Faraday As A Discoverer
by
John Tyndall
Part 2 out of 3
give it greater beauty.'
Footnote to Chapter 9
[1] See note, p. 77.
Chapter 10.
Magnetization of light.
But we must quit the man and go on to the discoverer: we shall
return for a brief space to his company by-and-by. Carry your
thoughts back to his last experiments, and see him endeavouring to
prove that induction is due to the action of contiguous particles.
He knew that polarized light was a most subtle and delicate
investigator of molecular condition. He used it in 1834 in
exploring his electrolytes, and he tried it in 1838 upon his
dielectrics. At that time he coated two opposite faces of a glass
cube with tinfoil, connected one coating with his powerful electric
machine and the other with the earth, and examined by polarized
light the condition of the glass when thus subjected to strong
electric influence. He failed to obtain any effect; still he was
persuaded an action existed, and required only suitable means to
call it forth.
After his return from Switzerland he was beset by these thoughts;
they were more inspired than logical: but he resorted to magnets and
proved his inspiration true. His dislike of 'doubtful knowledge'
and his efforts to liberate his mind from the thraldom of hypotheses
have been already referred to. Still this rebel against theory was
incessantly theorising himself. His principal researches are all
connected by an undercurrent of speculation. Theoretic ideas were
the very sap of his intellect--the source from which all his
strength as an experimenter was derived. While once sauntering with
him through the Crystal Palace, at Sydenham, I asked him what
directed his attention to the magnetization of light. It was his
theoretic notions. He had certain views regarding the unity and
convertibility of natural forces; certain ideas regarding the
vibrations of light and their relations to the lines of magnetic
force; these views and ideas drove him to investigation. And so it
must always be: the great experimentalist must ever be the habitual
theorist, whether or not he gives to his theories formal
enunciation.
Faraday, you have been informed, endeavoured to improve the
manufacture of glass for optical purposes. But though he produced a
heavy glass of great refractive power, its value to optics did not
repay him for the pains and labour bestowed on it. Now, however,
we reach a result established by means of this same heavy glass,
which made ample amends for all.
In November, 1845, he announced his discovery of the 'Magnetization
of Light and the Illumination of the Lines of Magnetic Force.'
This title provoked comment at the time, and caused misapprehension.
He therefore added an explanatory note; but the note left his meaning
as entangled as before. In fact Faraday had notions regarding the
magnetization of light which were peculiar to himself, and
untranslatable into the scientific language of the time. Probably
no other philosopher of his day would have employed the phrases just
quoted as appropriate to the discovery announced in 1845.
But Faraday was more than a philosopher; he was a prophet, and often
wrought by an inspiration to be understood by sympathy alone.
The prophetic element in his character occasionally coloured,
and even injured, the utterance of the man of science; but
subtracting that element, though you might have conferred on him
intellectual symmetry, you would have destroyed his motive force.
But let us pass from the label of this casket to the jewel it contains.
'I have long,' he says, 'held an opinion, almost amounting to
conviction, in common, I believe, with many other lovers of natural
knowledge, that the various forms under which the forces of matter
are made manifest have one common origin; in other words, are so
directly related and mutually dependent, that they are convertible,
as it were, into one another, and possess equivalents of power in
their action.... This strong persuasion,' he adds, 'extended to the
powers of light.' And then he examines the action of magnets upon
light. From conversation with him and Anderson, I should infer that
the labour preceding this discovery was very great. The world knows
little of the toil of the discoverer. It sees the climber jubilant
on the mountain top, but does not know the labour expended in
reaching it. Probably hundreds of experiments had been made on
transparent crystals before he thought of testing his heavy glass.
Here is his own clear and simple description of the result of his
first experiment with this substance:--'A piece of this glass, about
two inches square, and 0.5 of an inch thick, having flat and
polished edges, was placed as a diamagnetic[1] between the poles
(not as yet magnetized by the electric current), so that the
polarized ray should pass through its length; the glass acted as
air, water, or any other transparent substance would do; and if the
eye-piece were previously turned into such a position that the
polarized ray was extinguished, or rather the image produced by it
rendered invisible, then the introduction of the glass made no
alteration in this respect. In this state of circumstances, the
force of the electro-magnet was developed by sending an electric
current through its coils, and immediately the image of the
lamp-flame became visible and continued so as long as the
arrangement continued magnetic. On stopping the electric current,
and so causing the magnetic force to cease, the light instantly
disappeared. These phenomena could be renewed at pleasure, at any
instant of time, and upon any occasion, showing a perfect dependence
of cause and effect.'
In a beam of ordinary light the particles of the luminiferous ether
vibrate in all directions perpendicular to the line of progression;
by the act of polarization, performed here by Faraday, all
oscillations but those parallel to a certain plane are eliminated.
When the plane of vibration of the polarizer coincides with that of
the analyzer, a portion of the beam passes through both; but when
these two planes are at right angles to each other, the beam is
extinguished. If by any means, while the polarizer and analyzer
remain thus crossed, the plane of vibration of the polarized beam
between them could be changed, then the light would be, in part at
least, transmitted. In Faraday's experiment this was accomplished.
His magnet turned the plane of polarization of the beam through a
certain angle, and thus enabled it to get through the analyzer;
so that 'the magnetization of light and the illumination of the
magnetic lines of force' becomes, when expressed in the language of
modern theory, the rotation of the plane of polarization.
To him, as to all true philosophers, the main value of a fact was
its position and suggestiveness in the general sequence of
scientific truth. Hence, having established the existence of a
phenomenon, his habit was to look at it from all possible points of
view, and to develop its relationship to other phenomena. He proved
that the direction of the rotation depends upon the polarity of his
magnet; being reversed when the magnetic poles are reversed.
He showed that when a polarized ray passed through his heavy glass
in a direction parallel to the magnetic lines of force, the rotation
is a maximum, and that when the direction of the ray is at right
angles to the lines of force, there is no rotation at all. He also
proved that the amount of the rotation is proportional to the length
of the diamagnetic through which the ray passes. He operated with
liquids and solutions. Of aqueous solutions he tried 150 and more,
and found the power in all of them. He then examined gases; but
here all his efforts to produce any sensible action upon the
polarized beam were ineffectual. He then passed from magnets to
currents, enclosing bars of heavy glass, and tubes containing
liquids and aqueous solutions within an electro-magnetic helix.
A current sent through the helix caused the plane of polarization to
rotate, and always in the direction of the current. The rotation
was reversed when the current was reversed. In the case of magnets,
he observed a gradual, though quick, ascent of the transmitted beam
from a state of darkness to its maximum brilliancy, when the magnet
was excited. In the case of currents, the beam attained at once its
maximum. This he showed to be due to the time required by the iron
of the electro-magnet to assume its full magnetic power, which time
vanishes when a current, without iron, is employed. 'In this
experiment,' he says, 'we may, I think, justly say that a ray of
light is electrified, and the electric forces illuminated.' In the
helix, as with the magnets, he submitted air to magnetic influence
'carefully and anxiously,' but could not discover any trace of
action on the polarized ray.
Many substances possess the power of turning the plane of polarization
without the intervention of magnetism. Oil of turpentine and quartz
are examples; but Faraday showed that, while in one direction,
that is, across the lines of magnetic force, his rotation is zero,
augmenting gradually from this until it attains its maximum, when
the direction of the ray is parallel to the lines of force; in the
oil of turpentine the rotation is independent of the direction of
the ray. But he showed that a still more profound distinction
exists between the magnetic rotation and the natural one. I will
try to explain how. Suppose a tube with glass ends containing oil
of turpentine to be placed north and south. Fixing the eye at the
south end of the tube, let a polarized beam be sent through it from
the north. To the observer in this position the rotation of the
plane of polarization, by the turpentine, is right-handed. Let the
eye be placed at the north end of the tube, and a beam be sent
through it from the south; the rotation is still right-handed.
Not so, however, when a bar of heavy glass is subjected to the
action of an electric current. In this case if, in the first
position of the eye, the rotation be right-handed, in the second
position it is left-handed. These considerations make it manifest
that if a polarized beam, after having passed through the oil of
turpentine in its natural state, could by any means be reflected
back through the liquid, the rotation impressed upon the direct beam
would be exactly neutralized by that impressed upon the reflected
one. Not so with the induced magnetic effect. Here it is manifest
that the rotation would be doubled by the act of reflection.
Hence Faraday concludes that the particles of the oil of turpentine
which rotate by virtue of their natural force, and those which
rotate in virtue of the induced force, cannot be in the same
condition. The same remark applies to all bodies which possess a
natural power of rotating the plane of polarization.
And then he proceeded with exquisite skill and insight to take
advantage of this conclusion. He silvered the ends of his piece of
heavy glass, leaving, however, a narrow portion parallel to two
edges diagonally opposed to each other unsilvered. He then sent his
beam through this uncovered portion, and by suitably inclining his
glass caused the beam within it to reach his eye first direct, and
then after two, four, and six reflections. These corresponded to
the passage of the ray once, three times, five times, and seven
times through the glass. He thus established with numerical
accuracy the exact proportionality of the rotation to the distance
traversed by the polarized beam. Thus in one series of experiments
where the rotation required by the direct beam was 12degrees, that
acquired by three passages through the glass was 36degrees, while that
acquired by five passages was 60degrees. But even when this method of
magnifying was applied, he failed with various solid substances to
obtain any effect; and in the case of air, though he employed to the
utmost the power which these repeated reflections placed in his
hands, he failed to produce the slightest sensible rotation.
These failures of Faraday to obtain the effect with gases seem to
indicate the true seat of the phenomenon. The luminiferous ether
surrounds and is influenced by the ultimate particles of matter.
The symmetry of the one involves that of the other. Thus, if the
molecules of a crystal be perfectly symmetrical round any line
through the crystal, we may safely conclude that a ray will pass
along this line as through ordinary glass. It will not be doubly
refracted. From the symmetry of the liquid figures, known to be
produced in the planes of freezing, when radiant heat is sent
through ice, we may safely infer symmetry of aggregation, and hence
conclude that the line perpendicular to the planes of freezing is a
line of no double refraction; that it is, in fact, the optic axis of
the crystal. The same remark applies to the line joining the
opposite blunt angles of a crystal of Iceland spar. The arrangement
of the molecules round this line being symmetrical, the condition of
the ether depending upon these molecules shares their symmetry; and
there is, therefore, no reason why the wavelength should alter with
the alteration of the azimuth round this line. Annealed glass has
its molecules symmetrically arranged round every line that can be
drawn through it; hence it is not doubly refractive. But let the
substance be either squeezed or strained in one direction, the
molecular symmetry, and with it the symmetry of the ether, is
immediately destroyed and the glass becomes doubly refractive.
Unequal heating produces the same effect. Thus mechanical strains
reveal themselves by optical effects; and there is little doubt that
in Faraday's experiment it is the magnetic strain that produces the
rotation of the plane of polarization.[2]
Footnotes to Chapter 10
[1] 'By a diamagnetic,' says Faraday, 'I mean a body through which
lines of magnetic force are passing, and which does not by their
action assume the usual magnetic state of iron or loadstone.'
Faraday subsequently used this term in a different sense from that
here given, as will immediately appear.
[2] The power of double refraction conferred on the centre of a
glass rod, when it is caused to sound the fundamental note due to
its longitudinal vibration, and the absence of the same power in the
case of vibrating air (enclosed in a glass organ-pipe), seems to be
analogous to the presence and absence of Faraday's effect in the
same two substances.
Faraday never, to my knowledge, attempted to give, even in
conversation, a picture of the molecular condition of his heavy
glass when subjected to magnetic influence. In a mathematical
investigation of the subject, published in the Proceedings of the
Royal Society for 1856, Sir William Thomson arrives at the
conclusion that the 'diamagnetic' is in a state of molecular
rotation.
Chapter 11.
Discovery of diamagnetism--researches on magne-crystallic action.
Faraday's next great step in discovery was announced in a memoir on
the 'Magnetic Condition of all matter,' communicated to the Royal
Society on December 18, 1845. One great source of his success was
the employment of extraordinary power. As already stated, he never
accepted a negative answer to an experiment until he had brought to
bear upon it all the force at his command. He had over and over
again tried steel magnets and ordinary electro-magnets on various
substances, but without detecting anything different from the
ordinary attraction exhibited by a few of them. Stronger coercion,
however, developed a new action. Before the pole of an electro-magnet,
he suspended a fragment of his famous heavy glass; and observed that
when the magnet was powerfully excited the glass fairly retreated
from the pole. It was a clear case of magnetic repulsion. He then
suspended a bar of the glass between two poles; the bar retreated
when the poles were excited, and set its length equatorially or at
right angles to the line joining them. When an ordinary magnetic
body was similarly suspended, it always set axially, that is, from
pole to pole.
Faraday called those bodies which were repelled by the poles of a
magnet, diamagnetic bodies; using this term in a sense different
from that in which he employed it in his memoir on the magnetization
of light. The term magnetic he reserved for bodies which exhibited
the ordinary attraction. He afterwards employed the term magnetic
to cover the whole phenomena of attraction and repulsion, and used
the word paramagnetic to designate such magnetic action as is
exhibited by iron.
Isolated observations by Brugmanns, Becquerel, Le Baillif, Saigy,
and Seebeck had indicated the existence of a repulsive force
exercised by the magnet on two or three substances; but these
observations, which were unknown to Faraday, had been permitted to
remain without extension or examination. Having laid hold of the
fact of repulsion, Faraday immediately expanded and multiplied it.
He subjected bodies of the most varied qualities to the action of
his magnet:--mineral salts, acids, alkalis, ethers, alcohols,
aqueous solutions, glass, phosphorus, resins, oils, essences,
vegetable and animal tissues, and found them all amenable to
magnetic influence. No known solid or liquid proved insensible to
the magnetic power when developed in sufficient strength. All the
tissues of the human body, the blood--though it contains iron--
included, were proved to be diamagnetic. So that if you could
suspend a man between the poles of a magnet, his extremities would
retreat from the poles until his length became equatorial.
Soon after he had commenced his researches on diamagnetism, Faraday
noticed a remarkable phenomenon which first crossed my own path in
the following way: In the year 1849, while working in the cabinet of
my friend, Professor Knoblauch, of Marburg, I suspended a small
copper coin between the poles of an electro-magnet. On exciting the
magnet, the coin moved towards the poles and then suddenly stopped,
as if it had struck against a cushion. On breaking the circuit, the
coin was repelled, the revulsion being so violent as to cause it to
spin several times round its axis of suspension. A Silber-groschen
similarly suspended exhibited the same deportment. For a moment I
thought this a new discovery; but on looking over the literature of
the subject, it appeared that Faraday had observed, multiplied, and
explained the same effect during his researches on diamagnetism.
His explanation was based upon his own great discovery of
magneto-electric currents. The effect is a most singular one.
A weight of several pounds of copper may be set spinning between the
electro-magnetic poles; the excitement of the magnet instantly stops
the rotation. Though nothing is apparent to the eye, the copper,
if moved in the excited magnetic field, appears to move through a
viscous fluid; while, when a flat piece of the metal is caused to
pass to and fro like a saw between the poles, the sawing of the
magnetic field resembles the cutting through of cheese or butter.[1]
This virtual friction of the magnetic field is so strong, that copper,
by its rapid rotation between the poles, might probably be fused.
We may easily dismiss this experiment by saying that the heat is due
to the electric currents excited in the copper. But so long as we
are unable to reply to the question, 'What is an electric current?'
the explanation is only provisional. For my own part, I look with
profound interest and hope on the strange action here referred to.
Faraday's thoughts ran intuitively into experimental combinations,
so that subjects whose capacity for experimental treatment would, to
ordinary minds, seem to be exhausted in a moment, were shown by him
to be all but inexhaustible. He has now an object in view, the
first step towards which is the proof that the principle of
Archimedes is true of magnetism. He forms magnetic solutions of
various degrees of strength, places them between the poles of his
magnet, and suspends in the solutions various magnetic bodies.
He proves that when the solution is stronger than the body plunged in
it, the body, though magnetic, is repelled; and when an elongated
piece of it is surrounded by the solution, it sets, like a
diamagnetic body, equatorially between the excited poles. The same
body when suspended in a solution of weaker magnetic power than
itself, is attracted as a whole, while an elongated portion of it
sets axially.
And now theoretic questions rush in upon him. Is this new force a
true repulsion, or is it merely a differential attraction? Might not
the apparent repulsion of diamagnetic bodies be really due to the
greater attraction of the medium by which they are surrounded?
He tries the rarefaction of air, but finds the effect insensible.
He is averse to ascribing a capacity of attraction to space, or to
any hypothetical medium supposed to fill space. He therefore
inclines, but still with caution, to the opinion that the action of
a magnet upon bismuth is a true and absolute repulsion, and not
merely the result of differential attraction. And then he clearly
states a theoretic view sufficient to account for the phenomena.
'Theoretically,' he says, 'an explanation of the movements of the
diamagnetic bodies, and all the dynamic phenomena consequent upon
the action of magnets upon them, might be offered in the supposition
that magnetic induction caused in them a contrary state to that
which it produced in ordinary matter.' That is to say, while in
ordinary magnetic influence the exciting pole excites adjacent to
itself the contrary magnetism, in diamagnetic bodies the adjacent
magnetism is the same as that of the exciting pole. This theory of
reversed polarity, however, does not appear to have ever laid deep
hold of Faraday's mind; and his own experiments failed to give any
evidence of its truth. He therefore subsequently abandoned it, and
maintained the non-polarity of the diamagnetic force.
He then entered a new, though related field of inquiry. Having
dealt with the metals and their compounds, and having classified all
of them that came within the range of his observation under the two
heads magnetic and diamagnetic, he began the investigation of the
phenomena presented by crystals when subjected to magnetic power.
This action of crystals had been in part theoretically predicted by
Poisson,[2] and actually discovered by Plucker, whose beautiful
results, at the period which we have now reached, profoundly
interested all scientific men. Faraday had been frequently puzzled
by the deportment of bismuth, a highly crystalline metal. Sometimes
elongated masses of the substance refused to set equatorially,
sometimes they set persistently oblique, and sometimes even, like a
magnetic body, from pole to pole.
'The effect,' he says, 'occurs at a single pole; and it is then
striking to observe a long piece of a substance so diamagnetic as
bismuth repelled, and yet at the same moment set round with force,
axially, or end on, as a piece of magnetic substance would do.'
The effect perplexed him; and in his efforts to release himself from
this perplexity, no feature of this new manifestation of force
escaped his attention. His experiments are described in a memoir
communicated to the Royal Society on December 7, 1848.
I have worked long myself at magne-crystallic action, amid all the
light of Faraday's and Plucker's researches. The papers now before
me were objects of daily and nightly study with me eighteen or
nineteen years ago; but even now, though their perusal is but the
last of a series of repetitions, they astonish me. Every
circumstance connected with the subject; every shade of deportment;
every variation in the energy of the action; almost every
application which could possibly be made of magnetism to bring out
in detail the character of this new force, is minutely described.
The field is swept clean, and hardly anything experimental is left
for the gleaner. The phenomena, he concludes, are altogether
different from those of magnetism or diamagnetism: they would appear,
in fact, to present to us 'a new force, or a new form of force,
in the molecules of matter,' which, for convenience sake, he designates
by a new word, as 'the magne-crystallic force.'
He looks at the crystal acted upon by the magnet. From its mass he
passes, in idea, to its atoms, and he asks himself whether the power
which can thus seize upon the crystalline molecules, after they have
been fixed in their proper positions by crystallizing force, may not,
when they are free, be able to determine their arrangement?
He, therefore, liberates the atoms by fusing the bismuth. He places
the fused substance between the poles of an electro-magnet,
powerfully excited; but he fails to detect any action. I think it
cannot be doubted that an action is exerted here, that a true cause
comes into play; but its magnitude is not such as sensibly to
interfere with the force of crystallization, which, in comparison
with the diamagnetic force, is enormous. 'Perhaps,' adds Faraday,
'if a longer time were allowed, and a permanent magnet used, a
better result might be obtained. I had built many hopes upon the
process.' This expression, and his writings abound in such,
illustrates what has been already said regarding his experiments
being suggested and guided by his theoretic conceptions. His mind
was full of hopes and hypotheses, but he always brought them to an
experimental test. The record of his planned and executed experiments
would, I doubt not, show a high ratio of hopes disappointed to hopes
fulfilled; but every case of fulfilment abolished all memory of
defeat; disappointment was swallowed up in victory.
After the description of the general character of this new force,
Faraday states with the emphasis here reproduced its mode of action:
'The law of action appears to be that the line or axis of
MAGNE-CRYSTALLIC force (being the resultant of the action of all the
molecules) tends to place itself parallel, or as a tangent, to the
magnetic curve, or line of magnetic force, passing through the place
where the crystal is situated.' The magne-crystallic force,
moreover, appears to him 'to be clearly distinguished from the
magnetic or diamagnetic forces, in that it causes neither approach
nor recession, consisting not in attraction or repulsion, but in
giving a certain determinate position to the mass under its
influence.' And then he goes on 'very carefully to examine and prove
the conclusion that there was no connection of the force with
attractive or repulsive influences.' With the most refined ingenuity
he shows that, under certain circumstances, the magne-crystallic
force can cause the centre of gravity of a highly magnetic body to
retreat from the poles, and the centre of gravity of a highly
diamagnetic body to approach them. His experiments root his mind
more and more firmly in the conclusion that 'neither attraction nor
repulsion causes the set, or governs the final position' of the
crystal in the magnetic field. That the force which does so is
therefore 'distinct in its character and effects from the magnetic
and diamagnetic forms of force. On the other hand,' he continues,
'it has a most manifest relation to the crystalline structure of
bismuth and other bodies, and therefore to the power by which their
molecules are able to build up the crystalline masses.'
And here follows one of those expressions which characterize the
conceptions of Faraday in regard to force generally:--'It appears to
me impossible to conceive of the results in any other way than by a
mutual reaction of the magnetic force, and the force of the
particles of the crystals upon each other.' He proves that the
action of the force, though thus molecular, is an action at a
distance; he shows that a bismuth crystal can cause a freely
suspended magnetic needle to set parallel to its magne-crystallic
axis. Few living men are aware of the difficulty of obtaining
results like this, or of the delicacy necessary to their attainment.
'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 crystalline
aggregation, which we call at other times the attraction of
aggregation, and so often speak of as acting at insensible distances.'
Thus he broods over this new force, and looks at it from all
possible points of inspection. Experiment follows experiment,
as thought follows thought. He will not relinquish the subject as
long as a hope exists of throwing more light upon it. He knows full
well the anomalous nature of the conclusion to which his experiments
lead him. But experiment to him is final, and he will not shrink
from the conclusion. 'This force,' he says, 'appears to me to be
very strange and striking in its character. It is not polar, for
there is no attraction or repulsion.' And then, as if startled by
his own utterance, he asks--'What is the nature of the mechanical
force which turns the crystal round, and makes it affect a magnet?'...
'I do not remember,' he continues 'heretofore such a case of force
as the present one, where a body is brought into position only,
without attraction or repulsion.'
Plucker, the celebrated geometer already mentioned, who pursued
experimental physics for many years of his life with singular
devotion and success, visited Faraday in those days, and repeated
before him his beautiful experiments on magneto-optic action.
Faraday repeated and verified Plucker's observations, and concluded,
what he at first seemed to doubt, that Plucker's results and
magne-crystallic action had the same origin.
At the end of his papers, when he takes a last look along the line
of research, and then turns his eyes to the future, utterances quite
as much emotional as scientific escape from Faraday. 'I cannot,'
he says, at the end of his first paper on magne-crystallic action,
'conclude this series of researches without remarking how rapidly
the knowledge of molecular forces grows upon us, and how strikingly
every investigation tends to develop more and more their importance,
and their extreme attraction as an object of study. A few years ago
magnetism was to us an occult power, affecting only a few bodies,
now it is found to influence all bodies, and to possess the most
intimate relations with electricity, heat, chemical action, light,
crystallization, and through it, with the forces concerned in
cohesion; and we may, in the present state of things, well feel
urged to continue in our labours, encouraged by the hope of bringing
it into a bond of union with gravity itself.'
Supplementary remarks
A brief space will, perhaps, be granted me here to state the further
progress of an investigation which interested Faraday so much.
Drawn by the fame of Bunsen as a teacher, in the year 1848 I became
a student in the University of Marburg, in Hesse Cassel. Bunsen's
behaviour to me was that of a brother as well as that of a teacher,
and it was also my happiness to make the acquaintance and gain the
friendship of Professor Knoblauch, so highly distinguished by his
researches on Radiant Heat. Plucker's and Faraday's investigations
filled all minds at the time, and towards the end of 1849, Professor
Knoblauch and myself commenced a joint investigation of the entire
question. Long discipline was necessary to give us due mastery over it.
Employing a method proposed by Dove, we examined the optical
properties of our crystals ourselves; and these optical observations
went hand in hand with our magnetic experiments. The number of
these experiments was very great, but for a considerable time no
fact of importance was added to those already published. At length,
however, it was our fortune to meet with various crystals whose
deportment could not be brought under the laws of magne-crystallic
action enunciated by Plucker. We also discovered instances which
led us to suppose that the magne-crystallic force was by no means
independent, as alleged, of the magnetism or diamagnetism of the
mass of the crystal. Indeed, the more we worked at the subject, the
more clearly did it appear to us that the deportment of crystals in
the magnetic field was due, not to a force previously unknown, but
to the modification of the known forces of magnetism and
diamagnetism by crystalline aggregation.
An eminent example of magne-crystallic action adduced by Plucker,
and experimented on by Faraday, was Iceland spar. It is what in
optics is called a negative crystal, and according to the law of
Plucker, the axis of such a crystal was always repelled by a magnet.
But we showed that it was only necessary to substitute, in whole or
in part, carbonate of iron for carbonate of lime, thus changing the
magnetic but not the optical character of the crystal, to cause the
axis to be attracted. That the deportment of magnetic crystals is
exactly antithetical to that of diamagnetic crystals isomorphous
with the magnetic ones, was proved to be a general law of action.
In all cases, the line which in a diamagnetic crystal set equatorially,
always set itself in an isomorphous magnetic crystal axially.
By mechanical compression other bodies were also made to imitate the
Iceland spar.
These and numerous other results bearing upon the question were
published at the time in the 'Philosophical Magazine' and in
'Poggendorff's Annalen'; and the investigation of diamagnetism and
magne-crystallic action was subsequently continued by me in the
laboratory of Professor Magnus of Berlin. In December, 1851, after
I had quitted Germany, Dr. Bence Jones went to the Prussian capital
to see the celebrated experiments of Du Bois Reymond. Influenced, I
suppose, by what he there heard, he afterwards invited me to give a
Friday evening discourse at the Royal Institution. I consented, not
without fear and trembling. For the Royal Institution was to me a
kind of dragon's den, where tact and strength would be necessary to
save me from destruction. On February 11, 1853, the discourse was
given, and it ended happily. I allude to these things, that I may
mention that, though my aim and object in that lecture was to
subvert the notions both of Faraday and Plucker, and to establish in
opposition to their views what I regarded as the truth, it was very
far from producing in Faraday either enmity or anger. At the
conclusion of the lecture, he quitted his accustomed seat, crossed
the theatre to the corner into which I had shrunk, shook me by the
hand, and brought me back to the table. Once more, subsequently,
and in connection with a related question, I ventured to differ from
him still more emphatically. It was done out of trust in the
greatness of his character; nor was the trust misplaced. He felt my
public dissent from him; and it pained me afterwards to the quick to
think that I had given him even momentary annoyance. It was,
however, only momentary. His soul was above all littleness and
proof to all egotism. He was the same to me afterwards that he had
been before; the very chance expression which led me to conclude
that he felt my dissent being one of kindness and affection.
It required long subsequent effort to subdue the complications of
magne-crystallic action, and to bring under the dominion of
elementary principles the vast mass of facts which the experiments
of Faraday and Plucker had brought to light. It was proved by
Reich, Edmond Becquerel, and myself, that the condition of
diamagnetic bodies, in virtue of which they were repelled by the
poles of a magnet, was excited in them by those poles; that the
strength of this condition rose and fell with, and was proportional
to, the strength of the acting magnet. It was not then any property
possessed permanently by the bismuth, and which merely required the
development of magnetism to act upon it, that caused the repulsion;
for then the repulsion would have been simply proportional to the
strength of the influencing magnet, whereas experiment proved it to
augment as the square of the strength. The capacity to be repelled
was therefore not inherent in the bismuth, but induced. So far an
identity of action was established between magnetic and diamagnetic
bodies. After this the deportment of magnetic bodies, 'normal' and
'abnormal'; crystalline, amorphous, and compressed, was compared
with that of crystalline, amorphous, and compressed diamagnetic
bodies; and by a series of experiments, executed in the laboratory
of this Institution, the most complete antithesis was established
between magnetism and diamagnetism. This antithesis embraced the
quality of polarity,--the theory of reversed polarity, first
propounded by Faraday, being proved to be true. The discussion of
the question was very brisk. On the Continent Professor Wilhelm
Weber was the ablest and most successful supporter of the doctrine
of diamagnetic polarity; and it was with an apparatus, devised by
him and constructed under his own superintendence, by Leyser of
Leipzig, that the last demands of the opponents of diamagnetic
polarity were satisfied. The establishment of this point was
absolutely necessary to the explanation of magne-crystallic action.
With that admirable instinct which always guided him, Faraday had
seen that it was possible, if not probable, that the diamagnetic
force acts with different degrees of intensity in different
directions, through the mass of a crystal. In his studies on
electricity, he had sought an experimental reply to the question
whether crystalline bodies had not different specific inductive
capacities in different directions, but he failed to establish any
difference of the kind. His first attempt to establish differences
of diamagnetic action in different directions through bismuth, was
also a failure; but he must have felt this to be a point of cardinal
importance, for he returned to the subject in 1850, and proved that
bismuth was repelled with different degrees of force in different
directions. It seemed as if the crystal were compounded of two
diamagnetic bodies of different strengths, the substance being more
strongly repelled across the magne-crystallic axis than along it.
The same result was obtained independently, and extended to various
other bodies, magnetic as well as diamagnetic, and also to
compressed substances, a little subsequently by myself.
The law of action in relation to this point is, that in diamagnetic
crystals, the line along which the repulsion is a maximum, sets
equatorially in the magnetic field; while in magnetic crystals the
line along which the attraction is a maximum sets from pole to pole.
Faraday had said that the magne-crystallic force was neither
attraction nor repulsion. Thus far he was right. It was neither
taken singly, but it was both. By the combination of the doctrine
of diamagnetic polarity with these differential attractions and
repulsions, and by paying due regard to the character of the
magnetic field, every fact brought to light in the domain of
magne-crystallic action received complete explanation. The most
perplexing of those facts were shown to result from the action of
mechanical couples, which the proved polarity both of magnetism and
diamagnetism brought into play. Indeed the thoroughness with which
the experiments of Faraday were thus explained, is the most striking
possible demonstration of the marvellous precision with which they
were executed.
Footnotes to Chapter 11
[1] See Heat as a Mode of Motion, ninth edition, p. 75.
[2] See Sir Wm. Thomson on Magne-crystallic Action. Phil. Mag., 1851.
Chapter 12.
Magnetism of flame and gases--atmospheric magnetism
When an experimental result was obtained by Faraday it was instantly
enlarged by his imagination. I am acquainted with no mind whose
power and suddenness of expansion at the touch of new physical truth
could be ranked with his. Sometimes I have compared the action of
his experiments on his mind to that of highly combustible matter
thrown into a furnace; every fresh entry of fact was accompanied by
the immediate development of light and heat. The light, which was
intellectual, enabled him to see far beyond the boundaries of the
fact itself, and the heat, which was emotional, urged him to the
conquest of this newly-revealed domain. But though the force of his
imagination was enormous, he bridled it like a mighty rider, and
never permitted his intellect to be overthrown.
In virtue of the expansive power which his vivid imagination
conferred upon him, he rose from the smallest beginnings to the
grandest ends. Having heard from Zantedeschi that Bancalari had
established the magnetism of flame, he repeated the experiments and
augmented the results. He passed from flames to gases, examining
and revealing their magnetic and diamagnetic powers; and then he
suddenly rose from his bubbles of oxygen and nitrogen to the
atmospheric envelope of the earth itself, and its relations to the
great question of terrestrial magnetism. The rapidity with which
these ever-augmenting thoughts assumed the form of experiments is
unparalleled. His power in this respect is often best illustrated by
his minor investigations, and, perhaps, by none more strikingly than
by his paper 'On the Diamagnetic Condition of Flame and Gases,'
published as a letter to Mr. Richard Taylor, in the 'Philosophical
Magazine' for December, 1847. After verifying, varying, and
expanding the results of Bancalari, he submitted to examination
heated air-currents, produced by platinum spirals placed in the
magnetic field, and raised to incandescence by electricity. He then
examined the magnetic deportment of gases generally. Almost all of
these gases are invisible; but he must, nevertheless, track them in
their unseen courses. He could not effect this by mingling smoke
with his gases, for the action of his magnet upon the smoke would
have troubled his conclusions. He, therefore, 'caught' his gases in
tubes, carried them out of the magnetic field, and made them reveal
themselves at a distance from the magnet.
Immersing one gas in another, he determined their differential
action; results of the utmost beauty being thus arrived at. Perhaps
the most important are those obtained with atmospheric air and its
two constituents. Oxygen, in various media, was strongly attracted
by the magnet; in coal-gas, for example, it was powerfully magnetic,
whereas nitrogen was diamagnetic. Some of the effects obtained with
oxygen in coal-gas were strikingly beautiful. When the fumes of
chloride of ammonium (a diamagnetic substance) were mingled with the
oxygen, the cloud of chloride behaved in a most singular manner,--
'The attraction of iron filings,' says Faraday, 'to a magnetic pole
is not more striking than the appearance presented by the oxygen
under these circumstances.'
On observing this deportment the question immediately occurs to him,
--Can we not separate the oxygen of the atmosphere from its nitrogen
by magnetic analysis? It is the perpetual occurrence of such
questions that marks the great experimenter. The attempt to analyze
atmospheric air by magnetic force proved a failure, like the
previous attempt to influence crystallization by the magnet.
The enormous comparative power of the force of crystallization I
have already assigned as a reason for the incompetence of the magnet
to determine molecular arrangement; in the present instance the
magnetic analysis is opposed by the force of diffusion, which is
also very strong comparatively. The same remark applies to, and is
illustrated by, another experiment subsequently executed by Faraday.
Water is diamagnetic, sulphate of iron is strongly magnetic.
He enclosed 'a dilute solution of sulphate of iron in a tube,
and placed the lower end of the tube between the poles of a powerful
horseshoe magnet for days together,' but he could produce
'no concentration of the solution in the part near the magnet.'
Here also the diffusibility of the salt was too powerful for the
force brought against it.
The experiment last referred to is recorded in a paper presented to
the Royal Society on the 2nd August, 1850, in which he pursues the
investigation of the magnetism of gases. Newton's observations on
soap-bubbles were often referred to by Faraday. His delight in a
soap-bubble was like that of a boy, and he often introduced them
into his lectures, causing them, when filled with air, to float on
invisible seas of carbonic acid, and otherwise employing them as a
means of illustration. He now finds them exceedingly useful in his
experiments on the magnetic condition of gases. A bubble of air in
a magnetic field occupied by air was unaffected, save through the
feeble repulsion of its envelope. A bubble of nitrogen, on the
contrary, was repelled from the magnetic axis with a force far
surpassing that of a bubble of air. The deportment of oxygen in air
'was very impressive, the bubble being pulled inward or towards the
axial line, sharply and suddenly, as if the oxygen were highly
magnetic.'
He next labours to establish the true magnetic zero, a problem not
so easy as might at first sight be imagined. For the action of the
magnet upon any gas, while surrounded by air or any other gas, can
only be differential; and if the experiment were made in vacuo, the
action of the envelope, in this case necessarily of a certain
thickness, would trouble the result. While dealing with this
subject, Faraday makes some noteworthy observations regarding space.
In reference to the Torricellian vacuum, he says, 'Perhaps it is
hardly necessary for me to state that I find both iron and bismuth
in such vacua perfectly obedient to the magnet. From such
experiments, and also from general observations and knowledge, it
seems manifest that the lines of magnetic force can traverse pure
space, just as gravitating force does, and as statical electrical
forces do, and therefore space has a magnetic relation of its own,
and one that we shall probably find hereafter to be of the utmost
importance in natural phenomena. But this character of space is not
of the same kind as that which, in relation to matter, we endeavour
to express by the terms magnetic and diamagnetic. To confuse these
together would be to confound space with matter, and to trouble all
the conceptions by which we endeavour to understand and work out a
progressively clearer view of the mode of action, and the laws of
natural forces. It would be as if in gravitation or electric forces,
one were to confound the particles acting on each other with the
space across which they are acting, and would, I think, shut the
door to advancement. Mere space cannot act as matter acts, even
though the utmost latitude be allowed to the hypothesis of an ether;
and admitting that hypothesis, it would be a large additional
assumption to suppose that the lines of magnetic force are
vibrations carried on by it, whilst as yet we have no proof that
time is required for their propagation, or in what respect they may,
in general character, assimilate to or differ from their respective
lines of gravitating, luminiferous, or electric forces.'
Pure space he assumes to be the true magnetic zero, but he pushes
his inquiries to ascertain whether among material substances there
may not be some which resemble space. If you follow his experiments,
you will soon emerge into the light of his results. A torsion-beam
was suspended by a skein of cocoon silk; at one end of the beam was
fixed a cross-piece 1 1/2 inch long. Tubes of exceedingly thin glass,
filled with various gases, and hermetically sealed, were suspended
in pairs from the two ends of the cross-piece. The position of the
rotating torsion-head was such that the two tubes were at opposite
sides of, and equidistant from, the magnetic axis, that is to say
from the line joining the two closely approximated polar points of
an electro-magnet. His object was to compare the magnetic action of
the gases in the two tubes. When one tube was filled with oxygen,
and the other with nitrogen, on the supervention of the magnetic
force, the oxygen was pulled towards the axis, the nitrogen being
pushed out. By turning the torsion-head they could be restored to
their primitive position of equidistance, where it is evident the
action of the glass envelopes was annulled. The amount of torsion
necessary to re-establish equidistance expressed the magnetic
difference of the substances compared.
And then he compared oxygen with oxygen at different pressures.
One of his tubes contained the gas at the pressure of 30 inches of
mercury, another at a pressure of 15 inches of mercury, a third at a
pressure of 10 inches, while a fourth was exhausted as far as a good
air-pump renders exhaustion possible. 'When the first of these was
compared with the other three, the effect was most striking.'
It was drawn towards the axis when the magnet was excited, the tube
containing the rarer gas being apparently driven away, and the
greater the difference between the densities of the two gases,
the greater was the energy of this action.
And now observe his mode of reaching a material magnetic zero.
When a bubble of nitrogen was exposed in air in the magnetic field,
on the supervention of the power, the bubble retreated from the magnet.
A less acute observer would have set nitrogen down as diamagnetic;
but Faraday knew that retreat, in a medium composed in part of oxygen,
might be due to the attraction of the latter gas, instead of to the
repulsion of the gas immersed in it. But if nitrogen be really
diamagnetic, then a bubble or bulb filled with the dense gas will
overcome one filled with the rarer gas. From the cross-piece of his
torsion-balance he suspended his bulbs of nitrogen, at equal distances
from the magnetic axis, and found that the rarefaction, or the
condensation of the gas in either of the bulbs had not the slightest
influence. When the magnetic force was developed, the bulbs
remained in their first position, even when one was filled with
nitrogen, and the other as far as possible exhausted. Nitrogen,
in fact, acted 'like space itself'; it was neither magnetic nor
diamagnetic.
He cannot conveniently compare the paramagnetic force of oxygen with
iron, in consequence of the exceeding magnetic intensity of the
latter substance; but he does compare it with the sulphate of iron,
and finds that, bulk for bulk, oxygen is equally magnetic with a
solution of this substance in water 'containing seventeen times the
weight of the oxygen in crystallized proto-sulphate of iron, or 3.4
times its weight of metallic iron in that state of combination.'
By its capability to deflect a fine glass fibre, he finds that the
attraction of this bulb of oxygen, containing only 0.117 of a grain
of the gas, at an average distance of more than an inch from the
magnetic axis, is about equal to the gravitating force of the same
amount of oxygen as expressed by its weight.
These facts could not rest for an instant in the mind of Faraday
without receiving that expansion to which I have already referred.
'It is hardly necessary,' he writes, 'for me to say here that this
oxygen cannot exist in the atmosphere exerting such a remarkable and
high amount of magnetic force, without having a most important
influence on the disposition of the magnetism of the earth, as a
planet; especially if it be remembered that its magnetic condition
is greatly altered by variations of its density and by variations of
its temperature. I think I see here the real cause of many of the
variations of that force, which have been, and are now so carefully
watched on different parts of the surface of the globe. The daily
variation, and the annual variation, both seem likely to come under
it; also very many of the irregular continual variations, which the
photographic process of record renders so beautifully manifest.
If such expectations be confirmed, and the influence of the atmosphere
be found able to produce results like these, then we shall probably
find a new relation between the aurora borealis and the magnetism of
the earth, namely, a relation established, more or less, through the
air itself in connection with the space above it; and even magnetic
relations and variations, which are not as yet suspected, may be
suggested and rendered manifest and measurable, in the further
development of what I will venture to call Atmospheric Magnetism.
I may be over-sanguine in these expectations, but as yet I am sustained
in them by the apparent reality, simplicity, and sufficiency of the
cause assumed, as it at present appears to my mind. As soon as I
have submitted these views to a close consideration, and the test of
accordance with observation, and, where applicable, with experiments
also, I will do myself the honour to bring them before the Royal
Society.'
Two elaborate memoirs are then devoted to the subject of Atmospheric
Magnetism; the first sent to the Royal Society on the 9th of October,
and the second on the 19th of November, 1850. In these memoirs he
discusses the effects of heat and cold upon the magnetism of the
air, and the action on the magnetic needle, which must result from
thermal changes. By the convergence and divergence of the lines of
terrestrial magnetic force, he shows how the distribution of
magnetism, in the earth's atmosphere, is effected. He applies his
results to the explanation of the Annual and of the Diurnal Variation:
he also considers irregular variations, including the action of
magnetic storms. He discusses, at length, the observations at
St. Petersburg, Greenwich, Hobarton, St. Helena, Toronto, and the
Cape of Good Hope; believing that the facts, revealed by his
experiments, furnish the key to the variations observed at all these
places.
In the year 1851, I had the honour of an interview with Humboldt, in
Berlin, and his parting words to me then were, 'Tell Faraday that I
entirely agree with him, and that he has, in my opinion, completely
explained the variation of the declination.' Eminent men have since
informed me that Humboldt was hasty in expressing this opinion. In
fact, Faraday's memoirs on atmospheric magnetism lost much of their
force--perhaps too much--through the important discovery of the
relation of the variation of the declination to the number of the
solar spots. But I agree with him and M. Edmond Becquerel, who
worked independently at this subject, in thinking, that a body so
magnetic as oxygen, swathing the earth, and subject to variations of
temperature, diurnal and annual, must affect the manifestations of
terrestrial magnetism.[1] The air that stands upon a single square
foot of the earth's surface is, according to Faraday, equivalent in
magnetic force to 8160 lbs. of crystallized protosulphate of iron.
Such a substance cannot be absolutely neutral as regards the
deportment of the magnetic needle. But Faraday's writings on this
subject are so voluminous, and the theoretic points are so novel and
intricate, that I shall postpone the complete analysis of these
researches to a time when I can lay hold of them more completely
than my other duties allow me to do now.
Footnote to Chapter 12
[1] This persuasion has been greatly strengthened by the recent
perusal of a paper by Mr. Baxendell.
Chapter 13.
Speculations: nature of matter: lines of force
The scientific picture of Faraday would not be complete without a
reference to his speculative writings. On Friday, January 19, 1844,
he opened the weekly evening-meetings of the Royal Institution by a
discourse entitled 'A speculation touching Electric Conduction and
the nature of Matter.' In this discourse he not only attempts the
overthrow of Dalton's Theory of Atoms, but also the subversion of
all ordinary scientific ideas regarding the nature and relations of
Matter and Force. He objected to the use of the term atom:--'I have
not yet found a mind,' he says, 'that did habitually separate it
from its accompanying temptations; and there can be no doubt that
the words definite proportions, equivalent, primes, &c., which did
and do fully express all the facts of what is usually called the
atomic theory in chemistry, were dismissed because they were not
expressive enough, and did not say all that was in the mind of him
who used the word atom in their stead.'
A moment will be granted me to indicate my own view of Faraday's
position here. The word 'atom' was not used in the stead of
definite proportions, equivalents, or primes. These terms
represented facts that followed from, but were not equivalent to,
the atomic theory. Facts cannot satisfy the mind: and the law of
definite combining proportions being once established, the question
'why should combination take place according to that law?' is
inevitable. Dalton answered this question by the enunciation of the
Atomic Theory, the fundamental idea of which is, in my opinion,
perfectly secure. The objection of Faraday to Dalton might be urged
with the same substantial force against Newton: it might be stated
with regard to the planetary motions that the laws of Kepler
revealed the facts; that the introduction of the principle of
gravitation was an addition to the facts. But this is the essence
of all theory. The theory is the backward guess from fact to
principle; the conjecture, or divination regarding something, which
lies behind the facts, and from which they flow in necessary
sequence. If Dalton's theory, then, account for the definite
proportions observed in the combinations of chemistry, its
justification rests upon the same basis as that of the principle of
gravitation. All that can in strictness be said in either case is
that the facts occur as if the principle existed.
The manner in which Faraday himself habitually deals with his
hypotheses is revealed in this lecture. He incessantly employed
them to gain experimental ends, but he incessantly took them down,
as an architect removes the scaffolding when the edifice is complete.
'I cannot but doubt,' he says, 'that he who as a mere philosopher
has most power of penetrating the secrets of nature, and guessing by
hypothesis at her mode of working, will also be most careful for his
own safe progress and that of others, to distinguish the knowledge
which consists of assumption, by which I mean theory and hypothesis,
from that which is the knowledge of facts and laws.' Faraday
himself, in fact, was always 'guessing by hypothesis,' and making
theoretic divination the stepping-stone to his experimental results.
I have already more than once dwelt on the vividness with which he
realised molecular conditions; we have a fine example of this
strength and brightness of imagination in the present 'speculation.'
He grapples with the notion that matter is made up of particles, not
in absolute contact, but surrounded by interatomic space. 'Space,'
he observes, 'must be taken as the only continuous part of a body so
constituted. Space will permeate all masses of matter in every
direction like a net, except that in place of meshes it will form
cells, isolating each atom from its neighbours, itself only being
continuous.'
Let us follow out this notion; consider, he argues, the case of a
non-conductor of electricity, such for example as shell-lac, with
its molecules, and intermolecular spaces running through the mass.
In its case space must be an insulator; for if it were a conductor
it would resemble 'a fine metallic web,' penetrating the lac in
every direction. But the fact is that it resembles the wax of black
sealing-wax, which surrounds and insulates the particles of
conducting carbon, interspersed throughout its mass. In the case of
shell-lac, therefore, space is an insulator.
But now, take the case of a conducting metal. Here we have, as
before, the swathing of space round every atom. If space be an
insulator there can be no transmission of electricity from atom to
atom. But there is transmission; hence space is a conductor. Thus
he endeavours to hamper the atomic theory. 'The reasoning,' he says,
'ends in a subversion of that theory altogether; for if space be an
insulator it cannot exist in conducting bodies, and if it be a
conductor it cannot exist in insulating bodies. Any ground of
reasoning,' he adds, as if carried away by the ardour of argument,
'which tends to such conclusions as these must in itself be false.'
He then tosses the atomic theory from horn to horn of his dilemmas.
What do we know, he asks, of the atom apart from its force?
You imagine a nucleus which may be called a, and surround it by
forces which may be called m; 'to my mind the a or nucleus vanishes,
and the substance consists in the powers of m. And indeed what
notion can we form of the nucleus independent of its powers?
What thought remains on which to hang the imagination of an a
independent of the acknowledged forces?' Like Boscovich,
he abolishes the atom, and puts a 'centre of force' in its place.
With his usual courage and sincerity he pushes his view to its
utmost consequences. 'This view of the constitution of matter,'
he continues, 'would seem to involve necessarily the conclusion that
matter fills all space, or at least all space to which gravitation
extends; for gravitation is a property of matter dependent on a
certain force, and it is this force which constitutes the matter.
In that view matter is not merely mutually penetrable;[1] but each
atom extends, so to say, throughout the whole of the solar system,
yet always retaining its own centre of force.'
It is the operation of a mind filled with thoughts of this profound,
strange, and subtle character that we have to take into account in
dealing with Faraday's later researches. A similar cast of thought
pervades a letter addressed by Faraday to Mr. Richard Phillips,
and published in the 'Philosophical Magazine' for May, 1846. It is
entitled 'Thoughts on Ray-vibrations,' and it contains one of the
most singular speculations that ever emanated from a scientific
mind. It must be remembered here, that though Faraday lived amid
such speculations he did not rate them highly, and that he was
prepared at any moment to change them or let them go. They spurred
him on, but they did not hamper him. His theoretic notions were
fluent; and when minds less plastic than his own attempted to render
those fluxional images rigid, he rebelled. He warns Phillips
moreover, that from first to last, 'he merely threw out as matter
for speculation the vague impressions of his mind; for he gave
nothing as the result of sufficient consideration, or as the settled
conviction, or even probable conclusion at which he had arrived.'
The gist of this communication is that gravitating force acts in
lines across space, and that the vibrations of light and radiant
heat consist in the tremors of these lines of force. 'This notion,'
he says, 'as far as it is admitted, will dispense with the ether,
which, in another view is supposed to be the medium in which these
vibrations take place.' And he adds further on, that his view
'endeavours to dismiss the ether but not the vibrations.' The idea
here set forth is the natural supplement of his previous notion,
that it is gravitating force which constitutes matter, each atom
extending, so to say, throughout the whole of the solar system.
The letter to Mr. Phillips winds up with this beautiful
conclusion:--
'I think it likely that I have made many mistakes in the preceding
pages, for even to myself my ideas on this point appear only as the
shadow of a speculation, or as one of those impressions upon the
mind which are allowable for a time as guides to thought and
research. He who labours in experimental inquiries, knows how
numerous these are, and how often their apparent fitness and beauty
vanish before the progress and development of real natural truth.'
Let it then be remembered that Faraday entertained notions regarding
matter and force altogether distinct from the views generally held
by scientific men. Force seemed to him an entity dwelling along the
line in which it is exerted. The lines along which gravity acts
between the sun and earth seem figured in his mind as so many
elastic strings; indeed he accepts the assumed instantaneity of
gravity as the expression of the enormous elasticity of the 'lines
of weight.' Such views, fruitful in the case of magnetism, barren,
as yet, in the case of gravity, explain his efforts to transform
this latter force. When he goes into the open air and permits his
helices to fall, to his mind's eye they are tearing through the
lines of gravitating power, and hence his hope and conviction that
an effect would and ought to be produced. It must ever be borne in
mind that Faraday's difficulty in dealing with these conceptions was
at bottom the same as that of Newton; that he is in fact trying to
overleap this difficulty, and with it probably the limits prescribed
to the intellect itself.
The idea of lines of magnetic force was suggested to Faraday by the
linear arrangement of iron filings when scattered over a magnet.
He speaks of and illustrates by sketches, the deflection, both
convergent and divergent, of the lines of force, when they pass
respectively through magnetic and diamagnetic bodies. These notions
of concentration and divergence are also based on the direct
observation of his filings. So long did he brood upon these lines;
so habitually did he associate them with his experiments on induced
currents, that the association became 'indissoluble,' and he could
not think without them. 'I have been so accustomed,' he writes,
'to employ them, and especially in my last researches, that I may
have unwittingly become prejudiced in their favour, and ceased to be
a clear-sighted judge. Still, I have always endeavoured to make
experiment the test and controller of theory and opinion; but
neither by that nor by close cross-examination in principle, have I
been made aware of any error involved in their use.'
In his later researches on magne-crystallic action, the idea of
lines of force is extensively employed; it indeed led him to an
experiment which lies at the root of the whole question. In his
subsequent researches on Atmospheric Magnetism the idea receives
still wider application, showing itself to be wonderfully flexible
and convenient. Indeed without this conception the attempt to seize
upon the magnetic actions, possible or actual, of the atmosphere
would be difficult in the extreme; but the notion of lines of force,
and of their divergence and convergence, guides Faraday without
perplexity through all the intricacies of the question. After the
completion of those researches, and in a paper forwarded to the
Royal Society on October 22, 1851, he devotes himself to the formal
development and illustration of his favourite idea. The paper bears
the title, 'On lines of magnetic force, their definite character,
and their distribution within a magnet and through space.'
A deep reflectiveness is the characteristic of this memoir.
In his experiments, which are perfectly beautiful and profoundly
suggestive, he takes but a secondary delight. His object is to
illustrate the utility of his conception of lines of force.
'The study of these lines,' he says, 'has at different times been
greatly influential in leading me to various results which I think
prove their utility as well as fertility.'
Faraday for a long period used the lines of force merely as
'a representative idea.' He seemed for a time averse to going further
in expression than the lines themselves, however much further he may
have gone in idea. That he believed them to exist at all times
round a magnet, and irrespective of the existence of magnetic
matter, such as iron filings, external to the magnet, is certain.
No doubt the space round every magnet presented itself to his
imagination as traversed by loops of magnetic power; but he was
chary in speaking of the physical substratum of those loops. Indeed
it may be doubted whether the physical theory of lines of force
presented itself with any distinctness to his own mind.
The possible complicity of the luminiferous ether in magnetic phenomena
was certainly in his thoughts. 'How the magnetic force,' he writes,
'is transferred through bodies or through space we know not; whether
the result is merely action at a distance, as in the case of gravity;
or by some intermediate agency, as in the case of light, heat,
the electric current, and (as I believe) static electric action.
The idea of magnetic fluids, as applied by some, or of Magnetic centres
of action, does not include that of the latter kind of transmission,
but the idea of lines of force does.' And he continues thus:--
'I am more inclined to the notion that in the transmission of the
[magnetic] force there is such an action [an intermediate agency]
external to the magnet, than that the effects are merely attraction
and repulsion at a distance. Such an affection may be a function of
the ether; for it is not at all unlikely that, if there be an ether,
it should have other uses than simply the conveyance of radiations.'
When he speaks of the magnet in certain cases, 'revolving amongst
its own forces,' he appears to have some conception of this kind in
view.
A great part of the investigation completed in October, 1851, was
taken up with the motions of wires round the poles of a magnet and
the converse. He carried an insulated wire along the axis of a bar
magnet from its pole to its equator, where it issued from the magnet,
and was bent up so as to connect its two ends. A complete circuit,
no part of which was in contact with the magnet, was thus obtained.
He found that when the magnet and the external wire were rotated
together no current was produced; whereas, when either of them was
rotated and the other left at rest currents were evolved. He then
abandoned the axial wire, and allowed the magnet itself to take its
place; the result was the same.[2] It was the relative motion of
the magnet and the loop that was effectual in producing a current.
The lines of force have their roots in the magnet, and though they
may expand into infinite space, they eventually return to the magnet.
Now these lines may be intersected close to the magnet or at a
distance from it. Faraday finds distance to be perfectly immaterial
so long as the number of lines intersected is the same.
For example, when the loop connecting the equator and the pole of
his barmagnet performs one complete revolution round the magnet,
it is manifest that all the lines of force issuing from the magnet
are once intersected. Now it matters not whether the loop be ten feet
or ten inches in length, it matters not how it may be twisted and
contorted, it matters not how near to the magnet or how distant from
it the loop may be, one revolution always produces the same amount
of current electricity, because in all these cases all the lines of
force issuing from the magnet are once intersected and no more.
From the external portion of the circuit he passes in idea to the
internal, and follows the lines of force into the body of the magnet
itself. His conclusion is that there exist lines of force within
the magnet of the same nature as those without. What is more, they
are exactly equal in amount to those without. They have a relation
in direction to those without; and in fact are continuations of
them.... 'Every line of force, therefore, at whatever distance it
may be taken from the magnet, must be considered as a closed
circuit, passing in some part of its course through the magnet,
and having an equal amount of force in every part of its course.'
All the results here described were obtained with moving metals.
'But,' he continues with profound sagacity, 'mere motion would not
generate a relation, which had not a foundation in the existence of
some previous state; and therefore the quiescent metals must be in
some relation to the active centre of force,' that is to the magnet.
He here touches the core of the whole question, and when we can
state the condition into which the conducting wire is thrown before
it is moved, we shall then be in a position to understand the
physical constitution of the electric current generated by its
motion.
In this inquiry Faraday worked with steel magnets, the force of
which varies with the distance from the magnet. He then sought a
uniform field of magnetic force, and found it in space as affected
by the magnetism of the earth. His next memoir, sent to the Royal
Society, December 31, 1851, is 'on the employment of the Induced
Magnetoelectro Current as a test and measure of magnetic forces.'
He forms rectangles and rings, and by ingenious and simple devices
collects the opposed currents which are developed in them by
rotation across the terrestrial lines of magnetic force. He varies
the shapes of his rectangles while preserving their areas constant,
and finds that the constant area produces always the same amount of
current per revolution. The current depends solely on the number of
lines of force intersected, and when this number is kept constant
the current remains constant too. Thus the lines of magnetic force
are continually before his eyes, by their aid he colligates his
facts, and through the inspirations derived from them he vastly
expands the boundaries of our experimental knowledge. The beauty
and exactitude of the results of this investigation are
extraordinary. I cannot help thinking while I dwell upon them, that
this discovery of magneto-electricity is the greatest experimental
result ever obtained by an investigator. It is the Mont Blanc of
Faraday's own achievements. He always worked at great elevations,
but a higher than this he never subsequently attained.
Footnotes to Chapter 13
[1] He compares the interpenetration of two atoms to the
coalescence of two distinct waves, which though for a moment blended
to a single mass, preserve their individuality, and afterwards
separate.
[2] In this form the experiment is identical with one made twenty
years earlier. See page 34.
Chapter 14.
Unity and convertibility of natural forces:
theory of the electric current.
The terms unity and convertibility, as applied to natural forces,
are often employed in these investigations, many profound and
beautiful thoughts respecting these subjects being expressed in
Faraday's memoirs. Modern inquiry has, however, much augmented our
knowledge of the relationship of natural forces, and it seems worth
while to say a few words here, tending to clear up certain
misconceptions which appear to exist among philosophic writers
regarding this relationship.
The whole stock of energy or working-power in the world consists of
attractions, repulsions, and motions. If the attractions and
repulsions are so circumstanced as to be able to produce motion,
they are sources of working-power, but not otherwise. Let us for
the sake of simplicity confine our attention to the case of
attraction. The attraction exerted between the earth and a body at
a distance from the earth's surface is a source of working-power;
because the body can be moved by the attraction, and in falling to
the earth can perform work. When it rests upon the earth's surface
it is not a source of power or energy, because it can fall no
further. But though it has ceased to be a source of energy, the
attraction of gravity still acts as a force, which holds the earth
and weight together.
The same remarks apply to attracting atoms and molecules. As long
as distance separates them, they can move across it in obedience to
the attraction, and the motion thus produced may, by proper appliances,
be caused to perform mechanical work. When, for example, two atoms
of hydrogen unite with one of oxygen, to form water the atoms are
first drawn towards each other--they move, they clash, and then by
virtue of their resiliency, they recoil and quiver. To this
quivering motion we give the name of heat. Now this quivering
motion is merely the redistribution of the motion produced by the
chemical affinity; and this is the only sense in which chemical
affinity can be said to be converted into heat. We must not imagine
the chemical attraction destroyed, or converted into anything else.
For the atoms, when mutually clasped to form a molecule of water,
are held together by the very attraction which first drew them
towards each other. That which has really been expended is the pull
exerted through the space by which the distance between the atoms
has been diminished.
If this be understood, it will be at once seen that gravity may in
this sense be said to be convertible into heat; that it is in
reality no more an outstanding and inconvertible agent, as it is
sometimes stated to be, than chemical affinity. By the exertion of
a certain pull, through a certain space, a body is caused to clash
with a certain definite velocity against the earth. Heat is thereby
developed, and this is the only sense in which gravity can be said
to be converted into heat. In no case is the force which produces
the motion annihilated or changed into anything else. The mutual
attraction of the earth and weight exists when they are in contact
as when they were separate; but the ability of that attraction to
employ itself in the production of motion does not exist.
The transformation, in this case, is easily followed by the mind's
eye. First, the weight as a whole is set in motion by the attraction
of gravity. This motion of the mass is arrested by collision with
the earth; being broken up into molecular tremors, to which we give
the name of heat.
And when we reverse the process, and employ those tremors of heat to
raise a weight, as is done through the intermediation of an elastic
fluid in the steam-engine, a certain definite portion of the
molecular motion is destroyed in raising the weight. In this sense,
and this sense only, can the heat be said to be converted into
gravity, or more correctly, into potential energy of gravity. It is
not that the destruction of the heat has created any new attraction,
but simply that the old attraction has now a power conferred upon it,
of exerting a certain definite pull in the interval between the
starting-point of the falling weight and its collision with the earth.
So also as regards magnetic attraction: when a sphere of iron placed
at some distance from a magnet rushes towards the magnet, and has
its motion stopped by collision, an effect mechanically the same as
that produced by the attraction of gravity occurs. The magnetic
attraction generates the motion of the mass, and the stoppage of
that motion produces heat. In this sense, and in this sense only,
is there a transformation of magnetic work into heat. And if by the
mechanical action of heat, brought to bear by means of a suitable
machine, the sphere be torn from the magnet and again placed at a
distance, a power of exerting a pull through that distance, and
producing a new motion of the sphere, is thereby conferred upon the
magnet; in this sense, and in this sense only, is the heat converted
into magnetic potential energy.
When, therefore, writers on the conservation of energy speak of
tensions being 'consumed' and 'generated,' they do not mean thereby
that old attractions have been annihilated and new ones brought into
existence, but that, in the one case, the power of the attraction to
produce motion has been diminished by the shortening of the distance
between the attracting bodies, and that in the other case the power
of producing motion has been augmented by the increase of the
distance. These remarks apply to all bodies, whether they be
sensible masses or molecules.
Of the inner quality that enables matter to attract matter we know
nothing; and the law of conservation makes no statement regarding
that quality. It takes the facts of attraction as they stand, and
affirms only the constancy of working-power. That power may exist
in the form of MOTION; or it may exist in the form of FORCE, with
distance to act through. The former is dynamic energy, the latter
is potential energy, the constancy of the sum of both being affirmed
by the law of conservation. The convertibility of natural forces
consists solely in transformations of dynamic into potential, and of
potential into dynamic, energy, which are incessantly going on.
In no other sense has the convertibility of force, at present,
any scientific meaning.
By the contraction of a muscle a man lifts a weight from the earth.
But the muscle can contract only through the oxidation of its own
tissue or of the blood passing through it. Molecular motion is thus
converted into mechanical motion. Supposing the muscle to contract
without raising the weight, oxidation would also occur, but the
whole of the heat produced by this oxidation would be liberated in
the muscle itself. Not so when it performs external work; to do
that work a certain definite portion of the heat of oxidation must
be expended. It is so expended in pulling the weight away from the
earth. If the weight be permitted to fall, the heat generated by
its collision with the earth would exactly make up for that lacking
in the muscle during the lifting of the weight. In the case here
supposed, we have a conversion of molecular muscular action into
potential energy of gravity; and a conversion of that potential
energy into heat; the heat, however, appearing at a distance from
its real origin in the muscle. The whole process consists of a
transference of molecular motion from the muscle to the weight,
and gravitating force is the mere go-between, by means of which the
transference is effected.
These considerations will help to clear our way to the conception of
the transformations which occur when a wire is moved across the
lines of force in a magnetic field. In this case it is commonly
said we have a conversion of magnetism into electricity. But let us
endeavour to understand what really occurs. For the sake of
simplicity, and with a view to its translation into a different one
subsequently, let us adopt for a moment the provisional conception
of a mixed fluid in the wire, composed of positive and negative
electricities in equal quantities, and therefore perfectly
neutralizing each other when the wire is still. By the motion of
the wire, say with the hand, towards the magnet, what the Germans
call a Scheidungs-Kraft--a separating force--is brought into play.
This force tears the mixed fluids asunder, and drives them in two
currents, the one positive and the other negative, in two opposite
directions through the wire. The presence of these currents evokes
a force of repulsion between the magnet and the wire; and to cause
the one to approach the other, this repulsion must be overcome.
The overcoming of this repulsion is, in fact, the work done in
separating and impelling the two electricities. When the wire is
moved away from the magnet, a Scheidungs-Kraft, or separating force,
also comes into play; but now it is an attraction that has to be
surmounted. In surmounting it, currents are developed in directions
opposed to the former; positive takes the place of negative, and
negative the place of positive; the overcoming of the attraction
being the work done in separating and impelling the two
electricities.
The mechanical action occurring here is different from that
occurring where a sphere of soft iron is withdrawn from a magnet,
and again attracted. In this case muscular force is expended during
the act of separation; but the attraction of the magnet effects the
reunion. In the case of the moving wire also we overcome a
resistance in separating it from the magnet, and thus far the action
is mechanically the same as the separation of the sphere of iron.
But after the wire has ceased moving, the attraction ceases; and so
far from any action occurring similar to that which draws the iron
sphere back to the magnet, we have to overcome a repulsion to bring
them together.
There is no potential energy conferred either by the removal or by
the approach of the wire, and the only power really transformed or
converted, in the experiment, is muscular power. Nothing that could
in strictness be called a conversion of magnetism into electricity
occurs. The muscular oxidation that moves the wire fails to produce
within the muscle its due amount of heat, a portion of that heat,
equivalent to the resistance overcome, appearing in the moving wire
instead.
Is this effect an attraction and a repulsion at a distance? If so,
why should both cease when the wire ceases to move? In fact, the
deportment of the wire resembles far more that of a body moving in a
resisting medium than anything else; the resistance ceasing when the
motion is suspended. Let us imagine the case of a liquid so mobile
that the hand may be passed through it to and fro, without
encountering any sensible resistance. It resembles the motion of a
conductor in the unexcited field of an electro-magnet. Now, let us
suppose a body placed in the liquid, or acting on it, which confers
upon it the property of viscosity; the hand would no longer move
freely. During its motion, but then only, resistance would be
encountered and overcome. Here we have rudely represented the case
of the excited magnetic field, and the result in both cases would be
substantially the same. In both cases heat would, in the end, be
generated outside of the muscle, its amount being exactly equivalent
to the resistance overcome.
Let us push the analogy a little further; suppose in the case of the
fluid rendered viscous, as assumed a moment ago, the viscosity not
to be so great as to prevent the formation of ripples when the hand
is passed through the liquid. Then the motion of the hand, before
its final conversion into heat, would exist for a time as
wave-motion, which, on subsiding, would generate its due equivalent
of heat. This intermediate stage, in the case of our moving wire,
is represented by the period during which the electric current is
flowing through it; but that current, like the ripples of our
liquid, soon subsides, being, like them, converted into heat.
Do these words shadow forth anything like the reality? Such
speculations cannot be injurious if they are enunciated without
dogmatism. I do confess that ideas such as these here indicated
exercise a strong fascination on my mind. Is then the magnetic
field really viscous, and if so, what substance exists in it and the
wire to produce the viscosity? Let us first look at the proved
effects, and afterwards turn our thoughts back upon their cause.
When the wire approaches the magnet, an action is evoked within it,
which travels through it with a velocity comparable to that of light.
One substance only in the universe has been hitherto proved
competent to transmit power at this velocity; the luminiferous
ether. Not only its rapidity of progression, but its ability to
produce the motion of light and heat, indicates that the electric
current is also motion.[1] Further, there is a striking resemblance
between the action of good and bad conductors as regards electricity,
and the action of diathermanous and adiathermanous bodies as regards
radiant heat. The good conductor is diathermanous to the electric
current; it allows free transmission without the development of
heat. The bad conductor is adiathermanous to the electric current,
and hence the passage of the latter is accompanied by the
development of heat. I am strongly inclined to hold the electric
current, pure and simple, to be a motion of the ether alone; good
conductors being so constituted that the motion may be propagated
through their ether without sensible transfer to their atoms, while
in the case of bad conductors this transfer is effected, the
transferred motion appearing as heat.[2]
I do not know whether Faraday would have subscribed to what is here
written; probably his habitual caution would have prevented him from
committing himself to anything so definite. But some such idea
filled his mind and coloured his language through all the later
years of his life. I dare not say that he has been always
successful in the treatment of these theoretic notions. In his
speculations he mixes together light and darkness in varying
proportions, and carries us along with him through strong
alternations of both. It is impossible to say how a certain amount
of mathematical training would have affected his work. We cannot
say what its influence would have been upon that force of
inspiration that urged him on; whether it would have daunted him,
and prevented him from driving his adits into places where no theory
pointed to a lode. If so, then we may rejoice that this strong
delver at the mine of natural knowledge was left free to wield his
mattock in his own way. It must be admitted, that Faraday's purely
speculative writings often lack that precision which the
mathematical habit of thought confers. Still across them flash
frequent gleams of prescient wisdom which will excite admiration
throughout all time; while the facts, relations, principles, and
laws which his experiments have established are sure to form the
body of grand theories yet to come.
Footnotes to Chapter 14
[1] Mr. Clerk Maxwell has recently published an exceedingly
important investigation connected with this question. Even in the
non-mathematical portions of the memoirs of Mr. Maxwell,
the admirable spirit of his philosophy is sufficiently revealed.
As regards the employment of scientific imagery, I hardly know his
equal in power of conception and clearness of definition.
[2] One important difference, of course, exists between the effect
of motion in the magnetic field, and motion in a resisting medium.
In the former case the heat is generated in the moving conductor,
in the latter it is in part generated in the medium.
Chapter 15.
Summary.
When from an Alpine height the eye of the climber ranges over the
mountains, he finds that for the most part they resolve themselves
into distinct groups, each consisting of a dominant mass surrounded
by peaks of lesser elevation. The power which lifted the mightier
eminences, in nearly all cases lifted others to an almost equal
height. And so it is with the discoveries of Faraday. As a general
rule, the dominant result does not stand alone, but forms the
culminating point of a vast and varied mass of inquiry. In this
way, round about his great discovery of Magneto-electric Induction,
other weighty labours group themselves. His investigations on the
Extra Current; on the Polar and other Condition of Diamagnetic
Bodies; on Lines of Magnetic Force, their definite character and
distribution; on the employment of the Induced Magneto-electric
Current as a measure and test of Magnetic Action; on the Revulsive
Phenomena of the magnetic field, are all, notwithstanding the
diversity of title, researches in the domain of Magneto-electric
Induction.
Faraday's second group of researches and discoveries embrace the
chemical phenomena of the current. The dominant result here is the
great law of definite Electro-chemical Decomposition, around which
are massed various researches on Electro-chemical Conduction and on
Electrolysis both with the Machine and with the Pile. To this group
also belongs his analysis of the Contact Theory, his inquiries as to
the Source of Voltaic Electricity, and his final development of the
Chemical Theory of the pile.
His third great discovery is the Magnetization of Light, which I
should liken to the Weisshorn among mountains--high, beautiful,
and alone.
The dominant result of his fourth group of researches is the
discovery of Diamagnetism, announced in his memoir as the Magnetic
Condition of all Matter, round which are grouped his inquiries on
the Magnetism of Flame and Gases; on Magne-crystallic action, and on
Atmospheric Magnetism, in its relations to the annual and diurnal
variation of the needle, the full significance of which is still to
be shown.
These are Faraday's most massive discoveries, and upon them his fame
must mainly rest. But even without them, sufficient would remain to
secure for him a high and lasting scientific reputation. We should
still have his researches on the Liquefaction of Gases; on
Frictional Electricity; on the Electricity of the Gymnotus; on the
source of Power in the Hydro-electric machine, the last two
investigations being untouched in the foregoing memoir; on
Electro-magnetic Rotations; on Regelation; all his more purely
Chemical Researches, including his discovery of Benzol. Besides
these he published a multitude of minor papers, most of which, in
some way or other, illustrate his genius. I have made no allusion
to his power and sweetness as a lecturer. Taking him for all in
all, I think it will be conceded that Michael Faraday was the
greatest experimental philosopher the world has ever seen; and I
will add the opinion, that the progress of future research will
tend, not to dim or to diminish, but to enhance and glorify the
labours of this mighty investigator.
Chapter 16.
Illustrations of Character.
Thus far I have confined myself to topics mainly interesting to the
man of science, endeavouring, however, to treat them in a manner
unrepellent to the general reader who might wish to obtain a notion
of Faraday as a worker. On others will fall the duty of presenting
to the world a picture of the man. But I know you will permit me to
add to the foregoing analysis a few personal reminiscences and
remarks, tending to connect Faraday with a wider world than that of
science--namely, with the general human heart.
One word in reference to his married life, in addition to what has
been already said, may find a place here. As in the former case,
Faraday shall be his own spokesman. The following paragraph, though
written in the third person, is from his hand:--'On June 12, 1821,
he married, an event which more than any other contributed to his
earthly happiness and healthful state of mind. The union has
continued for twenty-eight years and has in no wise changed, except
in the depth and strength of its character.'
Faraday's immediate forefathers lived in a little place called
Clapham Wood Hall, in Yorkshire. Here dwelt Robert Faraday and
Elizabeth his wife, who had ten children, one of them, James
Faraday, born in 1761, being father to the philosopher. A family
tradition exists that the Faradays came originally from Ireland.
Faraday himself has more than once expressed to me his belief that
his blood was in part Celtic, but how much of it was so, or when the
infusion took place, he was unable to say. He could imitate the
Irish brogue, and his wonderful vivacity may have been in part due
to his extraction. But there were other qualities which we should
hardly think of deriving from Ireland. The most prominent of these
was his sense of order, which ran like a luminous beam through all
the transactions of his life. The most entangled and complicated
matters fell into harmony in his hands. His mode of keeping
accounts excited the admiration of the managing board of this
Institution. And his science was similarly ordered. In his
Experimental Researches, he numbered every paragraph, and welded
their various parts together by incessant reference. His private
notes of the Experimental Researches, which are happily preserved,
are similarly numbered: their last paragraph bears the figure 16,041.
His working qualities, moreover, showed the tenacity of the Teuton.
His nature was impulsive, but there was a force behind the impulse
which did not permit it to retreat. If in his warm moments he
formed a resolution, in his cool ones he made that resolution good.
Thus his fire was that of a solid combustible, not that of a gas,
which blazes suddenly, and dies as suddenly away.
And here I must claim your tolerance for the limits by which I am
confined. No materials for a life of Faraday are in my hands, and
what I have now to say has arisen almost wholly out of our close
personal relationship.
Letters of his, covering a period of sixteen years, are before me,
each one of which contains some characteristic utterance;--strong,
yet delicate in counsel, joyful in encouragement, and warm in
affection. References which would be pleasant to such of them as
still live are made to Humboldt, Biot, Dumas, Chevreul, Magnus, and
Arago. Accident brought these names prominently forward; but many
others would be required to complete his list of continental
friends. He prized the love and sympathy of men--prized it almost
more than the renown which his science brought him. Nearly a dozen
years ago it fell to my lot to write a review of his 'Experimental
Researches' for the 'Philosophical Magazine.' After he had read it,
he took me by the hand, and said, 'Tyndall, the sweetest reward of
my work is the sympathy and good will which it has caused to flow in
upon me from all quarters of the world.' Among his letters I find
little sparks of kindness, precious to no one but myself, but more
precious to me than all. He would peep into the laboratory when he
thought me weary, and take me upstairs with him to rest. And if I
happened to be absent, he would leave a little note for me, couched
in this or some other similar form:--
'Dear Tyndall,--I was looking for you, because we were at tea--
we have not yet done--will you come up?' I frequently shared his
early dinner; almost always, in fact, while my lectures were going on.
There was no trace of asceticism in his nature. He preferred the
meat and wine of life to its locusts and wild honey. Never once
during an intimacy of fifteen years did he mention religion to me,
save when I drew him on to the subject. He then spoke to me without
hesitation or reluctance; not with any apparent desire to 'improve
the occasion,' but to give me such information as I sought.
He believed the human heart to be swayed by a power to which science
or logic opened no approach, and, right or wrong, this faith, held in
perfect tolerance of the faiths of others, strengthened and
beautified his life.
From the letters just referred to, I will select three for
publication here. I choose the first, because it contains a passage
revealing the feelings with which Faraday regarded his vocation, and
also because it contains an allusion which will give pleasure to a
friend.
'Royal Institution. [ this is crossed out by Faraday ]
'Ventnor, Isle of Wight, June 28, 1854.
'My Dear Tyndall,--You see by the top of this letter how much habit
prevails over me; I have just read yours from thence, and yet I
think myself there. However, I have left its science in very good
keeping, and I am glad to learn that you are at experiment once
more. But how is the health? Not well, I fear. I wish you would
get yourself strong first and work afterwards. As for the fruits, I
am sure they will be good, for though I sometimes despond as regards
myself, I do not as regards you. You are young, I am old....
But then our subjects are so glorious, that to work at them rejoices
and encourages the feeblest; delights and enchants the strongest.
'I have not yet seen anything from Magnus. Thoughts of him always
delight me. We shall look at his black sulphur together. I heard
from Schonbein the other day. He tells me that Liebig is full of
ozone, i.e., of allotropic oxygen.
'Good-bye for the present.
'Ever, my dear Tyndall,
'Yours truly,
'M. Faraday.'
The contemplation of Nature, and his own relation to her, produced
in Faraday a kind of spiritual exaltation which makes itself
manifest here. His religious feeling and his philosophy could not
be kept apart; there was an habitual overflow of the one into the
other.
Whether he or another was its exponent, he appeared to take equal
delight in science. A good experiment would make him almost dance
with delight. In November, 1850, he wrote to me thus: --'I hope
some day to take up the point respecting the magnetism of associated
particles. In the meantime I rejoice at every addition to the facts
and reasoning connected with the subject. When science is a
republic, then it gains: and though I am no republican in other
matters, I am in that.' All his letters illustrate this catholicity
of feeling. Ten years ago, when going down to Brighton, he carried
with him a little paper I had just completed, and afterwards wrote
to me. His letter is a mere sample of the sympathy which he always
showed to me and my work.
'Brighton, December 9, 1857.
'My Dear Tyndall,--I cannot resist the pleasure of saying how very
much I have enjoyed your paper. Every part has given me delight.
It goes on from point to point beautifully. You will find many
pencil marks, for I made them as I read. I let them stand, for
though many of them receive their answer as the story proceeds, yet
they show how the wording impresses a mind fresh to the subject, and
perhaps here and there you may like to alter it slightly, if you
wish the full idea, i.e., not an inaccurate one, to be suggested at
first; and yet after all I believe it is not your exposition, but
the natural jumping to a conclusion that affects or has affected my
pencil.
'We return on Friday, when I will return you the paper.
'Ever truly yours,
'M. Faraday.'
The third letter will come in its proper place towards the end.
While once conversing with Faraday on science, in its relations to
commerce and litigation, he said to me, that at a certain period of
his career, he was forced definitely to ask himself, and finally to
decide whether he should make wealth or science the pursuit of his
life. He could not serve both masters, and he was therefore
compelled to choose between them. After the discovery of
magneto-electricity his fame was so noised abroad, that the
commercial world would hardly have considered any remuneration too
high for the aid of abilities like his. Even before he became so
famous, he had done a little 'professional business.' This was the
phrase he applied to his purely commercial work. His friend,
Richard Phillips, for example, had induced him to undertake a number
of analyses, which produced, in the year 1830, an addition to his
income of more than a thousand pounds; and in 1831 a still greater
addition. He had only to will it to raise in 1832 his professional
business income to 5000L. a year. Indeed double this sum would be
a wholly insufficient estimate of what he might, with ease, have
realised annually during the last thirty years of his life.
While restudying the Experimental Researches with reference to the
present memoir, the conversation with Faraday here alluded to came
to my recollection, and I sought to ascertain the period when the
question, 'wealth or science,' had presented itself with such
emphasis to his mind. I fixed upon the year 1831 or 1832, for it
seemed beyond the range of human power to pursue science as he had
done during the subsequent years, and to pursue commercial work at
the same time. To test this conclusion I asked permission to see
his accounts, and on my own responsibility, I will state the result.
In 1832, his professional business income, instead of rising to
5000L., or more, fell from 1090L. 4s. to 155L. 9s. From this it
fell with slight oscillations to 92L. in 1837, and to zero in 1838.
Between 1839 and 1845, it never, except in one instance, exceeded
22L.; being for the most part much under this. The exceptional year
referred to was that in which he and Sir Charles Lyell were engaged
by Government to write a report on the Haswell Colliery explosion,
and then his business income rose to 112L. From the end of 1845 to
the day of his death, Faraday's annual professional business income
was exactly zero. Taking the duration of his life into account,
this son of a blacksmith, and apprentice to a bookbinder, had to
decide between a fortune of 150,000L. on the one side, and his
undowered science on the other. He chose the latter, and died a
poor man. But his was the glory of holding aloft among the nations
the scientific name of England for a period of forty years.
The outward and visible signs of fame were also of less account to
him than to most men. He had been loaded with scientific honours
from all parts of the world. Without, I imagine, a dissentient
voice, he was regarded as the prince of the physical investigators
of the present age. The highest scientific position in this country
he had, however, never filled. When the late excellent and lamented
Lord Wrottesley resigned the presidency of the Royal Society, a
deputation from the council, consisting of his Lordship, Mr. Grove,
and Mr. Gassiot, waited upon Faraday, to urge him to accept the
president's chair. All that argument or friendly persuasion could
do was done to induce him to yield to the wishes of the council,
which was also the unanimous wish of scientific men. A knowledge of
the quickness of his own nature had induced in Faraday the habit of
requiring an interval of reflection, before he decided upon any
question of importance. In the present instance he followed his
usual habit, and begged for a little time.
On the following morning, I went up to his room and said on entering
that I had come to him with some anxiety of mind. He demanded its
cause, and I responded:--'Lest you should have decided against the
wishes of the deputation that waited on you yesterday.' 'You would
not urge me to undertake this responsibility,' he said. 'I not only
urge you,' was my reply, 'but I consider it your bounden duty to
accept it.' He spoke of the labour that it would involve; urged that
it was not in his nature to take things easy; and that if he became
president, he would surely have to stir many new questions, and
agitate for some changes. I said that in such cases he would find
himself supported by the youth and strength of the Royal Society.
This, however, did not seem to satisfy him. Mrs. Faraday came into
the room, and he appealed to her. Her decision was adverse, and I
deprecated her decision. 'Tyndall,' he said at length, 'I must
remain plain Michael Faraday to the last; and let me now tell you,
that if I accepted the honour which the Royal Society desires to
confer upon me, I would not answer for the integrity of my intellect
for a single year.' I urged him no more, and Lord Wrottesley had a
most worthy successor in Sir Benjamin Brodie.
After the death of the Duke of Northumberland, our Board of Managers
wished to see Mr. Faraday finish his career as President of the
Institution, which he had entered on weekly wages more than half a
century before. But he would have nothing to do with the
presidency. He wished for rest, and the reverent affection of his
friends was to him infinitely more precious than all the honours of
official life.
The first requisite of the intellectual life of Faraday was the
independence of his mind; and though prompt to urge obedience where
obedience was due, with every right assertion of manhood he
intensely sympathized. Even rashness on the side of honour found
from him ready forgiveness, if not open applause. The wisdom of
years, tempered by a character of this kind, rendered his counsel
peculiarly precious to men sensitive like himself. I often sought
that counsel, and, with your permission, will illustrate its
character by one or two typical instances.
In 1855, I was appointed examiner under the Council for Military
Education. At that time, as indeed now, I entertained strong
convictions as to the enormous utility of physical science to
officers of artillery and engineers, and whenever opportunity
offered, I expressed this conviction without reserve. I did not
think the recognition, though considerable, accorded to physical
science in those examinations at all proportionate to its
importance; and this probably rendered me more jealous than I
otherwise should have been of its claims.
In Trinity College, Dublin, a school had been organized with
reference to the Woolwich examinations, and a large number of
exceedingly well-instructed young gentlemen were sent over from
Dublin, to compete for appointments in the artillery and the
engineers. The result of one examination was particularly
satisfactory to me; indeed the marks obtained appeared so eloquent
that I forbore saying a word about them. My colleagues, however,
followed the usual custom of sending in brief reports with their
returns of marks. After the results were published, a leading
article appeared in 'The Times,' in which the reports were largely
quoted, praise being bestowed on all the candidates, except the
excellent young fellows who had passed through my hands.
A letter from Trinity College drew my attention to this article,
bitterly complaining that whereas the marks proved them to be the
best of all, the science candidates were wholly ignored. I tried to
set matters right by publishing, on my own responsibility, a letter
in 'The Times.' The act, I knew, could not bear justification from
the War Office point of view; and I expected and risked the
displeasure of my superiors. The merited reprimand promptly came.
'Highly as the Secretary of State for War might value the expression
of Professor Tyndall's opinion, he begged to say that an examiner,
appointed by His Royal Highness the Commander-in-Chief, had no right
to appear in the public papers as Professor Tyndall has done,
without the sanction of the War Office.' Nothing could be more just
than this reproof, but I did not like to rest under it. I wrote a
reply, and previous to sending it took it up to Faraday. We sat
together before his fire, and he looked very earnest as he rubbed
his hands and pondered. The following conversation then passed
between us:--
F. You certainly have received a reprimand, Tyndall; but the
matter is over, and if you wish to accept the reproof, you will
hear no more about it.
T. But I do not wish to accept it.
F. Then you know what the consequence of sending that letter will be?
T. I do.
F. They will dismiss you.
T. I know it.
F. Then send the letter!
The letter was firm, but respectful; it acknowledged the justice of
the censure, but expressed neither repentance nor regret. Faraday,
in his gracious way, slightly altered a sentence or two to make it
more respectful still. It was duly sent, and on the following day I
entered the Institution with the conviction that my dismissal was
there before me. Weeks, however, passed. At length the well-known
envelope appeared, and I broke the seal, not doubting the contents.
They were very different from what I expected. 'The Secretary of
State for War has received Professor Tyndall's letter, and deems the
explanation therein given perfectly satisfactory.' I have often
wished for an opportunity of publicly acknowledging this liberal
treatment, proving, as it did, that Lord Panmure could discern and
make allowance for a good intention, though it involved an offence
against routine. For many years subsequently it was my privilege to
act under that excellent body, the Council for Military Education.
On another occasion of this kind, having encouraged me in a somewhat
hardy resolution I had formed, Faraday backed his encouragement by
an illustration drawn from his own life. The subject will interest
you, and it is so sure to be talked about in the world, that no
avoidable harm can rise from its introduction here.
In the year 1835, Sir Robert Peel wished to offer Faraday a pension,
but that great statesman quitted office before he was able to
realise his wish. The Minister who founded these pensions intended
them, I believe, to be marks of honour which even proud men might
accept without compromise of independence. When, however, the
intimation first reached Faraday in an unofficial way, he wrote a
letter announcing his determination to decline the pension; and
stating that he was quite competent to earn his livelihood himself.
That letter still exists, but it was never sent, Faraday's
repugnance having been overruled by his friends. When Lord
Melbourne came into office, he desired to see Faraday; and probably
in utter ignorance of the man--for unhappily for them and us,
Ministers of State in England are only too often ignorant of great
Englishmen--his Lordship said something that must have deeply
displeased his visitor. All the circumstances were once
communicated to me, but I have forgotten the details. The term
'humbug,' I think, was incautiously employed by his Lordship, and
other expressions were used of a similar kind. Faraday quitted the
Minister with his own resolves, and that evening he left his card
and a short and decisive note at the residence of Lord Melbourne,
stating that he had manifestly mistaken his Lordship's intention of
honouring science in his person, and declining to have anything
whatever to do with the proposed pension. The good-humoured
nobleman at first considered the matter a capital joke; but he was
afterwards led to look at it more seriously. An excellent lady,
who was a friend both to Faraday and the Minister, tried to arrange
matters between them; but she found Faraday very difficult to move
from the position he had assumed. After many fruitless efforts,
she at length begged of him to state what he would require of Lord
Melbourne to induce him to change his mind. He replied, 'I should
require from his Lordship what I have no right or reason to expect
that he would grant--a written apology for the words he permitted
himself to use to me.' The required apology came, frank and full,
creditable, I thought, alike to the Prime Minister and the
philosopher.
Considering the enormous strain imposed on Faraday's intellect, the
boy-like buoyancy even of his later years was astonishing. He was
often prostrate, but he had immense resiliency, which he brought
into action by getting away from London whenever his health failed.
I have already indicated the thoughts which filled his mind during
the evening of his life. He brooded on magnetic media and lines of
force; and the great object of the last investigation he ever
undertook was the decision of the question whether magnetic force
requires time for its propagation. How he proposed to attack this
subject we may never know. But he has left some beautiful apparatus
behind; delicate wheels and pinions, and associated mirrors, which
were to have been employed in the investigation. The mere conception
of such an inquiry is an illustration of his strength and hopefulness,
and it is impossible to say to what results it might have led him.
But the work was too heavy for his tired brain. It was long before
he could bring himself to relinquish it and during this struggle he
often suffered from fatigue of mind. It was at this period,
and before he resigned himself to the repose which marked the last
two years of his life, that he wrote to me the following letter--
one of many priceless letters now before me--which reveals, more than
anything another pen could express, the state of his mind at the time.
I was sometimes censured in his presence for my doings in the Alps,
but his constant reply was, 'Let him alone, he knows how to take
care of himself.' In this letter, anxiety on this score reveals
itself for the first time.
'Hampton Court, August 1, 1864.
'My Dear Tyndall,--I do not know whether my letter will catch you,
but I will risk it, though feeling very unfit to communicate with a
man whose life is as vivid and active as yours; but the receipt of
your kind letter makes me to know that, though I forget, I am not
forgotten, and though I am not able to remember at the end of a line
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