Michael Faraday: His Life and Work, by Silvanus P. Thompson

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.

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:—

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.

[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.

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.

[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.

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.”

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.”

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 section54 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.

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.

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.”

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.

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.

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.