CHAPTER V.

SCIENTIFIC RESEARCHES: THIRD PERIOD.

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.”

In 1848 Faraday gave five Friday night discourses, three of them on the “Diamagnetic Condition of Flame and Gases.” In 1849 he gave two, one of them on Plücker’s researches. In 1850 he gave two, one of them being on the electricity of the air, the other on certain conditions of freezing water. He had meanwhile continued to work at magnetism. The twenty-third series dealt with the supposed diamagnetic polarity. It incidentally discussed the distortion produced in a magnetic field by a mass of copper in motion across it. The twenty-fourth series was on the possible relation of gravity to electricity. The paper concludes with the words: “Here end my trials for the present. The results are negative. They do not shake my strong feeling of the existence of a relation between gravity and electricity, though they give no proof that such a relation exists.” The next series (the twenty-fifth) was on the “Non-expansion of Gases by Magnetic Force” and on the “Magnetic Characters of Oxygen [which he had found to be highly magnetic], Nitrogen, and Space.” He had found that magnetically substances must be classed either along with iron and the materials that point axially, or else with bismuth and those that point equatorially, in the magnetic field. The best vacuum he could procure he regarded as the zero of these tests; but before adopting it as such, he verified by experiment that even in a vacuum a magnetic body still tends from weaker to stronger places in the magnetic field; while diamagnetic bodies tend from stronger to weaker. He then says we must consider the magnetic character and relation of _space_ free from any material substance. “Mere space cannot act as matter acts, even though the utmost latitude be allowed to the hypothesis of an ether.” He then proceeds as follows:--

[Sidenote: MORE NEW WORDS.]

Now that the true zero is obtained, and the great variety of material substances satisfactorily divided into two general classes, it appears to me that we want another name for the magnetic class, that we may avoid confusion. The word _magnetic_ ought to be general, and include _all_ the phenomena and effects produced by that power. But then a word for the subdivision opposed to the diamagnetic class is necessary. As the language of this branch of science may soon require general and careful changes, I, assisted by a kind friend, have thought that a word--not selected with particular care--might be provisionally useful; and as the magnetism of iron, nickel, and cobalt when in the magnetic field is like that of the earth as a whole, so that when rendered active they place themselves parallel to its axes or lines of magnetic force, I have supposed that they and their similars (including oxygen now) might be called paramagnetic bodies, giving the following division:--

{ paramagnetic Magnetic { { diamagnetic.

The “kind friend” alluded to was Whewell, as the following letter shows:--

[_Rev. W. Whewell to M. Faraday._]

July, 1850.

I am always glad to hear of your wanting new words, because the want shows that you are pursuing new thoughts--and your new thoughts are worth something--but I always feel also how difficult it is for one who has not pursued the train of thought to suggest the right word. There are so many relations involved in a new discovery, and the word ought not glaringly to violate any of them. The purists would certainly object to the opposition, or co-ordination, of _ferromagnetic_ and _diamagnetic_, not only on account of the want of symmetry in the relation of _ferro_ and _dia_, but also because the one is Latin and the other Greek.... Hence it would appear that the two classes of magnetic bodies are those which place their length _parallel_, or _according_, to the terrestrial magnetic lines, and those which place their length transverse to such lines. Keeping the preposition _dia_ for the latter, the preposition _para_, or _ana_, might be used for the former. Perhaps para would be best, as the word _parallel_, in which it is involved, would be a technical memory for it.... I rejoice to hear that you have new views of discovery opening to you. I always rejoice to hail the light of such when they dawn upon you.

The twenty-sixth series of researches opened with a consideration of magnetic “conducting power,” or permeability as we should now term it, and then branched off into a lengthy discussion of atmospheric magnetism. The subject was continued through the twenty-seventh series, which was completed in November, 1850. The gist of this is summed up in one of his letters to Schönbein:--

Royal Institution, November 19, 1850.

MY DEAR SCHÖNBEIN,--I wish I could talk with you, instead of being obliged to use pen and paper. I have fifty matters to speak about, but either they are too trifling for writing, or too important, for what can one discuss or say in a letter?... By the bye, I have been working with the oxygen of the air also. You remember that three years ago I distinguished it as a magnetic gas in my paper on the diamagnetism of flame and gases founded on Bancalari’s experiment. Now I find in it the cause of all the annual and diurnal, and many of the irregular, variations in the terrestrial magnetism. The observations made at Hobarton, Toronto, Greenwich, St. Petersburg, Washington, St. Helena, the Cape of Good Hope, and Singapore, all appear to me to accord with and support my hypothesis. I will not pretend to give you an account of it here, for it would require some detail, and I really am weary of the subject. I have sent in three long papers to the Royal Society, and you shall have copies of them in due time....

Ever, my dear Schönbein, most truly yours, M. FARADAY.

[Sidenote: PAPERS TO BE LET LOOSE.]

While writing out these researches for the Royal Society, he had been staying in Upper Norwood. He wrote thus of himself to Miss Moore at the end of August:--

We have taken a little house here on the hill-top, where I have a small room to myself, and have, ever since we came here, been deeply immersed in magnetic cogitations. I write, and write, and write, until three papers for the Royal Society are nearly completed, and I hope that two of them will be good if they justify my hopes, for I have to criticise them again and again before I let them loose. You shall hear of them at some of the Friday evenings. At present I must not say more. After writing, I walk out in the evening, hand-in-hand with my dear wife, to enjoy the sunset; for to me, who love scenery, of all that I have seen or can see there is none surpasses that of Heaven. A glorious sunset brings with it a thousand thoughts that delight me.

To De la Rive he wrote later as follows:--

[_M. Faraday to A. de la Rive._]

Royal Institution, February 4, 1851.

MY DEAR DE LA RIVE,--My wife and I were exceedingly sorry to hear of your sad loss. It brought vividly to our remembrance the time when we were at your house, and you, and others with you, made us so welcome. What can we say to these changes but that they show by comparison the vanity of all things under the sun? I am very glad that you have spirits to return to work again, for that is a healthy and proper employment of the mind under such circumstances.

With respect to my views and experiments, I do not think that anything shorter than the papers (and they will run to a hundred pages in the “Transactions”) will give you possession of the subject, because a great deal depends upon the comparison of observations in different parts of the world with the facts obtained by experiment, and with the deductions drawn from them; but I will try to give you an idea of the root of the matter. You are aware that I use the phrase _line of magnetic force_, to represent the presence of magnetic force, and the direction (of polarity) in which it is exerted; and by the idea which it conveys one obtains very well, and I believe without error, a notion of the distribution of the forces about a bar magnet, or between near flat poles presenting a field of equal force, or in any other case. Now, if circumstances be arranged so as to present a field of equal force, which is easily done, as I have shown by the electro-magnet, then if a sphere of iron or nickel be placed in the field, it immediately disturbs the direction of the lines of force, for they are concentrated within the sphere. They are, however, not merely concentrated, but _contorted_, for the sum of forces in any one section across the field is always equal to the sum of forces in any other section, and therefore their condensation in the iron or nickel cannot occur without this contortion. Moreover, the contortion is easily shown by using a small needle (one-tenth of an inch long) to examine the field, for, as before the introduction of the sphere of iron or nickel, it would always take up a position parallel to itself. Afterwards it varies in position in different places near the sphere. This being understood, let us then suppose the sphere to be raised in temperature. At a certain temperature it begins to lose its power of affecting the lines of magnetic force, and ends by retaining scarcely any. So that as regards the little needle mentioned above, it now stands everywhere parallel to itself within the field of force. This change occurs with iron at a very high temperature, and is passed through within the compass, apparently, of a small number of degrees. With nickel it occurs at much lower temperatures, being affected by the heat of boiling oil.

Now take another step. Oxygen, as I showed above, three years ago in the _Philosophical Magazine_ for 1847, vol. xxxi., pp. 410, 415, 416, is magnetic in relation to nitrogen and other gases. E. Becquerel, without knowing of my results, has confirmed and extended them in his paper of last year, and given certain excellent measures. In my paper of 1847 I showed also that oxygen (like iron and nickel) lost its magnetic power and its ability of being attracted by the magnet when heated (p. 417). And I further showed that the temperatures at which this took place were within the range of common temperature, for the oxygen of the air--_i.e._ the air altogether--is increased in magnetic power when cooled to 0° F. (p. 406). Now I must refer you to the papers themselves for the (to me) strange results of the incompressibility (magnetically speaking) of oxygen and the inexpansibility of nitrogen and other gases; for the description of a differential balance by which I can compare gas with gas, or the same gas at different degrees of rarefaction; for the determination of the true zero, or point between magnetic and diamagnetic bodies; and for certain views of magnetic conduction and polarity. You will there find described certain very delicate experiments upon diamagnetic and very weak magnetic bodies concerning their action on each other in a magnetic field of equal force. The magnetic bodies repel each other, and the diamagnetic bodies repel each other; but a magnetic and a diamagnetic body _attract_ each other. And these results, combined with the qualities of oxygen as just described, convince me that it is able to deflect the lines of magnetic force passing through it just as iron or nickel is, but to an infinitely smaller amount, and that its power of deflecting the lines varies with its temperature and degree of rarefaction.

[Sidenote: ATMOSPHERIC MAGNETISM.]

Then comes in the consideration of the atmosphere, and the manner in which it rises and falls in temperature by the presence and absence of the sun. The place of the great warm region nearly in his neighbourhood; of the two colder regions which grow up and diminish in the northern and southern hemispheres as the sun travels between the tropics; the effect of the extra warmth of the northern hemisphere over the southern; the effect of accumulation from the action of preceding months; the effect of dip and mean declination at each particular station; the effects that follow from the non-coincidence of magnetic and astronomical conditions of polarity, meridians, and so forth; the results of the distribution of land and water for any given place--for all these and many other things I must refer you to the papers. I could not do them justice in any account that a letter could contain, and should run the risk of leading you into error regarding them. But I may say that, deducing from the experiments and the theory what are the deviations of the magnetic needle at any given station, which may be expected as the mean result of the heating and cooling of the atmosphere for a given season and hour, I find such a general accordance with the results of observations, especially in the direction and generally in the amount for different seasons of the _declination_ variation, as to give me the strongest hopes that I have assigned the true physical cause of those variations, and shown the _modus operandi_ of their production.

And now, my dear de la Rive, I must leave you and run to other matters. As soon as I can send you a copy of the papers I will do so, and can only say I hope that they will meet with your approbation. With the kindest remembrances to your son,

Believe me to be, my dear friend, ever truly yours,

M. FARADAY.

This hope of explaining the variations of terrestrial magnetism by the magnetic properties of the oxygen of the air was destined to be illusory. At that time the cosmical nature of magnetic storms was unknown and unsuspected. To this matter we may well apply Faraday’s own words addressed to Tyndall respecting the alleged diamagnetic polarity, and the conflict of views between himself on the one hand and Weber and Tyndall on the other:--“It is not wonderful that views differ at first. Time will gradually sift and shape them. And I believe that we have little idea at present of the importance they may have ten or twenty years hence.”

[Sidenote: LINES OF MAGNETIC FORCE.]

In 1851, from July to December, Faraday was actively at work in the laboratory. The results constitute the material for the twenty-eighth and twenty-ninth (the last) series of the “Experimental Researches.” In these he returned to the subject with which the first series had opened in 1831: the induction of electric currents by the relative motion of magnets and conducting wires. These two memoirs, together with his Royal Institution lecture of January, 1852, “On the Lines of Magnetic Force,” and the paper “On the Physical Character of the Lines of Magnetic Force” (which he sent to the _Philosophical Magazine_, as containing “so much of a speculative and hypothetical nature”), should be read, and re-read, and read again, by every student of physics. They are reprinted at the end of the third volume of the “Experimental Researches.”

In the opening of the twenty-eighth memoir he says:--

From my earliest experiments on the relation of electricity and magnetism, I have had to think and speak of lines of magnetic force as representations of the magnetic power--not merely in the points of quality and direction, but also in quantity.... The direction of these lines about and amongst magnets and electric currents is easily represented and understood in a general manner by the ordinary use of iron filings.

A point equally important to the definition of these lines is, that they represent a determinate and unchanging amount of force. Though, therefore, their forms, as they exist between two or more centres or sources of power, may vary very greatly, and also the space through which they may be traced, yet the sum of power contained in any one section of a given portion of the lines is exactly equal to the sum of power in any other section[54] of the same lines, however altered in form or however convergent or divergent they may be at the second place.... Now, it appears to me that these lines may be employed with great advantage to represent the nature, condition, and comparative amount of the magnetic forces, and that in many cases they have, to the physical reasoner, at least, a superiority over that method which represents the forces as concentrated in centres of action, such as the poles of magnets or needles; or some other methods, as, for instance, that which considers north or south magnetisms as fluids diffused over the end, or amongst the particles, of a bar. No doubt any of these methods which does not assume too much will, with a faithful application, give true results. And so they all ought to give the same results, as far as they can respectively be applied. But some may, by their very nature, be applicable to a far greater extent, and give far more varied results, than others. For, just as either geometry or analysis may be employed to solve correctly a particular problem, though one has far more power and capability, generally speaking, than the other; or, just as either the idea of the reflexion of images or that of the reverberation of sounds may be used to represent certain physical forces and conditions, so may the idea of the attractions and repulsions of centres, or that of the disposition of magnetic fluids, or that of lines of force, be applied in the consideration of magnetic phenomena. It is the occasional and more frequent use of the latter which I at present wish to advocate.... When the natural truth, and the conventional representation of it, most closely agree, then are we most advanced in our knowledge. The emission and æther theories present such cases in relation to light. The idea of a fluid or of two fluids is the same for electricity; and there the further idea of a current has been raised, which, indeed, has such hold on the mind as occasionally to embarrass the science as respects the true character of the physical agencies, and may be doing so even now to a degree which we at present little suspect. The same is the case with the idea of a magnetic fluid or fluids, or with the assumption of magnetic centres of action of which the resultants are at the poles.

[Sidenote: THE FUNCTIONS OF THE ÆTHER.]

How the magnetic force 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 cases 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. Nevertheless, because a particular method of representing the forces does not include such a mode of transmission, the latter is not disproved, and that method of representation which harmonises with it may be the most true to nature. The general conclusion of philosophers seems to be that such cases are by far the most numerous. And for my own part, considering the relation of a vacuum to the magnetic force, and the general character of magnetic phenomena external to the magnet, I am more inclined to the notion that in the transmission of the force there is such an action, external to the magnet, than that the effects are merely attraction and repulsion at a distance. _Such an action may be a function of the æther, for it is not at all unlikely that if there be an æther, it should have other uses than simply the conveyance of radiations._[55]

He then proceeds to recount the experimental evidence of revolving magnets and loops of wire. Following out the old lines of so moving the parts of the system that the magnetic lines were “cut” by the copper conductors, and connecting the latter with a slow-period galvanometer, to test the resultant induction, he found that “the _amount_ of magnetic force” [or _flux_, as we should nowadays call it] “is determinate for the same lines of force, whatever the distance of the point or plane at which their power is exerted is from the magnet.” The convergence or divergence of the lines of force caused, _per se_, no difference in their amount. Obliquity of intersection caused no difference, provided the same lines of force were cut. If a wire was moving in a field of equal intensity, and with a uniform motion, then the current produced was proportional to the velocity of motion. The “quantity of electricity thrown into a current” was, _ceteris paribus_, “directly as the amount of curves intersected.” Within the magnet, running through its substance, existed lines of force of the _same nature_ as those without, exactly equal in _amount_ to those without, and were, indeed, _continuous_ with them. The conclusion must logically be that every line of force is a closed circuit.

Having thus established the exact quantitative laws of magneto-electric induction, he then advanced to make use of the induced current as a means of investigating the presence, direction, and amount of magnetic forces--in other words, to explore and measure magnetic fields. He constructed revolving rectangles and rings furnished with a simple commutator, to measure inductively the magnetic forces of the earth. Then he employed the induced current to test the constancy of magnets when placed near to other magnets in ways that might affect their power. Next he considers the fields of magnetic force of two or more associated magnets, and notes how their magnetic lines may coalesce when they are so placed as to constitute parts of a common magnetic circuit. The twenty-ninth series is brought to a close by a discussion of the experimental way of delineating lines of magnetic force by means of iron filings.

[Sidenote: THE ELECTROTONIC STATE.]

The paper on the “Physical Character of the Lines of Magnetic Force” recapitulated the points established in the twenty-ninth series of “Researches,” and emphasis is laid upon the logical necessity that time must be required for their propagation. The physical effects in a magnetic field, as equivalent to a tendency for the magnetic lines to shorten themselves, and to repel one another laterally, are considered, and are contrasted with the effects of parallel electric currents. Commenting on the mutual relation between the directions of an electric current and of its surrounding magnetic lines, he raises the question whether or not they consist in a state of tension of the æther. “Again and again,” he says, “the idea of an _electrotonic_ state has been forced on my mind. Such a state would coincide and become identified with that which would then constitute the physical lines of magnetic force.” Then he traces out the analogy between a magnet, with its “sphondyloid” (or spindle-form field) of magnetic lines, and a voltaic battery immersed in water, with its re-entrant lines of flow of circulating current. Incidentally, while discussing the principle of the magnetic circuit, he points out that when a magnet is furnished at its poles with masses of soft iron, it can both receive and retain a higher magnetic charge than it does without them, “for these masses carry on the physical lines of force, and deliver them to a body of surrounding space, which is either widened, and therefore increased, in the direction across the lines of force, or shortened in that direction parallel to them, or both; and both are circumstances which facilitate the conduction from pole to pole.”

[Sidenote: NOVELTY OF FARADAY’S VIEWS.]

Thus closed, with the exception of two fragmentary papers, one on “Physical Lines of Force,” and the other on “Some Points in Magnetic Philosophy,” in the years 1853 and 1854 respectively, the main life-work of Faraday, his “Experimental Researches.” Their effect in revolutionising electric science, if slow, was yet sure. Though the principle of the dynamo was discovered and published in 1831, nearly forty years elapsed before electric-lighting machinery became a commercial product. Though the dependence of inductive actions, both electromagnetic and electrostatic, upon the properties of the intervening medium was demonstrated and elaborated in these “Researches,” electricians for many years continued to propound theories which ignored this fundamental fact. French and German writers continued to publish treatises based on the ancient doctrines of action at a distance, and of imaginary electric and magnetic fluids. Von Boltzmann, a typical German of the first rank in science, says that until there came straight from England the counter-doctrines amidst which Faraday had lived, “we (in Germany and France) had all more or less imbibed with our mothers’ milk the ideas of magnetic and electric fluids acting direct at a distance.” And again, “The theory of Maxwell”--that is, Faraday’s theory thrown by Maxwell into mathematical shape--“is so diametrically opposed to the ideas which have become customary to us, that we must first cast behind us all our previous views of the nature and operation of electric forces before we can enter into its portals.” The divergence of view between Faraday and the Continental electricians is nowhere more clearly stated than by Faraday’s great interpreter, Maxwell, in the _apologia_ which he prefixed in 1873 to his “Treatise on Electricity and Magnetism,” wherein, speaking of the differences between this work and those recently published in Germany, he wrote:--

One reason of this is that before I began the study of electricity I resolved to read no mathematics on the subject till I had first read through Faraday’s “Experimental Researches on Electricity.” I was aware that there was supposed to be a difference between Faraday’s way of conceiving phenomena and that of the mathematicians. So that neither he nor they were satisfied with each other’s language. I had also the conviction that this discrepancy did not arise from either party being wrong. I was first convinced of this by Sir William Thomson [Lord Kelvin], to whose advice and assistance, as well as to his published papers, I owe most of what I have learned on this subject.

As I proceeded with the study of Faraday, I perceived that his method of conceiving the phenomena was also a mathematical one, though not exhibited in the conventional form of mathematical symbols. I also found that these methods were capable of being expressed in the ordinary mathematical forms, and thus compared with those of the professed mathematicians.

For instance, Faraday, in his mind’s eye, saw lines of force traversing all space where the mathematicians saw centres of force attracting at a distance. Faraday saw a medium where they saw nothing but distance. Faraday sought the seat of the phenomena in real actions going on in the medium; they were satisfied that they had found it in a power of action at a distance impressed on electric fluids.

When I had translated what I considered to be Faraday’s ideas into a mathematical form, I found that in general the results of the two methods coincided, so that the same phenomena were accounted for and the same laws of action deduced by both methods, but that Faraday’s methods resembled those in which we begin with the whole and arrive at the parts by analysis, while the ordinary mathematical methods were founded on the principle of beginning with the parts and building up the whole by synthesis.

I found, also, that several of the most fertile methods of research discovered by the mathematicians could be expressed much better in terms of ideas derived from Faraday than in their original form.

The whole theory, for instance, of potential, considered as a quantity which satisfies a certain partial differential equation, belongs essentially to the method which I have called of Faraday....

If by anything I have here written I may assist any student in understanding Faraday’s modes of thought and expression, I shall regard it as the accomplishment of one of my principal aims: to communicate to others the same delight which I have found myself in reading Faraday’s “Researches.”

Clerk Maxwell may also be credited with the remark that Faraday’s work had had the result of banishing the term “the electric fluid” into the limbo of newspaper science.

[Sidenote: ELECTRIC LIGHT IN LIGHTHOUSES.]

Faraday’s work for Trinity House continued during these last years of research work. He reported on such subjects as adulteration of white lead, impure oils, Chance’s lenses, lighthouse ventilation, and fog signals. Two systems of electric arc lighting for lighthouses--one by Watson, using batteries, the other by Holmes, using a magneto-electric machine--were examined in 1853 and 1854, but his report on them was adverse. He “could not put up in a lighthouse what has not been established beforehand, and is only experimental.” In 1856 he made five reports, in 1857 six, and in 1858 twelve reports to Trinity House, one of these being on the electric light at the South Foreland. In 1859 he reported on further trials in which Duboscq’s lamps were used. In 1860 he gave a final report on the practicability and utility of magneto-electric lighting, and expressed the hope it would be applied, as there was _now_ no difficulty. In 1861 he inspected the machinery as established at the Dungeness lighthouse. In 1862 he gave no fewer than seventeen reports, visiting Dungeness, Grisnez, and the South Foreland. In 1863 he again visited Dungeness. In 1864 he made twelve reports, and examined the drawings and estimates for establishing the electric light at Portland. His last report was in 1865, upon the St. Bees’ light, and he then retired from this service.

His Friday night discourses were still continued during these years. In 1855 he gave one on “Ruhmkorff’s Induction-coil.” In 1856 he gave one on a process for silvering glass, and on finely divided gold. This latter subject, the optical properties of precipitated gold, formed the topic of the Bakerian lecture of that year--his last contribution to the Royal Society. He gave another discourse on the same subject in 1857, and also one on the conservation of force. In 1856, when investigating the crystallisation of water, he discovered the phenomenon of regelation of ice. In virtue of this property two pieces of ice will freeze solidly together under pressure, even when the temperature of the surrounding atmosphere is above the freezing point. This discovery led on the one hand to the explanation of glacier motions; on the other to important results in thermodynamic theory. In 1859 he gave two discourses, one on ozone, the other on phosphorescence and fluorescence. He also gave two in 1860, on lighthouse illumination by electric light, and on the electric silk-loom. In 1861 he discoursed on platinum and on De la Rue’s eclipse photographs. The last of his Friday night discourses was given on June 20th, 1862. It was on Siemens’s gas furnaces. He had been down at Swansea watching the furnaces in operation, and now proposed to describe their principle. It was rather a sad occasion, for it was but too evident that his powers were fast waning. Early in the evening he had the misfortune to burn the notes he had prepared, and became confused. He concluded with a touching personal explanation how with advancing years his memory had failed, and that in justice to others he felt it his duty to retire.

At intervals he still attempted to work at research. In 1860 he sent a paper to the Royal Society on the relations of electricity to gravity, but, on the advice of Professor (afterwards Sir George) Stokes, it was withdrawn. He had also in contemplation some experiments upon the time required in the propagation of magnetism, and began the construction of a complicated instrument, which was never finished.

[Sidenote: HYPOTHESIS AND EXPERIMENT.]

His very last experiment, as recorded in his laboratory notebook, is of extraordinary interest, as showing how his mind was still at work inquiring into the borderland of possible phenomena. It was on March 12th, 1862. He was inquiring into the effect of a magnetic field upon a beam of light, which he was observing with a spectroscope to ascertain whether there was any change produced in the refrangibility of the light. The entry concludes: “Not the slightest effect on the polarised or unpolarised ray was observed.” The experiment is of the highest interest in magneto-optics. The effect for which Faraday looked in vain in 1862 was discovered in 1897 by Zeeman. That Faraday should have _conceived_ the existence of this obscure relation between magnetism and light is a striking illustration of the acuteness of mental vision which he brought to bear. Living and working amongst the appliances of his laboratory, letting his thoughts play freely around the phenomena, incessantly framing hypotheses to account for the facts, and as incessantly testing his hypotheses by the touchstone of experiment, never hesitating to push to their logical conclusion the ideas suggested by experiment, however widely they might seem to lead from the accepted modes of thought, he worked on with a scientific prevision little short of miraculous. His experiments, even those which at the time seemed unsuccessful, in that they yielded no positive result, have proved to be a mine of amazing richness. The volumes of his “Experimental Researches” are a veritable treasure-house of science.