Heroes of Science: Physicists Part 13

A few weeks later he obtained induction currents by means of the earth's magnetism only, first with a coil of wire wound upon an iron bar in which a strong current was produced when it was being quickly placed in the direction of the magnetic dip or being removed from that position, and afterwards with a coil of wire without an iron core. On February 8, 1832, he succeeded in obtaining a spark from the induced current. Unless the electro-motive force is very great, it is not possible to obtain a spark between two metallic surfaces which are separated by a sensible thickness of air. If, however, the circuit of a wire is broken _while_ the current is pa.s.sing, a little bridge of metallic vapour is formed, across which for an instant the spark leaps. The induced current being of such short duration, the difficulty was to break the circuit while it was flowing. Faraday wound a considerable length of fine wire around a short bar of iron; the ends of the wire were crossed so as just to be in contact with one another, but free to separate if exposed to a slight shock. The ends of the iron bar projected beyond the coil, and were held just over the poles of the magnet. On releasing the bar it fell so as to strike the magnetic poles and close the circuit of the magnet. An induced current was generated in the wire, but, while this was pa.s.sing, the shock caused by the bar striking the magnet separated the ends of the wire, thus breaking the circuit of the conductor, and a spark appeared at the gap. In this little spark was the germ of the electric light of to-day. Subsequently Faraday improved the apparatus, by attaching a little disc of amalgamated copper to one end of the wire, and bending over the other end so as just to press lightly against the surface of the disc. With this apparatus he showed the "magnetic spark" at the meeting of the British a.s.sociation at Oxford.

Faraday supposed that when a coil of wire was in the neighbourhood of a magnet, or near to a conductor conveying a current, the coil was thrown into a peculiar condition, which he called the _electro-tonic state_, and that the induced currents appeared whenever this state was a.s.sumed or lost by the coil. He frequently reverted to his conception of the electro-tonic state, though he saw clearly that, when the currents were induced by the relative motion of a wire and a magnet, the current induced depended on the rate at which the lines of magnetic force had been cut by the wire. Of his conception of lines of force filling the whole of s.p.a.ce, we shall have more to say presently.

It is sufficient to remark here that, in the electro-tonic state of Faraday, Clerk Maxwell recognized the number of lines of magnetic force enclosed by the circuit, and showed that the electro-motive force induced is proportional to the rate of change of the number of lines of force thus enclosed.

It is seldom that a great discovery is made which has not been gradually led up to by several observed phenomena which awaited that discovery for their explanation. In the case of electro-magnetic induction, however, there appears to have been but one experiment which had baffled philosophers, and the key to which was found in Faraday's discovery, while the complete explanation was given by Faraday himself. Arago had found that, if a copper plate were made rapidly to rotate beneath a freely suspended magnetic needle, the needle followed (slowly) the plate in its revolution, though a sheet of gla.s.s were inserted between the two to prevent any air-currents acting on the magnet. The experiment had been repeated by Sir John Herschel and Mr. Babbage, but no explanation was forthcoming. Faraday saw that the revolution of the disc beneath the poles of the magnet must generate induced currents in the disc, as the different portions of the metal would be constantly cutting the lines of force of the magnet. These currents would react upon the magnet, causing a mechanical stress to act between the two, which, as stated by Lenz, would be in the direction tending to oppose the _relative_ motion, and therefore to drag the magnet after the disc in its revolution. In the above figure the unfledged arrows show the general distribution of the currents in the disc, while the winged arrows indicate the direction of the disc's rotation. The currents in the semicircle A will repel the north pole and attract the south pole. Those in the semicircle B will produce the opposite effect, and hence there will be a tendency for the magnet to revolve in the direction of the disc, while the motion of the disc will be resisted. This resistance to the motion of a conductor in a magnetic field was noticed by Faraday, and, independently, by Tyndall, and it is sufficiently obvious in the power absorbed by dynamos when they are generating large currents.

Faraday's next series of researches was devoted to the experimental proof of the ident.i.ty of frictional and voltaic electricity. He showed that a magnet could be deflected and iodide of pota.s.sium decomposed by the current from his electrical machine, and came to the conclusion that the amount of electricity required to decompose a grain of water was equal to 800,000 charges of his large Leyden battery. The current from the frictional machine also served to deflect the needle of his galvanometer. These investigations led on to a complete series of researches on the laws of electrolysis, wherein Faraday demonstrated the principle that, however the strength of the current may be varied, the amount of any compound decomposed is proportional to the whole quant.i.ty of electricity which has pa.s.sed through the electrolyte. When the same current is sent through different compounds, there is a constant relation between the amounts of the several compounds decomposed. In modern language, Faraday's laws may be thus expressed:--

_If the same current be made to pa.s.s through several different electrolytes, the quant.i.ty of each ion produced will be proportional to its combining weight divided by its valency, and if the current vary, the quant.i.ty of each ion liberated per second will be proportional to the current._

This is the great law of electro-chemical equivalents. The amount of hydrogen liberated per second by a current of one ampere is about 00001038 gramme, or nearly one six-thousandth of a grain. This is the electro-chemical equivalent of hydrogen. That of any other substance may be found by Faraday's law.

From Faraday's results it appears that the pa.s.sage of the same amount of electricity is required in order to decompose one molecule of any compound of the same chemical type, but it does not follow that the same amount of energy is employed in the decomposition. For example, the combining weights of copper and zinc are nearly equal. Hence it will require the pa.s.sage of about the same amount of electricity to liberate a pound of copper from, say, the copper sulphate as to liberate a pound of zinc from zinc sulphate; but the work to be done is much less in the case of the copper. This is made manifest in the following way:--A battery, which will just decompose the copper salt slowly, liberating copper, oxygen, and sulphuric acid, will not decompose the zinc salt at all so as to liberate metallic zinc, but immediately on sending the current through the electrolyte, polarization will set in, and the opposing electro-motive force thus introduced will become equal to that of the battery, and stop the current before metallic zinc makes its appearance. In the case of the copper, polarization also sets in, but never attains to equality with the electro-motive force of the primary battery. In fact, in all cases of electrolysis, polarization produces an opposing electro-motive force strictly proportional to the work done in the cell by the pa.s.sage of each unit of electricity. If the strength of the battery be increased, so that it is able to decompose the zinc sulphate, and if this battery be applied to the copper sulphate solution, the latter will be _rapidly_ decomposed, and the excess of energy developed by the battery will be converted into heat in the circuit.

One important point in connection with electrolysis which Faraday demonstrated is that the decomposition is the result of the pa.s.sage of the current, and is not simply due to the attraction of the electrodes. Thus he showed that pota.s.sium iodide could be decomposed by a stream of electricity coming from a metallic point on the prime conductor of his electric machine, though the point did not touch the test-paper on which the iodide was placed.

It was in 1834 that Mr. Wm. Jenkin, after one of the Friday evening lectures at the Royal Inst.i.tution, called the attention of Faraday to a shock which he had experienced in breaking the circuit of an electro-magnet, though the battery employed consisted of only one pair of plates. Faraday repeated the experiment, and found that, with a large magnet in circuit, a strong spark could thus be obtained. On November 14, 1834, he writes, "The phenomenon of increased spark is merely a case of the induction of electric currents. If a current be established in a wire, and another wire forming a complete circuit be placed parallel to it, at the moment the current in the first is stopped it induces a current in the same direction in the second, itself then showing but a feeble spark. But if the second be away, it induces a current in its own wire in the same direction, producing a strong spark. The strong spark in the current when alone is therefore the equivalent of the current it can produce in a neighbouring wire when in company." The strong spark does, in fact, represent the energy of the current due to the self-induction of its circuit, which energy would, in part at least, be expended in inducing a current in a neighbouring wire if such existed.

His time from 1835 till 1838 was largely taken up with his work on electro-static induction. Faraday could never be content with any explanation based on direct action at a distance; he always sought for the machinery through which the action was communicated. In this search the lines of magnetic force, which he had so often delineated in iron filings, came to his aid. Faraday made many pictures in iron filings of magnetic fields due to various combinations of magnets. He employed gummed paper, and when the filings were arranged on the hard gummed surface, he projected a feeble jet of steam on the paper, which melted the gum and fixed the filings. Several of his diagrams were exhibited at the Loan Collection at South Kensington. He conceived electrical action to be transmitted along such lines as these, and to him the whole electric field was filled with lines pa.s.sing always from positive to negative electrification, and in some respects resembling elastic strings. The action at any place could then be expressed in terms of the lines of force that existed there, the electrifications by which these lines were produced being left out of consideration.

The acting bodies were thus replaced by the field of force they produced. He showed that it was impossible to call into existence a charge of positive electricity without at the same time producing an equal negative charge. From every unit of positive electricity he conceived a line of force to start, and thus, with the origin of the line, there was created simultaneously a charge of negative electricity on which the line might terminate. By the famous ice-pail experiment he showed that, when a charged body is inserted in a closed or nearly closed hollow conductor, an equal amount of the same kind of electricity appeared on the outside of the hollow conductor, while an equal amount of the opposite kind appeared on the interior surface of the conductor. With the ice-pail and the b.u.t.terfly-net he showed that there could be no free electricity on the interior of a conductor.

Lines of force cannot pa.s.s through the material of a conductor without producing electric displacement. Every element of electricity must be joined to an equal amount of the opposite kind by a line of force.

Such lines cannot pa.s.s through the conductor itself; hence the charge must be entirely on the outside of the conductor, so that every element of the charge may be a.s.sociated with an equal amount of the opposite electricity upon the surfaces of surrounding objects. Thus to Faraday every electrical action was an exhibition of electric induction. All this work had been done before by Henry Cavendish, but neither Faraday nor any one else knew about it at the time. From the fact that there could be no electricity in the interior of a hollow conductor, Cavendish deduced, in the best way possible, the truth of the law of inverse squares as applied to electrical attraction and repulsion, and thus laid the foundation of the mathematical theory of electricity. To Cavendish every electrical action was a displacement of an incompressible fluid which filled the whole of s.p.a.ce, producing no effect in conductors on account of the freedom of its motion, but producing strains in insulators by displacing the material of the body. Faraday, in his lines of force, saw, as it were, the lines along which the displacements of Cavendish's fluid took place.

Faraday thought that, if he could show that electric induction could take place along curved lines, it would prove that the action took place through a medium, and not directly at a distance. He succeeded in experimentally demonstrating the curvature of these lines; but his conclusions were not warranted, for if we conceive of two or more centres of force acting directly at a distance according to the law of inverse squares, the resultant lines of force will generally be curved. Of course, this does not prove the possibility of direct action at a distance, but only shows that the curvature of the lines is as much a consequence of the one hypothesis as of the other.

It soon appeared to Faraday that the nature of the dielectric had very much to do with electric induction. The capacity of a condenser, for instance, depends on the nature of the dielectric as well as on the configuration of the conductors. To express this property, Faraday employed the term "specific inductive capacity." He compared the electric capacity of condensers, equal in all other respects, but one possessing air for its dielectric, and the other having other media, and thus roughly determined the specific inductive capacities of several insulators. These results turned out afterwards to be of great value in connection with the insulation of submarine cables. Even now the student of electricity is sometimes puzzled by the manner in which specific inductive capacity is introduced to his notice as modifying the capacity of condensers, after learning that the capacity of any system of conductors can be calculated from its geometrical configuration; but the fact is that the intensity of all electrical actions depends on the nature of the medium through which they take place, and it will require more electricity to exert upon an equal charge a unit force at unit distance when the intervening medium has a high than when it possesses a low specific inductive capacity.

In 1835 Faraday received a pension from the civil list; in 1836 he was appointed scientific adviser to the Elder Brethren of the Trinity House. In the same year he was made a member of the Senate of the University of London, and in that capacity he has exerted no small influence on the scientific education of the country, for he was one of those who drew up the schedules of the various examinations.

In his early years, Faraday thought that all kinds of matter might ultimately consist of three materials only, and that as gases and vapours appeared more nearly to resemble one another than the liquids or solids to which they corresponded, so each might be subject to a still higher change in the same direction, and the gas or vapour become radiant matter--either heat, light, or electricity. Later on, Faraday clearly recognized the dynamical nature of heat and light; but his work was always guided by his theoretical conceptions of the "correlation of the physical forces." For a long time he had tried to discover relations between electricity and light; at length, on September 13, 1845, after experimenting on a number of other substances, he placed a piece of silico-borate of lead, or heavy-gla.s.s, in the field of the magnet, and found that, when a beam of polarized light was transmitted through the gla.s.s in the direction of the lines of magnetic force, there was a rotation of the plane of polarization. Afterwards it appeared that all the transparent solids and liquids experimented on were capable of producing this rotation in a greater or less degree, and in the case of all non-magnetic substances the rotation was in the direction of the electric current, which, pa.s.sing round the substance, would produce the magnetic field employed. Abandoning the magnet, and using only a coil of wire with the transparent substance within it, similar effects were obtained.

Thus at length a relation was found between light and electricity.

On November 4, employing a piece of heavy-gla.s.s and a new horseshoe magnet, Faraday noticed that the magnet appeared to have a directive action upon the gla.s.s. Further examination showed that the gla.s.s was repelled by the magnetic poles. Three days afterwards he found that all sorts of substances, including most metals, were acted upon like the heavy-gla.s.s. Small portions of them were repelled, while elongated cylinders tended to set with their lengths perpendicular to the lines of magnetic force. Such actions could be imitated by suspending a feebly magnetic body in a medium more magnetic than itself. Faraday, therefore, sought for some medium which would be absolutely neutral to magnetic action. Filling a gla.s.s tube with compressed oxygen, and suspending it in an atmosphere of oxygen at ordinary pressure, the compressed gas behaved like iron or other magnetic substances.

Faraday compared the intensity of its action with that of ferrous sulphate, and this led to an explanation of the diurnal variations of the compa.s.s-needle based on the sun's heat diminishing the magnetic _permeability_ of the oxygen of the air. Repeating the experiment with nitrogen, he found that the compressed gas behaved in a perfectly neutral manner when surrounded by the gas at ordinary pressure. Hence he inferred that in nitrogen he had found the neutral medium required.

Repeating his experiments in an atmosphere of nitrogen, it still appeared that most bodies were repelled by the magnetic poles, and set _equatorially_, or at right angles to the lines of force when elongated portions were tested. To this action Faraday gave the name of diamagnetism.

About a month after his marriage, Faraday joined the Sandemanian Church, to which his family had for several generations belonged, by confession of sin and profession of faith. Not unfrequently he used to speak at the meetings of his Church, but in 1840 he was elected an elder, and then he took his turn regularly in conducting the services.

The notes of his addresses he generally made on small pieces of card.

He had a curious habit of separating his religious belief from his scientific work, although the spirit of his religion perpetually pervaded his life. A lecture on mental education, given in 1854, at the Royal Inst.i.tution, in the presence of the late Prince Consort, he commenced as follows:--

"Before entering on this subject, I must make one distinction, which, however it may appear to others, is to me of the utmost importance.

High as man is placed above the creatures around him, there is a higher and far more exalted position within his view; and the ways are infinite in which he occupies his thoughts about the fears, or hopes, or expectations of a future life. I believe that the truth of that future cannot be brought to his knowledge by any exertion of his mental powers, however exalted they may be; that it is made known to him by other teaching than his own, and is received through simple belief of the testimony given. Let no one suppose for a moment that the self-education I am about to commend, in respect of the things of this life, extends to any considerations of the hope set before us, as if man by reasoning could find out G.o.d. It would be improper here to enter upon this subject further than to claim an absolute distinction between religious and ordinary belief. I shall be reproached with the weakness of refusing to apply those mental operations which I think good in respect of high things to the very highest. I am content to bear the reproach. Yet even in earthly matters I believe that 'the invisible things of Him from the creation of the world are clearly seen, being understood by the things that are made, even His eternal power and G.o.dhead;' and I have never seen anything incompatible between those things of man which can be known by the spirit of man which is within him, and those higher things concerning his future which he cannot know by that spirit."

On more than one occasion the late Prince Consort had discussed physical questions with Faraday, and in 1858 the Queen offered him a house on Hampton Court Green. This was his home until August 25, 1867.

He saw not only the magnetic spark, which he had first produced, employed in the lighthouses at the South Foreland and Dungeness, but he saw also his views respecting lines of electric induction examined and confirmed by the investigations of Thomson and Clerk Maxwell.

Of the ninety-five distinctions conferred upon him, we need only mention that of Commandant of the Legion of Honour, which he received in January, 1856.

JAMES CLERK MAXWELL.

The story of the life of James Clerk Maxwell has been told so recently by the able pen of his lifelong friend, Professor Lewis Campbell, that it is unnecessary, in the few pages which now remain to us, to attempt to give a repet.i.tion of the tale which would not only fail to do justice to its subject, but must of necessity fall far short of the merits of the (confessedly imperfect) sketch which has recently been placed within the reach of all. Looking back on the life of Clerk Maxwell, he seems to have come amongst us as a light from another world--to have but partly revealed his message to minds too often incapable of grasping its full meaning, and all too soon to have returned to the source from whence he came. There was scarcely any branch of natural philosophy that he did not grapple with, and upon which his vivid imagination and far-seeing intelligence did not throw light. He was born a philosopher, and at every step Nature partly drew aside the veil and revealed that which was hidden from a gaze less prophetic. A very brief sketch of the princ.i.p.al incidents in his life may, however, not be out of place.

James Clerk Maxwell was born in Edinburgh, on June 13, 1831. His father, John Clerk Maxwell, was the second son of James Clerk, of Penicuik, and took the name of Maxwell on inheriting the estate at Middlesbie. His mother was the daughter of R. H. Cay, Esq., of North Charlton, Northumberland. James was the only child who survived infancy.

Some years before his birth his parents had built a house at Glenlair, which had been added to their Middlesbie estate, and resided there during the greater part of the year, though they retained their house in Edinburgh. Hence it was that James's boyish days were spent almost entirely in the country, until he entered the Edinburgh Academy in 1841. As a child, he was never content until he had completely investigated everything which attracted his attention, such as the hidden courses of bell-wires, water-streams, and the like. His constant question was "What's the go o' that?" and, if answered in terms too general for his satisfaction, he would continue, "But what's the particular go of it?" This desire for the thorough investigation of every phenomenon was a characteristic of his mind through life.

From a child his knowledge of Scripture was extensive and accurate, and when eight years old he could repeat the whole of the hundred and nineteenth psalm. About this time his mother died, and thenceforward he and his father became constant companions. Together they would devise all sorts of ingenious mechanical contrivances. Young James was essentially a child of nature, and free from all conventionality. He loved every living thing, and took delight in petting young frogs, and putting them into his mouth to see them jump out. One of his attainments was to paddle on the duck-pond in a wash-tub, and to make the vessel go "without spinning"--a recreation which had to be relinquished on washing-days. He was never without the companionship of one or two terriers, to whom he taught many tricks, and with whom he seemed to have complete sympathy.

As a boy, Maxwell was not one to profit much by the ordinary teaching of the schools, and experience with a private tutor at home did not lead to very satisfactory results. At the age of ten, therefore, he was sent to the Edinburgh Academy, under the care of Archdeacon Williams, who was then rector. On his first appearance in this fashionable school, he was naturally a source of amus.e.m.e.nt to his companions; but he held his ground, and soon gained more respect than he had previously provoked ridicule. While at school in Edinburgh, he resided with his father's sister, Mrs. Wedderburn, and devoted a very considerable share of his time and attention to relieving the solitude of the old man at Glenlair, by letters written in quaint styles, sometimes backwards, sometimes in cypher, sometimes in different colours, so arranged that the characters written in a particular colour, when placed consecutively, formed another sentence. All the details of his school and home life, and the special peculiarities of the masters at the academy, were thus faithfully transmitted to his father, by whom the letters were religiously preserved. At thirteen he had evidently made progress in solid geometry, though he had not commenced Euclid, for he writes to his father, "I have made a tetrahedron, a dodecahedron, and two other hedrons whose names I don't know." In these letters to Glenlair he generally signed himself, "Your most obedient servant." Sometimes his fun found vent even upon the envelope; for example:--

"Mr. John Clerk Maxwell, "Postyknowswere, "Kirkpatrick Durham, "Dumfries."

Sometimes he would seal his letters with electrotypes of natural objects (beetles, etc.), of his own making. In July, 1845, he writes:--

I have got the eleventh prize for scholarship, the first for English, the prize for English verses, and the mathematical medal.

When only fifteen a paper on oval curves was contributed by him to the _Proceedings of the Royal Society of Edinburgh_. In the spring of 1847 he accompanied his uncle on a visit to Mr. Nicol, the inventor of the Nicol prism, and on his return he made a polariscope with gla.s.s and a lucifer-match box, and sketched in water-colours the chromatic appearances presented by pieces of unannealed gla.s.s which he himself prepared. These sketches he sent to Mr. Nicol, who presented him in return with a pair of prisms of his own construction. The prisms are now in the Cavendish Laboratory at Cambridge. Maxwell found that, for unannealed gla.s.s, pieces of window-gla.s.s placed in bundles of eight or nine, one on the other, answered the purpose very well. He cut the figures, triangles, squares, etc., with a diamond, heated the pieces of gla.s.s on an iron plate to redness in the kitchen fire, and then dropped them into a plate of iron sparks (scales from the smithy) to cool.

In 1847 Maxwell entered the University of Edinburgh, and during his course of study there he contributed to the Royal Society of Edinburgh papers upon rolling curves and on the equilibrium of elastic solids.

His attention was mostly devoted to mathematics, physics, chemistry, and mental and moral philosophy. In 1850 he went to Cambridge, entering Peterhouse, but at the end of a year he "migrated" to Trinity; here he was soon surrounded with a circle of friends who helped to render his Cambridge life a very happy one. His love of experiment sometimes extended to his own mode of life, and once he tried sleeping in the evening and working after midnight, but this was soon given up at the request of his father. One of his friends writes, "From 2 to 2.30 a.m. he took exercise by running along the upper corridor, _down_ the stairs, along the lower corridor, then _up_ the stairs, and so on until the inhabitants of the rooms along his track got up and laid _perdus_ behind their sporting-doors, to have shots at him with boots, hair-brushes, etc., as he pa.s.sed." His love of fun, his sharp wit, his extensive knowledge, and above all, his complete unselfishness, rendered him a universal favourite in spite of the temporary inconveniences which his experiments may have occasionally caused to his fellow-students.

An undergraduate friend writes, "Every one who knew him at Trinity can recall some kindness or some act of his which has left an ineffaceable impression of his goodness on the memory--for 'good' Maxwell was in the best sense of the word." The same friend wrote in his diary in 1854, after meeting Maxwell at a social gathering, "Maxwell, as usual, showing himself acquainted with every subject on which the conversation turned. I never met a man like him. I do believe there is not a single subject on which he cannot talk, and talk well too, displaying always the most curious and out-of-the-way information."

His private tutor, the late well-known Mr. Hopkins, said of him, "It is not possible for that man to think incorrectly on physical subjects."

In 1854 Maxwell took his degree at Cambridge as second wrangler, and was bracketed with the senior wrangler (Mr. E. J. Routh) for the Smith's prize. During his undergraduate course, he appears to have done much of the work which formed the basis of his subsequent papers on electricity, particularly that on Faraday's lines of force. The colour-top and colour-box appear also to have been gradually developing during this time, while the principle of the stereoscope and the "art of squinting" received their due share of attention.

Shortly after his degree, he devoted a considerable amount of time to the preparation of a ma.n.u.script on geometrical optics, which was intended to form a university text-book, but was never completed. In the autumn of 1855 he was elected Fellow of Trinity. About this time the colour-top was in full swing, and he also constructed an ophthalmoscope. In May, 1855, he writes:--

The colour trick came off on Monday, 7th. I had the proof-sheets of my paper, and was going to read; but I changed my mind and talked instead, which was more to the purpose. There were sundry men who thought that blue and yellow make green, so I had to undeceive them. I have got Hay's book of colours out of the University Library, and am working through the specimens, matching them with the top.

The "colour trick" came off before the Cambridge Philosophical Society.

While a Bachelor Fellow, Maxwell gave lectures to working men in Barnwell, besides lecturing in college. His father died in April, 1856, and shortly afterwards he was appointed Professor of Natural Philosophy in Marischal College, Aberdeen. This appointment he held until the fusion of the college with King's College in 1860. These four years were very productive of valuable work. During them the dynamical top was constructed, which ill.u.s.trates the motion of a rigid body about its axis of greatest, least, or mean moment of inertia; for, by the movement of certain screws, the axis of the top may be made to coincide with any one at will. The Adams Prize Essay on the stability of Saturn's rings belongs also to this period. In this essay Maxwell showed that the phenomena presented by Saturn's rings can only be explained on the supposition that they consist of innumerable small bodies--"a flight of brickbats"--each independent of all the others, and revolving round Saturn as a satellite. He compared them to a siege of Sebastopol from a battery of guns measuring thirty thousand miles in one direction, and a hundred miles in the other, the shots never stopping, but revolving round a circle of a hundred and seventy thousand miles radius. A solid ring of such dimensions would be completely crushed by its own weight, though made of the strongest material of which we have any knowledge. If revolving at such a rate as to balance the attraction of the planet at one part, the stress in other parts would be more than sufficient to crush or tear the ring.

Laplace had shown that a narrow ring might revolve about the planet and be stable if so loaded that its centre of gravity was at a considerable distance from its centre, and thought that Saturn's rings might consist of a number of such unsymmetrical rings--a theory to which some support was given by the many small divisions observable in the bright rings. Maxwell showed that, for stability, the ma.s.s required to load each of Laplace's rings must be four and a half times that of the rest of the ring; and the system would then be far too artificially balanced to be proof against the action of one ring on another. He further showed that, in liquid rings, waves would be produced by the mutual action of the rings, and that before long some of these waves would be sure to acquire such an amplitude as would cause the rings to break up into small portions. Finally, he concluded that the only admissible theory is that of the independent satellites, and that the _average_ density of the rings so found cannot be much greater than that of air at ordinary pressure and temperature.

While he remained at Aberdeen, Maxwell lectured to working men in the evenings, on the principles of mechanics. On the whole, it is doubtful whether Aberdeen society was as congenial to him as that of Cambridge or Edinburgh. He seems not to have been understood even by his colleagues. On one occasion he wrote:--

Gaiety is just beginning here again.... No jokes of any kind are understood here. I have not made one for two months, and if I feel one coming I shall bite my tongue.

But every cloud has its bright side, and, however Maxwell may have been regarded by his colleagues, he was not long without congenial companionships. An honoured guest at the home of the Princ.i.p.al, "in February, 1858, he announced his betrothal to Katherine Mary Dewar, and they were married early in the following June." Professor Campbell speaks of his married life as one of unexampled devotion, and those who enjoyed the great privilege of seeing him at home could more than endorse the description.

In 1860 Maxwell accepted the chair of Natural Philosophy at King's College, London. Here he continued his lectures to working men, and even kept them up for one session after resigning the chair in 1865.

On May 17, 1861, he gave his first lecture at the Royal Inst.i.tution, on "The Theory of the Three Primary Colours." This lecture embodies many of the results of his work with the colour-top and colour-box, to be again referred to presently. While at King's College, he was placed on the Electrical Standards Committee of the British a.s.sociation, and most of the work of the committee was carried out in his laboratory.

Here, too, he compared the electro-static repulsion between two discs of bra.s.s with the electro-magnetic attraction of two coils of wire surrounding them, through which a current of electricity was allowed to flow, and obtained a result which he afterwards applied to the electro-magnetic theory of light. The colour-box was perfected, and his experiments on the viscosity of gases were concluded during his residence in London. These last were described by him in the Bakerian Lecture for 1866.

After resigning the professorship at King's College, Maxwell spent most of his time at Glenlair, having enlarged the house, in accordance with his father's original plans. Here he completed his great work on "Electricity and Magnetism," as well as his "Theory of Heat," an elementary text-book which may be said to be without a parallel.