preserving the last magnetic poles, and sending the
current through the helix in the opposite direction, the
deflection was that shown in fig. 12.

In the following four experiments the helix was excited
first.

Operated upon by the helix alone, the suspended bar
set its length parallel to the convolutions, and perpendicu-
lar to the axis of the coil, as shown by the unbroken out-
line : the direction of the current was first that shown in
fig. 13, Plate la. When the magnet was excited, the bar
was deflected towards the dotted position.

Interrupting both currents and permitting the bar to

154 DIAMAGNETISM AND MAGNE-CKYSTALLIC ACTION.

come to rest, then reversing the current in the helix, the
bar set as before parallel to the coils. When the magnet
was excited, as in the last experiment, the deflection was
that shown in fig. 14.

Preserving the helix current as in the last experiment,
when the polarity of the magnet was reversed, the deflec-
tion was that shown in fig. 1 5.

Interrupting both, and reversing the current in the
helix ; when the magnet was excited as in the last experi-
ment, the deflection was that shown in fig. 16.

In a paper on the ' Polarity of Bismuth,' published
in the ' Philosophical Magazine,' ser. 4, vol. ii., and in
Poggendorffs Annalen, vol. Ixxxvii., an experiment of
mine is recorded showing the deportment exhibited by fig.
11, Plate I. of the present series. In a recent memoir on the
same subject, M. v. Feilitzsch ! states that he has sought
this result in vain. Sometimes he observed the deflection
at the moment of closing the circuit, but conceived that it
must be ascribed to the action of induced currents ; for
immediately afterwards a deflection in the opposite direc-
tion was observed, which deflection proved to be the per-
manent one.

I have repeated the experiment here referred to with
all possible care ; and the result is certainly that described
in the remarks which refer to fig. 11. This result agrees
in all respects with that described in my former paper.
With a view to quantitative measurement, a small gra-
duated circle was constructed and placed underneath the bar
of bismuth suspended within the helix. The effect, as will
be seen, is not one regarding which a mistake could be
made on account of its minuteness : operating delicately,
and choosing a suitable relation between the strength of

1 Poggendorff's Annalen, vol. xcii. p. 395.

COMBINATIONS OP MAGNET AND CURRENT. 155

the magnet and that of the helix,1 on sending a current
through the latter as in fig. 11, the bismuth bar was de-
flected so forcibly that the limit of its first impulsion
reached 120° on the graduated circle underneath. [An
action entirely due to the extreme caution bestowed upon
the experiment, in which power and delicacy were com-
bined.] The permanent deflection of the bar amounted to
60° in the same direction, and hence the deportment could
in no wise be ascribed to induced currents, which vanish
immediately. Before sending the current through the
helix, the bar was acted on by the magnet alone, and
pointed to zero.

Though it was not likely that the shape of the
poles could have any influence here, I repeated the experi-
ment, using the hemispherical ends of two soft iron cores
as poles : the result was the same.

A pair of poles with the right and left-hand edges
rounded off showed the same deportment.

A pair of poles presenting chisel edges to the helix
showed the same deportment.

Various other poles were made use of, some of which
appeared to correspond exactly with those figured by M.
v. Feilitzsch ; but no deviation from the described deport-
ment was observed. To test the polarity of the magnet,
a magnetic needle was always at hand : once or twice the
polarity of the needle became reversed, which, had it not
been noticed in time, would have introduced confusion
into the experiments. Here is a source of error against
which, however, M. v. Feilitzsch has probably guarded
himself. Some irregularity of crystalline structure may
also have influenced the result. With ' chemically pure
zinc ' M. v. Feilitzsch obtained the same deflection that I

1 In most of these experiments the helix was excited by ten cells,
the magnet by two.

156 DIAMAGNETISM AND MAGNE-CHYSTALLIC ACTION.

obtained with bismuth : now chemically pure zinc is dia-
magnetic,1 and hence its deportment is corroborative of
that which I have observed. M. v. Feilitzsch, however,
appears to regard the zinc used by him as magnetic;
but if this be the case, it cannot have been chemically
pure. It is necessary to remark that I have called the
north pole of the electro-magnet that which attracts the
south, or unmarked end of a magnetic needle ; and I
believe this is the custom throughout Germany.

Abnormal Paramagnetic Bar.

This bar consisted of compressed carbonate of iron dust,
and was suspended within the helix with the line of com-
pression, which was its shortest dimension, horizontal. As
in the cases already described, it was first acted upon
by the magnet alone. Having attained its position of
equilibrium, a current was sent through the helix, and the
subsequent deflection was observed.

The magnet being excited as shown by the letters
s and N in fig. 17, Plate I., the bar sets its length
equatorial ; on sending a current through the helix in the
direction of the arrow, the bar was deflected to the dotted
position.

Eeversing the current in the helix, but permitting the
magnet to remain as before, the deflection was that shown
in fig. 18.

Interrupting all, and reversing the polarity of the
magnet ; on sending the current through as in the last
case, the deflection was that shown in fig. 19.

Reversing the current, but preserving the last condition
of the magnet, the deflection was that shown in fig. 20.

In the subsequent four experiments the helix was
excited first.

1 Phil. Mag. vol. xxviii. p. 456.

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ABNORMAL BARS OF BOTH CLASSES. 157

It is to be remembered that whatever might be the
direction of the current through the helix alone, the bar
always set its length perpendicular to the axis of the
latter, and parallel to the coils.

When the direction of the helix current, and the
polarity of the magnet, were those shown in fig. 21, Plate
la, the deflection was to the dotted position.

Interrupting all, and reversing the current in the
helix; on exciting the magnet the deflection was that
shown in fig. 22.

Changing the polarity of the magnet, and preserving
the helix current in its former direction, the deflection
was that shown in fig. 23.

Interrupting all, and reversing the current through the
helix ; when the magnetism was developed the deflection
was that shown in fig. 24.

Abnormal Diamagnetic Bar.

This bar consisted of a prism of bismuth whose prin-
cipal planes of crystallisation were perpendicular to its
length : the mode of experiment was the same as. that
applied in the other cases.

Acted upon by the magnet alone, the bar set its length
from pole to pole : the magnetic excitation being that de-
noted by the letters N s, fig. 29, Plate la, a current was
sent through the helix in the direction of the arrow ; the
bar was deflected to the dotted position.

Reversing the current through the helix, the deflection
was that shown in fig. 30.

Interrupting both currents and reversing the magnetic
poles ; on sending a current through the helix as in the
last experiment, the deflection was that shown in fig. 31.

Reversing the current through the- helix, the deflection
was that shown in fig. 32.

158 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

In the subsequent four experiments the helix was ex-
cited first.

Sending a current through the helix in the direction
denoted by the arrow, the bar set its length at right
angles to the convolutions, and parallel to the axis of the
helix ; when the magnetism was excited as in fig. 25,
Plate I., the deflection was to the dotted position.

When the current was sent through the helix in an
opposite direction, the deflection was that shown in fig. 26.

Interrupting both currents, and reversing the poles of
the magnet ; on sending a current through the helix as in
the last experiment, the deflection was that shown in
fig. 27.

Reversing the current in the helix, the deflection
was that shown in fig. 28.

In all these cases the position of equilibrium due
to the first force was attained before the second force was
permitted to act.

It will be observed, on comparing the deportment
of the normal paramagnetic bar with that of the normal
diamagnetic one, that the position of equilibrium taken
up by the latter, when operated on by the helix alone, is
the same as that taken up by the former when acted on by
the magnet alone : in both cases the position is from pole
to pole of the magnet. A similar remark applies to
the abnormal para- and diamagnetic bars. It will render
the distinction between the deportment of both classes of
bodies more evident, if the position of the two bars, before
the application of the second force, be rendered one and
the same. When both the bars, acted on by one of the
forces, are axial, or both equatorial, the contrast or co-
incidence, as the case may be, of the deflections from
this common position, by the second force, will be strikingly
manifest.

ACTIONS THROUGHOUT ANTITHETICAL. 159

To effect the comparison in the manner here indicated,
the figures have been collected together and arranged upon
Plate I. and Plate la. The first column represents the
deportment of the normal paramagnetic bar under all the
conditions described ; the second column, that of the
normal diamagnetic bar ; the third shows the deportment
of the abnormal paramagnetic bar, and the fourth that
of the abnormal diamagnetic bar.

A comparison of the first two columns shows us that
the deportment of the normal magnetic bar is perfectly
antithetical to that of the normal diamagnetic one. When,
on the application of the second force, an end of the former
is deflected to the right, the same end of the latter is de-
flected to the left. When the position of equilibrium of the
magnetic bar, under the joint action of the two forces, is
from N.E. to S.W., then the position of equilibrium
for the diamagnetic bar is invariably from N.W. to S.E.
There is no exception to this antithesis, and I have been
thus careful to vary the conditions of experiment in all
possible ways, on account of the divergent results obtained
by other inquirers. In his recent memoirs upon this sub-
ject M. v. Feilitzsch states that he has found the deflection
of diamagnetic bodies, under the circumstances here de-
scribed, to be precisely the same as that of paramagnetic
bodies : this result is of course opposed to mine ; but when
it is remembered that the learned German worked con-
fessedly with the 'roughest apparatus,' and possessed no
means of eliminating the effects of structure, there seems
little difficulty in referring the discrepancy between us to
its proper cause.

The same perfect antithesis will be observed in the case
of the abnormal bars, on a comparison of the third and
fourth columns. In all cases then, whether we apply the
magnet singly, or the current singly, or the magnet and
current combined, the deportment of the normal dia-

160 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTIOX.

magnetic bar is opposed to that of the normal para-
magnetic one, and the deportment of the abnormal para-
magnetic bar is opposed to that of the abnormal dia-
magnetic one. But if we compare the normal para-
magnetic with the abnormal diamagnetic bar, we see that
the deportment of the one is identical with that of the
other.1 The same identity of action is observed when the
normal diamagnetic bar is compared with the abnormal
paramagnetic one. The necessity of taking molecular
structure into account in experiments of this nature could
not, I think, be more strikingly exhibited.

For each of the bars, under the operation of the two
forces, there is an oblique position of equilibrium : on the
application of the second force, the bar swings like a pendu-
lum beyond this position, oscillates through it, and finally
comes to rest there. Hence, if before the application of the
second force the bar occupy the axial position, the deflec-
tion, when the second force is applied, appears to be from
the axis to the equator ; but if it first occupy the
equatorial position, the deflection appears to be from the
equator to the axis.

It has been already shown that the repulsion of dia-
magnetic bodies is to be referred to a state of excitement
induced by the magnet, and it has been long known that
the attraction of paramagnetic bodies is due to the same
cause. The experiments just described exhibit to us bars
of both classes of bodies moving in the magnetic field :
such motions occur in virtue of the induced state of the

1 Identical to the eye, but not to the mind. T e notion appears to
be entertained by some, that, by changing molecular structure, I had
actually converted paramagnetic substances into diamagnetic ones, and
vice versa. No such change, however, can cause tJie mass of a diamag-
netic body suspended by its centre of gravity to be attracted, or the
mass of a paramagnetic body to be repelled. But by a change of mole-
cular structure, one of the forces may be so caused to apply itself that
it shall present to the eye all the directive phenomena exhibited by the
other.— J. T., May 5, 1855.

ATTACK ON PROBLEM VARIED. 161

body, and the relation of that state to the forces which
act upon it. We have seen that in all cases the anti-
thesis between both classes of bodies is maintained.
Whatever, therefore, the state of the paramagnetic bar
under magnetic excitement may be, a precisely antithe-
tical state would produce all the phenomena of the
diamagnetic bar. If the bar of iron be polar, a reverse
polarity on the part of bismuth would produce the effects
observed. From this point of view all the movements of
diamagnetic bodies become perfectly intelligible, and the
experiments to be recorded in the next chapter are not
calculated to invalidate the conclusion that diamagnetic
bodies possess a polarity opposed to that of magnetic
ones.

The phenomena to which we have thus far referred
consist in the rotations of elongated bars about their axes
of suspension. The same antithesis, however, presents
itself when we compare the motion of translation of a
paramagnetic body, within the coil, with that of a dia-
magnetic one. A paramagnetic sphere was attached to
the end of a horizontal beam and introduced into the
coil : the magnet being excited, the sphere could be made
to traverse the space within the coil in various directions,
by properly varying the current through the coil. A
diamagnetic sphere was submitted to the same examina-
tion, and it was found that the motions of both spheres,
when operated on by the same forces, were always in
opposite directions.

FURTHER COMPARISON OF PARAMAGNETIC AND DIAMAGNETTC-
PHENOMENA : — DIAMAGNETIC POLARITY.

On sending a current through a helix within which
is placed an iron bar, the latter is converted into a
magnet, one end of the bar thus excited being attracted,
and the other end repelled by the same magnetic pole.

162 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

In this twonesa of action consists what is called the
polarity of the bar : we will now consider whether a bar
of bismuth exhibits a similar duality.

Fig. 39, Plate II. represents, in plan, the disposition of
the apparatus used in the examination of this question. A u
is a helix, formed of covered copper wire one-fifteenth of an
inch in thickness : the length of the helix is 5 inches, the
external diameter 5 inches, and internal diameter 1 '5 inch.
Within this helix a cylinder of bismuth 6^ inches long
and 0-4 of an inch in diameter was suspended. The
suspension was effected by means of a light beam, from
two points of which, sufficiently distant from each other,
depended two silver wires each ending in a loop : into
these loops, II', the bar of bismuth was introduced, and
the whole was suspended by a number of fibres of unspun
silk from a suitable point of support. Fig. 39a is a side
view of the arrangement used for the suspension of the bar.
Before introducing the bar within the helix, it was first sus-
pended in a receiver, which protected it from air currents,
and in which it remained until the torsion of the suspending
fibre had exhausted itself : the bar was then removed, and
the beam, without permitting the silk to twist again, was
placed over the helix, the bismuth bar being then intro-
duced through the latter, and through the wire loops. From
the ends of this helix two wires passed to a current reverser
B, from which they proceeded to the poles of a voltaic
battery, c D and E F are two electro-magnetic helices, each
12 inches long, 5| inches external and 2 inches internal
diameter. The wire composing them is one-tenth of an
inch thick, and so coiled that the current could be sent
through four wires simultaneously. Within these helices
were introduced two cores of soft iron 2 inches thick and
] 4 inches long : the ends of the cores appear at p and p'.
The helices were so connected that the same current
excited both, thus developing the same magnetic strength

ACTION OF MAGNET OX DIAMAGNET. 163

in the poles p p'. From the ends of the helices wires
proceeded to a second current reverser n', and thence to a
second battery of considerably less power than the former.
By means of the reverser R' the polarity of the cores
could be changed; p' could be converted from a south
pole to a north pole, at the same time that p was con-
verted from north to south. Lastly, by a change of the
connections between the two helices, the cores could be
so excited as to make the poles of the same quality, both
north or both south.

The diameter of the cylindrical space, within which
the bismuth bar was suspended, was such as to permit of
a free play of the ends of the bar through the space of an
inch and a half. Having seen that the bar swung with-
out impediment, and that its axis coincided as nearly as
possible with the axis of the helix, A B, a current from the
battery was sent through the latter. The magnetism of
the cores p and p/ was then excited, and the action upon
the bismuth bar observed. M. v. Feilitzsch has attempted
a similar experiment to that here described, but without
success : when, however, sufficient power is combined with
sufficient delicacy, the success is complete, and the most
perfect mastery is obtained over the motions of the bar.

The helix above described as surrounding the bismuth
bar is the one which I have found most convenient for
these experiments; various other helices, however, were
tried, with a result equally certain, if less energetic. The
one first made use of was 4 inches long, 3 inches exterior
diameter, and three-quarters of an inch interior diameter,
with wire one-fifteenth of an inch in thickness, the bar
being suspended by a fibre which passed through a slit in
the helix: sending through this helix a current from a
battery of ten cells, and exciting the cores by a current
from one cell, the phenomena of repulsion and attraction
were exhibited with all desirable precision.

164 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

I will now describe the results obtained by operating
in the manner described. The bismuth bar being
suitably suspended, a current was sent through the
helix, so that the direction of the current in the upper
half was that indicated by the arrow in fig. 40, PI. Ha.
On exciting the magnet, so that the pole N was a north
pole and the pole s a south pole, the ends of the bar of
bismuth were repelled. The final position of the bar was
against the side of the helix most remote from the
magnets : it is shown by dots in the figure.

By means of the reverser R the current was now sent
through the helix in the direction shown in fig. 41 : the
bar promptly left its position, crossed the space in which
it could freely move, and came to rest as near the mag-
nets as the side of the helix would permit it. It was
manifestly attracted by the magnets.

Permitting the current in the helix to flow in the last
direction, the polarity of the iron cores was reversed. We
had then the state of things sketched in fig. 42. The
bismuth bar instantly loosed from the position it formerly
occupied, receded from the magnet, and took up finally
the position marked by the dots.

After this new position had been attained, the current
through the helix was reversed : the bar promptly sailed
across the field towards the magnets, and finally came to
rest in the dotted position, fig. 43. In all these cases,
when the bar was freely moving in any direction, under
the operation of the forces acting upon it, the reversion
either of the current in the helix, or of the polarity of the
cores, arrested the motion ; approach was converted into
recession, and recession into approach.

The ends of the helix in these experiments were not
far from the ends of the soft iron cores ; and it might
therefore be supposed that theu action was due to some
modification of the cores by the helix, or of the helix by

Rar of Jron,

flak II c
Bar of Bi'.rtrmth .

4O .

CRUCIAL EXPERIMENTS. 165

the cores. It is manifest that the magnets can have no
permanent effect upon the helix ; the current through
the latter, measured by a tangent galvanometer, is just as
strorjg when the cores are excited as when they are un-
excited. The helix may certainly have an effect upon the
cores, and this effect is either to enfeeble the magnetism
of the cores or to strengthen it ; but if the former, and
if the bar were the simple bismuth which it is when
no current operates on it, the action, though weakened,
would still be repulsive ; and if the latter, the increase
would simply augment the repulsion. The fact, however,
of the ends of the bar being attracted, proves that the bar
has been thrown into a peculiar condition by the current
circulating in the surrounding coil. Changing the direc-
tion of the current in the coil, we find that the self-same
magnetic forces which were formerly attractive are now
repulsive ; to produce this effect the condition of the bar
must have changed with the change of the current ; or,
in other words, the bar is capable of accepting two differ-
ent states of excitement, which depend upon the direction
of the current.

In order, however, to reduce as far as possible the
action of the helix upon the cores, I repeated the experi-
ments with the small helix referred to in fig. 6, page 146.
It will be remembered that this helix is but an inch in
length, and that the bismuth bar is 6^ inches long. I
removed the magnets further apart, so that the centres of
the cores were half an inch beyond the ends of the bismuth
bar, while the helix encircled only an inch of its central
portion : in this position, when the helix was excited,
there was no appreciable magnetism excited by it in
the dormant cores ; at least, if such were excited, it was
unable to attract the smallest iron nail. Here then we
had cores and helix sensibly independent of each other,
but the phenomena appeared as before. The bar could

166 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

be held by the cores against the side of the helix, with its
ends only a quarter of an inch distant from the ends of
the cores; on reversing either of the currents the ends
instantly receded, but the recession could be stopped by
again changing the direction of the current. With a
tranquil atmosphere, and an arrangement for reversing
the current without shock or motion, the bar obeyed in an
admirable manner the will of the experimenter, and, under
the operation of the forces indicated, exhibited all the
deflections sketched in figs. 40, 41, 42, and 43, Plate Ila.

That the motion of the bar could not be referred to the
action of induced currents was readily proved. The bar was
brought into the centre of the hollow cylinder in which it
swung, and held there, with the forces in action, until
all phenomena of induced currents had long passed away ;
the arrangement of the forces being that shown in fig. 40,
on releasing the bar it was driven from the cores, whereas
when the arrangement was that shown in fig. 41, it was
drawn towards them.

But it does not sufficiently express the facts to say
that the bar is capable of two different states of excite-
ment ; it must be added, that both states exist simul-
taneously in the excited bar. It has been already proved,
that the state corresponding to the action of one pole is
not that which enables an opposite pole to produce the
same action ; hence, when the two ends of the bar are
attracted or repelled, at the same time, by two opposite
poles, it is a proof that these two ends are in opposite
states. But if this be correct, we can test our conclusion
by reversing one of the poles : the direction of its force
being thereby changed, it ought to hold the other pole in
check and prevent all motion in the bar. This is the
case : if, in any one of the instances cited, the polarity of
either of the cores be altered ; if the south be converted
into a north, or the north into a south pole, thus making

DIAMAGiSETIC POLARITY PROVED. 167

both poles of the same quality, the repulsion of the one is
so nearly balanced by the attraction of the other, that the
bar remains without motion towards either of them.

To carry the argument a step further, let us fix our
attention for an instant upon fig. 40. The end of the
bar nearest to the reader is repelled by a south pole ; the
same end ought to be attracted by a north pole. In like
manner, the end of the bar most distant from the reader
is repelled by a north pole, and hence the state of that
end ought to fit it for attraction by a south pole. If,
therefore, our reasoning be correct, when we place a north
pole opposite to the near end of the bar, and on the same
side of it as the upper north pole, and a south pole
opposite the further end of the bar and on the same side
of it as the lower south pole, the simultaneous action of
these four poles ought to be more prompt and energetic
than when only two poles are used. This arrangement is
shown in Plate III. : the two poles to the right of the
bismuth bar must be of the same name, and the two to
the left of the bar of the opposite quality. If those to the
right be both north, those to the left must be both south,
and vice versa. The current reverser for the magnets
appears in front, that for the helix is hidden by the
figure. The above conclusion is perfectly verified by
experiments with this apparatus, and the twofold deflec-
tion of the bismuth bar is exhibited with remarkable
energy.1

The bar used in these cases is far heavier than those
commonly employed in experiments on diamagnetism,
but the dimensions stated do not mark the practical limit

1 With careful manipulation these experiments, and almost all the
others mentioned in this memoir, may be exhibited in the lecture-room.
By attaching indexes of wood to the bars of bismuth, and protecting
the indexes from air currents by glass shades, the motions may be
made visible to several hundred persons at the same time. See a
description of aPolymagnet, Phil. Mag., June 1855.— J. T.

168 DIAMAGNETISM AND MAQNE-CRYSTALLIC ACTIOX.

of the size of the bar. A solid bismuth cylinder, 14
inches long and 1 inch in diameter, was suspended in a
helix 5*7 inches long, 1*8 inch internal diameter, 4 inches
external diameter, and composed of copper wire 0-1 of an
inch in thickness. When a current of twenty cells was
sent through the helix, and the magnets (only two of
them were used) were excited by one cell, all the phe-
nomena exhibited by figs. 40, 41, 42, and 43 were dis-
tinctly exhibited.

A considerable difference is always necessary between
the strength of the current which excites the bismuth and
that which excites the cores, so as to prevent the induction
of the cores, which of itself would be followed by repul-
sion^ from neutralising, or perhaps inverting, the induc-
tion of the helix. When two magnets were used and the
bismuth was excited by ten cells, I found the magnetic
excitement by one or two cells to be most advantageous.
When the cores were excited by ten, or even five cells, the
action was always repulsive.

The deportment of paramagnetic bodies is so well
known, that it might be left to the reader to discern that
in all the cases described it is perfectly antithetical to
that of the diamagnetic body. I have nevertheless
thought it worth while to make the corresponding
experiments with an iron bar ; and to facilitate com-
parison, the results are placed in Plate Ila. side by side
with those obtained with the bar of bismuth. It must be
left to the reader to decide whether throughout this
inquiry the path of strict inductive reasoning has not
been adhered to : if this be the case, then the inference
appears unavoidable : —

That the diamagnetic force is a polar force, the
polarity of diamagnetic bodies being opposed to that of
paramagnetic ones under the same conditions of ex-
citement.

BETENTION OF DIAMAGNETIC POLARITY. ICO

NOTE.

I would gladly refer to M. Pliicker's results in connection with this
subject had I been successful in obtaining them ; I will here, how-
ever, introduce the description of his most decisive experiment in his
own words. (See Scien. Mem. New Ser. p. 336.)

' From considerations of which we shall speak afterwards, it appeared
to me probable that bismuth not only assumes polarity in the vicinity
of a magnetic pole, but that it also retains the polarity for some time
after the excitation has taken place ; or, in other words, that bismuth
retains a portion of its magnetism permanently, as steel, unlike soft
iron, retains a portion of the magnetism excited in it by induction.
My conjecture has been corroborated by experiment.

' I hung a bar of bismuth, 15 millims. long and 5 millims. thick,
between the pointed poles of the large electro-magnet ; it was sus-
pended horizontally from a double cocoon-thread (fig. 1). The distance
between the points was diminished until the bar could barely swing
freely between them. A little rod of glass was brought near to one of

the points, so that the bis-
muth bar, before the mag-
netism was excited, and in
consequence of the torsion,
leaned against the glass rod.
On exciting the magnet by a
current of three of Grove's

elements, the bismuth, prevented from assuming the equatorial position,
pressed more forcibly against the glass rod ; when the current was
interrupted, the bar remained still in contact with the rod, while its
free end vibrated round its position of equilibrium. The current
was closed anew and then reversed by a gyrotrope. In consequence of
this reversion, the bar of bismuth, loosening from the glass rod, moved
towards the axial position, but soon turned and pressed against the
glass as before, or in some cases having passed quite through the axial
position was driven round with the reversed ends into the equatorial.
.... This experiment, which was made with some care, proves
that the bismuth requires time to reverse its polarity.'

I have repeated this experiment with great care, and have obtained
in part the effect described : it is perfectly easy to produce the rotation
of the bar. The cause of this rotation, however, was in my case as
follows : — When the magnet was unexcited, the position of equili-
brium of the axis of the bar acted upon by the torsion of the fibre was
that shown by the dotted line in the figure ; when the magnetism was
developed, the repulsive force acting on the free end of the bar neces-
sarily pushed it beyond the dotted line— an action which was perfectly
evident when the attention was directed towards it. On reversing the
current, a little time was required to change the polarity of the iron

170 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

masses ; during this time the free end of the bismuth fell towards its
former position, and the velocity required was sufficient to carry it
quite beyond the pole points. The only difference between M. Pliicker
and myself is, that I obtained the same result by simply intercepting
the current as by reversing it. I may remark that I have submitted
ordinary bismuth to the most powerful and delicate tests, but as yet I
have never been able to detect in it a trace of that retentive power
ascribed to it by M. Pliicker.

ON W. WEBER'S THEORY OF DIAMAGNETIC POLARITY,1 AND
ON AMPERE'S THEORY OF MOLECULAR CURRENTS.

If we reflect upon the experiments recorded in the
foregoing pages from first to last ; on the inversion of
magne-crystallic phenomena by the substitution of a
magnetic constituent for a diamagnetic ; on the analogy
of the effects produced in magnetic and diamagnetic
bodies by compression ; on the antithesis of the rotating
actions described near the commencement ; on the in-
dubitable fact that diamagnetic bodies, like magnetic
ones, owe their phenomena to an induced condition into
which they are thrown by the influencing magnet, and
the intensity of which is a function of the magnetic
strength; on the circumstance that this excitation, like
that of soft iron, is of a dual character ; on the numerous
additional experiments which have been recorded, all
tending to show the perfect antithesis between the two
classes of bodies ; we can hardly fail to be convinced
that Faraday's first hypothesis of diamagnetic action is
the true one — that diamagnetic bodies operated on by
magnetic forces possess a polarity ' the same in kind as,
but the reverse in direction of that acquired by magnetic
bodies.' But if this be the case, how are we to conceive
of the physical mechanism of this polarity ? According
to Coulomb's and Poisson's theory, the act of magnetisa-

1 Poggendorff's Annalen, vol. Ixxxvii. p. 145, and Taylor's Scientific
Memoirs, New Ser. p. 163.

THEORY OF W. WEBER. 171

tion consists in the decomposition of a neutral magnetic;
fluid ; the north pole of a magnet, for example, possesses
an attraction for the south fluid of a piece of soft iron
submitted to its influence, draws the said fluid to-
wards it, and with it the material particles with which
the fluid is associated. To account for diamagnetic
phenomena this theory seems to fail altogether ; accord-
ing to it, indeed, the oft-used phrase, ' a north pole
exciting a north pole, and a south pole a south pole,'
involves a contradiction. For if the north fluid be sup-
posed to be attracted towards the influencing north pole,
it is absurd to suppose that its presence there could pro-
duce repulsion. The theory of Ampere is equally at a
loss to explain diamagnetic action ; for if we suppose the
particles of bismuth surrounded by molecular currents,
then according to all that is known of electro-dynamic
laws, these currents would set themselves parallel to, and
in the same direction as those of the magnet, and hence
attraction, and not repulsion, would be the result. The
fact, however, of this not being the case proves that these
molecular currents are not the mechanism by which dia-
magnetic induction is effected. The consciousness of this,
I doubt not, drove M. Weber to the assumption that the
phenomena of diamagnetism are produced by molecular
currents, not directed, but actually excited in the bismuth
by the magnet. Such induced currents would, according
to known laws, have a direction opposed to those of the
inducing magnet, and hence would produce the pheno-
mena of repulsion. To carry out the assumption here
made, M. Weber is obliged to suppose that the molecules
of diamagnetic bodies are surrounded by channels, in
which the induced molecular currents, once excited, con-
tinue to flow without resistance.

This theory, notwithstanding its great beauty, is so
extremely artificial, that I imagine the general conviction

172 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

of its truth cannot be very strong; but there is one con-
clusion flowing from it which appears to me to be in
direct opposition to experimental facts. The conclusion
is ' that the 'magnetism of two iron particles in the line
of magnetisation is increased by their reciprocal action ;
but that, on the contrary, the diamagnetism of two bis-
muth particles lying in this direction is diminished
by their reciprocal action.'' The reciprocal action of the
particles varies inversely as the cube of the distance
between them; at a distance expressed by the number 1,
for example, the enfeeblement is eight times what it
would be at the distance 2.

The conclusion, as regards the iron, is undoubtedly
correct ; but I believe experiment proves that the mutual
action of diamagnetic molecules, when caused to approach
each other, increases their repulsive action. I have had
massive iron moulds1 made and coated with copper
electrolytically ; into these fine bismuth powder has been
introduced and submitted to powerful hydraulic pressure.
No sensible fact can, I think, be more certain than that
the particles of this dust are brought into closer proximity
along the Kne in which the pressure is exerted, and this is
the line of strongest diamagnetisation. If a portion of
the compressed mass be placed upon the end of a torsion
beam and the amount of repulsion measured, it will be
found that the repulsion is a maximum when the line of
magnetisation coincides with the line of compression ; or,
in other words, with that line in which the particles are
packed most closely together ; if the bismuth were fixed,
and the magnet movable, the former would repel the
latter with a maximum force when the line of compres-
sion is parallel to the direction of magnetisation. It is a
stronger diamagnet in this direction than in any other.
Cubes of bismuth which, in virtue of their crystallisation,
1 For drawings of these moulds see a future page.

POLARITY BY INDUCED CURRENTS. 173

possessed a line of minimum magnetisation, have been
placed in those moulds and pressed closely together in
the direction of the said line : the approximation of the
particles thus affected has converted the direction spoken
of from one of minimum into one of maximum magne-
tisation. It would be difficult for me to say how many
diamagnetic bodies I have submitted to compression,
some massive, some in a state of powder, but in no single
instance have I discovered an exception to the law that
the line of compression of purely diamagnetic bodies is
the line of strongest diamagnetisation. The approxima-
tion of diamagnetic particles is therefore accompanied by
an augmentation of their power, instead of a diminution of
it, as supposed by the theory of M. Weber.

It is scarcely possible to reflect upon the discovery of
Faraday in all its bearings, without being deeply im-
pressed with the feeling that we know absolutely nothing
of the physical causes of magnetic action. We find the
magnetic force producing, by processes which are evidently
similar, two great classes of effects. We have a certain
number of bodies which are attracted by the magnet, and
a far greater number which are repelled by the same
agent. Supposing these facts to have been known to
Ampere, would he have satisfied his profound mind by
founding a theory which accounts for only the smaller
portion of them ? This theory is admirable as far as it
goes, but the generalisation is yet to come which shall
show the true relationship of phenomena, towards whose
connection the theory of Ampere furnishes at present no
apparent clue.

ON M. MATTEUCCI'S OBJECTIONS.

The foregoing memoir was on the point of leaving
my hands for the Royal Society, when accident, backed
by the kindness of Faraday, placed the Cours special

174 DIAMAGNETISM AND MAGNE-CUYSTALLIC ACTION.

of M. Matteucci, recently published in Paris, in my hands.
An evening's perusal of this valuable work induces
me to append the following remarks to the present
paper.

M. Matteucci honours the researches which bear my
name, and those which I published in connection with M.
Knoblauch, with a considerable share of his attention.
He corroborates all the experimental facts, but at the
conclusion states three objections to the manner in which
these facts have been explained. c La faveur,' writes the
learned Italian, ' avec laquelle les idees de MM. Tyndall
et Knoblauch ont ete accueillies m'imposent le devoir de
ne pas vous laisser ignorer les objections qui s'elevent
contre elles. La premiere consiste dans la difference tres-
grande et constante dans la force qui fait osciller entre les
poles une aiguille de bismuth cristallise, suivant que ses
clivages paralleles a sa longueur sont suspendus verticale-
ment ou dans un plan horizontal: cette difference me
parait inconciliable avec le resultat deja rapporte de 1'ex-
perience de M. Tyndall, sur lequel se fonde 1'explication
des phenomenes magneto-cristallises. Mais une objection
encore plus grave est celle du mouvement ^attraction l
vers les poles qui se manifeste dans les prismes de bismuth
cristallise dont les clivages sont perpendiculaires a leur
longueur. Pour rendre la consequence de cette derniere
experience encore plus evidente, j'ai fixe deux cubes de
bismuth, qui ont deux faces opposees naturelles et parall-
eles aux plans de clivage, aux extremites d'un petit levier
de verre, ou de sulphate de chaux, suspendu par un fil de
cocon au milieu du champ magnetique entre les extre-
mites polaires d'un electro-aimant (fig. 27a) ; lorsque
les deux cubes ont les clivages verticaux et perpendiculaires
a la longueur de 1'aiguille, au moment ou le circuit est

1 This is in reality not a 'movement of attract'ion.'1 — See Appendix
to the present paper. — J. T., May 1855.

MATTEUCCIS EXPERIMENTS AND OPINIONS.

175

FIG. 270.

ferme, 1'aiguille est attiree, quelle que soit la position
quelle occupe dans le champ magnetique, et se fixe en
equilit re dans la ligne

polaire II me

semble impossible d'ex-

pliquerces inouvements

du bismuth cristallise,

comme on a essaye de

le faire, par la force

repulsive de 1'aimant,

qui, suivant 1'experi-

ence de M. Tyndall,'

s'exerce avec pi as d'in-

tensite parallelement aux clivages que dans la direction

perpendiculaire a ces plans.

' Remarquons encore qu'on ne trouve pas constamment
1'accord qui devrait exister, selon les idees de MM.
Tyndall et Knoblauch, entre les phenomenes magneto-
cristallises et les effets produits par la compression dans le
bismuth, si 1'on considere ces plans de clivage et la ligne
suivant laquelle la compression a eu lieu comme jouissant
des memes proprietes.' 2

With regard to the first objection I may say that it is
extremely difficult to meet one so put ; it is simply an
opinion, and I can scarcely say more than that mine does
not coincide with it. I would gladly enter upon the
subject and endeavour to give the objection a scientific
form were the necessary time at my disposal, but this, I
regret to say, is not the case at present. I shall moreover
be better pleased to deal with the objection after it has
assumed a more definite form in the hands of its proposer,
for I entertain no doubt that it is capable of a sufficient
answer. The second objection M. Matteucci considers to

1 This was first proved by Mr. Faraday. — J. T.
* Court special sur rintroductimt, etc., p. 255.

176 DIAMAGNETISM AND MAGNE-CEYSTALLIC ACTION.

be a more grave one. The facts are as follows : — The
repulsion of a mass of crystallised bismuth depends upon
the direction in which the mass is magnetised. When the
magnetising force acts in a certain direction, the intensity
of magnetisation, and the consequent repulsion of the
mass, is a maximum. This is proved by placing the mass
upon the end of a torsion beam and bringing its several
directions successively into the line of the magnetic force.
Poisson would have called such a direction through the
mass a principal axis of magnetic induction, and it has
been elsewhere called a line of elective polarity. When a
sphere or cube of bismuth is freely suspended in the mag-
netic field, with the direction referred to horizontal, in all
positions, except two, the forces acting on the mass tend
to turn it ; those positions are, when the line of maximum
magnetisation is axial and when it is equatorial, the for-
mer being a position of unstable, and the latter a position
of stable equilibrium. When the above line is oblique to
the direction of magnetisation, the sphere or cube will
turn round its axis of suspension until the direction re-
ferred to has set itself at right angles to the line joining
the poles. Now if the direction of maximum magnetisa-
tion be transverse to an elongated mass of bismuth, such
a mass must, when the said direction recedes to the equator,
sets its length from pole to pole. The facts observed by
M. Matteucci seem to me to be a simple corroboration of
this deduction.

The third objection is directed against an imaginary
case, ' si 1'on considere les plans de clivage et la ligne de
compression comme jouissant des memes proprietes.' It
must be evident that a crystal like bismuth, possessing a
number of cleavages of unequal values, cannot be compared
in all respects with a body which has suffered pressure in
one direction only. I have no doubt whatever, that, by a
proper application of pressure in different directions, a

MUTUAL INDUCTION OP PARTICLES. 177

compressed mass might be caused to imitate to perfection
every one of the actions exhibited by crystallised bismuth.
Indeed, I would go further, and say, that I shall be happy
to undertake to reproduce, with bismuth powder, the de-
portment of any diamagnetic crystal whatever that M.
Matteucci may think proper to name.

In looking further over M. Matteucci's instructive
book, I find another point alluded to in a manner which
tempts me to make a few remarks in anticipation of a
fuller examination of the subject. The point refers to the
reciprocal action of the particles of magnetic and diamag-
netic bodies. It is easy to see, that if the attraction of a
bar of iron varies simply as the number of the molecules
attracted, then, inasmuch as the weight of the body varies
in the same ratio, and the moment of inertia as the weight,
the times of oscillation of two masses of the same length,
but possessing different numbers of attracting particles,
must be the same. Coulomb indeed mixed iron filings
with wax, so as to remove the particles out of the sphere
of their mutual inductive action, and proved that when
needles of equal lengths, but of different diameters, were
formed from the same mixture, the duration of an oscilla-
tion was the same for all. From this he inferred that the
attractive force is simply proportional to the number of
ferruginous particles ; but this could not be the case if
these particles exerted any sensible reciprocal action,
either tending to augment or diminish the induction due
to the direct action of the magnet. On account of such a
mutual action, two bars of solid iron, of the same length,
and of different diameters, have not the same time of
oscillation.

In examining the question whether the particles of
diamagnetic bodies exert a similar reciprocal action, M.
Matteucci fills quills of the same length, and of different
diameters, with powdered bismuth, and finds that there is

178 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

no difference between the duration of an oscillation of the
thick ones and the slender ones ; from this he infers that
there can be no reciprocal action among the particles of
the bismuth.

Now it is not to be imagined that even in Coulomb's
experiments with the iron filings the molecular induction
was absolutely nothing, but simply that it was so enfeebled
by the separation of the particles that it was insensible in
the experiments. This remark applies with still greater
force to M. Matteucci's experiments with the bismuth
powder ; for the enfeeblement of a force already so weak,
by the division of the diamagnetic mass into powder, must
of course practically extinguish all reciprocal action of
the particles, even supposing a weak action of the kind to
exist when the mass is compact.

I will not here refer to my own experiments on com-
pressed bismuth, but will take a result arrived at by
M. Matteucci himself while repeating and corroborating
these experiments. ' I made,' says M. Matteucci, ' two
cylinders of bismuth precisely of the same dimensions, the
one compressed, the other in its natural state, and found
that the compressed mass had a diamagnetic power dis-
tinctly superior to that of natural bismuth.' * Now M.
Matteucci, in his Cours special, has made his own choice
of a test of reciprocal molecular action ; he assumes that if
cylinders of the same length, but of different masses, have
equal times of oscillation, it is a conclusive proof that there
is no action of the kind referred to. This necessarily
implies the assumption, that were the times of oscillation
different, a reciprocal action would be demonstrated.
According to his experiments described in the Association
Eeport, the times of oscillation are different ; the dia-
inagnetism of the compressed cylinder is ' distinctly su-

1 Eeport of Brit. Assoo. for 1852, Transactions of Sections, p. 7.

CRUCIAL TESTS OF DIAMAGNETIC POLARITY. 179

perior ' to that of the uncompressed one : the diamag-
netic effect increases in a greater proportion than the
quantity of matter ; and hence, on M. Matteucci's own
principles, the result negatived by his experiments on
powdered bismuth is fairly established by those which he
has made with the compressed substance.

FURTHER REFLECTIONS.

Reflecting further on the subject of diamagnetic po-
larity, an experiment occurred to me which constitutes a
crucial test to which the conclusions arrived at in the fore-
going memoir may be submitted.

Two square prisms of bismuth, 0*43 of an inch long
and O2 of an inch wide, were laid across the ends of a thin
plate of cedar wood, and fastened there by white wax.
Another similar plate of wood was laid over the prisms,
and also attached to them by wax ; a kind of rectangular
box was thus formed, 1 inch long and
of the same width as the length of the
prisms, the ends of the box being formed
by the latter, while its sides were
open. Both plates of wood were pierced

r bection.

through at the centre, and in the

aperture thus formed a wooden pin was

fixed, which could readily be attached

to a suspending fibre. Fig. 1 represents the arrangement

both in plan and section.

The prisms first chosen were produced by the compres-
sion of fine bismuth powder, without the admixture of gum
or any other foreign ingredient, the compressed mass being
perfectly compact and presenting a surface of metallic
brilliancy. Placed on the end of a torsion balance, with a
magnetic pole brought to bear upon it, the repulsion of
such a mass is a maximum when the direction in

180 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

which the mass has been squeezed is in the continuation
of the axis of the magnet. A comparative view of the
repulsion in this direction, and in another perpendicular to
it, is given in the following table : —

Compressed Bismuth powder.
Repulsion

Strength of magnet line of pressure axial Line of pressure equatorial

5-8 22 13

8-4 46 31

10-0 67 46

11-9 98 67

We seejiere that the repulsion, when the line of pres-
sure is axial, exceeds what occurs when the same line is
equatorial by fully one-half the amount of the latter.
Now this can only be due to the more intense magnetisa-
tion, or rather diamagnetisation, of the bismuth along
the line of pressure ; and in the experiment now to be
described, I availed myself of this fact to render the
effect more decided.

The prisms of bismuth were so constructed that the line
of pressure was parallel to the length of each prism. The

rectangular box
above referred to
was suspended
from its centre
of gravity in the
magnetic field,
so that the two
prisms were in
the same hori-
zontal plane. Let
the position of
the box thus sus-
pended be that

shown in fig. 2. For the sake of simplicity, we will con-
fine our attention to the action of one of the poles M, which

FIG. 2.

ANOMALIES DEVISED. 181

may be either flat or rounded, upon the prism hf adjacent
to it, as indeed all the phenomena to be described can be
produced before a single pole. The direction of the force
emanating from N is represented by the arrows ; and if
this force be purely repulsive, the action upon every single
particle of the diamagnetic mass furnishes a ' moment'
which, in the position here assumed, tends to turn the rec-
tangular box in the direction marked by the full arrow
above. It is perfectly impossible that such a system of
forces could cause the box to turn in a direction opposed
to the arrow; yet this is the direction in which the box
turns when the magnetic force is developed.

Here, then, we have a mechanical effect which is ab-
solutely inexplicable on the supposition that the dia-
magnetic force is purely repulsive. But if the conclusions
arrived at in the foregoing memoir be correct, if the dia-
magnetic force be a polar force, then we must assume that
attraction and repulsion are developed simultaneously,
as in the case of ordinary magnetic phenomena. Let us
examine how this assumption will affect the analysis of the
experiment before us.

The marked end of a magnetic needle is pulled to-
wards the north magnetic pole of the earth ; and yet, if the
needle be caused to float upon a liquid, there is no motion
of its mass towards the terrestrial pole referred to. The
reason of this is known to be, that the south end of
the needle is repelled by a force equal to that by which
the north, or marked end, is attracted. These two equal
and opposite forces destroy each other as regards a motion
of translation, but they are effective in producing a
'motion of rotation. The magnetic needle, indeed, when
in a position oblique to the plane of the magnetic meridian,
is solicited towards that plane by a mechanical couple, and
if free to move, will turn and find its position of equilibrium
there.

182 DIAMAGNETISM AND MAGNE-C'RYSTALLIC ACTION.

Let such a needle, /A, be attached, as in fig. 3, to the
end of a light wooden beam, vw ; let the beam and needle

be suspended horizontally from
the point a, round which the
whole system is free to turn,
the weight of the needle being
balanced by a suitable counter-
poise, w ; let the north pole of
the earth be towards N. Sup-
posing the beam to occupy a
position oblique to the mag-
netic meridian, as in the figure,

the end /, or the marked end, of the needle is solicited
towards N by a force </>, and the tendency of this force
to produce rotation in the direction of the arrow is ex-
pressed by the product of <£ into the perpendicular drawn
from the centre of suspension a, to the line of direction of
the force. Setting this distance = d, we have the moment
of 0 in the direction stated,

The end fi of the needle is repelled by the magnetic pole
N with a force </>' : calling the distance of the direction of
this latter force from the axis of rotation, d', we have the
moment of <f>' in a direction opposed to the arrow,

= ^d'.

Now as the length of the needle may be considered a
vanishing quantity as compared with its distance from the
terrestrial pole, we have practically

<£ = <£'>
and consequently, as d is less than d',

The tendency to turn the lever in a direction opposed to
the arrow is therefore predominant ; the lever will obey

NECESSAEY INFERENCES. 183

this tendency, and move until the needle finds itself in
the magnetic meridian ; when this position is attained, the
predominance spoken of evidently ceases, and the system
will be in equilibrium. Experiment perfectly corroborates
this theoretic deduction.

In this case, the centre of gravity of the needle recedes
from the north magnetic pole as if it were repelled by the
latter; but it is evident that the recession is not due
either to the attraction or repulsion of the needle con-
sidered as a whole, but simply to the mechanical advan-
tage possessed by the force <£', on account of its greater
distance from the axis of rotation. If the force acting
upon every particle of the needle were purely attractive, it
is evident that no such recession could take place. Sup-
posing, then, that we were simply acquainted with the fact,
that the end / of the needle is attracted by the terrestrial
pole, and that we were wholly ignorant of the action of the
said pole upon the end h, the experiment here described
would lead us infallibly to the conclusion that the end h
must be repelled. For if it were attracted, or even if it
were neither attracted nor repelled, the motion of the bar
must be towards the pole N instead of in the opposite
direction.

Let us apply this reasoning to the experiment with
the bismuth prisms already described. The motion of the
magnetic needle in the case referred to is not more inex-
plicable, on the assumption of a purely attractive force,
than is the motion of our rectangular box on the assump-
tion of a purely repulsive one ; and if the above experi-
ment would lead to the conclusion that the end h of
the magnetic needle is repelled, the experiment with the
bismuth leads equally to the conclusion that the end / of
the prism hf, fig. 2, must be attracted by the pole N.
The assumption of such an attraction, or in other
words, of diamagnetic polarity, is alone capable of

184 DIAMAGNETISM iND MAGNE-CRYSTALLIC ACTION.

explaining the effect, and the explanation which it offers
is perfect.

On the hypothesis of diamagnetic polarity, the prism
hf turns a hostile end h to the magnetic pole N, and
a friendly pole / away from it. Let the repulsive force
acting upon the former be <£, and the attractive force
acting upon the latter <£'. It is manifest that if </> were
equal to <£', as in the case of the earth's action, or in other
words, if the field of force were perfectly uniform, then,
owing to the greater distance of <j/ from the axis of rota-
tion, from the moment at which the rectangular box quits
the equatorial position, which is one of unstable equili-
brium, to the moment when its position is axial, the
box would be incessantly drawn towards the position last
referred to.

But it will be retorted that the field of force is not
uniform, and that the end h, on account of its greater
proximity to the magnet, is more forcibly repelled thaji the
end / is attracted : to this I would reply, that it is only
in ' fields ' which are approximately uniform that the
effects can be produced ; but to produce motion to-
wards the pole, it is not necessary that the field should be
perfectly uniform : setting, as before, the distance of
the direction of the force <f> from the axis of rotation = d,
and that of the force <£' = d', a motion towards the pole N
will always occur whenever

d <j)*

To ascertain the diminution of the force on receding
from a polar surface such as that here used, I suspended a
prism of bismuth, similar to those contained in the
rectangular box, at a distance of 0'9 of an inch from the
surface of the pole. Here, under the action of the magnet
excited by a current of ten cells, the number of oscilla-

ANOMALIES EXPLAINED. 185

tions accomplished in a second was 1 7 ; at 0*7 of an
inch distant the number was 18; at O5 of an inch
distant the number was 19; at 0*3 distant the number
was 1 9'5 ; and at 0-2 distant the number was 20. The
forces at these respective distances being so very little
different from each other, it follows that a very slight
deviation of the box from the equatorial position is suf-
ficient to give the moment of <j>' a preponderance over
that of <£, and consequently to produce the exact effect
observed in the experiment.

The consistency of this reasoning is still further
shown when we operate in a field of force which
diminishes speedily in intensity as we recede from the
magnet. Such a field is the space immediately in front
of pointed poles. Suspending our rectangular box be-
tween the points, and causing the latter to approach until
the box has barely room to swing between them, it is im-
possible to produce the phenomena which we have just
described. The intensity with which the nearest points of
the bismuth bar are repelled so much exceeds the attrac-
tion of the more distant end, that the moment of attraction
is not able to cope successfully with the moment of repul-
sion ; the bars are consequently repelled en masse, and the
length of the box takes up a position at right angles
to the line which unites the poles.

It is manifest, however, that by increasing the distance
between the bismuth bar and the points acting upon it,
we diminish the difference of action upon the two ends
of the bar. When the distance is sufficient, we can pro-
duce, with the pointed poles, all the phenomena exhibited
between flat or rounded ones.

All the effects which have been described are produced
with great distinctness when, instead of compressed bis-
muth, two similar bars of the crystallised substance are
used, in which the planes of principal cleavage are parallel

186 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

to the length. Such bars are not difficult to procure, and
they ought to hang in the magnetic field with the planes
of cleavage vertical. It is unnecessary to describe the
experiments made with such bars ; they exhibit with
promptness and decision all the effects observed with the
compressed bismuth.

We have hitherto operated upon elongated masses of
bismuth ; but with the compressed substance, or with the
substance crystallised uniformly in planes, as in 'the case
last referred to, an elongation of the mass is not necessary
to the production of the effects described. Previous, how-
ever, to the demonstration of this proposition, I shall
introduce a kind of lemma, which will prepare the way for
the complete proof.

Diamagnetic bodies, like paramagnetic ones, vary con-
siderably in the intensity of their forces. Bismuth or
antimony, for example, exhibits the diamagnetic force with
greater energy than gold or silver, just
as iron or nickel exhibits the magnetic
force with greater energy than platinum
or chromium. Let two thin bars, «6,
cd, fig. 4, of two bodies of different dia-
magnetic powers be placed at right
angles to each other, so as to form a
cross ; let the cross be attached to the
end of a lever and suspended horizon-
tally from the point x, before the flat
or rounded pole N of a magnet. Let
the continuous line ab represent the
needle of the powerful diamagnetic
body, and the broken line cd that of
the feeble one. On the former a mechanical couple acts
in the directions denoted by the arrows at its ends ; and on
the latter a couple operates in the directions of the arrows

FIG. 4.

DIFFERENTIAL REPULSION ANALYZED. 187

at its ends. These two couples are evidently opposed to
each other ; but the former being, by hypothesis, the more
powerful of the two, it will overcome the latter. The me-
chanical advantage possessed by the attracted end a of the
more powerful bar, on account of its greater distance from
the axis of suspension #, will, in an approximately uniform
field of force which we here assume, cause the centre of
gravity of the cross to move towards the pole N.

In the formation of such a cross, however, it is not
necessary to resort to two different substances in order to
find two needles of different diamagnetic powers; for in
crystallised bodies, or in bodies subjected to mechanical
pressure, the diamagnetic force acts with very different
energies in different directions. Let a diamagnetic body
which has been forcibly compressed in one direction be
imagined ; let two needles be taken from such a mass,
the one with its length parallel, and the other with its
length perpendicular to the line of pressure. Two such
needles, though composed of the same chemical substance,
will behave exactly as the two bars of the cross in the
experiment last described : that needle whose length co-
incides with the line of pressure will bear the same rela-
tion to the other that the needle of the powerfully diamag-
netic substance bears to that of the feeble one. An
inspection of the table at page 1 80 will show that this
must be the case.

It is also shown in the following table, that in masses
of crystallised bismuth the diamagnetic repulsion acts with
very different energies in different directions. From a
bismuth crystal cubes were taken with the planes of
principal cleavage parallel throughout to two opposite
faces of each cube. The cubes were placed upon the ends
of a torsion balance, and the diamagnetic repulsion was
accurately measured when the force acted parallel to the
planes of cleavage. The cubes were then turned 90° round,

188 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

and the repulsion was measured when the force acted per-
pendicular to the planes referred to.

Cubes of crystallised Bismuth.

Eepulsion when the force was directed

Strength of magnet along the cleavage across the cleavage

3-6 11-7 8

5-7 34-8 23

8-4 78 53

10-0 118 76-5

11-9 153 110

It is manifest from this table that bismuth behaves as a
body of considerably superior diamagnetic power when the
force acts along the planes of cleavage.

Let two indefinitely thin needles be taken from such a
mass, the one with its length parallel, and the other with
its length perpendicular to the planes of cleavage ; it is
evident that if two such needles be formed into a cross and
subjected to experiment in the manner above described,
the former will act the part of the more powerfully dia-
magnetic needle, and produce similar effects in the
magnetic field.

We now pass on to the demonstration of the proposi-
tion, that it is not necessary that the crystallised masses
should be elongated to produce the effects exhibited by the
prisms in the experiments already recorded. Let us sup-
pose the ends of our rectangular box to be composed of
cubes, instead of elongated masses, of crystallised bismuth,
and let the planes of principal cleavage be supposed to be
parallel to the face 06, fig. 5. Let the continuous line de
represent an indefinitely thin slice of the cube passing
through its centre, and the dotted line gf a similar slice
in a perpendicular direction. These two slices manifestly
represent the case of the cross in fig. 4 ; and were they
alone active, the rectangular box, in a uniform field of
magnetic force, must turn in the direction of the arrow.

MATTEUCCIS OBJECTIONS ANSWERED.

189

Comparing similar slices, in pairs, on each side of those
two central slices, it is manifest that every pair parallel to
the line de represents a stronger mechanical couple than
every corresponding pair parallel to fg. The consequence

FIG. 5.

is, that a cube of crystallised bismuth suspended in the
manner described, in a sufficiently uniform field of mag-
netic force, will move in the same direction as the cross in
fig. 4 : its centre of gravity will therefore approach the
pole N — which was to be demonstrated.

This deduction is perfectly illustrated by experiment.
It is manifest that the effect of the pole s upon the cube
adjacent to it is to increase the moment of rotation of the
rectangular box: the same reasoning applies to it as to the
pole N.

Eeferring to fig. 27a, page 175, it will be seen that we
have here dealt with the second and gravest objection of
M. Matteucci, and converted the facts upon which the
objection is based into a proof of diamagnetic polarity, so
cogent that it alone would seem to be sufficient to decide
this important question.

Holding the opinion entertained by M. Matteucci re-

190 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

gardingthe non-polarity of diamagnetic force,1 bis objection
must bave appeared to him to be absolutely unanswerable :
I sbould be glad to believe tbat the remarks contained
in this Appendix furnish, in the estimation of this distin-
guished philosopher, a satisfactory explanation of the
difficulty which he has disclosed.

Let me, in conclusion, briefly direct the reader's atten-
tion to the body of evidence laid before him in the fore-
going pages. It has been proved that matter is repelled
by the pole of a magnet in virtue of an induced condition
into which the matter is thrown by such a pole. It is
shown that the condition evoked by one pole is not that
which is evoked by a pole of an opposite quality — that each
pole excites a condition peculiar to itself. A perfect anti-
thesis has been shown to exist between the deportment of
paramagnetic and diamagnetic bodies when acted on by a
magnet alone, by an electric current alone, or by a magnet
and an electric current combined. The perplexing phe-
nomena resulting from molecular structure have been laid
open, and the antithesis between paramagnetic and dia-
magnetic action traced throughout. It is further shown,
that whatever title to polarity the deportment of a bar of
soft iron, surrounded by an electric current, and acted on
by other magnets, gives to this substance, a bar of bismuth
possesses precisely the same title : the disposition of forces,
which in the former case produces attraction, produces in
the latter case repulsion, while the repulsion of the iron
finds its exact complement in the attraction of the bismuth.
Finally, we have a case adduced by M. Matteucci which
suggests a crucial experiment to which all our previous
reasoning has been submitted, by which its accuracy has
been proved, and the insufficiency of the assumption, that
the diamagnetic force is not polar, is reduced to demon-

1 ' H ne peut exister dans les corps diamagnetiques une polarite telle
qu'on la congoit dans le fer doux.' — Cours special, p. 201.

SUMMARY OF EVIDENCE. J91

stration. When we remember that against all this no
single experimental fact1 or theoretic argument which can
in any degree be considered as conclusive, has ever been
brought forward, nor do I believe can be brought forward,
the conclusion seems irresistible, that we have in the
agency by which bodies are repelled from the poles of a
magnet, a force of the same dual character as that by which
bodies are attracted; that, in short, 'diamagnetic bodies
possess a polarity the same in kind but the opposite in
direction to that possessed by magnetic ones.'

[The experiments and reasonings recorded in the foregoing
memoir left no shadow of doubt upon my mind as to the polar
character of the diamagnetic force. Throughout the most com-
plex series of actions, the doubleness of action to which the term
polarity has been applied, was manifested in a clear and conclu-
sive manner. Still I thought it would contribute to the final
settlement of the question if I were to take up the subject after
the method of Weber, and satisfy all the demands which had
been made upon him by the opponents of diamagnetic polarity.
Here, as in the foregoing enquiry, it was my wish to render the
experiments exhaustive, and to employ apparatus which should
place it definitely within the power of all investigators to sub-
ject the question to experimental demonstration. I devised a
scheme of experiment, but, previous to putting it into execution,
wrote to Prof. Weber asking him whether he did not think it
possible so to improve his apparatus as materially to exalt the
action. Weber's own experiments had been made with bismuth
solely. It was objected that his results were due to ordinary
induced currents, and he was called upon to produce the same

1 I refrain from alluding to the negative results obtained by Mr.
Faraday in repeating M. Weber's experiments ; for though admirably
suited to the exhibition of certain effects of ordinary induction, Mr.
Faraday himself has shown how unsuitable the apparatus employed
would be for the investigation of the question of diamagnetic polarity.
See Experimental Researches (2653, 2654), vol. iii. p. 143. — J. T., May
9, 1855.

192 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

effects with insulators. This demand it was my object to meet,
and I think it has been met by the experiments recorded in
the ' Fifth Memoir.' l— J. T., 1870.]

1 Professor Weber's practical reply to my question is given at page
198.

FIFTH MEMOIR.

FURTHER RESEARCHES ON THE POLARITY OF
THE DIAMAGNETIC FORCE.1

Introduction.

A TEAR ago I placed before the Royal Society the results
of an investigation ' On the Nature of the Force by which
Bodies are repelled from the Poles of a Magnet.'2 The
simultaneous exhibition of attraction and repulsion in the
case of magnetised iron or steel is the basis on which the idea
of the polarity of this substance is founded; and it resulted
from the investigation referred to, that a corresponding
duality of action was manifested by bismuth. In those ex-
periments the bismuth was the moveable object upon which
fixed magnets were caused to act, and from the deflection
of the bismuth its polarity was inferred. But, inasmuch as
such action is reciprocal, we ought also to obtain evidence
of diamagnetic polarity by reversing the conditions of
experiment — making the magnet the moveable object,
and inferring from its deflection the polarity of the mass
which produces the deflection. This experiment would
be complementary to those described in the communica-
tion just referred to, and existing circumstances invested

1 From the Philosophical Transactions for 1856, part i. ; having been
received by the Royal Society November 27, 1855, and read December
20, 1855.

2 Philosophical Transactions, 1855 ; and Phil. Mag. for September
1855.

194 DIAMAGNETISM AND MAGNE-CEYSTALLIC ACTION.

the question with a great degree of interest and im-
portance.

In fact, an experiment similar to that here indicated
was made by Professor W. Weber, previous to my inves-
tigation, and the result was such as to satisfy its author of
the reverse polarity of diamagnetic bodies. I will not
here enter into a minute description of the instrument and
mode of experiment by which this result was obtained ; for
the instrument made use of in the present enquiry being
simply a refinement of that employed by 'Weber, its ex-
planation will embrace the explanation of his apparatus.
For the general comprehension of the criticisms to which
Weber's results have been subjected, it is necessary, how-
ever, to remark, that in his experiments a bismuth bar,
within a vertical spiral of copper wire, through which
an electric current was transmitted, was caused to act
upon a steel magnet freely suspended outside the spiral.
When the two ends of the bar of bismuth were permitted
to act successively upon the suspended magnet, a motion
of the latter was observed, which indicated that the bis-
muth bar was polar, and that its polarity was the reverse
of that of iron.

Notwithstanding the acknowledged eminence of Weber
as an experimenter, this result failed to produce gene-
ral conviction. In his paper * On the Polar or other
Condition of Diamagnetic Bodies,' ' Faraday had shown
that results quite similar to those obtained by Weber,
in his first investigation with bismuth, were obtained in
a greatly exalted degree with gold, silver, and copper ; the
effect being one of induced currents and not of diamagnetic
polarity. He by no means asserted that his results had
the same origin as those obtained by Weber ; but as the
latter philosopher had made no mention of the source

1 Experimental ^Researches, 2640, Philosophical Transactions, 1850,
p. 171.

PREFATORY REMARKS. 195

of error which Faraday's experiments rendered mani-
fest, it was natural to suppose that it had been overlooked,
and the observed action attributed to a wrong cause. In
an article published in his ' Massbestimmungen ' in 1852,
Weber, however, with reference to this point, writes as
follows : — ' I will remark that the article transferred from
the Eeports of the Society of Sciences of Saxony to
PoggendorfFs Annalen was only a preliminary notice of
my investigation, the special discussion of which was
reserved for a subsequent communication. It will be
sufficient to state here, that in the experiments referred to
I sought to eliminate the inductive action by suitable
combinations; but it is certainly far better to set aside
this action altogether, as has been done in the experiments
described in the present memoir.'

One conviction grew and strengthened throughout
these discussions — this, namely, that in experiments on
diamagnetic polarity great caution is required to separate
the pure effects of diamagnetism from those of ordinary
induced currents. With reference to even the most recent
experiments of Weber, referred to at the conclusion of
the citation just made, it is strongly urged that there
is no assurance that the separation referred to has been
effected. In those experiments, as already stated, a cylin-
der of bismuth was suspended within a long vertical helix
of covered copper wire, and the action of the cylinder
upon a magnet suspended opposite to the centre or
neutral point of the helix was observed. To increase the
action, the position of the cylinder Avas changed at each
termination of the minute swing of the magnet, the
amplitude of the oscillations being thus increased, and the
effect rendered more sensible to the eye. Now, it is urged,
there is every reason to believe that in these motions of a
metallic mass within an excited helix induced currents
will be developed, which, acting upon the magnet, will

196 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

produce the motions observed. The failure indeed to
demonstrate the existence of diamagnetic polarity by other
means has, in the case of some investigators, converted
this belief into a certainty.

Among the number whom Weber's experiments have
failed to convince, Matteucci occupies a prominent place.
With reference to the question before us, this philosopher
writes as follows : — l

4 In reading the description of the experiments of M.
Weber, we are struck on beholding the effects produced by
moving the bismuth when there is no current in the spiral.
Although the direction of oscillation in this latter case
is opposed to that observed when the spiral is active,
still the fact excites doubts as to the correctness of the con-
clusions which have been drawn from these experiments.2
To deduce rigorously the demonstration of diamagnetic
polarity, it would be necessary to substitute for the mas-
sive bismuth, cylinders formed of insulated particles of
the 'metal,3 to vary the dimensions of the cylinder, and
above all, to compare the effects thus obtained with those
which would probably be obtained with cylinders of
copper and silver in a state of purity.

' We are obliged,' continues Matteucci, ' to make the
same remarks on another series of experiments executed by
this physicist with a view to obtain anew, by the effects

1 Court special sur V Induction, p. 206.

2 It is not my place to account for the effect here referred to. I
may, however, remark, that there appears to be no difficulty in referring
it to the ordinary action of a diamagnetic body upon a magnet. It is
the result which Brugmans published upwards of half a century ago ;
the peculiar form of this result in one of the series of experiments
quoted by M. Weber must, I think, be regarded as purely accidental.
-J, T.

8 Also in page 204 : — ' II fallait done, pour prouver si 1'influence
d'un corps diamagnetique produit sur un aimant une variation de sens
contraire a celle developpee dans le fer doux, operer avec ce corps prive
do conductililitt?

OPPONENTS OP LIAMAGNETIC POLARITY. 197

of induction, the proof of diamagnetic polarity. It is
astonishing, that after having sought to neutralise the
development of induced currents in the moving cylinders
of bismuth, by means of a very ingenious disposition
of the spiral — it is astonishing, I repeat, that no attempt
was made to prove by preliminary essays with metals
possessing a higher conductibility than bismuth, that
the same end could be obtained. I cannot leave you
[Matteucci is here addressing his pupils] ignorant that
the doubts which I have ventured to advance against the
experiments of M. Weber are supported by the negative
result which I have obtained in endeavouring to excite
diamagnetic polarity in bismuth by the discharge of the
Ley den jar.'

It will be seen in the following pages that the con-
ditions laid down by Matteucci for the rigorous demon-
stration of diamagnetic polarity are more than fulfilled.

The conclusions of Weber find a still more strenuous
opponent in his countryman Professor V. Feilitzsch, who
has repeated Weber's experiments, obtained his results, but
who denies the validity of his inferences. M. v. Feilitzsch
argues, that in the experiments referred to it is impossible
to shut out ordinary induction, and for the rigorous proof
of diamagnetic polarity he demands that the following
conditions shall be fulfilled.1 ' To render the experiment
free from the action of induced currents two ways are open.
The currents can be so guided that they shall mutually
neutralise each other's action upon the magnet, or the
induced currents can be completely got rid of by using,
instead of a diamagnetic conductor, a diamagnetic in-
sulator? To test the question, M. v. Feilitzsch resorted
to the latter method : instead of cylinders of bismuth
he made use of cylinders of wax, and also employed a prism
of heavy glass, but in neither case was he able to detect
1 Poggendorff's Annalen, xcii. 377.

198 DIAMAGNETISM AND MAGNE-CKYSTALLIC ACTION.

the slightest action upon the magnet. 'However the
motions of the prism might be varied, it was not possible
either to cause the motionless magnet to oscillate, or to
bring the magnet from a state of oscillation to one of rest.'
M. v. Feilitzsch pushes his experiments further, and nods
that when the bismuth is motionless within its spiral, the
position of the magnet is just the same as when the bis-
muth is entirely withdrawn ; hence his final conclusion,
that the deflection of the magnet in Weber's experiments
is due to induced currents, which are excited in the
bismuth by its mechanical motion up and down within
the spiral.

These divergent opinions upon a question of such vital
bearing upon the general theory of magnetic phenomena,
naturally excited in me the desire to make myself
acquainted with the exact value of Weber's experiments.
The most direct way of accomplishing this I considered to
be, to operate with an instrument similar to that made
use of by Weber himself; I therefore resolved to write to
the constructor of his apparatus, but previous to doing so
I wrote to M. Weber, enquiring whether his further reflec-
tions on the subject had suggested to him any desirable
modification of his instrument. In reply to my question
he undertook to devise for me an apparatus, surpassing
in delicacy any hitherto made use of. The design of M.
Weber was ably carried out by M. Leyser of Leipzig ; and
with the instrument thus placed in my possession, I have
been able to satisfy the severest conditions proposed by
those who saw in the results of Weber's experiments the
effects of ordinary induction.

Description of Apparatus.

A sketch of the instrument employed in the present
investigation is given in fig. 2. BO, B'O' is the outline of
a rectangular box, the front of which is removed so as

CORRESPONDENCE WITH WEBER. 199

to show the apparatus within. The back of the box is
prolonged, and terminates in two semicircular projections,
which have apertures at H and H'. Stout bolts of brass,
which have been made fast in solid masonry, pass through
these apertures, and the instrument, being secured to the
bolts by screws and washers, is supported in a vertical
position, being free from all disturbance save such as
affects the foundations of the Eoyal Institution. All the
arrangements presented to the eye in fig. 2 are made fast
to the back of the box, but are unconnected with the front,
so as to permit of the removal of the latter, w w' are two
boxwood wheels with grooved peripheries, which permit ot
motion being transferred from one wheel to the other by
means of a string ss'. Attached to this string are two
cylinders, WTI, op, of the body to be examined : in some
cases the cylinders are perforated longitudinally, the string
passing through the perforation, and the cylinders being
supported by knots on the string. H B, H'E' are two helices
of copper wire overspun with silk, and wound round two
brass reels, the upper ends of which protrude from H
to G, and from H' to G'. The internal diameter of each
helix is 0*8 of an inch, and its external diameter about 1*3
inch ; the length from H to E is 1 9 inches, and the centres
of the helices are 4 inches apart; the diameters of the
wheels ww' being also 4 inches. The cross bar G G' is of
brass, and through its centre passes the screw R. From
this screw depend a number of silk fibres which support an
astatic arrangement of two magnets, the front one of which,
s N, is shown in the figure. An enlarged horizontal section
of the instrument through the astatic system is shown in
fig. 4. The magnets are connected by a brass cross-piece, in
which is the point of suspension P, fig. 4; and the position
of the helices is shown to be between the magnets. It will
be seen that the astatic system is a horizontal one, and
not vertical, as in the ordinary galvanometer. The black
10

200 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

circle in front of the magnet s N, fig. 2, is a mirror,
which is shown in section at M, fig. 4 ; to balance the

FIG. 2.

FIG. 3.

Si

weight of this mirror, and adjust the magnets in a
horizontal position, a brass washer, w, is caused to move
along a screw, until a point is attained at which its weight
brings both the magnets into the same horizontal plane.

APPARATUS EMPLOYED. 201

There is also another adjustment, which permits of the
magnets being brought closer together or separated more
widely asunder.

The motions of this compound magnet are observed by
means of a distant scale and telescope, according to the
method applied to the magnetometer of Gauss. The
rectangle da, d'af, fig. 2, is the section of a copper
damper, which, owing to the electric currents induced in
it by the motion of the magnet, brings the latter rapidly
to rest, and thus expedites experiment.

It is well known that one end of a magnet attracts,
while the other end repels the same pole of a magnetic
needle ; and that between the two poles there is a neutral
point which neither attracts nor repels. The same is the
case with the helices H E, nV ; so that when a current is
sent through them, if the astatic magnet be exactly
opposite the neutral point, it is unaffected by the helices.
This is scarcely attainable in practice; a slight residual
action remains which draws the magnets against the
helices ; but this is very easily neutralised by disposing
an external portion of the circuit so as to act upon the
magnets in a direction opposed to that of the residual
action. Here then we have a pair of spirals which, when
excited, do not act upon the magnets, and which therefore
permit us to examine the pure action of any body, capable
of magnetic excitement, placed within them.

In the experiments to be described, it was arranged
that the current should always flow in opposite directions
through the two spirals ; so that if the cylinders within
them were polar, the two upper ends of these cylinders
should be poles of opposite names, and consequently the
two lower ends also opposite. Suppose the two cylinders
mn, op to occupy the central position indicated in fig. 2 :
then, even if the cylinders became polar through the
action of the surrounding current, the astatic magnets,

202 DIAMAGNET1SM AND MAGNE-CEYSTALLIC ACTION.

being opposite to the neutral points of the cylinders,
•would experience no action from the latter. But suppose
the wheel w' to be so turned that the two cylinders are
brought into the position shown in fig. 1, the upper end
o of op and the lower end n of win will act simultaneously
upon the suspended magnets. For the sake of illustra-
tion, let us suppose the ends o and n to be both north
poles, and that the section, fig. 4, is taken when the bars
are in the position shown in fig. 1. The right-hand pole
o will attract s' and repel N, which attraction and repulsion
will sum themselves together to produce a deflection of the
system of magnets. On the other hand, the left-hand pole
n, being also north, will attract s and repel N', which two
effects also sum themselves to produce a deflection in the
same direction as the former two. Hence, not only is the
action of terrestrial magnetism annulled by this arrange-
ment, but the moving force, due to the reciprocal action
of the magnets and the bodies within the helices, is
increased fourfold. By turning the wheel in the other
direction, we bring the cylinders into the position shown
in fig. 3, and thus may study the action of the ends m
and p upon the magnets.

The screw R is employed to raise or lower the magnets.
At the end, £, of the screw is a small torsion circle which
can be turned independently ; by means of the latter the
suspending fibre can be twisted or untwisted without
altering the level of the magnets.

The front is attached to the box by brass hasps, and
opposite to the mirror M a small plate of glass is intro-
duced, through which the mirror is observed; the magnets
within the box being thus effectually protected from the
disturbances of the external air. A small handle to turn
the wheel w' accompanied the instrument from its maker;
but in the experiments, I used, instead of it, a key attached
to the end of a rod 10 feet long ; with this rod in my right

NEW EXPERIMENTS. 203

hand, and the telescope and scale before me, the experi-
ments were completely under my own control. Finally,
the course of the current through the helices was as
follows: — Proceeding from the platinum pole of the
battery it entered the box along the wire w, fig. 2, which
passed through the bottom of the box ; thence through
the helix to H', returning to E' ; thence to the second
helix, returning to E, from which it passed along the wire
w' to the zinc pole of the battery. A commutator was
introduced in the circuit, so that the direction of the
current could be varied at pleasure.

Experiments. — Deportment of Diamagnetic Bodies.

A pair of cylinders of chemically pure bismuth, 3
inches long and 0*7 of an inch in diameter, accompanied
the instrument from Germany. These were first tested,
commencing with a battery of one cell of Grove. Matters
being as sketched in fig. 2, when the current circulated
in the helices and the magnet had come to rest, the cross
wire of the telescope cut the number 482 on the scale.
Turning the wheel V so as to bring the cylinders into
the position fig. 1, the magnet moved promptly, and after
some oscillations took up a new position of equilibrium ;
the cross wire of the telescope then cut the figure 468 on
the scale. Reversing the motion so as to place the cylinders
again central, the former position 482 was resumed ; and
on turning further in the same direction, so as to place
the cylinders as in fig. 3, the position of equilibrium of
the magnet was at the number 493. Hence by bringing
the two ends n and o to bear upon the astatic magnet, the
motion was from greater to smaller numbers, the position
of rest being then fourteen divisions less than when the
bars were central. By bringing the ends m and p to bear
upon the magnet, the motion was from smaller to greater

204 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

numbers, the position of rest being eleven divisions more
than when the bars were central.

As the positions here referred to will be the subject of
frequent reference, for the sake of convenience I will call
the position of the cylinders sketched in fig. 1, Position 1 ;
that sketched in fig. 2, Position 2 ; and that sketched
in fig. 3, Position 3. The results which we have just
described, tabulated with reference to these terms, would

then stand thus : —

L

Bismuth Cylinders. — Length 3 inches ; diameter 0'7.
Position 1. 468 Position 2. 482 Position 3. 493

In changing therefore from position 1 to position 3, a
deflection corresponding to twenty-five divisions of the
scale was produced.

Wishing to place myself beyond the possibility of
illusion as regards the fact of deflection, I repeated the ex-
periment with successive batteries of two, three, and four
cells. The following are the results : —

n.

2 cells 8 cplls 4 cells

Position 1. 450 439 425

Position 2. 462 450 437

Position 3. 473 462 448

In all the cases cited we observe the same result. From
position 2 to position 1 the motion is from larger to smaller
numbers ; while from position 2 to position 3 the motion
is from smaller to larger numbers.

It may at first sight appear strange that the amount
of the deflection did not increase with the battery power ;
the reason, in part, is that the magnet, when the current
circulated, was held in a position free from the spirals, by
forces emanating partly from the latter and partly from a
portion of the external circuit. When the current increased,
the magnetisation of the bismuth increased also, but so did

ACTION OF DIAMAGNETS ON MAGNETS. 205

the force which held the magnets in their position of equi-
librium. To remove them from this position, a greater
amount of force was necessary than when only the residual
action of a feeble current held them there. This fact,
coupled with the circumstance that less heat was developed,
and less disturbance caused by air currents, when a feeble
battery was used, induced me for some time to experiment
with a battery of two cells. Subsequent experience
however enabled me to change this for five cells with
advantage.

Notwithstanding the improbability of the argument,
it may still be urged that these experiments do not prove
beyond a doubt that the bismuth cylinders produce the ob-
served motion of the magnets, in virtue of their excitement
by the voltaic current; for it is not certain that these
cylinders would not produce the same motion wholly inde-
pendent of the current. Something of this kind has
already occurred to M. Leyser,1 and why not to others ?

In answer to this, I reply, that if the case be as here
suggested, the motion of the magnets will not be changed
when the current in the helices flows in the opposite
direction. Here is the experiment.

m.

Position 1. 670 Position 2. 742 Position 3. 704

We observe here that in passing from position 2 to position
1 the motion is from smaller to larger numbers ; while in
passing from position 2 to position 3 the motion is from
larger to smaller numbers. This is the opposite result to
that obtained when the current flowed in the opposite
direction ; and it proves that the polarity of the bismuth
cylinders depends upon the direction of the surrounding
current, changing as the latter changes. It was pleasant

1 Scientific Memoirs, New Series, vol. i. page 184.

206 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

to observe the prompt and steady march of the magnet
as the cylinders were shifted in the helices. When the
magnets, operated on by two ends of the bars of bismuth,
were moving in any direction, by bringing the two op-
posite ends into action, the motion could be promptly
checked ; the magnets could be brought to rest, or their
movement converted into one in the opposite direction.

I may add to the above a series of results obtained
some days subsequently in the presence of Professors
Faraday, De la Eive, and Marcet.

IV.

Bismuth Cylinders.
Position 1. 670 Position 2. 650 Position 3. 630

The difference between positions 1 and 3 amounts here to
forty divisions of the scale ; subsequent experience enabled
me to make it still greater.

It was found by experiment, that when the motion was
from lower to higher numbers it denoted that the poles
N N', fig. 4, were repelled from the spirals, and the poles
s s' attracted towards them. When, on the contrary, the
motion was from larger to smaller numbers, it indicated
that the poles N N' were attracted and the poles s s' re-
pelled. In the position fig. 1, therefore, of Tables III.
and IV. the poles N N' were repelled by the ends n'o of the
bismuth cylinders, and the poles s s' attracted ; while in
the position fig. 3, the poles N N' were attracted by the
ends mp, and the poles s s' repelled ; the ends n and o,
therefore, acted as two north poles, while the ends m and
p acted as two south poles. Now the direction of the
current in the experiments recorded in the two tables
referred to was that shown by the arrows in fig. 4. Stand-
ing in front of the instrument, the direction in the adjacent
face of the spiral H'E' was from right to left, while it was

DIAMAGNETS OF BISMUTH. 207

from left to right in H E. Hence, the polarity of the
bismuth cylinders was the reverse of that which would be
excited in cylinders of iron under the same circumstances.
This assertion, however, shall be transferred, before we
conclude, from the domain of deduction to that of fact.

Let us now urge against these experiments all that ever
has been urged against the experiments of Weber by the
opponents of diamagnetic polarity. The bismuth cylinders
are metallic conductors, and, in moving them through the
spirals, induced currents will be excited in these conductors.
The motion observed may not, after all, be due to diamag-
netic polarity, but to the currents thus excited. I reply,
that in all cases the number set down marks the perma-
nent position of the magnet. Were the action due to
induced currents, these, being momentary, could only
impart a shock to the magnet, which, on the disappearance
of the currents, would return to its original position. But
the deflection is permanent, and is therefore due to an
enduring cause. In his paper on 'Supposed Diamagnetic
Polarity, Faraday rightly observes: — 'If the polarity
exists, it must be in the particles, and for the time per-
manent, and therefore distinguishable from the momentary
polarity of the mass due to induced temporary currents,
and it must also be distinguishable from ordinary mag-
netic polarity by its contrary direction.' These are the
precise characteristics of the force made manifest by the
experiments now under consideration.

Further, the strength of induced currents depends on
the conducting power for electricity of the mass in which
they are formed. Expressing the conducting power of
bismuth by the number 1-8, that of copper would be ex-
pressed by 73'G,1 the conductivity of the latter being
therefore forty times that of the former. Hence arises the
demand, made by the opponents of diamagnetic polarity,
1 Philosophical Magazine, Series 4, vol. vii. p. 37.

208 DIAMAGNETISM AND MAGNE-CKYSTALLIC ACTION.

to have the experiments repeated with cylinders of copper ;
for if the effect be due to induced currents, they will
show themselves in copper in a greatly increased degree.
The following is the result of a series of experiments made
with two copper cylinders, of the same dimensions as the
bismuth ones already described : —

V.

Cylinders of Copper.
Position 1. 754 Position 2. 754 Position 3. 755

If the effects obtained with bismuth were due to induced
currents, we ought to have the same effects forty times
multiplied in the case of copper, in place of which we
have scarcely any sensible effect at all.

Bismuth is the only substance which has hitherto pro-
duced an appreciable action in experiments of this nature ;
another illustration, however, is furnished by the metal
antimony, which possesses a greater conductive power, but
a less diamagnetic power than bismuth. The following
results were obtained with this substance : —

VI.

Cylinders of Antimony. — Length 3 inches; diameter 0'7.

Current direct l Current reversed2
Position 1. 693 244

Position 2. 688 252

Position 3. 683 261

On comparing these numbers with those already obtained
with bismuth, we observe that for like positions the actions
of both metals are alike in direction. We further observe
that the results are determined, not by the relative con-
ducting powers of the two metals, but by their relative
diamagnetic powers. If the former were the determining
cause, we should have greater deflections with antimony
than with bismuth, which is not the case ; if the latter,
we should have less deflections, which is the case.
1 As in III. and IV. » As in I. and II.

DIAMAGNETS OF HEAVY GLASS AND CALC-SPAR. 200

The third and severest condition proposed by those
who object to the experiments of Weber is to substitute
insulators for conductors. I call this condition severe for
the following reasons: — according to the experiments of
Faraday,1 when bismuth and sulphur are submitted to the
same magnetising force, the repulsion of the former being
expressed by the number 1968, that of the latter is ex-
pressed by 118. Hence an action which, with the means
hitherto employed by Faraday and others, was difficult of
detection in the case of bismuth, must wholly escape such
means of observation in the case of sulphur. The same
remarks apply, in a great measure, to all other insulators.

But the admirable apparatus made use of in this
investigation has enabled me to satisfy tnis condition also.
To Faraday I am indebted for the loan of two prisms
of the self-same heavy glass with which he made the dis-
covery of diamagnetism. The bismuth cylinders were
withdrawn from the helices and the prisms of glass put in
their places. It was now necessary to have a perfectly
steady magnet, the expected result being so small as to be
readily masked by, or confounded with, a motion arising
from some extraneous disturbance. The feeble warmth
developed in the helices by an electric current from two
cells was found able to create air currents of sufficient
power to defeat all attempts to obtain the pure action
of the prisms. To break up these air currents I stuffed
all unfilled spaces of the box with old newspapers, and
found the expedient to answer perfectly. With a fresh
battery, which delivered a constant current throughout the
duration of an experiment, the magnet was admirably
steady,2 and under these favourable conditions the follow-
ing results were obtained : —

1 Phil. Mag. March 1853, p. 222.

z It was necessary, however, to select a portiou of the day when
Albemarle Street was free from cabs and carriages, as the shaking of

210 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION1.

vn.

Prisms of Heavy Glass. — Length 3 inches ; width 0*6 ; depth 0-5.

Current direct Current direct Current direct

Position 1. 664 Position 2. 662 Position 3. 660

Thus in passing from position 1 to 3, or vice versa, a
permanent deflection corresponding to four divisions of the
scale was produced. By raising or lowering the respective
prisms at the proper moments the amplitude of the oscil-
lations could be considerably augmented, and, when at
a maximum, could be speedily extinguished by reversing
the motions of the prisms. In six different series of
experiments made with this substance the same in-
variable result was obtained. It will be observed that
the deflections are, in all cases^ identical in direction
with those produced by bismuth under the same circum-
stances.

The following results were afterwards obtained with
the same prisms in the presence of M. de la Kive ; the

current was 'direct'

VIII.

Position 1. 652 Position 2. 650 Position 3. 648

On the negative result arrived at with this substance, it
will be remembered that Von Feilitzsch bases one of his
arguments against the conclusions of Weber.

Calcareous spar was next submitted to experiment.
Two cylinders of the transparent crystal were prepared and
examined in the manner already described. The results

are as follows : —

IX.

Cylinders of Calcareous Spar. — Length 3 inches ; diameter 0-7.

Current direct Current direct Current direct

Position 1. 699-5 Position 2. 698-5 Position 3. 697'5

Here, as in the other cases, the deflection was permanent, and

the entire building, by the rolling of these vehicles, rendered the
magnets unsteady.

STATUARY MARBLE, PHOSPHORUS, SULPHUR, ETC. 211

could be augmented by the suitable raising or lowering of
the respective cylinders. The action is small, but perfectly
certain. The magnet was steady and moved promptly and
invariably in the directions indicated by the numbers. It
will be observed that the deflections are the same in kind
as those produced by bismuth.

The intrusion of other employments compelled me
to postpone the continuation of these experiments for
several weeks. On taking up the subject again, my first
care was to assure myself that the instrument retained its
sensibility. Subsequent to the experiments last recorded
it had been transported over several hundred miles of
railway, and hence the possibility of a disturbance of its
power. The following experiments, while they corroborate
the former ones, show that the instrument retained its
power and delicacy unimpaired : —

X.

Bismuth Cylinders.

Current direct Current reversed

Position 1. 612 264

Position 2. 572 230

Position 3. 526 200

The deflections, it will be observed, are the same in kind
as before; but by improved manipulation the effect is
augmented. In passing from position 1 to 3 we have
here a deflection amounting in one case to 64, and in the
other to 86 divisions of the scale.

To Mr. Noble I am indebted for two cylinders of pure
statuary marble ; the examination of these gave the follow-
ing results : —

XI.

Cylinders of Statuary Marble.— Length 4 inches; diameter 0'7-

Current direct Current reversed

Position 1. 601 215

Position 2. 508 218

Position 3. 51)6 220

212 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

Here, in passing from position 1 to 3, we have a permanent
deflection corresponding to five divisions of the scale. As
in all other cases, the impulsion of the magnet might be
augmented by changing the position of the cylinders at the
limit of each swing. The deflections are the same in kind
as those produced by bismuth, which ought to be the case,
for marble is diamagnetic.

An upright iron stove influenced by the earth's
magnetism becomes a magnet, with its bottom a north
and its top a south pole. Doubtless, though in an im-
mensely feebler degree, every erect marble statue is a true
diamagnet, with its head a north pole and its feet a south
pole. The same is certainly true of a man as he stands
upon the earth's surface, for all the tissues of the human
body are diamagnetic.

A pair of cylinders of phosphorus enclosed in thin glass
tubes were next examined.

XH.

Cylinders of Phosphorus. — Length 3-5 inches ; diameter 0-63.

Current direct Current reversed
Series I. Series II.

Position 1. 620 670 224

Position 2. 618 668 226

Position 3. 616 666 228

The change of the bars from position 1 to 3 is in this
case accompanied by permanent deflection corresponding
to four divisions of the scale. The deflection and polarity
is that of a diamaguetic body. The magnet was remark-
ably steady during these experiments, and the consequent
clearness and sharpness of the result pleasant to observe.

XIII.

Cylinders of Sulphur. — Length 6 inches ; diameter 0'7.

Current direct Current reversed
Position 1. 658-5 222

Position 2. 657 223-5

Position 3. 655.5 225'5

LIQUID DIAMAGXETS. 213

XIV.

Cylinders of Nitre. — Length 3-5 inches ; diameter OT.

Current direct Current reversed
Position 1. 648-5 263

Position 3. 647 265

Finally, as regards solid diamagnetic bodies, a series
of experiments was made with wax ; this also being one of
the substances whose negative deportment is urged by
Von Feilitzsch against Weber.

XV.

Cylinders of Wax. — Length 4 inches ; diameter 0-7.

Current direct Current reversed
Position 1. 624-5 240

Position 3. 623 241

The action is very small, but it is nevertheless perfectly
certain, and proves the polarity of the wax. The argument
founded on the negative deportment of this substance must
therefore give way. When we consider the feebleness of
the action with so delicate a means of examination, the
failure of Von Feilitzsch to obtain the effect, with an in-
strument constructed by himself, will not excite surprise.

Thus, in the case of seven insulating bodies, the
existence of diamagnetic polarity has been proved. The
list might be augmented without difficulty ; but sufficient
I trust has been done to remove the scruples of those who
saw in Weber's results an action produced by induced
currents.

Polarity of Diamagnetic Liquids.

A portion of the subject hitherto untouched by experi-
menters, but one of great interest, has reference to the
polar condition of liquids while under magnetic influence.

The first liquid examined was distilled water ; it was
enclosed in thin glass tubes, corked at the ends ; and by

214 DIAMAGNETISM AND MAGNE-CKYSTALLIC ACTION.

means of a loop passing round the cork, the tubes were
attached to the string passing round the wheels ww7.
Previous to use, the corks were carefully cleansed, so that
any impurity contracted in cutting, or by contact with
ferruginous matters, was completely removed. The follow-
ing are the results obtained with this liquid : —

XVI.

Cylinders of Distilled Water. — Length 4 inches ; diameter 0'65.

Current direct Current reversed
Position 1. 605 246

Position 2. 603 248

Position 3. 601 250

The experiment was many times repeated, but always
with the same result ; indeed, the polarity of the water
is as safely established as that of iron. Pure water is dia-
magnetic, and the deflections produced by it are the same
as those of all the other diamagnetic bodies submitted to
examination.

From the position which it occupies in Faraday's list,1
I had also some hopes of proving the polarity of sulphide
of carbon. The following results were obtained : —

XVH.

Cylinders of Bisulphide of Carbon. — Length 4 inches ; diameter 0'65.

Current direct Current reversed
Position 1. 631 210

Position 2. 629 213

Position 3. 626 216

As in the case of distilled water, we observe a deflection in
one direction when the current is ' direct,' and in the other
when it is ' reversed,' the action in the first case, in passing
from position 1 to 3, amounting to five, and in the latter
case to six divisions of the scale. The polarity of the
substance is therefore established, and it is that of dia-
magnetic bodies.

1 Phil. Mag. March 1853, p. 222.

MAGNETS OP SLATE. 215

Deportment of Magnetic Bodies.

Thus far we have confined our examination to diamag-
netic substances : turn we now to the deportment of
magnetic bodies when submitted to the same conditions of
experiment. Here we must select substances suitable for
examination, for all are not so. Cylinders of iron, for ex-
ample, of the same size as our diamagnetic cylinders, would,
through the intensity of their action, quite derange the
apparatus ; so that we are obliged to have recourse to bodies
of smaller size or of feebler magnetic capacity. Besides,
the remarks of writers on this subject render it of im-
portance to examine, whether bodies through which the
magnetic constituents are very sparingly distributed pre-
sent a veritable polarity the same as that exhibited by iron
itself.

Slate rock usually contains from eight to ten per cent,
of oxide of iron, and a fragment of the substance presented
to the single pole of an electro-magnet is attracted by the
pole. A cylinder of slate from the Penrhyn quarries near
Bangor was first examined. It was not found necessary to
increase the effect by using two cylinders, and the single
one used was suspended in the right-hand helix nV.
The deportment of the substance was as follows : —

xvm.

Cylinder of Penrhyn Slate. — Length 4 inches ; diameter 0'7.

Current direct Current reversed
Position 1. 620 280

Position 2. 647 240

Position 3. 667 198

Comparing these deflections with those obtained with
diamagnetic bodies, we see that they are in the opposite
direction. With the direct current a change from position
1 to 3 is followed, in the case of diamagnetic bodies, by a

216 DIAMAGNETISM AND MAGNE-CRYSTALL1C ACTION.

motion from higher to lower numbers ; while in the present
instance the motion is from lower numbers to higher. In
the former case the north poles of the astatic magnet
are attracted, in the latter they are repelled. We also see
that a direct current acting on diamagnetic bodies pro-
duces the same deflection as a reverse current on magnetic
ones. Thus, as promised at page 207, the opposite po-
larities of diamagnetic and magnetic bodies are transferred
from the region of deduction to that of fact.

XIX.

Cylinder of Caermarthen Slate. — Length 4 inches ; diameter O7.

Current direct Current reversed
Position 1. 664 300

Position 2. 690 235

Position 3. 720 185

The deflections in this case are also indicative of magnetic
polarity.

These two cylinders were so taken from the rock that
the axis of each lay in the plane of cleavage. The
following experiments, made with a cylinder of the same
size, show the capability of a rock of this structure to be
magnetised across the planes of cleavage.

XX.

Cylinder of Slate : axis of cylinder perpendicular to cleavage.

Current direct Current reversed
Position 1. 655 240

Position 2. 678 205

Position 3. 695 192

Chloride of iron was next examined : the substance, in
powder, was enclosed in a single glass tube, which was
attached to the string passing round the wheels w w' of
the instrument.

MAGNETS OF IRON COMPOUNDS. 217

XXI.

Cylinder of powdered Chloride of Iron. — Length 3-8 inches ;
diameter 0-5.

Current direct Current reversed
Position 1. 185 990

Position 2. — 230

Position 3. 990 185

The deflection here indicates magnetic polarity. The
action was very powerful. When swiftly moving in any
direction, a change in the position of the cylinder instantly
checked the magnet in its course, brought it to rest, or
drove it forcibly in the opposite direction. The numbers
1 85 and 990 mark indeed the utmost \imit between which
it was possible for the magnet to move; here it rested
against the helices.

Two glass tubes were filled with red oxide of iron and
examined. The action of the poles of these cylinders
upon the magnets was so strong, as to efface, by the
velocity imparted to the magnets, all distinct impression
of the numbers on the scale. By changing the position of
the tubes within the helices, the magnets could be driven
violently through the field of view, or could be held
rigidly against the respective helices. As in all other
cases, the centres of the cylinders were neutral points,
and the two ends of each were poles of opposite qualities.
The polarity was the same as that of iron.

A small quantity of iron filings was kneaded thoroughly
in wax, and a cylinder formed from the mass. Its deport-
ment was also very violent, and its polarity was just as
clear and pronounced as that of a solid cylinder of iron
could possibly be.

Sulphate of iron was next examined : the crystallised
substance was enclosed in two glass tubes and tested in
the usual manner.

218 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

XXII.

Cylinders of Sulphate of Iron. — Length 4-5 inches ; diameter 0'7.

Current direct Current reversed
Position 1. 510 610

Position 2. 600 370

Position 3. 700 220

The red ferroprussiate of potassa is a magnetic salt ;
with this substance the following results were obtained : —

XXIII.

Cylinders of red Ferroprussiate of Potassa. — Length 4'5 inches;
diameter 0*65.

Current direct Current reversed
Position 1. 610 250

Position 2. 630 220

Position 3. 655 197

In this case also the crystallised salt was enclosed in
glass tubes.

Two glass tubes were next filled with carbonate of iron
in powder ; the following are the results : —

XXIV.

Cylinders of Carbonate of Iron. — Length 4 inches ; diameter 0'5.

Current direct Current direct Current direct

Position 1. 185 Position 2. 620 Position 3. 740

In all these cases the deflections show that the cylinders
of powder are true magnets, being polar after the manner
of iron.

Polarity of Magnetic Liquids.

As the complement of the experiments made with
diamagnetic liquids, we now pass on to the examination
of the polarity of magnetic liquids. A concentrated
solution of sulphate of iron was enclosed in two glass
tubes and submitted to examination.

XXV.

Sulphate of Iron Solution in tubes. — Length 4 inches ; diameter 0-65.

Current direct Current direct Current direct

Position 1. 648 Position 2. 600 Position 3. 648

LIQUID MAGNETS: SUMMARY. 219

A solution of muriate of nickel, examined in the same
manner, gave the following results : —

XXVI.

Muriate of Nickel Solution in tubes. — Length 3-6 inches;
diameter 0'65.

Current direct Current reversed
Position 1. 605 224

Position 2. 632 200

Position 3. 650 185

A solution of muriate of cobalt yielded as follows : —
xxvn.

Muriate of Cobalt solution in tubes. — Length 3-6 inches ;
diameter 0'65.

Current direct Current reversed
Position 1. 630 262

Position 2. 645 235

Position 3. 660 202

In all these cases we have ample evidence of a polar
action the reverse of that exhibited by diamagnetic
liquids. These are the first experiments in which the
action of either liquid magnets, or liquid diamagnets, upon
a suspended steel magnet has been exhibited.

Thus far then the following substances have been sub-
mitted to examination : —

Diamagnetic bodies Magnetic bodies

Bismuth. Penrhyn slate.

Antimony. Slate, axis parallel to cleavage.

Heavy glass. Slate, axis perpendicular to

cleavage.

Calcareous spar. Chloride of iron.

Statuary marble. Sulphate of iron.

Phosphorus. Carbonate of iron.

Sulphur. Ferrocyanide of potassium.

Nitre. Oxide of iron.

Wax. Iron filings.

Liquids Liquid!

Distilled water. Sulphate of iron.

Bisulphide of carbon. Muriate of nickel.

Muriate of cobalt.

220 DIAMAGNETISM AND MAGNE-CEYSTALLIC ACTION.

Every substance in each of these lists has been proved
to be polar under magnetic influence, the polarity of the
diamagnetic bodies being invariably opposed to that of
the magnetic ones.

In his investigation on the supposed polarity of dia-
magnetic bodies, Faraday made use of a core of six-
penny pieces, and obtained with it the results he sought.
Wishing to add the testimony of silver as a good con-
ductor to that of copper, two cylinders were formed o
sixpenny pieces, covered with paper, and submitted to
experiment. The following are the results obtained : —

XXVHI.

Silver Cylinders (sixpenny pieces).

Current direct Current direct Current direct

Position 1. 721 Position 2. 774 Position 3. 804

The action here was prompt and energetic, strongly
contrasted with the neutrality of copper ; but the deflec-
tion was permanent, and could not therefore be the result
of induced currents. Further, it was a deflection which
showed magnetic polarity, whereas pure silver is feebly
diamagnetic. The cylinders were removed and examined
between the poles of an electro-magnet ; they proved to
be magnetic.

On observing this deportment of the silver, I tried the
copper cylinders once more. The results with a direct

current were, —

XXIX.

Position 1. 7G6 Position 2. 767 Position 3. 768

Here almost the same neutrality as before is evidenced.

Deeming that the magnetism of the cores of silver
coins was due to m^netic impurity attaching itself to
the paper which covered them, a number of fourpenny
pieces were procured, washed in ammonia and water, and
enclosed in thin glass tubes. The following were the
results : — -

SILVER COINS AND BISMUTH POWDER. 221

XXX.

Silver Cylinders (fourpenny pieces).

Current direct Current direct Current direct

Position 1. 490 Position 2. 565 Position 3. 660

Here also we have a very considerable action indicative of
magnetic polarity. On examining the cylinders between
the poles of an electro-magnet, they were found decidedly
magnetic. This, therefore, appears to be the common
character of our silver coins. [They doubtless contain a
trace of iron.] The tubes which contained the pieces were
sensibly neutral.

Knowing the difficulty of demonstrating the exist-
ence of diamagnetic polarity in ordinary insulators, Mat-
teucci suggested that insulated fragments of bismuth
ought to be employed, the insulation being effected by a
coat of lac or resin. I constructed a pair of cylinders in
accordance with the suggestion of M. Matteucci. The
following are the results they yielded with a direct
current : —

Position 1. 730 Position 2. 750 Position 3. 768

Here we have a very marked action, but the polarity indi-
cated is magnetic polarity. On subsequent examination,
the cylinders proved to be magnetic. This was due to
impurities attaching themselves to the resin.

But the resin may be done away with and the pow-
dered metal still rendered an insulator. This thought
was suggested to me by an experiment of Faraday,
which I will here describe. Eeferring to certain effects
obtained in his investigations on supposed diamagnetic
polarity, he writes thus : — ' If the effect were produced by
induced currents in the mass, division of the mass would
stop these currents and so alter the effect ; whereas, if
produced by a true diamagnetic polarity, division of
the mass would not affect the polarity seriously or in its

222 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

essential nature. Some copper filings were therefore
digested for a few days in dilute sulphuric acid to remove
any adhering iron, then well washed and dried, and after-
wards warmed and stirred in the air, until it was seen by
the orange colour that a very thin film of oxide had formed
upon them ; they were finally introduced into a glass tube
and employed as a core. It produced no effect whatever,
but was as inactive as bismuth.' (Exper. Eesear. 2658.)

Now when bismuth is powdered and exposed to the
action of the air, it very soon becomes tarnished, even
without heating. A quantity of such powder was pre-
pared, and its conducting power for electricity tested.
The clean ends of two copper wires proceeding from a
battery of Grove were immersed in the powder ; but
though the wires were brought as near as possible to
each other, short of contact, not the slightest action
was observed upon a galvanometer placed in the circuit.
When the wires touched, the needle of the galvanometer
flew violently aside, thus proving that the current was
ready, but that the powder was unable to conduct it.
Two glass tubes were filled with the powder and sub-
mitted to experiment. The following results were ob-
tained : —

XXXII.

Cylinders of Bismuth Powder.
Length 3 inches. Diameter O7.

Current direct Current reversed
Position 1. 640 230

Position 2. 625 245

Position 3. 596 260

These deflections are the same in kind as those obtained
with the cylinders of massive bismuth. We have here no
cessation of action. The division of the mass does not
affect the result seriously or in its essential nature, and
hence the deportment exhibits the characteristics of *a
true diamagnetic polarity.'

FARADAY S APPARATUS : A COMPARISON. 223

In summing up the results of his enquiry on this
subject, Mr. Faraday writes thus : — * Finally, I am
obliged to say that I can find no experimental evidence
to support the hypothetical view of diamagnetic polarity,
either in my own experiments, or in the repetition of
those of Weber, Keich, and others. ... It appears
to me also, that, as magnetic polarity conferred by iron
or nickel in small quantity, and in unfavourable states, is
far more easily indicated by its effects upon an astatic
needle, or by pointing between the poles of a strong
horseshoe magnet, than by any such arrangement as mine
or Weber's or Reich's, so diamagnetic polarity would be
'much more easily distinguished in the same way? I was
struck, on reading this passage, to find how accurately the
surmise has been fulfilled by the instrument with which
the foregoing experiments were made. In illustration of
the powers of this instrument, as compared with that
made use of by Mr. Faraday, I may be permitted to
quote the following result from his paper on supposed
diamagnetic polarity so often referred to : — ' A thin glass
tube, 5^- inches by three-quarters of an inch, was filled
with a saturated solution of proto-sulphate of iron, and
employed as an experimental core ; the velocity given to
the machine at this and all average times was such as to
cause five or six approaches and withdrawals of the core in
one second ; yet the solution produced no sensible indica-
tion on the galvanometer.' Referring to Table XXV., it
will be seen that the instrument made use of in the
present enquiry has given with a solution of protosulphate
of iron a deflection amounting to no less than one hundred
divisions of the scale. Mr. Faraday proceeds : — ' A tube
filled with small crystals of protosulphate of iron caused

the needle to move about 2° Bed oxide of

iron produced the least possible effect.' In the experi-
ments recorded in the foregoing pages, the crystallised
11

224 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

sulphate of iron gave a deflection of nearly two hundred
divisions of the scale, while the red oxide gave a deflec-
tion as wide as the helices would permit, which corre-
sponds to about eight hundred divisions of the scale.
The correctness of Faraday's statement regarding the
inferiority of the means first devised to investigate this
subject, is thus strikingly illustrated. It might be
added, that red ferroprussiate of potash and other sub-
stances, which have given me powerful effects, produced
no sensible impression in experiments made with Faraday's
instrument.

Thus have we seen the objections raised against
diamagnetic polarity fall away one by one, and a body
of evidence accumulated in its favour, which places it
among the most firmly established truths of science. This
I cannot help thinking is, in great part, to be attributed
to the bold and sincere questioning of the principle when
it seemed questionable. The cause of science is more truly
served, even by the denial of what may be a truth, than by
the indolent acceptance of it on insufficient grounds. Such
denials drive us to a deeper communion with Nature, and,
as in the present instance, compel us through severe and
laborious enquiry to strive after certainty, instead of resting
satisfied, as we are prone to do, with mere probable
conjecture.

Royal Institution, November 1855.

SIXTH MEMOIR

ON TEE RELATION OF DIAMAGNETIC POLARITY
TO MAGNE-CRYSTALLIC ACTION.*

[COMPLETION OF ARGUMENT.]

In a communication presented to the Eoyal Society some
weeks ago, the fact of diamagnetic polarity was established
for a great variety of substances, including insulators, such
as phosphorus, sulphur, calcareous spar, statuary marble,
heavy glass, and nitre. The demonstration was also extended
to distilled water and other liquids ; the conditions proposed
by the opponents of diamagnetic polarity for its rigorous
demonstration being thereby fulfilled. The importance
of the principle is demonstrated by the fruitfulness of
its consequences ; for by it we obtain a clear insight of
effects which, without it, would remain standing enigmas
in science, being connected by no known tie with the
ordinary laws of mechanics. Many of the phenomena of
magne-crystallic action are of this paradoxical character.
For the sake of those who see no clear connection between
these and the other effects of magnetism, as well as for the
sake of completeness, I will here endeavour to indicate in
a simple manner, and from my own point of view, the
bearing of the question of polarity upon that of magne-
crystallic action. I will commence with the elementary
phenomena, and select for illustration, as I proceed, cases
of real difficulty which have been actually encountered
by those who have worked experimentally at the subject.
1 Phil. Mag., vol. ii. p. 1 23.

226 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

To free our thoughts from all effects except those
which are purely magne-crystallic, we will for the pre-
sent operate with spheres. Let a sphere of carbonate
of lime be suspended before the pole s, fig. 1, of an
electro-magnet, so that the axis of the crystal shall be
horizontal. Let the line ah mark any position of the
axis inclined to the direction of the force emanating
from s (marked by the large arrow) ; and let the dotted
line dc make an equal angle with the direction of the
force at the other side. As the sphere is diamagnetic,
the face of it which is turned towards s will, according to
the principles established in the foregoing memoirs, be
hostile to s, while that turned from s will be friendly

FIG. 1.

to s ; and, if the sphere were homogeneous, the tendency
to set ab at right angles to the direction of the force would
be exactly neutralised by the tendency to set cd in the
same position : the sphere would consequently stand still.
But the case is otherwise when the intensity of diamagnet-
isation along ab is greater than along cd, which I have
elsewhere proved to be the fact.1 If, adopting a line of
argument already pursued, we suppose the sphere to vanish,
with the exception of two thin needles taken along the lines
mentioned, the hostile pole at a will be stronger than that
at c, and the friendly pole at b will be stronger than that
at d ; hence, the ends a and b being acted upon by a
mechanical couple of superior power, the line ab will

1 Phil. Mag., S. 4, vol. ii. p. 176, and at p. 63 of this volume.

COMPLETION OP ARGUMENT. 227

recede from its inclined position, and finally set itself at
right angles to the direction of the force. Whatever be
the inclination of the line ab to the magnetic axis, this
superiority will belong to its couple ; the entire sphere will
therefore turn in the manner here indicated, and finally
set with the axis of the crystal equatorial. This is the
result established by experiment.

For the diamagnetic calcium, contained in this crystal,
let the magnetic element, iron, be substituted. Each mole-
cule of the crystal becomes thereby magnetic; we have
carbonate of iron in place of carbonate of lime ; and the
axis which, in the latter substance, is that of maximum

Fia. 2.

S

repulsion, is that of maximum attraction in the former.
This, I think, is one of the most suggestive points * that
researches in magne-crystallic action have hitherto estab-
lished, namely, that the same arrangement of molecules
influences the paramagnetic and diamagnetic forces in
the same way, intensifying both in the same direction.
Let us suppose, then, that the sphere of carbonate of iron
is suspended as in fig. 2, the line ab being the axis of the
crystal. I have already shown this line to be that in
which the magnetic induction is most intense.2 Compar-
ing, as before, the lines ab and cd, the friendly pole a is
stronger than c, and the hostile pole b is stronger than d ;

1 For its bearing upon the question of a magnetic medium see Phil.
Mag., vol. ix. p. 208, and further on in this volume.

8 Phil. Mag. S. 4, vol. ii. p. 177 and at p. 65 of this volume.

228 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

a residual ' couple ' therefore acts upon ab in the direction
indicated by the arrows, which must finally set this line
parallel to the direction of the lines of force. This is also
the result which experiment exhibits.

We will now apply the principle of polarity to some
of the more complicated forms of magne-crystallic action.
Some highly paradoxical effects were adduced by Faraday,
in proof of the assertion that the magne-crystallic force
is neither attraction nor repulsion. I cannot bring the
subject in a fairer manner before the reader than by quot-
ing Faraday's own description of the phenomena referred
to. Here it follows : —

' Another very striking series of proofs that the effect
is not due to attraction or repulsion was obtained in the
following manner : — A skein of fifteen filaments of cocoon
silk, about 14 inches long, was made fast above, and then
a weight of an ounce or more hung to the lower end ; the
middle of this skein was about the middle of the magnetic
field of the electro-magnet, and the square weight below
rested against the side of a block of wood so as to give a
steady silken vertical axis without swing or revolution. A
small strip of card, about half an inch long and the tenth
of an inch broad, was fastened across the middle of this
axis by cement ; and then a small prismatic crystal of
sulphate of iron O3 of an inch long and O'l in thickness,
was attached to the card, so that the length and also the
magne-crystallic axis were in the horizontal plane ; all the
length was on one side of the silken axis, so that as the
crystal swung round, the length was radius to the circle
described, and the magne-crystallic axis parallel to the
tangent.

' When the crystal was made to stand between the
flat-faced poles, the moment the magnet was excited it
moved, tending to stand with its length equatorial, or its
magne-crystallic axis parallel to the lines of force. When

COMPLETION OF ARGUMENT. 229

one pole was removed and the experiment repeated, the
same effect took place, but not so strongly as before ;
finally, when the pole was brought as near to the crystal
as it could be without touching it, the same result
occurred, and with more strength than in the last case.
In the two latter experiments, therefore, the crystal of
sulphate of iron, though a magnetic body, and strongly
attracted by such a magnet as that used, actually receded
from the pole of the magnet under the influence of the
magne-crystallic condition.

'If the pole s be removed, and that marked N be
retained l for action on the crystal, then the latter
approaches the pole urged by both the magnetic and
magne-crystallic forces ; but if the crystal be revolved
90° to the left, or 180° to the right, round the silken
axis, so as to come into the contrary or opposite position,
then this pole repels or rather causes the removal to a
distance of the crystal, just as the former did. The
experiment requires care, and I find that conical poles
are not good ; but with attention I could obtain the
results with the utmost readiness.

' The sulphate of iron was then replaced by a crystalline
plate of bismuth, placed, as before, on one side of the silk
suspender, and with its magne-crystallic axis horizontal.2
Making the position the same as that which the crystal
had in relation to the N pole in the former experiment,
so that to place its axis parallel to the lines of magnetic
force it must approach this magnetic pole, and then
throwing the magnet into an active state, the bismuth

1 The figures will be given and explained further on.

8 It will be borne in mind that Faraday calls the line in a crystal
which sets from pole to pole, the magne-crystallic axis of the crystal,
whether the latter is paramagnetic or diamagnetic. In bodies of the
former class, however, the ' axis ' sets from pole to pole because the
attraction along it is a maximum ; while in bodies of the latter class,
the 'axis 'sets from pole to pole because the repulsion along the line
perpendicular to it is a maximum.

230 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

moved accordingly and did approach the pole, against
its diamagnetic tendency, but under the influence of the
magne-crystallic force.

' Hence a proof that neither attraction nor repulsion
governs the set This force, then, is dis-
tinct in its character and effects from the magnetic and
diamagnetic forms of force.'

These experiments present grave difficulties, and,
without invoking the aid of diamagnetic polarity, they
are inexplicable. That principle once established, they
follow from it as the simplest mechanical consequences.
I will now endeavour to apply the idea of a force which is
both attractive and repulsive, or in other words of a polar
force, to the solution of these difficulties.

For the sake, once more, of disencumbering the
mind of all considerations save those which belong to
pure magne-crystallic action, we will suppose the bodies
experimented with to be spherical.

FIG. 3.

Let the dot at £, fig. 3, be the intersection of the
vertical silken axis with Faraday's strip of card ; and on
the end of the strip, let the sphere of sulphate of iron be
placed with its magne-crystallic axis ab at right angles
to the length of the strip. This line, as I have already
shown,1 is that of most intense magnetisation through the

1 Phil. Mag., S. 4, vol. ii. p. 178, and at p. 66 of this volume.

COMPLETION OF ARGUMENT.

231

crystal. The forces acting on the sphere in its present
position are exactly similar to those acting upon the
carbonate of iron in fig. 2. A residual ' couple ' will
apply itself at the extremities of ab, as indicated by the
arrows, and would, if the sphere were free to turn round
its centre of gravity, set the line ab parallel to the lines
of force. But the sphere is here rigidly connected
with a lever moveable round its own axis of suspension,
and it is easy to state the mechanical result that must
follow from this arrangement. To obtain the ' moments '
of the two forces acting upon a and 6, we have to multiply
each of them by the distance of its point of application
from the axis x. Now in front of a flat pole such as
that made use of by Faraday in these experiments, the
force diminishes very slowly as we recede from the pole.
The consequence is that the attraction of a does not so
far exceed the repulsion of b as to prevent the product of
the latter into xz from exceeding that of the former into
xy, and consequently the paramagnetic sphere must recede
from the pole.1 Faraday's result is thus explained.

FIG. 4.

N

In Ins next experiment, Faraday removed the pole s
and allowed the pole N to act upon the crystal as in
fig. 4. In this case it will be seen that the end nearest

1 [Calling the attraction a, the force with which the sphere tends
to turn towards the magnet is equal to a x xy. Calling the repulsion r,
the force with which the sphere tends to retreat from the magnet is
rxxz. If a be not much greater than r, the product r x xz will exceed
a x xy, and the sphere, though magnetic, must retreat as if repelled by
the pole.

232 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

the pole, and therefore the most strongly attracted, is also at
the greatest distance from the axis of rotation. Hence the
sphere must approach the pole, as in Faraday's experiment.
When the strip of card is revolved 90°, we have the
state of things shown in fig. 5 ; and when it is revolved
180°, we have the state of things shown in fig. 6. It is

FIG. 5. FIG. 6.

N

manifest, for the mechanical reasons already assigned, that
the crystal, in both these cases, must recede from the pole.
Faraday's difficulty thus disappears.

Substituting for the sphere of sulphate of iron a sphere
of bismuth with its magne-crystallic axis cd, fig. 7, per-

FIG. 7.

N

pendicular to the strip of card, the bismuth is found by
Faraday to approach the pole when the magnet is excited.
The line ab, perpendicular to that called the magne-crys-
tallic axis, has been shown by Faraday himself to be that

COMPLETION OF ARGUMENT. 233

of greatest diamagnetic intensity ; the mass is therefore
under the influence of forces precisely similar to those acting
on the carbonate of lime in fig. 1 . A ' residual couple,' as
denoted by the arrows, will act at the extremities of the
line ab. The absolute repulsion of a in the field of force
here assumed, does not differ much from the absolute attrac-
tion of b ; but the latter force acts at the end of a much
longer lever, and consequently the sphere is drawn towards
the excited pole. I cannot help remarking here upon the
severe faithfulness with which these results are recorded,
and on the inestimable value of such records to scientific
progress. The key to their solution being once found, the
investigator may proceed confidently to the application of
his principles, without fear of check or perplexity arising
from the imperfection of his data.

In all these cases we have assumed that the magnetic
force diminishes slowly as we recede from the pole. This
is essential to the production of the effects. The exact
expression of the condition is, that the advantage due
to the proximity of the part of the mass nearest the pole,
must be less than that arising from the greater leverage
possessed by the force acting on the more distant parts.
When the shape of the poles is such that the diminution
of the force with the increase of distance is too speedy
for the above condition to be fulfilled, the phenomena no
longer exhibit themselves. It is plain that the diminu-
tion of the force as we recede from a pointed pole must
be more rapid than when we recede from a magnetised
surface, and hence it is that Faraday finds that ' conical
poles are not good.' It is also essential that the length
of the lever which supports the magne-crystallic body
shall bear a sensible ratio to the distance between the
two points of application of the magnetic force. If the
lever be long, recession will take place in cases where,
with a shorter lever, approach would be observed.

234 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

It is well known that a piece of soft iron is attracted
most strongly by the angles and corners of a magnet, and
hence it is sometimes inferred that the magnetic force
emanating from these edges and corners is more intense
than that issuing from the central parts of the polar
surfaces. Such experiments, however, when narrowly
criticised, do not justify the inference drawn from them.
They simply show that the difference between attraction
and repulsion, on which the final attraction depends, is
greater at the edges than elsewhere ; but they do not
enable us to infer the absolute strength of either the
attraction or the repulsion, or, in other words, of the force
of magnetisation. The fact really is, that while the at-
traction of the mass is nearly absent in the central por-
tion of a magnetic field bounded by two flat poles,
the magnetisation is really stronger there than between
the edges. This is proved by the following experiment : —

I suspended a cube of crystallised bismuth from a
fibre of cocoon silk ; when the magnet was excited, the cube
set its planes of principal cleavage equatorial. When
drawn aside from this position and liberated, it oscillated
to and fro through it. Between the upper edges of the
moveable poles the number of oscillations performed in a
minute was seventy-six ; in the centre of the field the
number performed was eighty-eight, and between the lower
edges eighty. A cube of magnetic slate, similarly sus-
pended, oscillated in the centre of the field forty-nine
times, and between the edges only forty times, in fifteen
seconds. In the former position there was no sensible tend-
ency of the cube to move towards either pole ; but in the
latter position, though the magnetisation was considerably
less intense, the cube was with difficulty prevented from
moving up to one or the other of the edges. The reason
of all this manifestly is, that while the forces in the centre
of the field nearly neutralise each other as regards the

COMPLETION OF ARGUMENT. 235

translation of the mass, they are effective in producing its
oscillation ; while between the edges, though the absolute
forces acting on the north and south poles of the excited
substances are less intense, the difference of these forces,
owing to the speedier diminution of the force with the
distance, is greater than in the centre of the field. It is
therefore an error to infer, that, because the attraction of
the mass is greater at the edges and corners than in the
centre of the field, the magnetising force of the former
must therefore be more intense than that of the latter.1

There is another interesting and delicate experiment
of Faraday's to which I am anxious to apply the prin-
ciple of diamagnetic polarity : the experiment was made
with a view of proving that ' the magne-crystallic force
is a force acting at a distance.' ' The crystal,' writes
Faraday, ' is moved by the magnet at a distance, and the
crystal can also move the magnet at a distance. To pro-
duce the latter result, I converted a steel bodkin, 3 inches
long, into a magnet, and then suspended it vertically by a
cocoon filament from a small horizontal rod, which again
was suspended by its centre and another length of cocoon
filament, from a fixed point of support. In this manner
the bodkin was free to move on its own axis, and could also
describe a circle about 1^ inch in diameter ; and the latter
motion was not hindered by any tendency of the needle
to point under the earth's influence, because it could take
any position in the circle and yet remain parallel to itself.

' When a crystal of bismuth was fixed on a support with
the magne-crystallic axis in a horizontal direction, it
could be placed near the lower pole of the magnet in any
position ; and being then left for two or three hours, or
until by repeated examination the magnetic pole was found
to be stationary, the place of the latter could be examined,

1 Some important consequences resulting from this experiment are
intended for a future communication.

236 DIAMAGNET1SM AND HAGNE-CRYSTALLIC ACTION.

and the degree and direction in which it was affected by
the bismuth ascertained. . . . The effect produced was
small ; but the result was, that if the direction of the
magne-crystallic axis made an angle of 10°, 20°, or 30°
with the line from the magnetic pole to the middle of the
bismuth crystal, then the pole followed it, tending to bring
the two lines into parallelism ; and this it did whichever
end of the magne-crystallic axis was towards the pole, or
whichever side it was inclined to. By moving the bismuth
at successive times, the deviation of the magnetic pole
could be carried up to 60°. The crystal, therefore, is able
to react upon the magnet at a distance. But though it
thus takes up the character of a force acting at a distance,
still it is due to that power of the particles which makes
them cohere in regular order, and gives the mass its crys-
talline aggregation ; which we call at other times the at-
traction of aggregation, and so often speak of as acting at
insensible distances.'

The disposition of this important experiment will be
manifest from fig. 8, where cd is the magne-crystallic axis

of a sphere of bismuth, or the
line in which the diamagnetic
... induction is least intense; and
s'n' the direction of the prin-
cipal cleavage, or that of most
intense diamagnetisation. Let
n be the point of the bodkin,
say its north pole, the crystal
will be excited by the influence
of this pole, and the resultant
action will be the same as if it
were exclusively * diamagnetised '

\ along the line s'n'. At the end

^ nearest to the pole of the bodkin

a repelled pole n' will be excited in the bismuth ; at the

COMPLETION OF ARGUMENT. 237

most distant end an attracted pole s' will be excited. Let
the repulsive force tending to separate n from n' be re-
presented by the line np and let the attraction exerted
between s' and n be represented by the line nq ; the
arrangement is such that the force of s' acts more nearly
in the direction of the tangent than that of n' ; the latter
may be decomposed into two, one acting along the circle
and the other across it: the latter component exerts a
pressure against the axis of suspension ; the former only is
effective in causing the pole n to move ; so that the whole,
or nearly the whole, of the attraction has to compete with
a comparatively small component of the repulsion. The
former therefore preponderates, and the pole n approaches
the crystal. It is manifest that as the angle which the
line jfrom n to the centre of the crystal makes with the
magne-crystallic axis, increases, the component of repul-
sion which acts in the direction of a tangent to the curve,
augments also ; and that at a certain point this component
must become preponderant. Beyond an angle of 30° it is
to be presumed that Mr. Faraday did not obtain the effect.
Removing the crystal, and placing a small magnet in the
position of the line sf n', with its poles arranged as in the
figure, the same phenomena would be produced.1

As finally illustrative of the sufficiency of the principle
of polarity to explain the most complicated phenomena
of magne-crystallic action, let us turn to the consideration
of those curious effects of rotation first observed by M.
Pliicker, and illustrated by thirty-seven cases brought
forward in the Eakerian Lecture for 1 855. The effects, it
will be remembered, consisted of the turning of elongated
paramagnetic bodies suspended between pointed poles
from the axial to the equatorial position, and of elongated

1 As there are no measurements given of the distances between the
crystal and the pole, it is of course impossible to do more than indicate
generally the theoretic solution of the experiment.

238 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

diamagnetic bodies, from the equatorial to the axial
position, when the distance between the suspended body
and the poles was augmented. This is a subject of con-
siderable difficulty to many, and I therefore claim the in-
dulgence of those who have paid more than ordinary at-
tention to it, if in this explanation I should appear to
presume too far on the reader's want of acquaintance with
the question. Let us then suppose an elongated crystal
of tourmaline, staurolite, ferrocyanide of potassium, or
beryl, cr, to be suspended between the conical poles N, s,
fig. 9, of an electro-magnet ; supposing the position be-
tween the poles to be the oblique one shown in the figure,
let us inquire what are the forces acting upon the crystal

FIG. 9.

in this position. In the case of all paramagnetic crystals
which exhibit the phenomenon of rotation, it will be borne
in mind that the line of most intense magnetisation is at
right angles to the length of the crystal. Let sn be any
transverse line near the end of the crystal ; fixing our at-
tention for the present on the action of the pole N, we
find that a friendly pole is excited at s and a hostile pole at
n : let us suppose s and n to be the points of application
of the polar force, and, for the sake of simplicity, let us
assume the distance from the point cf the pole N to s to
be half of the distance from N to n. We will further
suppose the action of the pole to be that of a magnetic
point, to which, in reality, it approximates ; then, inas-

COMPLETION OF ARGUMENT. 239

much as the quantities of north and south magnetism are
equal, we have simply to apply the law of inverse squares
to find the difference between the two forces. Calling
that acting on 8 unity, that acting on n will be £. Op-
posed to this difference of the absolute forces is the differ-
ence of their moments of rotation ; the force acting on n
is applied at a greater distance from the axis of rotation,
but it is manifest that to counterbalance the advantage
enjoyed by s, on account of its greater proximity, the dis-
tance x z would require to be four times that of x y.
Taking the figure as the correct sketch-plan of the poles
and crystal, it is plain that this condition is not fulfilled,
and that hence the end of the crystal will be drawn towards

FIG. 10.

the pole N. What we have said of the pole N is equally
applicable to the pole s, so that such a crystal suspended
between two such poles, in the manner here indicated, will
set its length along the line which unites them.

While the crystal retains the position which it occupied
in fig. 9, let the poles be removed further apart, say to
ten times their former distance. The ratio of the two
forces acting on the two points of application s and n will
be now as the square of 11 to the square of 10, or as 6 : 5
nearly. Taking fig. 10, as in the former case, to be the
exact sketch of the crystal, it is manifest that the ratio of
x z to x y is greater than that of 6 to 5,1 the advantage,

1 At a distance, moreover, the whole mass of the pole, not its point
alone, comes into play.

240 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

on account of greater leverage, possessed by the force act-
ing on n is therefore greater than that which greater
proximity gives to s, and the consequence is that the
crystal will recede from the pole, and its position of rest
between two poles placed at this distance apart will be at
right angles to the line which joins them. It is needless
for me to go over the reasoning in the case of a diamag-
netic body whose line of strongest diamagnetisation is
perpendicular to its length. Reversing the direction of
the arrows in the last two figures, we should have the
graphic representation of the forces acting upon such a
body ; and a precisely analogous mode of reasoning would
lead us to the conclusion, that when the polar points are
near the crystal, the latter will be driven towards the
equatorial position, while where they are distant, the
crystal will be drawn into the axial position. In this way
the law of action laid down empirically in the Bakerian
Lecture for 18,55 is deduced a priori from the polar cha-
racter of both the magnetic and diamagnetic forces. The
most complicated effects of magne-crystallic action are
thus reduced to mechanical problems of extreme simplicity ;
and, inasmuch as these actions are perfectly inexplicable
except on the assumption of diamagnetic polarity, they
add their evidence in favour of this polarity to that already
furnished in such abundance.

Perhaps as remarkable an illustration as could be
chosen of the apparently perplexing character of certain
magnetic phenomena, but of their real simplicity when
the exact nature of the force producing them is understood,
is furnished by the following experiment. I took a quan-
tity of pure bismuth powder and squeezed it between two
clean copper plates until the powder became a compact
mass. A fragment of the mass suspended before the
pointed pole of a magnet was forcibly repelled ; and when
suspended in the magnetic field with the direction of
pressure horizontal, in accordance with results already

COMPLETION OF ARGUMENT. 241

sufficiently well known, it set its line of pressure equa-
torial.

A second quantity of the bismuth powder was taken,
and with it was mixed powdered carbonate of iron,
amounting to -^ths per cent, of the whole ; the mass was
still strongly diamagnetic, but the line of compression,
instead of setting equatorial as in the former instance,
set decidedly axial.

A portion of the mixed powder was next taken, in
which the magnetic constituent amounted to 1 per cent.
The mass was still diamagnetic, but the line of compres-
sion set axial ; it did so when the influence of exterior
form was quite neutralised, so that the effect must be re-
ferred solely to the compression of the mass. With 2 per
cent, of carbonate of iron powder the mass was magnetic,
and set, with increased energy, its line of compression axial ;
with 4 per cent, of carbonate of iron the same effect was
produced in a still more exalted degree.

Now, why should the addition of a quantity of carbonate
of iron powder, which is altogether insufficient to convert
the mass from a diamagnetic into a paramagnetic one, be
able to overturn the tendency of the diamagnetic body to
set its line of compression equatorial ? The question is
puzzling at first sight, but the difficulty vanishes on re-
flection. The repulsion of the bismuth, when suspended
before a pointed pole, depends upon its general capacity
for diamagnetic induction, while its position as a magne-
crystal between flat poles depends on the difference be-
tween its capacities in two different directions. The
diamagnetic capacity of the substance may be very great
while its capacity in different directions may be nearly
alike, or quite so : the former, in the case before us, came
into play before the pointed pole ; but between the flat
poles, where the directive, and not the translative energy
is great, the carbonate of iron powder, whose directive

242 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

power, when compressed, far exceeds that of bismuth, de-
termined the position of the body. In this simple way a
number of perplexing results obtained with bodies formed
of a mixture of paramagnetic and diamagnetic constituents,
may be shown capable of satisfactory explanation.

Finally, inasmuch as the set of the mass in the mag-
netic field depends upon the difference of its excitement
in different directions, it follows that any circumstance
which affects all directions of a magne-crystallic mass in
the same degree will not disturb the differential action
upon which its deportment depends. This seems to me to
be the explanation of the results recently obtained by Mr.
Faraday with such remarkable uniformity, namely, that,
no matter what the medium may be in which the magne-
crystallic body is immersed, whether air or liquid, para-
magnetic or diamagnetic, it requires, in all cases, the same
amount of force to turn it from the position which it takes
up in virtue of its structure.1

I have thus dwelt upon instances of magne-crystallic
action which have revealed themselves in actual practice,
as affording the best examples for the application of
the key which the demonstration of the polarity of the
diamagnetic force places in our possession ; and I believe
it has been shown that these phenomena, which were in
the highest degree paradoxical when first announced, are
deducible with perfect ease and certainty from the action
of polar forces. The whole domain of magne-crystallic
action is thus transferred from a region of mechanical
enigmas to one in which our knowledge is as clear and
sure as it is regarding the most elementary phenomena
of magnetic action.

1 I need hardly draw attention to the suggestive beauty of this
experiment.— J. T., 1870.

1. LETTER FROM PROFESSOR W. WEBER.

The honoured name of Prof. Wilhelm Weber has been
mentioned more than once in the foregoing Memoirs. To
him I forwarded a copy of the Bakerian Lecture for 1855,
giving, at the same time, a sketch of some experiments
which I had then executed with the instrument already
referred to as designed for me by himself. He favoured
me, in reply, with the following interesting communica-
tion : —

Gottingen, September 25, 1855.

* MY DEAR SIR, — Accept my best thanks for your kind
communication of September 3 ; I am gratified to learn
that the apparatus executed by M. Leyser in Leipzig for
the demonstration of diamagnetic polarity has so com-
pletely fulfilled your expectations. This intelligence is all
the more agreeable to me, inasmuch as before the apparatus
was sent away, it was not in my power to go to Leipzig
and test the instrument myself.

' It gave me great pleasure to learn that Mr. Faraday
and M. De la Rive have had an opportunity of witnessing
the experiments, and of convincing themselves as to the
facts of the case.

' It was also of peculiar interest to me to learn that
you had succeeded in establishing the polarity of the self-
same heavy glass with which Faraday first discovered dia-
magnetism. This is the best proof that these experiments
do not depend upon the conductive power of bismuth for
electricity.

244 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

* I have read with great interest your memoir " On the
Diamagnetic Force," &c. contained in the "Philosophical
Transactions," vol. cxlv. It has been your care to separate
the fact of diamagnetic polarity from the theory, and to
place the former beyond the region of doubt. Allow me,
with reference to this subject, to direct your attention to
a passage at page 39 of your memoir, which you adduce
as a conclusion from my theory ; the passage runs as
follows : —

' " The magnetism of two iron particles in the line
of magnetisation is increased by their reciprocal action ;
but, on the contrary, the diamagnetism of two bismuth
particles lying in this direction is diminished by their
reciprocal action."

* This proposition is by no means a necessary assump-
tion of my theory, but is rather a direct consequence of
diamagnetic polarity, if the facts be such as both you and
I affirm them to be. What, therefore, you have adduced
against the above conclusion must be regarded as an
argument against diamagnetic polarity itself. The dia-
magnetic reciprocal action of the bismuth particles in
the line of magnetisation is necessarily opposed to the
action of the exciting magnetic force. The latter must
be enfeebled, because the diamagnetic is opposed to the
magnetic reciprocal action of iron particles which lie in
the line of magnetisation, through which latter it is known
the action of the exciting magnetic force is increased.
Hence also the modification produced in bismuth by
magnetic excitement, whatever it may be, must be weak-
ened, because the force of excitation is diminished.

* (I believe, however, that this argument against dia-
magnetic polarity may also be surmounted. The phe-
nomenon which you have observed must be referred to
other circumstances, also connected with the compression
of the bismuth. For the diamagnetic reciprocal action is,

LETTEE FROM PROFESSOR WEBER. 245

as I have sbown, much too weak to produce an effect
which could be compared in point of magnitude with the
reciprocal action produced in the case of iron.)

' I take this opportunity of adding a few remarks for
the purpose of setting my theory of diamagnetic polarity
in a more correct light.

* My theory assumes : — 1, that the fact of diamagnetic
polarity is granted ; 2, that in regard to magnetic phe-
nomena, Poisson's theory of two magnetic fluids, and
Ampere's theory of molecular currents, are equally ad-
missible. Whoever denies the first fact, or rejects the
theory of Ampere, cannot, I am ready to confess, accept
my theory.

t But supposing that you do not reject Ampere's theory
of permanent molecular currents, but are disposed to enter
upon the inner connection and true significance of the
theory, you will easily recognise that it is by no means an
arbitrary assumption of 'mine, that in bismuth molecular
currents are excited, when the exciting magnetic force is
augmented or diminished ; but that the excitation of such
molecular currents is a necessary conclusion from, the
theory of Ampere, which conclusion Ampere himself could
not make, because the laws of voltaic induction, discovered
by Faraday, were unknown to him. In all cases where
molecular currents exist, by increase or diminution of the
magnetic exciting force, molecular currents must be
excited, which either add their action to, or subtract it
from, the action of those already present.

' Finally, permit me to make a few remarks on the
following words of your memoir : —

' " To carry out the assumption here made, M. Weber
is obliged to suppose that the molecules of diamagnetic
bodies are surrounded by channels, in which the induced
currents, once excited, continue to flow without resist-
ance."

246 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

' The assumption of channels which surround the
molecules, and in which the electric fluids move without
resistance, is an assumption contained in the theory of
Ampere, and is by no means added by me for the purpose
of explaining diamagnetic polarity. A permanent mole-
cular current without such a channel involves a manifest
contradiction, according to the law of Ohm.

* I may further observe, that I do not wonder that you
regard a theory which is built upon the assumption of such
channels, as " so extremely artificial that you imagine the
general conviction of its truth cannot be very strong." In
a certain sense I quite agree with you, but I only wish to
convince you that this objection applies really to the theory
of Ampere,1 and only applies to mine in so far as it is
built upon the former. (You may perhaps find less ground
for objecting to the specialty of such an assumption, if
you separate the simple fundamental conception, which
recommends itself particularly by a certain analogy of the
molecules to the heavenly bodies in space, from those ad-
ditions which Ampere was forced to make, in order to
apply the mathematical methods at his command, and
to make the subject one of strict calculation. He was
necessitated to reduce the case to that of linear currents,
which necessarily demand channel-shaped bounds, if every
possibility of a lateral outspreading is to be avoided.)

' To place my theory of diamagnetic polarity in a truer
light, I am anxious also to convince you that this theory
is by no means based upon new assumptions (hypotheses),
but that it only rests upon such conclusions as may be
drawn from the theory of Ampere, when the laws of voltaic
induction discovered by Faraday, and the laws of electric
currents by Ohm, are suitably connected with it. I affirm,
that, even if Faraday had not discovered diamagnetism,
by the combination of Ampere's theory with Faraday's

1 This is quite true. — J. H.

LETTER FROM PROFESSOR WEBER. 247

laws of voltaic induction, and Ohm's laws of the electric
current, as shown in iny memoir, the said discovery might
possibly have been made.

1 In respect, however, to the artificiality of the theory
of Ampere, I hope that mathematical methods may he
found whereby the limitation before mentioned to the case
of linear currents may be set aside, and with it the ob-
jection against channel-form beds. All our molecular
theories are still very artificial. I, for my part, find less
to object to in this respect in the theory of Ampere than
in other artificialities of our molecular theories ; and for
this reason, that in Ampere's case the nature of the
artificiality is placed clearly in view, and hence also a way
opened towards its removal.1

' To Mr. Faraday I beg of you to present my sincerest
respect.

* Believe me, dear Sir,

' Most sincerely yours,

'WILHELM WEBER.'
' Professor Tyndall.'

The foregoing letter possessed more than a private
interest, and I therefore laid it before the readers of the
1 Philosophical Magazine' for December 1855. On one
point in it only did I ask permission to make a remark,
and that was the proposition, that the diminution of the
excitement of a row of bismuth particles in the line of
magnetisation by their reciprocal action is * a, direct con-
sequence of diamagnetic polarity.' M. Weber (I believe)

1 In Heat as a Mode of Motion, 4th edition, and elsewhere, I write
thus : — ' Whether we see rightly or wrongly — whether our insight be
real or imaginary— it is of the utmost importance in science to aim at
perfect clearness in the description of all that comes, or seems to come,
within the range of the intellect. For if we are right, clearness of
utterance forwards the cause of right ; while if we are wrong, it ensures
the speedy correction of error.' It is needless to say more to show how
heartily I subscribe to the view of Professor Weber. — J. T.

12

N

nQsnQ:

248 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

founds this proposition on the following considerations : —
Let a series of bismuth particles lie in the axial line be-
tween the magnetic poles N and s : the polarity excited
in these particles by the direct action of the poles will be

that shown in the figure,
beiDg the reverse of that
g of iron particles under the
same circumstances. But
as the end n of the right-
hand particle tends to excite a magnetism like its own in
the end s' of the left-hand particle, and vice versa, this
action is opposed to that of the magnet, and hence the
magnetism of such a row of particles is enfeebled by their
reciprocal action.

Now it appears to me that there is more assumed in
this argument than experiment at present can bear out.
There are no experimental grounds for the assumption,
that what we call the north pole of a bismuth particle
exerts upon a second bismuth particle precisely the same
action that the north pole of an iron particle would exert.
Magnetised iron repels bismuth; but whatever the fact
may be, the conclusion is scarcely warranted, that
therefore magnetised bismuth will repel bismuth. Sup-
posing it were asserted that magnetised iron attracts iron
and repels bismuth, while magnetised bismuth attracts
bismuth and repels iron, would there be anything essen-
tially impossible, self- contradictory, or absurd involved in
the assertion ? I think not. And yet if even the possible
correctness of such an assertion be granted, the proposi-
tion above referred to becomes untenable. It will be ob-
served that it is against a conclusion rather than a fact
that I contend. With regard to the fact, I should be
sorry to express a positive opinion ; for this is a subject on
which I am at present seeking instruction, which may lead

REMARKS ON M. WEBER'S LETTER. 249

me either to M. Weber's view or the opposite. Be that as
it may, the result cannot materially affect the respect I
entertain for every opinion emanating from my distin-
guished correspondent on this and all other scientific
subjects.

250 DIAMAGNETiSM AND MAGNE-CKYSTALLIC ACTION.

2. FARADAY ON MEDIA.

In the foregoing letter Professor Weber remarks : —
'It has been your care to separate the fact of diamag-
netic polarity from the theory, and to place the former
beyond the region of doubt.' Indeed the fact was, at the
time here referred to, the point in question. With regard
to the theory, which lies at the root of magnetic theory
generally, we have not made up our minds about it to
the present hour. The fact, however, as we have seen, en-
ables us to explain those numerous phenomena of magne-
crystallic action which Faraday found so bewildering.
With regard to theory M. E. Becquerel had, at an early
stage of the controversy, regarded the phenomena of
diamagnetism as illustrations of the principle of Archi-
medes. Bismuth, M. Becquerel assumed, was apparently
repelled because of the greater attraction of the etherial
medium in which it was immersed, as light wood under
water is apparently repelled by the earth. Later on,
Faraday made some beautiful experiments on the influence
of media, and founded upon them arguments of funda-
mental import as regards diamagnetism. The paper from
which the following is an extract will be found in the
'Philosophical Magazine' for February, 1855.

' Let us now consider for a time the action of different
media, and the evidence they give in respect of polarity.
If a weak solution of protosulphate of iron^m, be put

1 Let I contain 4 grains, m 8 grains, n 16 grains, and o 32 grains of
crystallised protosulphate of iron in each cubic inch of water.

FARADAY ON MEDIA. 251

into a selected thin glass tube about an inch long, and
one-third or one-fourth of an inch in diameter, and
sealed up hermetically, and be then suspended horizon-
tally between the magnetic poles in the air, it will point
axially, and behave in other respects as iron ; if. instead of
air between the poles, a solution of the same kind as m,
but a little stronger, n, be substituted, the solution in the
tube will point equatorially, or as bismuth. A like solu-
tion somewhat weaker than m, to be called I, enclosed in a
similar tube, will behave like bismuth in air but like iron
in water. Now these are precisely the actions which have
been attributed to polarity, and by which the assumed
reversed polarities of paramagnetic and diamagnetic bodies
have been considered as established ; but when examined,
how will ideas of polarity apply to these cases, or they to
it? The solution I points and acts like bismuth in air
and like iron in water ; are we then to conclude that it
has reverse polarity in these cases? and if so, what are the
reasons and causes for such a singular contrast in that
which must be considered as dependent upon its internal
or molecular state ?

* In the first place, no want of magnetic continuity of
parts can have anything to do with the inversion of the
phenomena ; for it has been shown sufficiently by former
experiments,1 that such solutions are as magnetically con-
tinuous in character as iron itself.

' In the next place, I think it is impossible to say that
the medium interposed between the magnet and the sus-
pended cylinder of fluid can cut off, or in any way affect
the direct force of the former on the latter, so as to change
the direction of its internal polarity. Let the tube be
filled with the solution m, then if it be surrounded by the
solution I, it will point as iron ; if the stronger solution n
surround it, it will point as bismuth ; and with sufficient
1 Phil. Mag. 1846, vol. xxix. p. 254.

252 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

FIG. 3.

care a succession of these fluids may be arranged as indi-
cated in figs. 2, 3, where the outlines between the poles
FIG. 2. represent the forms of thin glass troughs,
and the letters the solutions in them. In
fig. 2 we see that the action on m is the
same as that on m', and the pointing of
the two portions is the same, i.e. equato-
rial ; neither has the action on m been
altered by the power of the poles having
to traverse n, m! and n' ; and in fig. 3 we
see, that, under like circumstances of the
power, mf points as bismuth and m as
iron, though they are the same solution
with each other and with the former m mf
solutions. No cutting off of power by
the media could cause these changes ; re-
petitions of position in the first case, and
inversions in the second. All that could be expected from
any such interceptions would be perhaps diminutions of
action, but not inversions of polarity ; and every consider-
ation indicates that all the portions of these solutions in
the field at once have like polarity, i.e. like direction of
force through them, and like internal condition ; each so-
lution in its complex arrangement being affected exactly
in the same way and degree as if it filled the whole of the
magnetic field, although in these particular arrangements
it sometimes points like iron, and at other times like
bismuth.

* These motions and pointings of the, same or of diffe-
rent solutions, contain every action and indication which
is supposed to distinguish the contrary polarities of para-
magnetic and diamagnetie bodies from each other, and
the solutions I and m in air repeat exactly the phenomena
presented in air by phosphorus and platinum, which are
respectively diamagnetie and paramagnetic substances.

FARADAY ON MEDIA. 253

But we know that these actions are due to the differential
result of the masses of the moving or setting solution and
of that (or the air) surrounding it. No structural or in-
ternal polarity, having opposite directions, is necessary to
account for them. If, therefore, it is still said that the
solution m has one polarity in I and the reverse polarity
in 7i, that would be to make the polarity depend upon
the mass of m independently of its particles ; for it can
hardly be supposed that the particles of m are more
affected by the influence upon them of the surrounding
medium (itself under like inductive action only, and
almost insensible as a magnet) than they are by the domi-
nant magnet.1 It would be also to make the polarity of
m as much, or more, dependent upon the surrounding
medium than upon the magnet itself; — and it would be,
to make the masses of m and I and even their form the
determining cause of the polarity ; which would remove
polarity altogether from dependence upon internal mole-
cular condition, and, I think, destroy the last remains of
the usual idea. For my own part, I cannot conceive that
when a little sphere of m in the solution I is attracted
upon the approach of a given magnetic pole, and repelled
under the action of the same pole when it is in the solu-
tion 7i, its particles are in the two cases polar in two
opposite directions ; or that if for a north magnetic pole
it is the near side of the particles of m when in I that
assume the south state, it is the further side which acquires
the same state when the solution I is changed for n. Nor
can I think that when the particles of m have the same

1 If the polarity of the inner mass of solution is dependent upon
that of the outer, and cannot be affected but through it, then why is
nnt air and space admitted as being in effective magnetic relation to
the bodies surrounded by them ? How else could a distant body be
acted upon by a magnet, if the inner solution of sulphate of iron is so
acted on ? Are we to assume one mode of action by con< iguous masses
of particles in one case, and another through distance in another case ?

254 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

polar state in both solutions, the whole, as a mass, can have
the opposite states.

' These differential results run on in one uninterrupted
course from the extreme of paramagnetic bodies to the
extreme of diarnagnetic bodies ; and there is no substance
within the series which, in association with those on each
side of it, may not be made to present in itself the
appearances and action which are considered as indicating
the opposite polarities of iron and bismuth. How then
is their case, in the one or the other condition, to be dis-
tinguished from the assumed polarity conditions of bismuth
or of iron ? — only, I think, by assuming other points which
beg the whole question. In the first place, it must be, or
is assumed, that no magnetic force exists in the space
around a magnet when it is in a vacuum, it being denied
that the power either crosses or reaches a locality in that
space until some material substance, as the bismuth or
iron, is there. It is assumed that the space is in a
state of magnetic darkness, an assumption so large, con-
sidering the knowledge we have of natural powers, and
especially of dual forces, that there is none larger in any
part of magnetic or electric science, and is the very point
which of all others should be held in doubt and pursued
by experimental investigation. It is as if one should say,
there is no light or form of light in the space between
the sun and the earth, because that space is invisible to
the eye. Newton himself durst not make a like assump-
tion even in the case of gravitation, but most carefully
guards himself and warns others against it, and Euler1
seems to follow him in this matter. Such an assumption,
however, enables the parties who make it to dismiss the
consideration of differential effects when bodies are placed
in a vacuum, and to divide the bodies into the well-known

1 Letters, &c. translated. Letter LXVI1L, or pp. 260-262.

FARADAY ON MEDIA. 255

double series of paramagnetic and diamagnetic substances.
But in the second place, even then, those who assume the
reverse polarity of diamagnetic bodies, must assume also
that the state set up in them by conduction is less favour-
able to either the exercise or the transmission of the mag-
netic force than the original unpolarised state of the bis-
muth ; an assumption which is, I think, contrary to the
natural action and final stable condition into which the
physical forces tend to bring all bodies subject to them.
That a magnet acting on a piece of iron should so deter-
mine and dispose of the forces as to make the magnet and
iron mutually accordant in their action, I can conceive ;
but that it should throw the bismuth into a state which
would make it repel the magnet, whereas if unaffected it
should be so far favourable as to be at least indifferent,
is what I cannot imagine to myself. In the third place,
those who rest their ideas on magnetic fluids, must
assume that in all diamagnetic cases, and in them only,
the fundamental idea of their mutual action must not
only be set aside but inverted, so that the hypothesis would
be at war with itself ; and those who assume that electric
currents are the cause of magnetic effects, would have to
give up the law of their inducing action (as far as we
know it) in all cases of diamagnetism, at the very same
moment when, if they approached the diamagnetic bismuth
in the form of a spiral to the pole, they would have a
current produced in it according to that law.'

256 DIAMAGNJETISM AND MAGNE-CKYSTALLIC ACTION.

3. ON THE EXISTENCE OF A MAGNETIC
MEDIUM IN SPACE.

* These motions and pointings,' says Faraday, in the
foregoing extract, 'contain every action and indication
which is supposed to distinguish the contrary polarities of
paramagnetic and diamagnetic bodies.' In the following
letter I ventured to draw his attention to certain pheno-
mena which the motions and pointings referred to did not
seem to cover. Faraday, it will be observed, here passes
from the fact of diamagnetic polarity, which is irrefutable,
to the theory of magnetism in general. It was probably
the perusal of Faraday's remarks that caused M. Weber
to emphasise the distinction between fact and theory in
his letter to me.

MY DEAR MR. FARADAY, — Few, I imagine, who read
your memoir in the last number of the * Philosophical
Magazine,' will escape the necessity of reconsidering
their views of magnetic action. We are so accustomed to
regard the phenomena of this portion of science through
the imagery with which hypothesis has invested them, that
it is extremely difficult to detach symbols from facts, and
to view the latter in their purity. This duty, however, is
now forced upon us ; for the more we reflect upon the
results of recent scientific research, the more deeply must
we be convinced of the impossibility of reconciling these
results with our present theories. In the downfall of
hypotheses thus pending, the great question of a universal

ON A MAGNETIC MEDIUM IN SPACE. 257

magnetic medium has presented itself to your mind.
Your researches incline you to believe in the existence of
such a medium, and lead you, at the same time, to infer
the perfect identity of magnetism and diamagnetism.

In support and illustration of your views, you appeal
to the following beautiful experiments : — Three solutions
of proto-sulphate of iron are taken ; the first, £, contains 4
grains; the second, m, 8 grains: and the third, n, 16
grains of the salt to a cubic inch of water. Enclosed in
hollow globules of glass, all these solutions, when suspended
in the air before the pole of a magnet, are attracted by the
pole. You then place a quantity of the medium solution,
m, in a proper vessel, immerse in it the globule containing
the strong solution, 71, and find that the latter is still
attracted ; but that when the globule containing the
solution I is immersed, the latter is repelled by the mag-
netic pole. Substituting elongated tubes for spheres, you
find that when a tube containing a solution of a certain
strength is suspended in a weaker solution, between the
two poles of a magnet, the tube sets from pole to pole ;
but that when the solution without the tube is stronger
than that within it, the tube recedes from the pole and
sets equatorial.

Here then, you state, are the phenomena of diamag-
netism. It is maintained by some, that, to account for
these phenomena, it is necessary to assume, in the case of
diamagnetic bodies, the existence of a polarity the reverse
of that of iron. But nobody will affirm that the mere
fact of its being suspended in a stronger solution reverses
the polarity of a magnetic liquid : — to account for the
repulsion of the weak solution, when submerged in a
stronger one, no such hypothesis is needed ; why then
should it be thought necessary in the case of so-called
diamagnetic bodies ? It is only by denying that space
holds a medium which bears the same relation to

258 DIAMAGNET1SM AND MAGNE-CRYSTALLIC ACTION.

diamagnetic bodies that the stronger magnetic solution
bears to the weaker one, that the hypothesis of a distinct
diamagnetic polarity is at all rendered necessary.

The effects upon which the foregoing striking argu-
ment is based are differential ones, and are embraced, as
already observed by M. E. Becquerel, by the so-called
principle of Archimedes. This principle, in reference to
the case before us, affirms that the body immersed in the
liquid is attracted by a force equal to the difference of the
attractions exerted upon the liquid and the body immersed
in it. Hence, if the attraction of the liquid be less than
that of the immersed body, the latter will approach the
pole ; if the former attraction be the greater, the immersed
body recedes from the pole, and is apparently repelled.
The action is the same as that of gravity upon a body
plunged in water ; if the body be more forcibly attracted
bulk for bulk, than the water, it sinks ; if less forcibly
attracted, it rises ; the mechanical effect being the same as
if it were repelled by the earth.

The question then is, are all magnetic phenomena the
result of a differential action of this kind ? Does space
contain a medium less strongly attracted than soft iron,
and more strongly attracted than bismuth, thus permitting
of the approach of the former, but causing the latter to
recede from the pole of a magnet ? If such a medium
exists, then diamagnetism, as you incline to believe,
merges into ordinary magnetism, and ' the polarity of the
magnetic force,' in iron and in bismuth, is one and the
same.

Pondering upon this subject a few evenings ago, and
almost despairing of seeing it ever brought to an experi-
mental test, a thought occurred to me which, when it first
presented itself, seemed to illuminate the matter. Such
illuminations vanish in nine cases out of ten before the
test of subsequent criticism ; but the thought referred to,

ON A MAGNETIC MEDIUM IN SPACE. 259

having thus far withstood the criticism brought to bear
upon it, I am emboldened to submit it to you for con-
sideration.

I shall best explain myself by assuming that a medium
of the nature described exists in space, and pursuing this
assumption to its necessary consequences.

Let a cube, formed from the impalpable dust of
carbonate of iron,1 which has been forcibly compressed in
one direction, be placed upon the end of a torsion beam,
and first let the line in which the pressure has been exerted
be in the direction of the beam. Let a magnet, with its
axis .at right angles to the beam, and hence also at right
angles to the line of pressure, be brought to bear upon
the cube. The cube will be attracted, and the amount of
this attraction, at any assigned distance, maybe accurately
measured by the torsion of the wire from which the beam
depends. Let this attraction, expressed in degrees of
torsion, be called a,. Let the cube now be turned round
90°, so that the line of pressure shall coincide with the
direction of the axis of the magnet, and let the attraction
d in this new position be determined as in the former
instance. On comparison it will be found that d exceeds
a ; or, in other words, that the attraction of the cube is
strongest when the force acts parallel to the line of com-
pression.

Instead of carbonate of iron we might choose other
substances of a much feebler magnetic capacity, with
precisely the same result. Let us now conceive the
magnetic capacity of the compressed cube to diminish
gradually, and thus to approach the capacity of the
medium in which, according to our assumption, the
carbonate of iron is supposed to be immersed. If it were
a perfectly homogeneous cube, and attracted with the

1 For an ample supply of this most useful mineral I am indebted to '
the kindness of J. Kenyon Blackwell, Esq., F.G.S.

260 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

same force in all directions, we should at length arrive
at a point, when the magnetic weight of the cube, if I
may use the term, would be equal to that of the medium,
and we should then have a substance which, as regards
magnetism, would be in a condition similar to that of a
body withdrawn from the action of gravity in Plateau's
experiments. Such a body would be neither attracted nor
repelled by the magnet. In the compressed cube, however,
the magnetic weight varies with the direction of the force.
Supposing the magnetic weight, when the force acts along
the line of compression, to be equal to that of the medium,
then if the force acted across the line of compression, the
magnetic weight of the cube would be less than that of the
medium. Acted upon in the former direction, the cube
would be a neutral body ; acted upon in the latter direc-
tion, it would be a diamagnetic body, If the magnetic
capacity of the cube diminish still further it will, accord-
ing to your hypothesis, become wholly diamagnetic. Now
it is evident, supposing the true magnetic excitement to
continue, that the cube, when acted on by the magnet in
the direction of compression, will approach nearer to the
magnetic weight of the medium in which we suppose it
immersed, than when the action is across the said line ;
and, hence, the repulsion of the cube, when the force acts
along the line of compression, must be less than when the
force acts across it.

Reasoning thus from the assumption of a magnetic
medium in space, we arrive at a conclusion which can be
brought to the test of experiment. So far as I can see at
present, the assumption is negatived by this test ; for in
diamagnetic bodies the repulsion along the line in which
the pressure is exerted is proved by experiment to be a
maximum. An ordinary magnetic excitement could not,
it appears to me, be accompanied by this effect.

The subject finds further, and perhaps clearer, elucida-

ON A MAGNETIC MEDIUM IN SPACE. 261

tion in the case of isomorphous crystals. It is not, I think,
questioned at present, that the deportment of crystals in
the magnetic field depends upon their molecular struc-
ture ; nor will it, I imagine, be doubted, that the molecular
structure of a complete crystal of carbonate of iron is the
same as that of an isomorphous crystal of carbonate of
lime. In the architecture of the latter crystal, calcium
simply takes the place which iron occupies in the former.
Now a crystal of carbonate of iron is attracted most forcibly
when the attracting force acts parallel to the crystal-
lographic axis. Let such a crystal be supposed to diminish
gradually in magnetic capacity, until finally it attains a
magnetic weight, in a direction parallel to its axis, equal
to that of the medium in which we assume it to be im-
mersed. Such a crystal would be indifferent, if the force
acted parallel to its axis, but would be repelled, if the
force acted in any other direction. If the magnetic weight
of the crystal diminish a little further, it will be repelled
in all directions, or, in other words, will become diamag-
netic ; but it will then follow, that the repulsion in the
direction of the axis, if the nature of the excitement re-
main unchanged, will be less than in any other direction.
In other words, a diamagnetic crystal of the form of car-
bonate of iron will, supposing magnetism and diamagne-
tism to be the same, be repelled with a minimum force
when the repulsion acts parallel to the axis. Here, as
before, we arrive at a conclusion which is controverted by
experiment ; for the repulsion of a crystal of carbonate of
lime is a maximum when the repelling force acts along
the axis of the crystal. Hence I would infer that the
excitement of carbonate of iron cannot be the same as that
of carbonate of lime.

Such are the reflections which presented themselves to
my mind on the evening to which I have referred. I now
submit them to you as a fraction of that thought which

262 DIAMAGNET1SM AND MAGNE-CRYSTALLIC ACTION.

your last memoir upon this great question will assuredly
awaken.

Believe me,

Dear Mr. Faraday,

Yours very faithfully,

JOHN TYNDALL.
ROYAL INSTITUTION :
February, 1855.

[To this letter Faraday wrote a brief reply, Phil. Mag., vol. ix. p. 253.
I fear I failed to make clear to him the gist of my argument. Further
communications on this subject were published by Prof. Williamson
and Dr. Hirst in the Philosophical Magazine.]

4. FARAD ATS LETTER TO MATTEUCCI.

THE following charming letter, extracted from Dr. Bence
Jones's ' Life and Letters of Faraday,' shows the views of
diamagnetic polarity entertained by Faraday in 1855. It
was written prior to the publication of the Bakerian
Lecture for that year ; but I have no reason to believe
that the views here expressed were ever changed.

' November 2, 1855.

* MY DEAR MATTEUCCI, — When I received your last of
October 23, I knew that Tyndall would return from the
country in a day or two, and so waited until he came. I
had before that told him of your desire to have a copy
of his paper, and I think he said he would send it to you ;
I have always concluded he did so, and therefore thought
it best to continue the same open practice and show
him your last letter, note and all.

'As I expected, he expressed himself greatly obliged by
your consideration, and I have no doubt will think on, and
repeat, your form of experiment ; but he wished you to
have no difficulty on his account. I conclude he is quite
assured in his own mind, but does not for a moment object
to counter views, or to their publication ; and I think feels
a little annoyed that you should imagine for a moment
that he would object to or be embarrassed by your
publication. I think in that respect he is of my mind,
that we are all liable to error, but that we love the truth,

264 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

and speak only what at the time we think to be truth ; and
ought not to take offence when proved to be in error, since
the error is not intentional ; but be a little humbled and so
turn the correction of the error to good account. I cannot
help thinking that there are many apparent differences
amongst us, which are not differences in reality. I differ
from Tyndall a good deal in phrases, but when I talk
with him I do not find that we differ in facts. That
phrase polarity in its present undefined state is a great
mystifier.

4 Well ! I am content, and I suppose he is, to place our
respective views before the world, and there leave them.
Although often contradicted, I do not think it worth while
reiterating the expressions once set forth or altering them,
until I either see myself in the wrong or misrepresented,
and even in the latter case I let many a misrepresentation
pass. Time will do justice in all these cases.

4 One of your letters asks me, what do you conceive the
nature of the lines of magnetic force to be? I think it
wise not to answer that question by an assumption, and
therefore have no further account to give of such* physical
lines than that already given in my various papers. See
that referred to already in the " Philosophical Magazine"
(3301-3305) ; and I would ask you to read also 3299, the
last paragraph in a paper in the " Philosophical Magazine,"
June 1852, which expresses truly my present state of
mind.

* But a physical line of force may be dealt with experi-
mentally without our knowing its intimate physical nature.
A ray of light is a -physical line of force ; it can be proved
to be such by experiments made whilst it was thought to be
an emission, and also by other experiments made since it has
been thought to be an undulation. Its physical character is
not proved either by the one view or the other (one of which
must be, and both may be wrong), but it is proved by the

FARADAY'S LETTER TO MATTEUCCI. 265

time it takes in propagation, and by its curvatures, inflec-
tions, and physical affections. So with other physical lines
of force, as the electric current ; we know no more of the
physical nature of the electric lines of force than we do of
the magnetic lines of force ; we fancy, and we form
hypotheses, but unless these hypotheses are considered
equally likely to be false as true, we had better not form
them ; and therefore I go with Newton when he speaks of
the physical lines of gravitating force (3305 note), and
leave that part of the subject for the consideration of my
readers.

' The use of lines of magnetic force (without the physi-
cal) as true representations of nature, is to me delightful,
and as yet never failing ; and so long as I can read your
facts, and those of Tyndall, Weber, and others by them,
and find they all come into one harmonious whole, without
any contradiction, I am content to let the erroneous ex-
pressions, by which they seem to differ, pass unnoticed. It
is only when a fact appears that they cannot represent that
I feel urged to examination, though that has not yet hap-
pened. All Tyndall's results are to me simple conse-
quences of the tendency of paramagnetic bodies to go from
weaker to stronger places of action, and of diamagnetic
bodies to go from stronger to weaker places of action, com-
bined with the true polarity or direction of the lines of
force in the places of action. . . .

'These principles, or rather laws, explain to me all
those movements obtained by Tyndall against which your
note is directed, and therefore I do not see in his ex-
periments any proofs of a denned or inverse polarity in
bismuth, beyond what we had before. He has worked out
well the antithetical relations of paramagnetic and dia-
magnetic bodies, and distinguished mixed actions which
by some have been much confused ; but the true nature of
polarity, and whether it is the same or reversed in the two

266 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

classes, is to my mind not touched. What a quantity I
have written to you, all of which has no doubt been in
your own mind, and tried by your judgment ! Forgive me
for intruding it.

' Ever truly yours,

' M. FARADAI.'

The circumstances in which this letter originated are
these. On the receipt of my paper, * On the Nature of the
Force by which Bodies are repelled from the Poles of
a Magnet,' Matteucci undertook to repeat the experiments
there recorded, but failed to obtain the results. He con-
sidered the memoir a tissue of error from beginning to
end, and thought my character as a scientific man so
gravely compromised that he wrote to ask Faraday for
advice as to how he ought to act under the circumstances.
Faraday showed me Matteucci's letter, and the result of
our conversation regarding it is stated by Faraday himself.
Weeks, it may have been months, elapsed without my
hearing anything further about the matter ; when at length
a terse, frank letter reached me direct from Matteucci, the
substance of which was this : — ' I have written to Faraday,
to Grove, and to Wheatstone, stating that your experi-
ments were wrong. I now wish to give you the op-
portunity of correcting me, and of saying to these gentle-
men that I have repeated all your experiments and found
them true to the letter.'

I think it probable that as regards diamagnetic polarity,
Faraday and myself were sometimes looking at two different
things. I looked to that doubleness of action in which
the term polarity originated, and which causes electricity,
as well as magnetism, to be regarded as a polar force.
Faraday, I doubt not, had his mind fixed upon his lines of
magnetic force. To this conception, however, though it
formed the guiding light of his researches, he never gave

AMENDE HONORABLE. 267

a mechanical form. Hence arose his difficulty in dealing
with the phenomena exhibited by crystals in the magnetic
field. Refusing the clue of polarity, and holding magne-
crystallic phenomena to be products of a new force which
•was neither attractive nor repulsive, his difficulty was
insurmountable. His thoughts, nevertheless, dwelt in the
profoundest depths of the subject. His great discovery of
the rotation of the plane of polarisation had connected
the force of magnetism with the luminiferous ether ; and
this future investigators will probably prove to be the
domain of all magnetic action.1 In the sense, however,
in which the term polarity, as applied to magnetic phe-
nomena, has been hitherto understood — in other words, as
a matter of fact — the polarity of the diamagnetic force is,
I submit, conclusively demonstrated.

1 A conclusion to which the researches of Thomson and Maxwell
even now distinctly point.

208 DIAMAGtfETiSM AND MAGNE-CRYSTALLIC ACTION.

5. CHANGE OF FORM BY MAGNETISATION.

WISHING in 1855 to make the comparison of magnetic
and diamagnetic phenomena as thorough as possible, I
sought to determine whether the act of magnetisation
produces any change of dimensions in the case of bismuth,
as it is known to do in the case of iron. The action, if
any, was sure to be infinitesimal, and I therefore cast
about for a means of magnifying it. The idea which
appeared most promising was to augment in the first in-
stance by a lever the small amount of change expected,
and to employ the augmented effect to turn the axis of a
rotating mirror. By making the axis small enough it was
plain that an infinitesimal amount of rectilinear motion
might be caused to produce a considerable amount of
angular motion. This I proposed to observe by a tele-
scope and scale after the method of Gauss. I consulted
Mr. Becker, and, thanks to his great intelligence and
refined mechanical skill, I became the possessor of the
apparatus now to be described.

A B (fig. 3) is the upper surface of a massive block of
Portland stone. It is 21 inches wide, 13 inches deep, and
29 inches high. In it are firmly fixed two cylindrical
brass pillars, c c, 1 inch in diameter and 35 inches in
height. Over the pillars pass the two clamps, o o', and
from the one to the other passes a cylindrical cross bar,
1 1 inches long and | of an inch wide. This cross bar is
capable of two motions ; the first up and down the two
pillars c c, parallel to itself ; the second being a motion

\

270 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION".

round its own axis. To this cross piece is attached the
magnifying apparatus A.

The bar to be examined Is set upright between the
two pillars; being fixed firmly into a leaded screw im-
bedded in the Portland stone. It is surrounded by an
electro-magnetic helix B. On the top of the bar I rests
one end of a small cylindrical brass rod, with pointed
steel ends. This rod fits accurately into a brass collar,
moving up and down in it with the least possible friction.
The other point of the rod presses against a plate of agate
very close to a pivot round which the plate can turn. The
agate plate is attached to a brass lever 2'1 inches long,
whose fulcrum is the pivot just mentioned. Any motion
of the point against which the rod presses is magnified
about fifty times at the end of the lever. From this end
passes a piece of fine steel fibre round the axis of a rotating
mirror, which turns as the end of the lever moves. The
mirror rotates with its axis. For accurate experiments an
illuminated vertical scale is placed at a distance of about
twelve feet from the mirror, which is observed through a
telescope placed beside the scale. The magnifying appa-
ratus is shown in detail in fig. 2, where M is the mirror ;
s and s' two centre-screws, whose points constitute the
pivot round which the lever turns ; E is a small counter-
weight; T T is the cross-piece to which the magnifying
apparatus is attached. A naked section of the magnify-
ing apparatus is given in fig. 1. I is the bar to be mag-
netised, F the brass rod with the pointed steel ends?
divested of its collar, one of its ends pressing against the
plate of agate near the pivot a?, and the other resting upon
the bar of iron at y. From the end L of the lever the
steel fibre passes round the axis a of the mirror M. When
the bar I changes its length, the motion at L turns the
mirror ; and when I resumes its primitive length, the

APPARATUS FOR TESTING CHANGE OP FORM. 271

mirror is brought back to its first position by the spiral
hair-spring shown in the figure.

Biot found it impossible to work at his experiments on
sound during the day in Paris ; he was obliged to wait
for the stillness of night. With the instrument just
described I found it almost equally difficult to make ac-
curate experiments in London. Take a single experiment
in illustration. The mirror was fixed so as to cause the
cross-hair of the telescope to cut the number 727 on the
scale; a cab passed while I was observing — the mirror
quivered, obliterating the distinctness of the figure, and
the scale slid apparently through the field of view and
became stationary at 694. I went upstairs for a book ; a
cab passed, and on my return I found the cross-hair at
686. A heavy waggon then passed, and shook the scale
down to 420. Several carriages passed subsequently, after
which the figure on the scale was 350. In fact, so sensi-
tive is the instrument that long before the sound of a cab
is heard its approach is heralded by the quivering of the
figures on the scale.

Various alterations which were suggested by the ex-
periments were carried out by Mr. Becker, and the longer
I worked with it the more mastery I obtained over it ;
but I did not work with it sufficiently long to perfect
its arrangement. Some of the results, however, may be
stated here.

At the beginning of a series of experiments the scale
was properly fixed, and the pressure of the pointed verti-
cal rod F, fig. 1, on the end of the iron bar, I, so regulated
as to give the mirror a convenient position ; then, before
the bar was magnetised, the figure cut by the cross-hair
of the telescope was read off. The circuit was then esta-
blished, and a new number, depending on the altered
length of the bar by its magnetisation, started into view.
Then the circuit was interrupted, and the return of the
13

272 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

mirror towards its primitive position was observed. The
mirror, as stated, was drawn back to its first position by
the spiral hair-spring shown in fig. 1. Here are some of
the results : —

Figure of scale.

Bar immagnetised .... 577
„ magnetised ..... 470
„ unmagnetised .... 517

Here the magnetisation of the bar produced an elon-
gation expressed by 107 divisions of the scale, while the
interruption of the circuit produced only a shrinking of
47 divisions. There was a tendency »on the part of the
bar, or of the mirror, to persist in the condition super-
induced by the magnetism. The passing of a cab in this
instance caused the scale to move from 517 to 534 — that
is, it made the shrinking 64 instead of 47. Tapping the
bar produced the same effect.

The bar employed here was a wrought-iron square core,
1*2 inch a side and two feet long.

The following tables will sufficiently illustrate the
performance of the instrument in its present condition.
In each case are given the figures observed before closing,
after closing, and after interrupting the circuit. Attached
to each table, also, are the lengthening produced by mag-
netising and the shortening consequent on the interruption
of the circuit : —

Circuit.

Scale
10 cells.

Circuit.

Scale
20 cells.

Open .
Closed
Broken

. 647
- 516
. 581

131 elongation.
65 return.

Open .
Closed .
Broken

. 653
. 475
. 579

188 elongation.
144 return.

V

Open .
Closed.
Broken

. 637
. 509
. 579

128 elongation.
70 return.

Open .
Closed .
Broken

. 638
. 452

. 568

186 elongation.
116 return.

Open .
Closed.
Broken

. 632
. 491

. 568

141 elongation.
77 return.

Open .
Closed .
Broken

. 632
. 472
. 561

160 elongation.
89 return.

CHANGE OF FORM BY MAGNETISATION. 273

These constitute but a small fraction of the number of
experiments actually made. There are, I may add, very
decided indications that the amount of elongation de-
pends on the molecular condition of the bar. For example,
a bar taken from a mass used in the manufacture of a
great gun at the Mersey Iron-works suffered changes on
magnetisation and demagnetisation considerably less than
those recorded here.1 With bars of bismuth, however strong
might be the magnetism, no change whatever was observed.

1 I owe these bars to the liberality of the proprietors of the Mersey
Iron-works, through the friendly intervention of Mr. Mallet

274 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

6. THE POLYMAGNET.

THE polymagnet consists of an arrangement of two horse-
shoe electro-magnets, a helix of covered copper wire dis-
posed between them, and suitable means of suspension.

A section of one of the electro-magnets and its
surrounding spirals is given, fig. 1. a6, cd are two cylin-
drical cores of soft iron, which are united by a cross-piece
of the same material, ef. Through the cross-piece pass
the strong screws g and h into the cores, and by them the
ends 6 and d of the cores, which are accurately planed
so as to ensure perfect contact with the cross-piece, are
attached to the latter. The diameter of the cores is 1-125
inch, and their distance apart, from centre to centre, 4-85
inches ; the cross-piece ef is drawn in proportion.

Round each core is a helix of copper wire, overspun
with cotton, saturated with shell-lac. In winding the
helix, two lengths of wire, one covered with red cotton
and the other with green, are laid side by side and coiled
as a single wire. The diameter of the wire is 0*1 of an

O

inch, and the weight of it which surrounds each limb of
the magnet is 12 Ibs. For all four limbs, therefore, a
weight of 48 Ibs. is made use of.

The second electro-magnet is in every respect similar
to the one just described.

Fig. 2 is a front view of a flat helix of covered copper
wire, intended to be placed between the two electro-
magnets ; it has an internal diameter, a&, of 1 inch ; an
external diameter, cc£, of 8 inches, and measures along its

ly. 1.

Fy. 2

IT
9"

THE POLYMAGNET. 275

axis 1*15 inch. The diameter of its wire is 0-065 of an
inch, and its weight is 6 Ibs. ; it is wound so as to form a
double coil, as in the case of the electro-magnets. The
radial strips, and central and surrounding ring seen in the
figure, are of brass, and hold the coils of the helix com-
pactly together.

Fig. 3 represents a stout slab of mahogany which
supports the apparatus. a&, cd are hollows cut in the slab
to receive the cross-pieces of the two electro-magnets ;
from e to f the slab is cut quite through, the cross-pieces
merely resting on the portions between / and 6, / and rf,
&c. The small apertures at x x' show where the screws
enter which attach the cross-piece to the slab of wood.
The central aperture at g shows where the pin g" of the
helix, fig. 2, enters, the helix being supported on the cen-
tral portion of the board. Eight and left are two projec-
tions for the reception of two current reversers, which will
be described immediately. The apertures 1, 2, 3, 4 are
for the reception of pins projecting from the bottom of a
glass case intended to cover the whole apparatus.

When the magnets and central helix are fixed in their
places and looked down upon, their appearance is that
represented in fig. 4 ; at a and c the tops of the cores are
seen, the movable soft iron poles which belong to them
being removed ; the two ends of the other electro-magnet
bear two such poles, each formed from a parallelepiped 4*5
inches long, 2 inches wide, and 1-25 inch high, having one
end bevelled off so as to render it pointed, the other end
being suffered to remain flat. The distance between those
movable masses may be varied, and the body to be exa-
mined may be suspended either between surfaces or
points, according to the nature of the experiment. The
horizontal projections of the current reversers are seen to
the right and left in fig. 4.

Simplicity and efficiency being the objects aimed at, a

276 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

current reverser was devised which fulfils these conditions.
A front view of the instrument is given in fig. 5, and its
horizontal projection in fig. 6. Q is the section of a
quadrant of box-wood, which is capable of being turned
by the handle HP ; ab is the section of a strip of brass
laid on the periphery of the quadrant ; cd is a shorter
strip similarly laid on ; between b and c is a gap, formed
of the wood of the quadrant itself, or of a piece of ivory or
glass inlaid ; 8 and s' are two brass springs, which are
shown resting upon the strips of brass ab and cd ; M M',
fig. 5, are two clamps secured to the wooden pillars o and
o' by screws s, which pass up through the latter. The plan,
fig. 6, corresponds to the section, fig. 5. From 6, fig. 6, a
strip of brass crosses to c', while a second strip crosses from
c to b', the strips being insulated from each other at n.
Supposing, then, the two clamps M and L to be connected
with the two poles of a galvanic battery, the current entering
at M would flow along the spring 8 to 6, thence to c'} and
finally along the spring s' to the clamp I/ : in like manner
the current entering at L would attain the clamp M'. In
this position of things the handle of the instrument leans
to the left, as in fig. 5. If the current is to be interrupted,
the handle is set vertical ; for when the handle is in this
position, the spring s' rests upon the non-conducting sur-
face 6c, and the circuit is broken. If it be desired to send
the current direct from L to I/, and from M to M', the
handle is turned to the right ; the two springs s 8' rest
then upon the self-same strip of brass a&, and there is
direct metallic communication between L and L', and be-
tween M and M'. This reverser has been tested practically,
and found extremely convenient. It is very similar to an
instrument devised by Professor Keusch, but simpler and
more easily constructed.

The whole instrument, surrounded by its glass case, is
shown in perspective in fig. 8. The magnets are visible,

liy. 6.

THE POLYMAGNET. 277

with the movable poles resting upon them ; in the centre
is seen the helix sketched in fig. 2, and within the helix a
bismuth bar supported by several fibres of unspun silk
attached to the central rod which passes through the top
of the glass case. The manner of suspension of the bis-
muth will be understood from the drawing, certain practical
artifices which suggest themselves when the drawing is
attentively inspected being introduced to facilitate the
placing of the axis of the bar accurately along the axis
of the surrounding helix. The current reversers are seen
without the case; two opposite sides of the latter can
be opened by the handles h and h', so that free and easy
access to the interior is always secured.

Experiments to be made with the Polymagnet.

1. All the experiments that are usually made with an
upright electro-magnet.

2. The various portions of the instrument may with
great facility be lifted separately out of the case. Fig. 1
shows one of the electro-magnets thus removed. A rope
can be passed through a ring r in the cross-piece. Ad-
jacent to the screws g and h are two perforated plates of
brass which are attached to the brass reels of the helices.
By passing a pin through the holes shown in the figure,
the helices are prevented from slipping off the cores when
the magnet is turned upside down. Attaching the rope
to a hook in the ceiling, or to a strong frame made for
the purpose, experiments on the lifting power of the mag-
net may be made.

3. While one of the magnets is suspended as last
described, the other, which is of exactly the same size,
can be brought up against it, the free ends of the four
cores being thus in contact. The same current being sent
through both magnets, we have the mutual attraction of
two electro-magnets, instead of the attraction of an electro-

278 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

magnet for an armature, as supposed in the last experi-
ment. The arrangement just described is indeed precisely
that devised by M. Pouillet in the construction of a power-
ful electro-magnet for the Faculty of Sciences at Paris. To
the cross-piece of the second magnet a ring is also attached,
from which weights can be suspended.

4. The cross-pieces can be removed by withdrawing
the screws g and h, and the helices may be made use of
singly with their corresponding bar-magnets. As two
wires surround each coil, one of them may be used to
exhibit the induced currents developed by the other. The
phenomena of the extra-current may also be studied, and
the remarkable effect produced on the spark of the extra-
current by connecting the two ends of one of the wires of
the other helix, may be exhibited.

5. If an ordinary feebly magnetic bar be suspended
between one pair of poles, and an ordinary diamagnetic
bar between the other pair, on sending the same current
round both magnets, the former sets itself parallel, while
the latter sets itself perpendicular to the polar line. The
phenomena of magnetism and diamagnetisra are thus made
to address the eye simultaneously.

6. In the same way, if a normal magnetic bar be sus-
pended between one pair of poles, and an abnormal mag-
netic bar between the other pair, the antithesis of their de-
portment may be made manifest. The same antithesis
is exhibited when we compare a normal diamagnetic bar
with an abnormal one.

7. And when between one pair of poles is suspended a
normal magnetic bar, and between the other pair an ab-
normal diamagnetic one, the apparent identity of deport-
ment of both bars is rendered evident at once. The
same identity is shown when we compare the abnormal
magnetic bar with the normal diamagnetic one.

8. Causing the points to face each other, instead of the

THE POLYMAGNET. 270

flat ends of the poles, and observing the directions given
in the Bakerian Lecture for 1855, the curious phenomena
of rotation on raising or lowering the body from between
the points, first observed by M. Pliicker, and explained in
the paper referred to, may be exhibited.

9. To show that a bar of bismuth, suspended within a
helix and acted upon by magnets, presents phenomena
exactly analogous to those of soft iron, only always in
opposite directions, let the flat helix be mounted between
the two electro-magnets. The bar of bismuth used in ex-
periments with the instrument just described is 6 inches
long and 0*4 of an inch in diameter. Suspended so as
to swing freely within the helix, its ends, to which the
diamagnetic excitement is freely propagated from the
centre, where the bar is surrounded by the flat coil, lie
between the movable poles which rest upon the electro-
magnetic cores. Four poles are thus brought simul-
taneously to bear upon the bar of bismuth, and its action
is thereby rendered both prompt and energetic. The two
poles to the right of the bar must both be of the same
name, and the two to the left of the bar of the opposite
quality. If those to the right be both north, those to
the left must be both south, and vice versa. On sending a
current from 10 or 15 cells round the helix, and excit-
ing the magnets by a battery of 4 or 5 cells, the current
reversers place the deflections of the bar entirely under
the experimenter's control. Changing, by means of its
reverser, the direction of the current in the helix, a
change of deflection is produced ; the same is effected if
the polarity of the magnets be changed by the reverser
which belongs to them.

10. To those acquainted with what has been done of
late years in diamagnetism, numerous other experiments
will suggest themselves. The antithesis of two isomorphous
crystals, one magnetic and the other diamagnetic, the

280 DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION.

general phenomena of magnecrystallic action, and the
analogous effects produced by pressure, may all be ex-
hibited.

1 1. By mounting two helices of the electro-magnet,
one upon the other, a coil of double length is obtained,
and two such coils may be formed from the four just
described. For the additional expense of the iron merely,
a single electro-magnet, far more powerful than either of
the others, because excited by twice the quantity of coil,
may be obtained.

The instrument above described was constructed by
Mr. Becker, of Newman Street, and its cost is about 24£.
It was not my intention originally to have so much wire
round the electro-magnets ; and the effects may also be
obtained with a smaller central coil. I have no doubt that
with 8 Ibs. of wire round each limb of the electro-magnets,
and a central coil weighing 4 Ibs., the experiments might
be exhibited with perfect distinctness. A sensible dimi-
nution of cost would of course accompany this diminution
of material and labour.

7. STEEL MOULDS FOR COMPRESSION.

THE steel moulds employed in my experiments on com-
pression are here represented. To prevent all magnetic
contamination they were coated galvano-plastically with
copper.

FIG. 1. FIG. 2. FIG. 3.

In fig. 1, A', B', c' represent the three parts of the
mould used for forming cubes of compressed bismuth,
whether of solid metal or in powder. Fig. 3, A, B, c,
represent the three parts of the mould employed to form

282 STEEL MOULDS FOR COMPRESSION.

rectangular bars. In fig. 2, x, the three parts of fig. 1
are put together. In fig. 2, T, the three parts of fig. 3
are put together. In experimenting, B' or B is first set
upon its base, c' or c ; the solid or the powder is then
placed within B' or B, the plunger A' or A is then intro-
duced, and the whole squeezed between the plates of a
small hydraulic press. The compressed substance is of
course firmly jammed in the mould, and to remove it a
perforated base (not shown in the figure) is employed, on
which B' or B is placed, and the squeezed metal forced
out by the plunger A' or A, acted on by the hydraulic
press. The drawings are half the linear size of the moulds
themselves.

INDEX.

AMP

AMPERE, his theory of molecular
currents, 134, 170, 171, 245

Antimony, deportment of, in the
magnetic field, 15, 16, 19, 208

Apple, deportment of slices of, in
the magnetic field, 21

Archimedes, principle of, 258

Arsenic, deportment of, in the mag-
netic field, 15

Attraction, ratio of, to magnetis-
ing force, 50

— measured, 65

BARYTA, "sulphate of, form and
cleavage of, 7

— deportment of, in the magnetic
field, 7

— calorific conduction of, 87
Becquerel, M. Edmond, his experi-
ments on bars of bismuth, sul-
phur, and wax, 58

Beryl, cleavage of, 31

— rotation of, when the poles are
removed to a distance, 43, 44, 129

Bismuth, diamagnetism of, 1

— Faraday's experiments on, 14-
16, 113

— dough, deportment of, 22, 33,
37,44

— repulsion of measured, 54

— magne-crystallic axis of, 67, 68

— and of bismuth powder, 26, 69,
70, 72

— reversal of magne-crystallic ac-
tion of, by mechanical action,74

— induced currents in, excited by
diamagnetisation, 90, 91

— PoggendorfTs experiments on
the polarity of, 95

— his experiments repeated, 98

— M. von Feilitsch's theory, 93

CAL

Bismuth, dual or polar induction
of, 142, 143, 144, 163

— state of a bar of, under mag-
netic influence, 137, 138

— oscillation of, between poles,
137

— strength of magnet and re-
pulsions of, 139

— M. Pliicker's experiments, 169
note

— further experiments on com-
pressed powder, 178, 179, 180

analysis of repulsion along

and across the cleavage, 187

— further proof of polarity of
magnetised bismuth, 205

— polarity of insulators, 209

— application of 'couples' to Fara-
day's experiments on magne-
crystallic action, 226 et seq.

his experiments explained,

228 et seq.

— experiments showing the dis-
tribution of force between flat
poles, 234

— translative and directive power,
235

Borax, deportment of, in magnetic
field, 12

— ring-system of, 13

Bread, compressed, deportment of,

in magnetic field, 77
Breunnerite, deportment of, in

magnetic field, 5
Brewster's classification of topaz, 10

— list of crystals tested, 12
Brugmans' observations, 1, 1 12, 196

CALCAREOUS spar, ratio of re-
pulsion of, to magnetising force,
57

284

INDEX.

CAL

DIA

Calcareous spar, differential repul-
sion of, 63, 64

— diamagnetic action of, 94

— polarity of, 210

Calcite, differential conduction of

heat, 87
Calorific conduction and magnetic

induction, 87
Carbon, bisulphide of, diamagnetic

polarity of, 214

Cherry-tree bark, M. Plucker's ex-
periments with, 48
Cleavages of crystals, 30, 31, 33,

34, 38, 68, 74, 75, 76
Cobalt, muriate of, polarity of a

solution of, 219
Ccelestine, form and deportment

of, in magnetic field, 8
Coercive force, 135
Compression, remarks on the effect

of, 86

Copper, polarity of, 208
Coulomb, his theory of magnetism,

170

— experiments with iron filings,
177, 178

Couples, action of, in the magnetic

field, 226 et seq.
Crystals, Prof. Plucker's laws of

the magnetic action of, 2

— examination of these laws, 3, 4

— Faraday's experiments, 14, 15

— his conclusion, 17-19

— application of the principle of
elective polarity to, 29

— influence of cleavage, 33

— and of proximity of aggrega-
tion, 35

— examination of Plucker's second
law, 38

— influence of pointed and flat
poles, 39

— local attraction and repulsion,
40

— rotation of, when the poles are
removed to a distance, 40

Crystals, modification of force by
structure, 45

— compressed, 75

— experiments on various crystals,
84

— calorific conduction of crystals,
87,88

— relation of diamagnetic polarity
to magne-crystallic action, 225

— action of ' couples ' in the mag-
netic field, 226

— and of magne-crystallic axis on
needle, 235-237

Cyanite, deportment of, in the
magnetic field, 8, 113 note

DE LA RIVE, statement of
Pliicker'a views, 113 note

— propagation of heat through
wood, 115

Diamagnetic bodies, tendency to
go from stronger to weaker
places of action, 100

Diamagnetism discovered by
Faraday, 1

— M. Edmond Becquerel'p me-
moir on, 58

— an induced state, 109

— comparative view of paramag-
netic and diamagnetic pheno-
mena, 134, 161

— state of diamagnetic bodies
under magnetic influence, 134

— law of diamagnetic induction,
136

duality of diamagnetic ex-
citement, 142, 161

separate and joint action of

a magnet and a voltaic cur-
rent, 145, 153, 156, et scq.

— antithesis of magnetism and
diamagnetism, 159, 165

— action of electro-magnet on
electro-diamagnet, 162

— Weber's theory of diamagnetic
polarity, 170, 244, et seq.

INDEX.

285

DIA

IRQ

Diamagnetism, M. Matteucci's ob-
jections, 173

— further reflections on diamag-
netic polarity, 179

— further researches on the pola-
rity of the diamagnetic force,193

description of the apparatus

used, 198-203

action of diamagnets on

magnets, 205

and of magnetised bis-
muth, copper, and antimony,
204-208

polarity of diamagnetic li-
quids, 213, %14

— on the relation of diamagnetic
polarity to magne-crystallic
action, 225 et seq.

Dichroite, deportment of, in mag-
netic field, 7, 83, 85
Diopside, diamagnetism of, 8, 10
Dolomite, deportment of, in the
magnetic field, 5

ELECTIVE polarity, line of, 23-25

— application of the principle of,
to crystals, 29

Electric currents, Ohm's laws of,

246
Electro-magnet of University of

Berlin, 74

FAKADAY proves all bodies to
be subject to magnetic influ-
ence, 1, 170

— his suggestion of the term
' para-magnetism,' 1 note

— his experiments on the deport-
ment of crystals in the magnetic
field, 14, 15

— his definition of magne-crystal-
lic force, 17-19

— discussion of his hypothesis,
21 et seq.

— his verification of Plucker's re-
sults between pointed and flat
poles, 41

Faraday, his optic axis force, 19, 61

— his experiments on the polarity
of the diamagnetic force, 90, 91,
194, 207, 223

— his letter to Matteucci on
diamagnetic polarity, quoted,
263-266

— his experiments on magne-cry-
stallic action explained, 228, 230

— his proof that the magne-cry-
stallic force is a force acting at
a distance, 235

— his answer to Prof. Tyndall on
the existence of a magnetic
medium in space, 256

Feilitzsch, M. von, his theory of
diamagnetic action, 100

— on the polarity of bismuth, 154

— conditions proposed by him for
the proof of diamagnetic pola-
rity, 197

GLASS, heavy, its part in Fara-
day's discovery of the diamag.
netic force, 111

polarity of, 210

Grit, stratified, deportment of, 46

Gutta-percha model, deportment
of, in magnetic field, 43

HEAT, conducted by crystals dif-
ferently in different directions,
87

ICELAND SPAE, heated in mag-
netic field, 19. See calcareous
spar and carbonate of lime.

molecular arrangement of,

35

polarity of, 210

Mitscherlich's line of great-
est expansion, 36

Iron, its law of attraction, 50

— action of magnet alone on, 145

— action of voltaic current, 146

— action of magnet and current
combined, 148, 152

286

INDEX.

IRQ

MED

Iron, carbonate of, deportment of,
in the magnetic field, 5, 64, 65,
80, 130

— models of, 25, 26

— rotation of, in the magnetic
field, 130

— ratio of strengths of magnet to
attractions of bars of, 1 40

— powder mixed with bismuth
compressed, 241

Iron, chloride of, magnetic de-
portment of, 217

Iron, oxide of, deportment of, in
the magnetic field, 6

Iron, sulphate of, action of, in the
magnetic field, 66, 80, 86, 217, 218

— polarity of solution of, 218

JOULE, Mr., his experiments on
diamagnetic bodies, 141, 142

KNOBLAUCH referred to, 61
67, 113, 115, 174

Koike, M., his investigation on the
distribution of the magnetic
force between two flat poles, 132

LEYSER, M., apparatus con-
structed by, for testing diamag-
netic polarity, 198, 205, 243

Lime, carbonate of, optic axis force
of, 61

— antithesis of, to carbonate of
iron, 65

— strength of magnet and ratio
of repulsions of spheres of, 141

— magne-crystallic action of a
sphere of, 226

Liquids, diamagnetic, polarity of,

213
-r and of magnetic, 218, 219

MAGNE-CRYSTALLIC force, Fa-
raday's definition of, 17

— his conclusion from his expe-
riments, 18

Magne-crystallic force, discussion
of Faraday's hypothesis, 22

— action, 61

— reversal of, by mechanical ac-
tion, 74

— Poisson's prediction of, 82
Magnesia, sulphate of, deportment

of, in the magnetic field, 8, 29
Magnetic action, all bodies subject
to, 1

— Pliicker's laws, 2

— examination of these laws, 2-16

— Faraday's conclusions, 17

— new magnetic forces, 19

— local attraction au,drepulsion,41

— induction and calorific conduc-
tion, 87

— imaginary magnetic matter,109
Magnetism, para- and dia-, 1

— comparison of magnetism and
diamagnetism, 51

— rotation of magnetic and dia-
magnetic bodies, 123, 131, 181

— distribution of magnetic force
between two flat poles, 131

— laws of magnetic induction,
135

— antithesis of magnetism and
diamagnetism, 159

— effect of magnetic and dia-
magnetic couples, 181

Magneto-crystallic force, 17
Marble, statuary, polarity of, 211
Matteucci, his objection to the
experimental proof of diamag-
netic polarity, 174

— Faraday's letter to him on dia-
magnetic polarity, 263

— conditions proposed by him for
the rigorous demonstration of
diamagnetic polarity, 196

Media, evidence of the action of dif-
ferent, in respect of polarity, 250

— letter to Faraday on the exist-
ence of a magnetic medium in
space, 256

— Faraday's answer, 2G2 note

INDEX.

287

MIT

REP

Mitscberlich, M., on the expansion

of crystals by heat, 36
Models, deportment of, in the

magnetic field, 33 et seq.
Molecular currents generated by

magnetisation in diamagnetic

bodies, Weber's theory, 02, 245
Ampere's theory, 170, 171,

245
Moulds, steel, for compression, 281

NICKEL, sulphate of, deportment
of, in the magnetic field, 12

— ring-system of, 13

— line of maximum force, 23

— process for discovering the
cleavage of, 30

— muriate of, polarity of a solu
tion of, 219

Nitre, polarity of, 213. See salt-
petre

OHM, M., theory of molecular
currents, 246

— theory of the distribution of
electricity, 133

Optic axis force, 61

PARAMAGNETISM, 119

— comparative view of paramag-
netic and diamagnetic pheno-
mena, 134, 161 et seq.

— separate and joint action of a
magnet and voltaic current, 145-
148

Penny, deportment of a, in the

magnetic field, 20
Phosphorus, polarity of, 212
Pliicker,his laws of magne- crystal-
lie action, 2, 3, 4

— forces in cherry-tree bark, 48

— his experiments with tourma-
line, and other bodies, 3, 39

— examination of his law, 4
examples which disobey the

law, 11

PI iicker,examination of his law that
magnetic attraction decreases in
a quicker ratio than the repul-
sion of the optic axis, 38, 39

— Faraday's verification of M.
Plucker's results, 41

— summary of the forces emanat-
ing from the poles of a magnet,
113 note

- theory of induction in para-
magnetic and diamagnetic
bodies, 142

— his experiment on the retention
of diamagnetic polarity of, 169
note

— rotation of bodies in magnetic
field, 38, 42, 123

Poggendorff, his experiments on
the polarity of bismuth, 95

Poisson, his prediction of magne-
crystallic action, 82

— his view of the act of magneti-
sation, 134, 135, 170

Polarity, experiments proving the
sufficiency of, to explain the
most complicated phenomena of
magne-crystallic action, 225
— various views of polarity,244-247
Polymagnet, description of the,
274-277

— experiments to be made with
the, 277-280

Potassa, red ferroprussiate of,
magnetic polarity of, 218

Potassium, yellow f errocyanide of,
deportment of, in the magnetic
field, 13, 14, 129

QUARTZ, deportment of, in the
magnetic field, 3, 11, 88

REICH, his experiments on polar-
ity, 89-90, 106, 107, 145
Repulsion of planes of cleavage, 25

— M. Plucker's law of, 47

— ratio of repulsion to magnetis-
ing force, 54

288

INDEX.

REP

Repulsion, differential, 61 et scq.

— superior repulsion of the line
of compression in bismuth, 75

Rock-crystal, deportment of, in the
magnetic field, 11, 34, 88

— conduction of heat in, 87
Rotation of bodies in the mag-
netic field, 38, 42, 123

— law of, 129

SALTPETRE, deportment of, 37,
126. See nitre

Sand-paper, deportment of models
in magnetic field, 32, 43

rotation of models on the

removal of the poles to a dis-
tance, 43

Scapolite, deportment of, 31

Schneider's purified bismuth, 53

Senarmont, M. de, his experiments
on calorific conduction of crys-
tals, 87, 88, 115

Selenite, deportment of, in the
magnetic field, 88

Shale, deportment of, in the mag-
netic field, 77,

Silver, magnetic polarity of im-
pure cylinders of, 220, 221

Slate rock, polarity of, 216, 217

Soda nitrate, deportment of, in the
magnetic field, 5

Steel moulds for compression, 281

Strontia, sulphate of (coelestine),
form of, 8

— deportment of, in the magnetic
field, 8

Sugar, deportment of, in the mag-
netic field, 11

Sulphur, ratio of repulsion of, to
magnetising force, 55, 141

— - diamagnetic polarity of, 212

THOMSON, Sir William, his re-

ZIR

marks on experiments with
powdered crystals, 71-72

— on Poisson's prediction of
magne-crystallic action, 82

— his imaginary magnetic matter,
109

Tin, compressed carbonate of, 128
Topaz, deportment of, in the mag-
netic field, 8, 9, 10

— deportment of, 3
Torsion-balance, the 51-54, 62, 67,

69, 83, 143

Tourmaline, magne - crystallic
action of, 3

— experiment to show the de-
crease of force with increase of
distance, 39

— calorific conduction of, 87

WATER, distilled, diamagnetic
polarity of, 214

Wax, white compressed, deport-
ment of, in the magnetic field,
76, 85

— djamagretic polarity of, 213
Weber, Prof. W., his experiments

on the polarity of the diamag-
netic force, 89, 90, 91, 118

— his hypotheses, 92, 118, 171,
194, 243, 256

— remarks on his. theory, 170
Wertheim, M., on velocity of sound

through wood, 115

— on action of compresse*& glass
on light, 116

Wiedemann, M., on electric con-
duction of crystals, 115

Wood, magnetic deportment of,
119-122, 132

ZINC, sulphate of, deportment of,
in the magnetic field, 8

— process for discovering the
cleavage of, 30

Zircon, deportment of, 8

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