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DIAMAGNETISM 

AND 

MAGNE-CEYSTALLIC ACTION 



PROFESSOR JOHN TYOTALL'S WORKS. 



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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 
subject 2 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 ., .^ - 
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 


0-364 


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 


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 


0-364 


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 


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 


0-364 


2-8 


1-68 


1-82 


25 


0-466 


5-0 


2-21 


2-33 


30 


9-577 


8-0 


2-83 


2-83 


35 


0-700 


11-2 


3-35 


37,0 


37 30 


0-767 


14-5 


3-81 


3-83 


57 


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 30 , 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} 7 stal 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 Berlin 1 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 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. 
Weber 2 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 excited 1 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 

2 HM' ; 

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 



168 2 
204 2 
253 2 
275 2 
313 2 
347 2 
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 78 2 

136 135 2 

184 191* 

226 226* 

259 259 

287 291* 

341 322' 

377 359 2 

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 385 2 , and in 
bismuth a repulsion of 386 2 . 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 74 2 

135 133* 

179 ISO 2 

214 218 2 

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 

95 2 
15S 2 
224' 
264 2 
316 2 



Table VIII. 



Spheres of Carbonate of Lime. 
Strength of Batio of 

repulsions 
134 2 
173 2 
212 2 
264 2 



magnet 
134 
172 
213 
259 
310 



311 2 



Table IX. 

Spheres of Carbonate of Iron. 



Strength of magnet 

66 

89 
114 
141 



Ratio of attractions 

66 Z 

89 2 

114 2 

141 2 



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 I 2 

2 2 2 
4 4 2 

' 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. 






I ill ii '1 
' i 
lllilllllillllliilllllllllH 



k 

I 






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I 





I 

I 
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Hi 




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\\nrn 



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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. 






111" 




I 










1 



I 



'II f/^pwi 

HI ** LW&\ J" 





1 






\ 



I 















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 the u 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 moulds 1 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 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 fact 1 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 reversed 2 
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 ACTION 1 . 

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 ww 7 . 
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 s f 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 Euler 1 
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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