Lord Kelvin Part 13

In order to understand this argument, the reader must bear in mind some fundamental facts of the flow of heat in a solid. Let him imagine a slab of any uniform material, say sandstone or marble, the two parallel faces of which are continually maintained at two different temperatures, uniform over each face. For example, steam may be continually blown against one face, while ice-cold water is made to flow over the other.

Heat will flow across the slab from the hotter face to the colder. It will be found that the rate of flow of heat per unit area of face, that is per square centimetre, or per square inch, is proportional to the difference of the temperatures in the slab at the two faces, and inversely proportional to the thickness of the slab. In other words, it is proportional to the fall of temperature from one face to the other taken per unit of the thickness, that is, to the "gradient of temperature" from one face to the other. Moreover, comparing the flow in one substance with the flow in another, we find it different in different substances for the same gradient of temperature. Thus we get finally a flow of heat across unit area of the slab which is equal to the gradient of temperature multiplied by a number which depends on the material: that number is called the "conductivity" of the substance.

Now, borings made in the earth show that the temperature increases inwards, and the same thing is shown by the higher temperatures found in deeper coal mines. By means of thermometers sunk to different depths, the rate of increase of temperature with depth has been determined.

Similar observations show that the daily and annual variations of temperature caused by the succession of day and night, and summer and winter, penetrate to only a comparatively small depth below the surface--three or four feet in the former case, sixty or seventy in the latter. Leaving these variations out of account, since the average of their effects over a considerable interval of time must be nothing, we have in the earth a body at every point of the crust of which there is a gradient of increasing temperature inwards. The amount of this may be taken as one degree of Fahrenheit's scale for every 50 feet of descent.

This gradient is not uniform, but diminishes at greater depths.

Supposing the material of uniform quality as regards heat-conducting power, the mathematical theory of a cooling globe of solid material (or of a straight bar which does not lose heat from its sides) gives on certain suppositions the gradients at different depths. The surface gradient of 1 F. in 50 feet may be taken as holding for 5000 feet or 6000 feet or more.

This gradient of diminution of temperature outwards leads inevitably to the conclusion that heat must be constantly flowing from the interior of the earth towards the surface. This is as certain as that heat flows along a poker, one end of which is in the fire, from the heated end to the other. The heat which arrives at the surface of the earth is radiated to the atmosphere or carried off by convection currents; there is no doubt that it is lost from the earth. Thus the earth must be cooling at a rate which can be calculated on certain a.s.sumptions, and it is possible on these a.s.sumptions to calculate backwards, and determine the interval of time which must have elapsed since the earth was just beginning to cool from a molten condition, when of course life cannot have existed on its surface, and those geological changes which have effected so much can hardly have began.

Considering a globe of uniform material, and of great radius, which was initially at one temperature, and at a certain instant had its surface suddenly brought to, let us say, the temperature of melting ice, at which the surface was kept ever after, we can find, by Fourier's mathematical theory of the flow of heat, the gradient of temperature at any subsequent time for a point on the surface, or at any specified distance within it. For a point on the surface this gradient is simply proportional to the initial uniform temperature, and inversely proportional to the square root of the product of the "diffusivity" of the material (the ratio of the conductivity to the specific heat) by the interval of time which has elapsed since the cooling was started. Taking a foot as the unit of length, and a year as the unit of time, we find the diffusivity of the surface strata to be 400. If we take the initial temperature as 7000 degrees F.--which is high enough for melting rock--and take the interval of time which has elapsed as 100,000,000 years, we obtain at the surface a gradient approximately equal to that which now exists. A greater interval of time would give a lower gradient, a smaller interval would give a higher gradient than that which exists at present. A lower initial temperature would require a smaller interval of time, a higher initial temperature a longer interval for the present gradient.

With the initial temperature of 7,000 degrees F., an interval of 4,000,000 years would give a surface gradient of 1 F. in 10 ft. Thus, on the a.s.sumption made, the surface gradient of temperature has diminished from 1?10 to 1?50 in about 96,000,000 years. After 10,000 years from the beginning of the cooling the gradient of temperature would be 2 F. per foot. But, as Thomson showed, such a large gradient would not lead to any sensible augmentation of the surface temperature, for "the radiation from earth and atmosphere into s.p.a.ce would almost certainly be so rapid" as to prevent this. Hence he inferred that conducted heat, even at that early period, could not sensibly affect the general climate.

Two objections (apart from the a.s.sumptions already indicated) will readily occur to any one considering this theory, and these Thomson answered by antic.i.p.ation. The first is, that no natural action could possibly bring the surface of a uniformly heated globe instantaneously to a temperature 7000 lower, and keep it so ever after. In reply to this Thomson urged "that a large ma.s.s of melted rock, exposed freely to our earth and sky, will, after it once becomes crusted over, present in a few hours, or a few days, or at most a few weeks, a surface so cool that it can be walked over with impunity. Hence, after 10,000 years, or indeed, I may say, after a single year, its condition will be sensibly the same as if the actual lowering of temperature experienced by the surface had been produced in an instant, and maintained constant ever after." The other objection was, that the earth was probably never a uniformly heated solid 7000 F. above the present surface temperature as a.s.sumed for the purpose of calculation. This Thomson answers by giving reasons for believing that "the earth, although once all melted, or melted all round its surface, did, in all probability, really become a solid at its melting temperature all through, or all through the outer layer which has been melted; and not until the solidification was thus complete, or nearly so, did the surface begin to cool."

Thomson was inclined to believe that a temperature of 7000 F. was probably too high, and results of experiments on the melting of basalt and other rocks led him to prefer a much reduced temperature. This, as has already been pointed out, would give a smaller value for the age of the earth. In a letter on the subject published in Nature (vol. 51, 1895) he states that he "is not led to differ much" from an estimate of 24,000,000 years founded by Mr. Clarence King (_American Journal of Science_, January 1893) on experiments on the physical properties of rocks at high temperatures.

It is to be observed that the a.s.sumptions made above that the physical constants of the material are constant throughout the earth, and at all temperatures, are confessedly far from the truth. Nevertheless Thomson strongly held that the uncertainty of the data can at most extend the earth's age to some value between 20,000,000 and 200,000,000 of years, and that the enormously long periods which were wont to be asked for by geologists and biologists for the changes of the earth's surface and the development of its flora and fauna, cannot possibly be conceded.

In Nature for January 3, 1895, Professor John Perry suggested that very possibly the conductivity of the material composing the interior of the earth was considerably higher than that of the surface strata. If this were so, then, as can be shown without difficulty, the attainment of the present gradient would be very greatly r.e.t.a.r.ded, and therefore the age of the earth correspondingly increased. The question then arose, and was discussed, as to whether the rocks and other materials at high temperatures were more or less conducting than at low temperatures, and experiments on the subject were inst.i.tuted and carried out. On the whole, the evidence seemed to show that the conductivity of most substances is diminished, not increased, by the rise of temperature, and so far as it went, therefore, the evidence was against Professor Perry's suggestion. On the other hand, he contended that the inside of the earth may be a ma.s.s of great rigidity, partly solid and partly fluid, possessing a "quasi-conductivity" which might greatly increase the period of cooling. The subject is a difficult one both from a mathematical and from the physical point of view, and further investigation is necessary, especially of the behaviour of materials under the enormous stresses which they undoubtedly sustain in the interior of the earth.

After the publication of the paper on Geological Time a reply to it was made by Professor Huxley, in an address to the Geological Society of London, delivered on February 19, 1869. He adopted the role of an advocate retained for the defence of geology against what seems to have been regarded as an unwarranted attack, made by one who had no right to offer an opinion on a geological question. For, after a long and eloquent "pleading," he concludes his address with the words: "My functions, as your advocate, are at an end. I speak with more than the sincerity of a mere advocate when I express the belief that the case against us has entirely broken down. The cry for reform which has been raised from without is superfluous, inasmuch as we have long been reforming from within with all needful speed; and the critical examination of the grounds upon which the very grave charge of opposition to the principles of Natural Philosophy has been brought against us, rather shows that we have exercised a wise discrimination in declining to meddle with our foundations at the bidding of the first pa.s.ser-by who fancies our house is not so well built as it might be." To this Thomson rejoined in an address ent.i.tled "Of Geological Dynamics,"

also delivered to the Geological Society of Glasgow on April 5, 1869; and to this, with Professor Huxley's address, the reader must be referred for the objection, brought against Thomson's arguments, and the replies which were immediately forthcoming. This is not the place to discuss the question, but reference may be made to an interesting paper on the subject in the _Glasgow Herald_ for February 22, 1908, by Professor J. W. Gregory, in which the suggestion of Professor Perry, of a nearer approach to uniformity of temperature in the interior of the earth than Thomson had thought possible, is welcomed as possibly extending the interval of time available to a period sufficient for all purposes. In Professor Gregory's opinion, "Lord Kelvin in one respect showed a keener insight than Huxley, who, referring to possible changes in the rate of rotation of the earth, or in the heat given forth from the sun or in the cooling of the earth, declared that geologists are Gallios, 'who care for none of these things.' An ever-increasing school of geologists now cares greatly for these questions, and reveres Lord Kelvin as one of the founders of the geology of the inner earth."

After all, the problem is not one to be dealt with by the geologist or biologist alone, but to be solved, so far as it can be solved at all, by a consideration of all relevant evidence, from whatsoever quarter it may come. It will not do in these days for scientific men to shut themselves up within their special departments and to say, with regard to branches of science which deal with other aspects of nature and other problems of the past, present and future of that same earth on which all dwell and work, that they "care for none of these things." This is an echo of an old spirit, not yet dead, that has done much harm to the progress of science. The division of science into departments is unavoidable, for specialisation is imperative; but it is all the more necessary to remember that the divisions set up are more or less arbitrary, and that there are absolutely no frontiers to be guarded and enforced. Chemistry, physiology, and physics cannot be walled off from one another without loss to all; and geology has suffered immensely through its having been regarded as essentially a branch of natural history, the devotees of which have no concern with considerations of natural philosophy. Lord Kelvin's dignified questions were unanswerable. "Who are the occupants of 'our house,' and who is the 'pa.s.ser-by'? Is geology not a branch of physical science? Are investigations, experimental and mathematical, of underground temperature not to be regarded as an integral part of geology?... For myself, I am anxious to be regarded by geologists not as a mere pa.s.ser-by, but as one constantly interested in their grand subject, and anxious in any way, however slight, to a.s.sist them in their search for truth."

CHAPTER XIII

BRITISH a.s.sOCIATION COMMITTEE ON ELECTRICAL STANDARDS

When Professor Thomson began his work as a teacher in the University of Glasgow, there was, as has already been noticed, great vagueness of specification of physical quant.i.ties. Few of the formal definitions of units of measurement, now to be found in the pages of every elementary text book, had been framed, and there was much confusion of quant.i.ties essentially distinct, a confusion which is now, to some extent at least, guarded against by the adoption of a definite unit, with a distinctive name for each magnitude to be measured. Thus rate of working, or activity, was confused with work done; the condition for maximum activity in the circuit of a battery or dynamo was often quoted as the condition of greatest efficiency, that is of greatest economy of energy, although it was exactly that in which half the available energy was wasted.

Partly as a consequence of this vagueness of specification, there was a great want of knowledge of the values of physical constants; for without exact definitions of quant.i.ties to be determined, such definitions as would indicate units for their measurement, related to ordinary dynamical units according to a consistent scheme, it was impossible to devise satisfactory experimental methods to do for electricity and magnetism what had been done by Regnault and others for heat.

The first steps towards the construction of a complete system of units for the quant.i.tative measurement of magnetic and electric quant.i.ties were taken by Gauss, in his celebrated paper ent.i.tled _Intensitas vis magneticae terrestris ad mensuram absolutam revocata_, published in 1832.

In this he showed how magnetic forces could be expressed in absolute units, and thus be connected with the absolute dynamical units which Gauss, in the same paper, based on chosen fundamental units of length, ma.s.s, and time. Thus the modern system of absolute units of dynamical quant.i.ties, and its extension to magnetism, are due to the practical insight of a great mathematician, not to the experimentalists or "practicians" of the time.

Methods of measuring electric quant.i.ties in absolute units were described by W. Weber, in Parts II and III of his _Elecktrodynamische Maa.s.sbestimmungen_, published in 1852. These were great steps in advance, and rendered further progress in the science of absolute measurement comparatively easy. But they remained the only steps taken until the British a.s.sociation Committee began their work. We have already (pp. 74-76) referred to the great importance of that work, not only for practical applications but also for the advancement of science.

But it was not a task which struck the imagination or excited the wonder of the mult.i.tude. For the realisation of standards of resistance, for example, involved long and tedious investigations of the effects of impurities on the resistance of metals, and the variation of resistance caused by change of temperature and lapse of time. Then alloys had to be sought which would have a temperature effect of small amount, and which were stable and durable in all their properties.

The discoveries of the experimentalist who finds a new element of hitherto undreamed-of properties attract world-wide attention, and the glory of the achievement is deservedly great. But the patient, plodding work which gives a universal system of units and related standards, and which enables a great physical subject like electricity and magnetism to rise from a mere enumeration of qualitative results to a science of the most delicate and exact measurement, and to find its practical applications in all the affairs of daily life and commerce, is equally deserving of the admiration and grat.i.tude of mankind. Yet it receives little or no recognition.

The construction of a standard of resistance was the first task undertaken by the committee; but other units, for example of quant.i.ty of electricity, intensity of electric field and difference of potential, had also to be defined, and methods of employing them in experimental work devised. It would be out of place to endeavour to discuss these units here, but some idea of the manner in which their definitions are founded on dynamical conceptions may be obtained from one or two examples. Therefore we shall describe two simple experiments, which will ill.u.s.trate this dynamical foundation. An account has been given in Chapter XI of the series of electrometers which Thomson invented for the measurement of differences of electric potential. These all act by the evaluation in terms of ordinary dynamical units of the force urging an electrified body from a place of higher towards a place of lower potential.

Some indication of the meaning of electrical quant.i.ties has been given in Chapter IV. Difference of electric potential between two points in an electric field was there defined as the dynamical work done in carrying a unit of positive electricity against the forces of the field from the point of lower to the point of higher potential. Now by the definition of unit quant.i.ty of electricity given in electrical theory--that quant.i.ty which, concentrated at a point at unit distance from an equal quant.i.ty also concentrated at a point, is repelled with unit force--we can find, by the simple experiment of hanging two pith b.a.l.l.s (or, better, two hollow, gilded beads of equal size) by two fine fibres of quartz, a metre long, say, electrifying the two b.a.l.l.s as they hang in contact, and observing the distance at which they then hang, the numerical magnitude in absolute units of a charge of electricity, and apply that to finding the charge on a large spherical conductor and the potential at points in its field also in absolute units. If m be the ma.s.s of a ball, g gravity in cm. sec. units, d the distance in cms. of the centres of the b.a.l.l.s apart, and l the length in cms. of a thread, the charge q, say, on each ball is easily found to be v[mgd?v{4^(l - d)}]. Thus the charge is got in absolute centimetre-gramme-second units in terms of the ma.s.s m obtained by ordinary weighing, and l and d obtained by easy and exact measurements.

If one of the b.a.l.l.s be now taken away without discharging the other, and the latter be placed in the field of a large electrified spherical conductor, the fibre will be deflected from the vertical by the force on the ball. Let the two centres be now on the same level. That force is got at once from the angle of deflection (which is easily observed), the charge on the ball, and the value of m. The electric field-intensity is obtained by dividing the value of the force by q. The field intensity multiplied by D, the distance apart in cms. of the centres of the ball and the conductor, gives the potential at the centre of the ball in C.G.S. units. Multiplication again by D gives the charge on the conductor.

When it made its first Report in 1862 (to the meeting at Cambridge) the committee consisted of Professors A. Williamson, C. Wheatstone, W.

Thomson, W. H. Miller, Dr. A. Matthiessen, and Mr. F. Jenkin. At the next meeting, at Newcastle, it had been augmented by the addition of Messrs. Balfour Stewart, C. W. Siemens, Professor Clerk Maxwell, Dr.

Joule, Dr. Esselbach, and Sir Charles Bright. The duty with which the committee had been charged was that of constructing a suitable standard of resistance. A reference to the account given in Chapter X above, of the derivation of what came to be called the electromagnetic unit of difference of potential, or electromotive force, by means of a simple magneto-electric machine--a disk turning on a uniform magnetic field, or the simple rails and slider and magnetic field arrangement there described--will show how from this unit and the electromagnetic unit of current (there also defined) the unit of resistance is defined. It is the resistance of the circuit of slider, rails, and connecting wire, when with this electromagnetic unit of electromotive force the unit of current is made to flow.

This was one clear and definite way of defining the unit of current, and of attaining the important object of connecting the units in such a way that the rate of working in a circuit, or the energy expended in any time, should be expressed at once in ordinary dynamical units of activity or energy. A considerable number of proposals were discussed by the committee; but it was finally determined to take the basis here indicated, and to realise a standard of resistance in material of constant and durable properties, which should have some simple multiple of the unit of resistance, in the system of dynamical units based on the centimetre as unit of length, the gramme as unit of ma.s.s, and the second as unit of time--the so-called C.G.S. system. The comparison of the different metals and alloys available was a most important but exceedingly laborious series of investigations, carried out mainly by Dr. Matthiessen and Professor Williamson.

Professor Thomson suggested to the committee the celebrated method of determining the resistance of a circuit by revolving a coil, which formed the main part of the circuit about a vertical axis in the earth's magnetic field. An account of the experiments made with this method is contained in the Report of 1863. They were carried out at King's College, London, where Maxwell was then Professor of Experimental Physics, by Maxwell, Balfour Stewart, and Fleeming Jenkin. The theoretical discussion and the description of the experiments was written by Maxwell, the details of the apparatus were described by Jenkin.

The principle of the method is essentially the same as that of the simple magneto-electric machine, to which reference has just been made.

Two parallel coils of wire were wound in channels cut round rings of bra.s.s, which, however, were cut across by slots filled with vulcanite, to prevent induced currents from circulating in the bra.s.s. These coils were mounted in a vertical position and could be driven as a rigid system, at a constant measured speed, about a vertical axis pa.s.sing through the centre of the system. Between the coils at this centre was hung, from a steady support, a small magnetic needle by a single fibre of silk; and a surrounding screen prevented the needle and suspension from being affected by currents of air.

The ends of the coil were connected together so that the whole revolved as a closed circuit about the vertical axis. When the coil system was at right angles to the magnetic meridian there was a magnetic induction through it of amount AH, where A denotes the effective area of the coils, and H the horizontal component of the earth's magnetic field. By one half-turn the coil was reversed with reference to this magnetic induction, and as the coil turned an induced current was generated, which depended at any instant on the rate at which the magnetic induction was varying at the instant, on the inductive electromotive force due to the varying of the current in the coil itself, and on the resistance of the circuit. A periodic current thus flowed in one direction _relatively to the coil_ in one half-turn from a position perpendicular to the magnetic meridian, and in the opposite direction in the next half-turn. But as the position of the coil was reversed in every half-turn as well as the current in it, the current flowed on the whole in the same average direction relatively to the needle, and but for self-induction would have had its maximum value always when the plane of the coil was in the magnetic meridian.

The needle was deflected as it would have been by a certain average current, and the deflection was opposed by the action of the earth's horizontal magnetic field H. But this was the field cut by the coil as it turned, and therefore (except for a small term depending on the turning of the coil in the field of the needle) the value of H did not appear in the result, and did not require to be known.

Full details of the theory of this method and of the experiments carried out to test it will be found in various memoirs and treatises[23]; but it must suffice here to state that the resistance of the coil was determined in this way, by a large series of experiments, before and after every one of which the resistance was compared with that of a German-silver standard. The resistance of this standard therefore became known in absolute units, and copies of it, or multiples or sub-multiples of it, could be made.

A unit called the B.A. unit, which was intended to contain 10^9 C.G.S.

electromagnetic units of resistance, was constructed from these experiments, and copies of it were soon after to be found in nearly all the physical laboratories of the world. Resistance boxes were constructed by various makers, in which the coils were various multiples of the B.A. unit, so that any resistance within a certain range could be obtained by connecting these coils in series (which was easily done by removing short circuiting plugs), and thus the absolute units of current electromotive force and resistance came into general use.

In 1881 Lord Rayleigh and Professor Schuster carried out a very careful repet.i.tion of the British a.s.sociation experiments with the same apparatus at the Cavendish Laboratory, and obtained a somewhat different result. They found that the former result was about 1.17 per cent. too small. Lord Rayleigh next carried out an independent set of experiments by the same method with improved apparatus, and found that this percentage error must be increased to about 1.35.

It may be noticed here that the simple disk machine, of Thomson's ill.u.s.tration of the absolute unit of electromotive force, has been used by Lorenz to give a method of determining resistance which is now recognised as the best of all. It is sketched here that the reader may obtain some idea of later work on this very important subject; work which is a continuation of that of the original British a.s.sociation Committee by their successors. A circuit is made up of a standard coil of wire, the ends of which are made to touch at the circ.u.mference and near the centre of the disk, which is placed symmetrically with respect to a cylindrical coil, and within it. A current is sent round this coil from a battery, and produces a magnetic field within the coil, the lines of magnetic force of which pa.s.s across the plane of the disk. This current, or a measured fraction of it, is also made to flow through the standard coil. The disk is now turned at a measured speed about its axis, so that the electromotive force due to the cutting of the field tends to produce a current in the standard coil of wire. The electromotive force of the disk is made to oppose the potential difference between the ends of this coil due to the current, so that no current flows along the disk or the wires connecting it with the standard coil. The magnetic field within the coil can be calculated from the form and dimensions of the coil and the current in it (supposed for the moment to be known), and the electromotive force of the disk is obtained in terms of its dimensions and its speed and the field intensity. But this electromotive force, which is proportional to the current in the coil, is equal to the product of the resistance of the wire and the same current, or a known fraction of it. Thus the current appears on both sides of the equation and goes out, and the value of the resistance is found in absolute units.

Lord Rayleigh obtained, by this method, a result which showed that the B.A. unit was 1.323 per cent. too small; and exact experiments have been made by others with concordant results. Values of the units have been agreed on by International Congresses as exact enough for general work, and with these units all electrical researches, wherever made, are available for use by other experimenters.

A vast amount of work has been done on this subject during the last forty years, and though the value of the practical unit of resistance--10^9 C.G.S. units, now called the "ohm"--is taken as settled, and copies can now be had in resistance boxes, or separately, adjusted with all needful accuracy, at the National Physical Laboratory and at the Bureau of Standards at Washington, and elsewhere, experiments are being made on the exact measurement of currents; while a careful watch is kept on the standards laid up at these places to see whether any perceptible variation of their resistance takes place with lapse of time.

The British a.s.sociation Committee also worked out a complete system of units for all electrical and magnetic quant.i.ties, and gave the first systematic statement of their relations, that is, of the so-called dimensional equations of the quant.i.ties. This will be found in the works to which reference has already been made (p. 251).

CHAPTER XIV

THE BALTIMORE LECTURES

The Baltimore Lectures were delivered in 1884 at Johns Hopkins University, soon after the Montreal meeting of the British a.s.sociation.

The subject chosen was the Wave Theory of Light; and the idea underlying the course was to discuss the difficulties of this theory to "Professorial fellow-students in physical science." A stenographic report of the course was taken by Mr. A. S. Hathaway, and was published soon after. The lectures were revised by Lord Kelvin, and the book now known as _The Baltimore Lectures_ was published just twenty years later (in 1904) at the Cambridge University Press. It is absolutely impossible in such a memoir as the present to give any account of the discussions contained in the lectures as now published. The difficulties dealt with can for the most part only be understood by those who are acquainted with the wave theory of light in its details, and such readers will naturally go direct to the book itself.

Some of the difficulties, however, were frequently alluded to in Lord Kelvin's ordinary lectures, and all his old students will remember the animation with which he discussed the apparent anomaly of a medium like the luminiferous ether, which is of such enormous rigidity that (on the elastic solid theory) a wave of transverse oscillation is propagated through it with a speed of 3 10^10 centimetres (186,000 miles) per second, and yet appears to offer no impediment to the slow motion of the heavenly bodies. For Lord Kelvin adopted the elastic solid theory of propagation of light as "the only tenable foundation for the wave theory of light in the present state of our knowledge," and dismissed the electromagnetic theory (his words were spoken in 1884, it is to be remembered) with the statement of his strong view that an electric displacement perpendicular to the line of propagation, accompanied by a magnetic disturbance at right angles to both, is inadmissible.

And he goes on to say that "when we have an electromagnetic theory of light," electric displacement will be seen as in the direction of propagation, with Fresnelian vibrations perpendicular to that direction.

In the preface, of date January 1904, the insufficiency of the elastic solid theory is admitted, and the question of the electromagnetic theory again referred to. He says there that the object of the Baltimore Lectures was to ascertain how far the phenomena of light could be explained within the limits of the elastic solid theory. And the answer is "everything _non-magnetic; nothing magnetic_." But he adds, "The so-called electromagnetic theory of light has not helped us. .h.i.therto,"

and that the problem is now fully before physicists of constructing a "comprehensive dynamics of ether, electricity, and ponderable matter which shall include electrostatic force, magnetostatic force, electromagnetism, electrochemistry, and the wave theory of light."