Chapter V: Scientific Researches: Third Period (2)

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

[Sidenote: MORE NEW WORDS.]

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

{ paramagnetic
Magnetic {
{ diamagnetic.

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

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

July, 1850.

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

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

Royal Institution, November 19, 1850.

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

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

[Sidenote: PAPERS TO BE LET LOOSE.]

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

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

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

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

Royal Institution, February 4, 1851.

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

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

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

[Sidenote: ATMOSPHERIC MAGNETISM.]

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

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

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

M. FARADAY.

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

[Sidenote: LINES OF MAGNETIC FORCE.]

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

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

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

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

[Sidenote: THE FUNCTIONS OF THE ÆTHER.]

How the magnetic force is transferred through bodies or through
space we know not--whether the result is merely action at a
distance, as in the case of gravity, or by some intermediate
agency, as in the cases of light, heat, the electric current,
and, as I believe, static electric action. The idea of magnetic
fluids, as applied by some, or of magnetic centres of action,
does not include that of the latter kind of transmission,
but the idea of lines of force does. Nevertheless, because a
particular method of representing the forces does not include
such a mode of transmission, the latter is not disproved, and
that method of representation which harmonises with it may be
the most true to nature. The general conclusion of philosophers
seems to be that such cases are by far the most numerous. And
for my own part, considering the relation of a vacuum to the
magnetic force, and the general character of magnetic phenomena
external to the magnet, I am more inclined to the notion that
in the transmission of the force there is such an action,
external to the magnet, than that the effects are merely
attraction and repulsion at a distance. _Such an action may be
a function of the æther, for it is not at all unlikely that if
there be an æther, it should have other uses than simply the
conveyance of radiations._[55]

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

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

[Sidenote: THE ELECTROTONIC STATE.]

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

[Sidenote: NOVELTY OF FARADAY’S VIEWS.]

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

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

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

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

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

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

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

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

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

[Sidenote: ELECTRIC LIGHT IN LIGHTHOUSES.]

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

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

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

[Sidenote: HYPOTHESIS AND EXPERIMENT.]

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