Chapter V: Scientific Researches: Third Period (1)

Throughout the fruitful ten years of Faraday’s middle period two magistral ideas had slowly grown up in his mind, and as he let his thought play about the objects of his daily activities, these ideas possessed and dominated him as no newly suggested idea could have done. They were the correlation and inter-convertibility of the forces of nature, and the optical relations of magnetism and electricity.

During the period of enforced rest, from 1839 to 1844, these ideas had been ever with him. His was a mind which during times of quiet brooding did not cease to advance. In silence his thoughts arranged themselves in readiness for the next period of activity, and his work, when it began again, was all the more fruitful for the antecedent period of cogitation.

[Sidenote: OPTICAL ANALYSIS.]

On August 30th, 1845, Faraday for the sixth time set to work in his laboratory to search for the connection between light and electricity for which he had so often looked, and about which he had so boldly speculated. He began by looking for some effect to be produced on polarised light by passing it through a liquid which was undergoing electrolysis. What effect precisely he expected to observe is unknown. Doubtless he had an open mind to perceive effects of any kind had such occurred. Earlier in the century the phenomena of polarised light had been worked out in great detail, through a host of beautiful phenomena, by Arago, Biot, Brewster, and others; and their discoveries had shown that this agent is capable of revealing in transparent substances details of structure which otherwise would be quite invisible. Placed between two Nicol prisms or two slices of tourmaline, to serve respectively as “polariser” and “analyser,” thin sheets of transparent crystal--selenite or mica--were made to reveal the fact that they possessed an axis of maximum elasticity. For when the analyser and polariser were set in the “crossed” position, where the one would cut off all the luminous vibrations that the other would transmit, no light would be visible to the observer, unless in the intervening space there were interposed some substance endowed with one of two properties, either that of resolving some part of the vibrations into an oblique direction or else that of rotating the plane of the vibrations to right or to left. If either of these things is done, light appears through the analyser. It is thus that structure is observed in horn and in starch grains. It is thus that the strains in a piece of compressed glass are made visible. It is thus that crystalline structures generally can be studied. It is thus that the discovery was made of the substances which possess the strange property of twisting or rotating the plane of polarisation of light--namely, quartz crystal, solutions of sugar and of certain alkaloids, and certain other liquids, such as turpentine. Such was the agent which Faraday proposed to employ to detect whether electric forces impress any quality resembling that of _structure_ upon transparent materials.

The notes begin with the words:--

“I have had a glass trough made 24 inches long, 1 inch wide and about 1½ deep, in which to decompose electrolites and, whilst under decomposition, along which I could pass a ray of light in different conditions and afterwards examine it.”

He put into this trough two platinum electrodes and a solution of sulphate of soda, but could find no effects. Eight pages of the notebook are filled with details all leading to negative results. For ten days he worked at these experiments with liquid electrolytes. The substances used were distilled water, solution of sugar, dilute sulphuric acid, solution of sulphate of soda (using platinum electrodes), and solution of sulphate of copper (using copper electrodes). The current was sent along the ray, and perpendicular to it in two directions at right angles with each other. The ray was made to rotate, by altering the position of the polariser (in this case a black-glass mirror at the proper angle), so that the plane of polarisation might be varied. The current was used as a continuous current, as a rapidly intermitting current, and as a rapidly alternating induction current; _but in no case was any trace of action perceived_.

[Sidenote: A DIFFICULT RESEARCH.]

Then he turned to solid dielectrics to see if under electric strain they would yield any optical effect. He had indeed so far back as 1838 tried the experiment of coating two opposite faces of a glass cube with metal foil plates that were then electrified by a powerful electric machine. But the experiment had no result. This experiment he now repeats with a score of elaborate variations, trying both crystalline and non-crystalline dielectrics. Rock-crystal, Iceland spar, flint glass, heavy-glass, turpentine, and air, had a beam of polarised light passed through them, and at the same time “lines of electrostatic tension” were, by means of the coatings, Leyden jars, and the electric machine, directed across these bodies, both parallel to the polarised ray and across it, both in and across the plane of polarisation; but again without any visible effect. Then he tries on the same bodies, and on water, the “tension” of a rapidly alternating induced current, but still with the same negative result. Professor Tyndall stated that from conversation with Faraday, and with his faithful assistant Anderson, he inferred that the labour expended on this preliminary and apparently fruitless research was very great. It occupies many pages of the laboratory notebook. That thirty-two years later Dr. Kerr succeeded in finding this optical effect of electrostatic strain for which Faraday vainly sought, is no reflection upon Faraday’s powers of observation. Had there been no Faraday there had doubtless been no discovery by Kerr.

So far the quest had been carried on either with electric currents flowing through the transparent substance or else with mere statical electric forces, and a whole fortnight had been spent without result. Now another track is taken, and it leads straight to success. He substitutes magnetic for electric forces.

[Sidenote: MAGNETO-OPTIC DISCOVERY.]

“13th Sept. 1845.

“To-day worked with lines of magnetic force, passing them across different bodies transparent in different directions, and at the same time passing a polarized ray of light through them, and afterwards examining the ray by a Nichol’s Eye-piece or other means. The magnets were Electro-magnets one being our large cylinder Electro-magnet and the other a temporary iron core put into the helix on a frame. This was not nearly so strong as the former. The current of 5 cells of Grove’s battery was sent through both helices at once and the magnets were made and unmade by putting in or stopping off the electric current.” Air, flint-glass, rock-crystal, calcareous spar, were examined, but without effect. And so he worked on through the morning, trying first one specimen, then another, altering the directions of the poles of his magnets, reversing their polarity, changing the position of his optical apparatus, increasing the battery-power of his magnetising current. Then he bethinks him of that “heavy-glass”--the boro-silicate of lead--which had cost him nearly four years of precious labour during the first period of his scientific life. The entry in the notebook is characteristic.

“A piece of heavy glass, which was 2 inches by 1·8 inches and 0·5 of an inch thick, being a silico-borate of lead, was experimented with. It gave no effects when the _same magnetic poles_ or the _contrary_ poles were on opposite sides (as respects the course of the polarised ray);--nor when the same poles were on the same side either with the constant or intermitting current; =BUT= when contrary magnetic poles were on the same side there _was an effect produced on the polarised ray_, and thus magnetic force and light were proved to have relations to each other. This fact will most likely prove exceedingly fertile, and of great value in the investigation of conditions of natural force.

“The effect was of this kind. The glass, a result of one of my old experiments on optical glass, had been exceedingly well annealed so that it did not in any degree affect the polarized ray. The two magnetic poles were in a horizontal plane, and the piece of glass put up flat against them so that the polarized ray could pass through its edges and be examined by the eye at a Nicholl’s eye piece. In its natural state the glass had no effect on the polarized ray but on making contact at the battery so as to render the cores N and S magnets instantly the glass acquired a certain degree of _power of depolarizing the ray_ which it retained steadily as long as the cores were magnets but which it lost the instant the electric current was stopped. Hence it was a permanent condition and as was expected did not sensibly appear with an intermitting current.

“The effect was not influenced by any jogging motion or any moderate pressure of the hands on the glass.

“The heavy glass had tinfoil coatings on its two sides but when these were taken off the effect remained exactly the same.

“A mass of soft iron on the outside of the _heavy glass_ greatly _diminished_ the effect [see Fig. 17]....

“All this shews that it is when the _polarized ray_ passes _parallel_ to the _lines of magnetic induction_ or rather to the _direction of the magnetic curves_, that the glass manifests its power of affecting the ray. So that the heavy glass in its magnetized state corresponds to the cube of rock crystal: the direction of the magnetic curves in the piece of glass corresponding to the direction of the optic axis in the crystal (see Exp. Researches 1689–1698)....

“Employed our large _ring electro-magnet_ which is very powerful and has of course the poles in the right [position] only they are very close not more than [0·5] of an inch apart. When the _heavy glass_ was put up against it the effect was produced better than in any former case....

[Sidenote: ENOUGH FOR TO-DAY.]

“Have got enough for to-day.”

The description which he published in the “Researches” of the first successful experiment is as follows:--

“A piece of this glass about 2 inches square and 0·5 of an inch thick, having flat and polished edges, was placed as a _diamagnetic_[47] 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 indifferent substance would do; and if the eye-piece [_i.e._ analyzer] 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 this glass made no alteration in that respect. In this state of circumstances the force of the electromagnet 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 phænomena could be renewed at pleasure, at any instant of time, and upon any occasion, showing a perfect dependence of cause and effect.”

He paused for four days in order to procure more powerful electromagnets, for the effect which he had observed was exceedingly slight: “A person looking for the phænomenon for the first time would not be able to see it with a weak magnet.”

The entry in the notebook begins again:--

“18th Sept. 1845.

“Have now borrowed and received the Woolwich magnet.”

[Sidenote: AN EXCELLENT DAY’S WORK.]

This was a more powerful electromagnet than that at the Institution. With this he sets to work with such energy that twelve pages of the laboratory book are filled in one day. His thoughts had ripened during the five days, and he advanced rapidly from point to point. The first experiment with the Woolwich magnet brings out another point, of which at once he grasped the significance:--

“Heavy Glass (original, or 174[48]) when placed thus produced a very fine effect. The brightness of the image produced rose gradually not instantly, due to this that the iron cores do not take their full intensity of magnetic state at once but require time, and so the magnetic curves rise in intensity. In this way the effect is one by which an optical examination of the electromagnet can be made--and the time necessary clearly shewn.”

He next ascertains definitely that the phenomenon is one of rotatory polarisation--that is to say, the action of the magnet is to twist and rotate the plane of polarisation through a definite angle depending on the strength of the magnet and the direction of the exciting current. He finds the direction of the rotation, and verifies it by comparison with the ordinary optical rotation produced by turpentine and by a solution of sugar, winding up with the words:--

“_An excellent day’s work._”

For four days he went on accumulating proofs, and now succeeding with the very substances with which he formerly failed. On September 26th he tried the conjoint effect of a magnetic and an electric field. He also tried the effect of a current running along a transparent liquid electrolytically whilst the magnet was in operation. The only results appeared to be those due to the magnet alone. For six days in October the experiments were continued. He noted, as a desideratum, a transparent oxide of iron. “With some degree of curiosity and hope” he “put gold leaf into the magnetic lines, but could perceive no effect.” He was instinctively looking for the phenomenon which Kundt later discovered as a property of thin transparent films of iron. He entered amongst the speculative suggestions in his notebook the query: “Does this [magnetic] force tend to make iron and oxide of iron transparent?” On October 3rd he tried experiments on light reflected from the surface of metals placed in the magnetic field. He indeed obtained an optical rotation by reflection at the surface of a polished steel button, but the results were inconclusive owing to imperfection of the surface. It was reserved for Dr. Kerr to rediscover and follow up this effect. On October 6th he looked for mechanical and magnetic effects on pieces of heavy-glass and on liquids in glass bulbs placed between the poles of his magnet, but found none. He also looked for possible effects of rapid motion given to the diamagnetic while jointly subject to the action of magnetism and the light, but found none.

[Sidenote: UNFULFILLED EXPECTATIONS.]

On October 11th he thinks he has got hold of another new fact when experimenting on liquids in a long glass tube, the record of it filling three pages. But two days afterwards he finds it only a disturbing effect due to the communication of heat to the liquid from the surrounding magnetising coil. He seems to regret the loss of the new fact, but adds: “As to the other phenomenon of circular polarization, that comes out constant, clear, and beautiful.”

Then, with that idea of the correlation of forces always in his head, there recurs to him the notion that if magnetism or electric currents can affect a beam of light, there must be some sort of converse phenomenon, and that in some way or other light must be able to electrify or to magnetise. Thirty-one years before, when visiting Rome with Davy, he had witnessed the experiments of Morichini on the alleged magnetic effect of violet light, and had remained unconvinced. His own idea is very different. And October 14th being a bright day with good sunlight, he makes the attempt. Selecting a very sensitive galvanometer, he connects it to a spiral of wire 1 inch in diameter, 4·2 inches long, of 56 convolutions, and then directs a beam of sunlight along its axis. He tries letting the beam pass alternately through the coil while the outside is covered, and then along the exterior while the interior is shaded. But still there is no effect. Then he inserts an unmagnetised steel bar within the coil, and rotates it while it is exposed to the sun’s rays. Still there is no effect, and the sun goes down on another of the unfulfilled expectations. But had he lived to learn of the effect of light in altering the electric resistance of selenium discovered by Mayhew, of the photoelectric currents discovered by Becquerel, of the discharging action of ultra-violet light discovered by Hertz, of the revivifying effect of light on recently demagnetised iron discovered by Bidwell, he would have rejoiced that such other correlations should have been found, though different from that which he sought. On November 3rd he receives a new horseshoe magnet, with which he hoped to find some optical effect on air and other gases, but again without result. That the magnetism of the earth does actually rotate the plane of polarisation of sky light was the discovery of Henri Becquerel so late as 1878.

Faithful to his own maxim: “Work, finish, publish,” Faraday lost no time in writing out his research. It was presented to the Royal Society on November 6th, but the main result was verbally mentioned on November 3rd at the monthly meeting of the Royal Institution, and reported in the _Athenæum_ of November 8th, 1845.

But even before the memoir was thus given to the world another discovery had been made. For on November 4th with the new magnet he repeated an experiment which a month previously had been without result. So preoccupied was he over the new event that he did not even go to the meeting of the Royal Society on November 20th, when his paper on the “Action of Magnets on Light” was read. What that new discovery was is well told by Faraday himself in the letter which he sent to Professor A. de la Rive on December 4th:--

[Sidenote: FRESH MAGNETIC DISCOVERY.]

[_Faraday to Professor Aug. de la Rive._]

Brighton, December 4, 1845.

MY DEAR FRIEND,-- * * * I count upon you as one of those whose
free hearts have pleasure in my success, and I am very grateful
to you for it. I have had your last letter by me on my desk
for several weeks, intending to answer it; but absolutely I
have not been able, for of late I have shut myself up in my
laboratory and wrought, to the exclusion of everything else. I
heard afterwards that even your brother had called on one of
these days and been excluded.

Well, a part of this result is that which you have heard,
and my paper was read to the Royal Society, I believe, last
Thursday, for I was not there; and I also understand there have
been notices in the _Athenæum_, but I have not had time to see
them, and I do not know how they are done. However, I can refer
you to the _Times_ of last Saturday (November 29th) for a very
good abstract of the paper. I do not know who put it in, but it
is well done, though brief. To that account, therefore, I will
refer you.

For I am still so involved in discovery that I have hardly
time for my meals, and am here at Brighton both to refresh
and work my head at once, and I feel that unless I had been
here, and been careful, I could not have continued my labours.
The consequence has been that last Monday I announced to our
members at the Royal Institution another discovery, of which I
will give you the pith in a few words. The paper will go to the
Royal Society next week, and probably be read as shortly after
as they can there find it convenient.

Many years ago I worked upon optical glass, and made a vitreous
compound of silica, boracic acid, and lead, which I will
now call heavy glass, and which Amici uses in some of his
microscopes; and it was this substance which enabled me first
to act on light by magnetic and electric forces. Now, if a
square bar of this substance, about half an inch thick and
two inches long, be very freely suspended between the poles
of a powerful horse-shoe electro-magnet, immediately that
the magnetic force is developed, the bar points; but it does
not point from pole to pole, but equatorially or across the
magnetic lines of force--_i.e._ east and west in respect of
the north and south poles. If it be moved from this position
it returns to it, and this continues as long as the magnetic
force is in action. This effect is the result of a still
simpler action of the magnet on the bar than what appears by
the experiment, and which may be obtained at a single magnetic
pole. For if a cubical or rounded piece of the glass be
suspended by a fine thread six or eight feet long, and allowed
to hang very near a strong magneto-electric pole (not as yet
made active), then on rendering the pole magnetic the glass
will be repelled, and continue repelled until the magnetism
ceases. This effect or power I have worked out through a great
number of its forms and strange consequences, and they will
occupy two series of the “Experimental Researches.” It belongs
to _all matter_ (not magnetic, as iron) without exception,
so that every substance belongs to the one or the other
class--magnetic or diamagnetic bodies. The law of action in
its simple form is that such matter tends to go from strong to
weak points of magnetic force, and in doing this the substance
will go in either direction along the magnetic curves, or in
either direction across them. It is curious that amongst the
metals are found bodies possessing this property in as high a
degree as perhaps any other substance. In fact, I do not know
at present whether heavy glass, or bismuth, or phosphorus is
the most striking in this respect. I have very little doubt
that you have an electro-magnet strong enough to enable you to
verify the chief facts of pointing equatorially and repulsion,
if you will use bismuth carefully examined as to its freedom
from magnetism, and making of it a bar an inch and a half long,
and one-third or one-fourth of an inch wide. Let me, however,
ask the favour of your keeping this fact to yourself for two
or three weeks, and preserving the date of this letter as a
record. I ought (in order to preserve the respect due to the
Royal Society) not to write a description to anyone until the
paper has been received or even read there. After three weeks
or a month I think you may use it, guarding, as I am sure you
will do, my right. And now, my dear friend, I must conclude,
and hasten to work again. But first give my kindest respects to
Madame de la Rive, and many thanks to your brother for his call.

Ever your obedient and affectionate friend,

M. FARADAY.

[Sidenote: MAGNETIC EXPERIMENTS.]

The discovery of diamagnetism which Faraday thus announced was in itself a notable achievement. As Tyndall points out, the discovery itself was in all probability due to Faraday’s habit of not regarding as final any negative result of an experiment until he had brought to bear upon it the most powerful resources at his command. He had tried the effects of ordinary magnets on brass and copper and other materials commonly considered as non-magnetic. But when, for the purpose of the preceding research on the relation of magnetism to light, he had deliberately procured electromagnets of unusual power, he again tried what their effect might be upon non-magnetic stuffs. Suspending a piece of his heavy glass near the poles in a stirrup of writing-paper slung upon the end of a long thread of cocoon silk, he found it to experience a strong mechanical action when the magnet was stimulated by turning on the current. His precision of description is characteristic:--

I shall have such frequent occasion to refer to two chief
positions of position across the magnetic field, that, to
avoid periphrasis, I will here ask leave to use a term or two
conditionally. One of these directions is that from pole to
pole, or along the lines of magnetic force, I will call it the
axial direction; the other is the direction perpendicular to
this, and across the line of magnetic force and for the time,
and as respects the space between the poles, I will call it the
_equatorial_ direction.

Note the occurrence in the above passage for the first time of the term “the magnetic field.” Faraday’s description of the discovery continues as follows:--

The bar of silicated borate of lead or heavy glass already
described as the substance in which magnetic forces were first
made effectually to bear on a ray of light, and which is 2
inches long, and about 0·5 inch wide and thick, was suspended
centrally between the magnetic poles, and left until the effect
of torsion was over. The magnet was then thrown into action
by making contact at the voltaic battery. Immediately the bar
moved, turning round its point of suspension, into a position
across the magnetic curve or line of force, and, after a few
vibrations, took up its place of rest there. On being displaced
by hand from this position it returned to it, and this occurred
many times in succession.

Either end of the bar indifferently went to either side of the
axial line. The determining circumstance was simply inclination
of the bar one way or the other to the axial line at the
beginning of the experiment. If a particular or marked end of
the bar were on one side of the magnetic or axial line when
the magnet was rendered active, that end went further outwards
until the bar had taken up the equatorial position....

Here, then, we have a magnetic bar which points east and
west in relation to north and south poles--_i.e._ points
perpendicularly to the lines of magnetic force....

[Sidenote: DIAMAGNETIC LAWS.]

To produce these effects of pointing across the magnetic
curves, the form of the heavy glass must be long. A cube or
a fragment approaching roundness in form will not point, but
a long piece will. Two or three rounded pieces or cubes,
placed side by side in a paper tray, so as to form an oblong
accumulation, will also point.

Portions, however, of any form are _repelled_; so if two pieces
be hung up at once in the axial line, one near each pole, they
are repelled by their respective poles, and approach, seeming
to attract each other. Or if two pieces be hung up in the
equatorial line, one on each side of the axis, then they both
recede from the axis, seeming to repel each other.

From the little that has been said, it is evident that the
bar presents in its motion a complicated result of the force
exerted by the magnetic power over the heavy glass, and that
when cubes or spheres are employed a much simpler indication of
the effect may be obtained. Accordingly, when a cube was thus
used with the two poles, the effect was repulsion or recession
from either pole, and also recession from the magnetic axis on
either side.

So the indicating particle would move either along the magnetic
curves or across them, and it would do this either in one
direction or the other, the only constant point being that its
tendency was to move from stronger to weaker places of magnetic
force.

This appeared much more simply in the case of a single magnetic
pole, for then the tendency of the indicating cube or sphere
was to move outwards in the direction of the magnetic lines
of force. The appearance was remarkably like a case of weak
electric repulsion.

The cause of the pointing of the bar, or any oblong arrangement
of the heavy glass, is now evident. It is merely a result of
the tendency of the particles to move outwards, or into the
positions of weakest magnetic action.

* * * * *

When the bar of heavy glass is immersed in water, alcohol,
or æther, contained in a vessel between the poles, all the
preceding effects occur--the bar points and the cube recedes
exactly in the same manner as in air.

The effects equally occur in vessels of wood, stone, earth,
copper, lead, silver, or any of those substances which belong
to the diamagnetic class.

I have obtained the same equatorial direction and motions of
the heavy glass bar as those just described, but in a very
feeble degree, by the use of a good common steel horseshoe
magnet.

Then he goes on to enumerate the many bodies of all sorts: crystals, powders, liquids, acids, oils; organic bodies such as wax, olive-oil, wood, beef (fresh and dry), blood, apple, and bread, all of which were found to be diamagnetic. On this he remarks:--

It is curious to see such a list as this of bodies presenting
on a sudden this remarkable property, and it is strange to find
a piece of wood, or beef, or apple, obedient to or repelled by
a magnet. If a man could be suspended with sufficient delicacy
after the manner of Dufay, and placed in the magnetic field, he
would point equatorially, for all the substances of which he is
formed, including the blood, possess this property.

[Sidenote: THE MAGNETIC BRAKE.]

A few bodies were found to be feebly magnetic, including paper, sealing-wax, china ink, asbestos, fluorspar, peroxide of lead, tourmaline, plumbago, and charcoal. As to the metals, he found iron, cobalt, and nickel to stand in a distinct class. A feeble magnetic action in platinum, palladium, and titanium was suspected to be due to traces of iron in them. Bismuth proved to be the most strongly diamagnetic, and was specially studied. The repellent effect between bismuth and a magnet had indeed been casually observed twice in the prior history of science, first by Brugmans, then by Le Baillif. Faraday, with characteristic frankness, refers to his having a “vague impression” that the repulsion of bismuth by a magnet had been observed before, though unable at the time of writing to recollect any reference. His own experiments ran over the whole range of substances, however, and demonstrated the universal existence in greater or less degree of this magnetic nature. Certain differences observed between the behaviour of bismuth and of heavy glass on the one hand, and of copper on the other hand, though all are diamagnetic, led him to note and describe some of the pseudo-diamagnetic effects which occur in copper and silver, in consequence of the induction in them of eddy-currents, from which heavy-glass and bismuth are, by their inferior electric conductivity, comparatively free. He described the beautiful and now classical experiment of arresting, by turning on the exciting current, the rotation of a copper cylinder spinning between the poles of an electromagnet.

Faraday continued to prosecute this newest line of research, and at the end of December, 1845, presented another memoir (the twenty-first series of the Experimental Researches) to the Royal Society. He had now examined the salts of iron, and had found that every salt and compound containing iron in the basic part was magnetic, both in the solid and in the liquid state. Even prussian-blue and green bottle-glass were magnetic. The solutions of the salts of iron were of special importance, since they furnish the means of making a magnet which is for the time liquid, transparent, and, within certain limits, adjustable in strength. His next step was to examine how bodies behaved when immersed in some surrounding medium. A weak solution of iron, enclosed in a very thin glass tube, though it pointed axially when hung in air, pointed equatorially when immersed in a stronger solution. A tube full of air pointed axially, and was attracted as if magnetic when surrounded with water. Substances such as bismuth, copper, and phosphorus are, however, highly diamagnetic when suspended _in vacuo_. Such a view would make _mere space_ magnetic. Hence Faraday inclined at first to the opinion that diamagnetics had a specific action antithetically distinct from ordinary magnetic action. Several times he pointed out that all the phenomena resolve themselves simply into this, that a portion of such matter as is termed diamagnetic tends to move from stronger to places or points of weaker force in the magnetic field. He does, indeed, hazard the suggestion that the phenomena might be explained on the assumption that there was a sort of diamagnetic polarity--that magnetic induction caused in them a contrary state to that which it produced in ordinary magnetic matter. But his own experiments failed to support this view, and, in opposition to Weber and Tyndall, he maintained afterwards the non-polar nature of diamagnetic action.

In 1846 Faraday gave two Friday night discourses on these magnetic researches, one on the cohesive force of water, and one on Wheatstone’s electromagnetic chronoscope. At the conclusion of the last-named he said that he was induced to utter a speculation which had long been gaining strength in his mind, that perhaps those vibrations by which radiant energies, such as light, heat, actinic rays, etc., convey their force through space are not mere vibrations of an æther, but of the lines of force which, in his view, connect different masses, and so was inclined, in his own phrase, “to dismiss the æther.” In one of his other discourses he made the suggestion that we might “perhaps hereafter obtain magnetism from light.”

[Sidenote: THOUGHTS ON RAY VIBRATIONS.]

The speculation above referred to is of such intrinsic importance, in view of the developments of the last decade, that it compels further notice. Faraday himself further expanded it in a letter to Richard Phillips, which was printed in the _Philosophical Magazine_ for May, 1846, under the title “Thoughts on Ray-vibrations.” In this avowedly speculative paper Faraday touched the highest point in his scientific writings, and threw out, though in a tentative and fragmentary way, brilliant hints of that which his imagination had perceived, as in a vision;--the doctrine now known as the electromagnetic theory of light. At the dates when the earlier biographies of Faraday appeared, neither that doctrine nor this paper had received the recognition due to its importance. Tyndall dismisses it as “one of the most singular speculations that ever emanated from a scientific man.” Bence Jones just mentions it in half a line. Dr. Gladstone does not allude to it. It therefore seems expedient to give here some extracts from the letter itself:--

THOUGHTS ON RAY-VIBRATIONS.

_To Richard Phillips, Esq._

DEAR SIR,--At your request, I will endeavour to convey to you a
notion of that which I ventured to say at the close of the last
Friday evening meeting ...; but, from first to last, understand
that I merely threw out, as matter for speculation, the vague
impressions of my mind, for I gave nothing as the result of
sufficient consideration, or as the settled conviction, or even
probable conclusion at which I had arrived.

The point intended to be set forth for the consideration
of the hearers was whether it was not possible that the
vibrations--which in a certain theory are assumed to account
for radiation and radiant phenomena--may not occur in the lines
of force which connect particles, and consequently masses, of
matter together--a notion which, as far as it is admitted, will
dispense with the æther, which, in another view, is supposed to
be the medium in which these vibrations take place.

* * * * *

Another consideration bearing conjointly on the hypothetical
view, both of matter and radiation, arises from the comparison
of the velocities with which the radiant action and certain
powers of matter are transmitted. The velocity of light through
space is about 190,000 miles[49] a second. The velocity of
electricity is, by the experiments of Wheatstone, shown to be
as great as this, if not greater. The light is supposed to be
transmitted by vibrations through an æther which is, so to
speak, destitute of gravitation, but infinite in elasticity;
the electricity is transmitted through a small metallic wire,
and is often viewed as transmitted by vibrations also. That
the electric transference depends on the forces or powers of
the matter of the wire can hardly be doubted when we consider
the different conductibility of the various metallic and
other bodies, the means of affecting it by heat or cold, the
way in which conducting bodies by combination enter into the
constitution of non-conducting substances, and the contrary,
and the actual existence of one elementary body (carbon)
both in the conducting and non-conducting state. The power
of electric conduction, being a transmission of force equal
in velocity to that of light, appears to be tied up in and
dependent upon the properties of the matter, and is, as it
were, existent in them.

* * * * *

[Sidenote: LATERAL VIBRATIONS.]

In experimental philosophy we can, by the phenomena presented,
recognise various kinds of lines of force. Thus there are the
lines of gravitating force, those of electrostatic induction,
those of magnetic action, and others partaking of a dynamic
character might be perhaps included. The lines of electric
and magnetic action are by many considered as exerted through
space like the lines of gravitating force. For my own part, I
incline to believe that when there are intervening particles
of matter--being themselves only centres of force--they take
part in carrying on the force through the line, but that when
there are none the line proceeds through space. Whatever the
view adopted respecting them may be, we can, at all events,
affect these lines of force in a manner which may be conceived
as partaking of the nature of a shake or lateral vibration.
For suppose two bodies, A B, distant from each other, and
under mutual action,[50] and therefore connected by lines of
force, and let us fix our attention upon one resultant of force
having an invariable direction as regards space; if one of
the bodies move in the least degree right or left, or if its
power be shifted for a moment within the mass (neither of these
cases being difficult to realise if A or B be either electric
or magnetic bodies), then an effect equivalent to a lateral
disturbance will take place in the resultant upon which we are
fixing our attention, for either it will increase in force
whilst the neighbouring resultants are diminishing, or it will
fall in force while they are increasing.

* * * * *

The view which I am so bold as to put forth considers,
therefore, radiation as a high species of vibration in the
lines of force which are known to connect particles, and also
masses, of matter together. It endeavours to dismiss the æther,
but not the vibrations. The kind of vibration which, I believe,
can alone account for the wonderful, varied, and beautiful
phenomena of polarisation is not the same as that which occurs
on the surface of disturbed water or the waves of sound in
gases or liquids, for the vibrations in these cases are direct,
or to and from the centre of action, whereas the former are
lateral. It seems to me that the resultant of two or more lines
of force is in an apt condition for that action, which may be
considered as equivalent to a _lateral_ vibration; whereas a
uniform medium like the æther does not appear apt, or more apt
than air or water.

The occurrence of a change at one end of a line of force easily
suggests a consequent change at the other. The propagation of
light, and therefore probably of all radiant action, occupies
_time_; and that a vibration of the line of force should
account for the phenomena of radiation, it is necessary that
such vibration should occupy time also.

* * * * *

[Sidenote: THE SHADOW OF A SPECULATION.]

And now, my dear Phillips I must conclude. I do not think I
should have allowed these notions to have escaped from me had
I not been led unawares, and without previous consideration,
by the circumstances of the evening on which I had to appear
suddenly[51] and occupy the place of another. Now that I have
put them on paper, I feel that I ought to have kept them much
longer for study, consideration, and perhaps final rejection;
and it is only because they are sure to go abroad in one
way or another, in consequence of their utterance on that
evening, that I give them a shape, if shape it may be called,
in this reply to your inquiry. One thing is certain, that any
hypothetical view of radiation which is likely to be received
or retained as satisfactory must not much longer comprehend
alone certain phenomena of light, but must include those of
heat and of actinic influence also, and even the conjoined
phenomena of sensible heat and chemical power produced by
them. In this respect a view which is in some degree founded
upon the ordinary forces of matter may perhaps find a little
consideration amongst the other views that will probably
arise. 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 on 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.

I am, my dear Phillips,
Ever truly yours,
M. FARADAY.

_Royal Institution_,
_April 15, 1846_.

If it be thought that too high a value has here been set upon a document which its author himself only claimed to be “the shadow of a speculation,” let that value be justified out of the mouth of the man who eighteen years later enriched the world with the mathematical theory of the propagation of electric waves, the late Professor Clerk Maxwell. In 1864 he published in the _Philosophical Transactions_ a “Dynamical Theory of the Electromagnetic Field,” in which the following passages occur:--

We have therefore reason to believe, from the phenomena of
light and heat, that there is an æthereal medium filling
space and permeating bodies capable of being set in motion,
and of transmitting that motion to gross matter, so as to
heat it and affect it in various ways.... Hence the parts
of this medium must be so connected that the motion of one
part depends in some way on the motion of the rest; and at
the same time these connections must be capable of a certain
kind of elastic yielding, since the communication of motion
is not instantaneous, but occupies time. The medium is
therefore capable of receiving and storing up two kinds of
energy--namely, the “actual” energy depending on the motion of
its parts, and “potential” energy, consisting of the work which
the medium will do in recovering from displacement in virtue of
its elasticity.

The propagation of undulations consists in the continual
transformation of one of these forms of energy into the other
alternately, and at any instant the amount of energy in the
whole medium is equally divided, so that half is energy of
motion and half is elastic resilience.

* * * * *

In order to bring these results within the power of symbolic
calculation, I then express them in the form of the general
equations of the electromagnetic field.

* * * * *

The general equations are next applied to the case of a
magnetic disturbance propagated through a non-conducting field,
and it is shown that the only disturbances which can be so
propagated are those which are transverse to the direction
of propagation, and that the velocity of propagation is
the velocity _v_, found from experiments such as those of
Weber, which expresses the number of electrostatic units of
electricity which are contained in one electromagnetic unit.
This velocity is so nearly that of light, that it seems we have
strong reason to conclude that light itself (including radiant
heat and other radiations, if any) is an electromagnetic
disturbance in the form of waves propagated through the
electromagnetic field according to electromagnetic laws....
Conducting media are shown to absorb such radiations rapidly,
and therefore to be generally opaque.

[Sidenote: ELECTROMAGNETIC THEORY OF LIGHT.]

The conception of the propagation of transverse magnetic
disturbances to the exclusion of normal ones is distinctly set
forth by Professor Faraday in his “Thoughts on Ray Vibrations.”
_The electromagnetic theory of light, as proposed by him, is
the same in substance as that which I have begun to develop
in this paper_,[52] except that in 1846 there were no data to
calculate the velocity of propagation.

During the rest of this year (1846) and the next Faraday did very little research, though he continued his Royal Institution lectures and his reports for Trinity House. Amongst the latter in 1847 was one on a proposal to light buoys by incandescent electric lamps containing a platinum wire spiral. He was compelled, indeed, to rest by a recurrence of brain troubles, giddiness, and loss of memory. Honours however, continued to be heaped upon him both abroad and at home, as the following extract from Bence Jones shows:--

In 1846, for his two great discoveries, the Rumford and the
Royal Medals were both awarded to him. This double honour
will probably long be unique in the annals of the Royal
Society. In former years he had already received the Copley
and Royal Medals for his experimental discoveries. As his
medals increased it became remarkable that he--who kept his
diploma-book, his portraits and letters of scientific men, and
everything he had in the most perfect order--seemed to take
least care of his most valuable rewards. They were locked up in
a box, and might have passed for old iron. Probably he thought,
as others did afterwards, that their value, if seen, might lead
to their loss.

[Sidenote: CRYSTALLINE FORCES.]

Between the twenty-first and twenty-second series of “Experimental Researches” nearly three years elapsed. In the autumn of 1848 the matter which claimed investigation was the peculiar behaviour of bismuth in the magnetic field. Certain anomalies were observed which were finally traced to the crystalline nature of the metal, for it appeared that when in that state the crystals themselves--to adopt modern phraseology--showed a greater magnetic permeability in a direction perpendicular to their planes of cleavage than in any direction parallel to those planes. Hence when a crystalline fragment was hung in a _uniform_ magnetic field (where the diamagnetic tendency to move from a strong to a weak region of the field was eliminated), it tended to point in a determinate direction. Faraday expressed it that the structure of the crystal showed a certain “axiality,” and he regarded these effects as presenting evidence of a “magnecrystallic” force, the law of action being that the line or axis of magnecrystallic force tended to place itself parallel to the lines of the magnetic field in which the crystal was placed. Arsenic, antimony, and other crystalline metals were similarly examined. The subject was an intricate one, and there are frequent obscurities in the phraseology tentatively adopted for explaining the phenomena. In one place Faraday rather pathetically laments his imperfect mathematical knowledge. This seems like an echo of his inability to follow the analytical reasoning of Poisson, who, starting from a hypothesis about the supposed “magnetic fluids” being movable within the particles of a body, supposing that these particles were non-spherical and were symmetrically arranged, had predicted (in 1827) that a portion of such a substance would, when brought into the neighbourhood of a magnet, act differently, according to the different positions in which it might be turned about its centre. But this very inability to follow Poisson’s refined analysis gave a new direction to Faraday’s thoughts, and caused him to conceive the idea of magnetic permeabilities differing in different directions, an idea which, as Sir William Thomson (the present Lord Kelvin) showed in 1851,[53] is equally susceptible of mathematical treatment by appropriate symbols. Lord Kelvin has also spoken (_op. cit._, p. 484) of the matter as follows: “The singular combination of mathematical acuteness with experimental research and profound physical speculation which Faraday, though not a ‘mathematician,’ presented is remarkably illustrated by his use of the expression ‘_conducting power of a magnetic medium for lines of force_.’” Tyndall has given a succinct summary of these researches--in which also he took a part--from which the following extract must suffice:--

And here follows one of those expressions which characterise
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 crystal 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 magnecrystallic 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, and so
often spoken of as acting at _insensible_ distances.” Thus he
broods over this new force, and looks at it from all 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.”

Plücker, 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 Plücker’s observations,
and concluded, what he at first seemed to doubt, that Plücker’s
results and magnecrystallic action had the same origin.

[Sidenote: MAGNETISM AND CRYSTALLISATION.]

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
magnecrystallic 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,
crystallisation, 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.”