this ball; a single puff of my breath moves it a little way from its
position of rest; it swings back towards me, and when it reaches the
limit of its swing I puff again. It now swings further; and thus by
timing the puffs I can so accumulate their action as to produce
oscillations of large amplitude. The ivory ball here has absorbed the
motion which my breath communicated to the air. I now bring the ball
to rest. Suppose, instead of the breath, a wave of air to strike
against it, and that this wave is followed by a series of others which
succeed each other exactly in the same intervals as my puffs; it is
obvious that these waves would communicate their motion to the ball
and cause it to swing as the puffs did. And it is equally manifest
that this would not be the case if the impulses of the waves were not
properly timed; for then the motion imparted to the pendulum by one
wave would be neutralised by another, and there could not be the
accumulation of effect obtained when the periods of the waves
correspond with the periods of the pendulum. So much for the
particular impulses absorbed by the pendulum. But if such a pendulum
set oscillating in air could produce waves in the air, it is evident
that the waves it would produce would be of the same period as those
whose motions it would take up or absorb most completely, if they
struck against it. Perhaps the most curious effect of these timed
impulses ever described was that observed by a watchmaker, named
Ellicott, in the year 1741. He left two clocks leaning against the
same rail; one of them, which we may call A, was set going; the other,
B, not. Some time afterwards he found, to his surprise, that B was
ticking also. The pendulums being of the same length, the shocks
imparted by the ticking of A to the rail against which both clocks
rested were propagated to B, and were so timed as to set B going.
Other curious effects were at the same time observed. When, the
pendulums differed from each other a certain amount, set B going, but
the reaction of B stopped A. Then B set A going, and the re-action of
A stopped B. When the periods of oscillation were close to each
other, but still not quite alike, the clocks mutually controlled each
other, and by a kind of compromise they ticked in perfect unison.
But what has all this to do with our present subject? The varied
actions of the universe are all modes of motion; and the vibration of
a ray claims strict brotherhood with the vibrations of our pendulum.
Suppose aethereal waves striking upon atoms which oscillate in the
same periods as the waves, the motion of the waves will be absorbed by
the atoms; suppose we send our beam of white light through a sodium
flame, the atoms of that flame will be chiefly affected by those
undulations which are synchronous with their own periods of vibration.
There will be on the part of those particular rays a transference of
motion from the agitated aether to the atoms of the volatilised metal,
which, as already defined, is absorption.
The experiment justifying this conclusion is now for the first time to
be made before a public audience. I pass a beam through our two
prisms, and the spectrum spreads its colours upon the screen. Between
the lamp and the prism I interpose a snapdragon light. Alcohol and
water are here mixed with common salt, and the metal dish that holds
them is heated by a spirit-lamp. The vapour from the mixture ignites
and we have a monochromatic flame. Through this flame the beam from
the lamp is now passing; and observe the result upon the spectrum. You
see a shady band cut out of the yellow,--not very dark, but
sufficiently so to be seen by everybody present.
But let me exalt this effect. Placing in front of the electric lamp
the intense flame of a large Bunsen's burner, a platinum capsule
containing a bit of sodium less than a pea in magnitude is plunged
into the flame. The sodium soon volatilises and burns with brilliant
incandescence. The beam crosses the flame, and at the same time the
yellow band of the spectrum is clearly and sharply cut out, a band of
intense darkness occupying its place. On withdrawing the sodium, the
brilliant yellow of the spectrum takes its proper place, while the
reintroduction of the flame causes the band to reappear.
Let me be more precise: The yellow colour of the spectrum extends
over a sensible space, blending on one side with the orange and on the
other with the green. The term 'yellow band' is therefore somewhat
indefinite. This vagueness may be entirely removed. By dipping the
carbon-point used for the positive electrode into a solution of common
salt, and replacing it in the lamp, the bright yellow band produced by
the sodium vapour stands out from the spectrum. When the sodium flame
is caused to act upon the beam it is that particular yellow band that
is obliterated, an intensely black streak occupying its place.
An additional step of reasoning leads to the conclusion that if,
instead of the flame of sodium alone, we were to introduce into the
path of the beam a flame in which lithium, strontium, magnesium,
calcium, &c, are in a state of volatilisation, each metallic vapour
would cut out a system of bands, corresponding exactly in position
with the bright bands of the same metallic vapour. The light of our
electric lamp shining through such a composite flame would give us a
spectrum cut up by dark lines, exactly as the solar spectrum is cut up
by the lines of Fraunhofer.
Thus by the combination of the strictest reasoning with the most
conclusive experiment, we reach the solution of one of the grandest of
scientific problems--the constitution of the sun. The sun consists of
a nucleus surrounded by a flaming atmosphere. The light of the
nucleus would give us a continuous spectrum, like that of our common
carbon-points; but having to pass through the photosphere, as our beam
had to pass through the flame, those rays of the nucleus which the
photosphere can itself emit are absorbed, and shaded spaces,
corresponding to the particular rays absorbed, occur in the spectrum.
Abolish the solar nucleus, and we should have a spectrum showing a
bright line in the place of every dark line of Fraunhofer. These
lines are therefore not absolutely dark, but dark by an amount
corresponding to the difference between the light of the nucleus
intercepted by the photosphere, and the light which issues from the
latter.
The man to whom we owe this noble generalisation is Kirchhoff,
Professor of Natural Philosophy in the University of Heidelberg;
[Footnote: Now Professor in the University of Berlin.] but, like
every other great discovery, it is compounded of various elements. Mr.
Talbot observed the bright lines in the spectra of coloured flames.
Sixteen years ago Dr. Miller gave drawings and descriptions of the
spectra of various coloured flames. Wheatstone, with his accustomed
ingenuity, analysed the light of the electric spark, and showed that
the metals between which the spark passed determined the bright bands
in the spectrum of the spark. Masson published a prize essay on these
bands; Van der Willigen, and more recently Plucker, have given us
beautiful drawings of the spectra, obtained from the discharge of
Ruhmkorff's coil. But none of these distinguished men betrayed the
least knowledge of the connection between the bright bands of the
metals and the dark lines of the solar spectrum. The man who came
nearest to the philosophy of the subject was Angstrom. In a paper
translated from Poggendorff's 'Annalen' by myself, and published in
the 'Philosophical Magazine' for 1855, he indicates that the rays
which a body absorbs are precisely those which it can emit when
rendered luminous. In another place, he speaks of one of his spectra
giving the general impression of a reversal of the solar spectrum.
Foucault, Stokes, and Thomson, have all been very close to the
discovery; and, for my own part, the examination of the radiation and
absorption of heat by gases and vapours, some of the results of which
I placed before you at the commencement of this discourse, would have
led me in 1859 to the law on which all Kirchhoff's speculations are
founded, had not an accident withdrawn me from the investigation. But
Kirchhoff's claims are unaffected by these circumstances. True, much
that I have referred to formed the necessary basis of his discovery;
so did the laws of Kepler furnish to Newton the basis of the theory of
gravitation. But what Kirchhoff has done carries us far beyond all
that had before been accomplished. He has introduced the order of law
amid a vast assemblage of empirical observations, and has ennobled our
previous knowledge by showing its relationship to some of the most
sublime of natural phenomena.
********************
XV. ELEMENTARY MAGNETISM.
A LECTURE TO SCHOOLMASTERS.
WE have no reason to believe that the sheep or the dog, or indeed any
of the lower animals, feel an interest in the laws by which natural
phenomena are regulated. A herd may be terrified by a thunderstorm;
birds may go to roost, and cattle return to their stalls, during a
solar eclipse; but neither birds nor cattle, as far as we know, ever
think of enquiring into the causes of these things. It is otherwise
with Man. The presence of natural objects, the occurrence of natural
events, the varied appearances of the universe in which he dwells
penetrate beyond his organs of sense, and appeal to an inner power of
which the senses are the mere instruments and excitants. No fact is
to him either original or final. He cannot limit himself to the
contemplation of it alone, but endeavours to ascertain its position in
a series to which uniform experience assures him it must belong. He
regards all that he witnesses in the present as the efflux and
sequence of something that has gone before, and as the source of a
system of events which is to follow. The notion of spontaneity, by
which in his ruder state he accounted for natural events, is
abandoned; the idea that nature is an aggregate of independent parts
also disappears, as the connection and mutual dependence of physical
powers become more and more manifest: until he is finally led to
regard Nature as an organic whole--as a body each of whose members
sympathises with the rest, changing, it is true, from age to age, but
changing without break of continuity in the relation of cause and
effect.
The system of things which we call Nature is, however, too vast and
various to be studied first-hand by any single mind. As knowledge
extends there is always a tendency to subdivide the field of
investigation. Its various parts are taken up by different minds, and
thus receive a greater amount of attention than could possibly be
bestowed on them if each investigator aimed at the mastery of the
whole. The centrifugal form in which knowledge, as a whole, advances,
spreading ever wider on all sides, is due in reality to the exertions
of individuals, each of whom directs his efforts, more or less, along
a single line. Accepting, in many respects, his culture from his
fellow-men--taking it from spoken words or from written books--in some
one direction, the student of Nature ought actually to touch his work.
He may otherwise be a distributor of knowledge, but not a creator, and
he fails to attain that vitality of thought, and correctness of
judgment, which direct and habitual contact with natural truth can
alone impart.
One large department of the system of Nature which forms the chief
subject of my own studies, and to which it is my duty to call your
attention this evening, is that of physics, or natural philosophy.
This term is large enough to cover the study of Nature generally, but
it is usually restricted to a department which, perhaps, lies closer
to our perceptions than any other. It deals with the phenomena and
laws of light and heat--with the phenomena and laws of magnetism and
electricity--with those of sound--with the pressures and motions of
liquids and gases, whether at rest or in a state of translation or of
undulation. The science of mechanics is a portion of natural
philosophy, though at present so large as to need the exclusive
attention of him who would cultivate it profoundly. Astronomy is the
application of physics to the motions of the heavenly bodies, the
vastness of the field causing it, however, to bed regarded as a
department in itself. In chemistry physical agents play important
parts. By heat and light we cause atoms and molecules to unite or to
fall asunder. Electricity exerts a similar power. Through their
ability to separate nutritive compounds into their constituents, the
solar beams build up the whole vegetable world, and by it the animal
world. The touch of the self-same beams causes hydrogen and chlorine
to; unite with sudden explosion, and to form by their combination a
powerful acid. Thus physics and chemistry intermingle. Physical
agents are, however, employed by the chemist as a means to an end;
while in physics proper the laws and phenomena of the agents
themselves, both qualitative and quantitative, are the primary objects
of attention.
My duty here to-night is to spend an hour in telling how this subject
is to be studied, and how a knowledge of it is to be imparted to
others. From the domain of physics, which would be unmanageable as a
whole, I select as a sample the subject of magnetism. I might readily
entertain you on the present occasion with an account of what natural
philosophy has accomplished. I might point to those applications of
science of which we hear so much in the newspapers, and which are so
often mistaken for science itself. I might, of course, ring changes
on the steam-engine and the telegraph, the electrotype and the
photograph, the medical applications of physics, and the various other
inlets by which scientific thought filters into practical life. That
would be easy compared with the task of informing you how you are to
make the study of physics the instrument of your pupil's culture; how
you are to possess its facts and make them living seeds which shall
take root and grow in the mind, and not lie like dead lumber in the
storehouse of memory. This is a task much heavier than the mere
recounting of scientific achievements; and it is one which, feeling my
own want of time to execute it aright, I might well hesitate to
accept.
But let me sink excuses, and attack the work before me. First and
foremost, then, I would advise you to get a knowledge of facts from
actual observation. Facts looked at directly are vital; when they
pass into words half the sap is taken out of them. You wish, for
example, to get a knowledge of magnetism; well, provide yourself with
a good book on the subject, if you can, but do not be content with
what the book tells you; do not be satisfied with its descriptive
woodcuts; see the operations of the force yourself. Half of our book
writers describe experiments which they never made, and their
descriptions often lack both force and truth; but, no matter how
clever or conscientious they may be, their written words cannot supply
the place of actual observation. Every fact has numerous radiations,
which are shorn off by the man who describes it.
Go, then, to a philosophical instrument maker, and give a shilling or
half a crown for a straight bar-magnet, or, if you can afford it,
purchase a pair of them; or get a smith to cut a length of ten inches
from a bar of steel an inch wide and half an inch thick; file its ends
smoothly, harden it, and get somebody like myself to magnetise it.
Procure some darning needles, and also a little unspun silk, which
will give you a suspending fibre void of torsion. Make little loop
of paper, or of wire, and attach your fibre to it. Do it neatly. In
the loop place a darning-needle, and bring the two ends or poles, as
they are called, of your bar-magnet successively up to the ends of the
needle. Both the poles, you find, attract both ends of the needle.
Replace the needle by a bit of annealed iron wire; the same effects
ensue. Suspend successively little rods of lead, copper, silver,
brass, wood, glass, ivory, or whalebone; the magnet produces no
sensible effect upon any of the substances. You thence infer a
special property in the case of steel and iron. Multiply your
experiments, However, and you will find that some other substances,
besides iron and steel, are acted upon by your magnet. A rod of the
metal nickel, or of the metal cobalt, from which the blue colour used
by painters is derived, exhibits powers similar to those observed with
the iron and steel.
In studying the character of the force you may, however, confine
yourself to iron and steel, which are always at hand. Make your
experiments with the darning-needle over and over again; operate on
both ends of the needle; try both ends of the magnet. Do not think
the work dull; you are conversing with Nature, and must acquire over
her language a certain grace and mastery, which practice can alone
impart. Let every movement be made with care, and avoid slovenliness,
from the outset. Experiment, as I have said, is the language by which
we address Nature, and through which she sends her replies; in the use
of this language a lack of straightforwardness is as possible, and as
prejudicial, as in the spoken language of the tongue. If, therefore,
you wish to become acquainted with the truth of Nature, you must from
the first resolve to deal with her sincerely.
Now remove your needle from its loop, and draw it from eye to point
along one of the ends of the magnet; resuspend it, and repeat your
former experiment. You now find that each extremity of the magnet
attracts one end of the needle, and repels the other. The simple
attraction observed in the first instance, is now replaced by a _dual_
force. Repeat the experiment till you have thoroughly observed the
ends which attract and those which repel each other.
Withdraw the magnet entirely from the vicinity of your needle, and
leave the latter freely suspended by its fibre. Shelter it as well as
you can from currents of air, and if you have iron buttons on your
coat, or a steel penknife in your pocket, beware of their action. If
you work at night, beware of iron candlesticks, or of brass ones with
iron rods inside. Freed from such disturbances, the needle takes up a
certain determinate position. It sets its length nearly north and
south. Draw it aside and let it go. After several oscillations it
will again come to the same position. If you have obtained your
magnet from a philosophical instrument maker, you will see a mark on
one of its ends. Supposing, then, that you drew your needle along the
end thus marked, and that the point of your needle was the last to
quit the magnet, you will find that the point turns to the south, the
eye of the needle turning towards the north. Make sure of this, and
do not take the statement on my authority.
Now take a second darning-needle like the first, and magnetise it in
precisely the same manner: freely suspended it also will turn its eye
to the north and its point to the south. Your next step is to examine
the action of the two needles which you have thus magnetised upon each
other.
Take one of them in your hand, and leave the other suspended; bring
the eye-end of the former near the eye-end of the latter; the
suspended needle retreats: it is repelled. Make the same experiment
with the two points; you obtain the same result, the suspended needle
is repelled. Now cause the dissimilar ends to act on each other--you
have attraction--point attracts eye, and eye attracts point. Prove
the reciprocity of this action by removing the suspended needle, and
putting the other in its place. You obtain the same result. The
attraction, then, is mutual, and the repulsion U mutual. You have
thus demonstrated in the clearest manner the fundamental law of
magnetism, that like poles repel, and that unlike poles attract, each
other. You may say that this is all easily understood without doing;
but _do it_, and your knowledge will not be confined to what I have
uttered here.
I have said that one end of your bar magnet has a mark upon it; lay
several silk fibres together, so as to get sufficient strength, or
employ a thin silk ribbon, and form a loop large enough to hold your
magnet. Suspend it; it turns its marked end towards the north. This
marked end is that which in England is called the north pole. If a
common smith has made your magnet, it will be convenient to determine
its north pole yourself, and to mark it with a file. Vary your
experiments by causing your magnetised darning-needle to attract and
repel your large magnet; it is quite competent to do so. In
magnetising the needle, I have supposed the point to be the last to
quit the marked end of the magnet; the point of the needle is a south
pole. The end which last quits the magnet is always opposed in
polarity to the end of the magnet with which it, has been last in
contact.
You may perhaps learn all this in a single hour; but spend several at
it, if necessary; and remember, understanding it is not sufficient:
you must obtain a manual aptitude in addressing Nature. If you speak
to your fellow-man you are not entitled to use jargon. Bad
experiments are jargon addressed to Nature, and just as much to be
deprecated. Manual dexterity in illustrating the interaction of
magnetic poles is of the utmost importance at this stage of your
progress; and you must not neglect attaining this power over your
implements. As you proceed, moreover, you will be tempted to do more
than I can possibly suggest. Thoughts will occur to you which you will
endeavour to follow out: questions will arise which you will try to
answer. The same experiment may be twenty different things to twenty
people. Having witnessed the action of pole on pole, through the air,
you will perhaps try whether the magnetic power is not to be screened
off. You use plates of glass, wood, slate, pasteboard, or
gutta-percha, but find them all pervious to this wondrous force. One
magnetic pole acts upon another through these bodies as if they were
not present. Should you ever become a patentee for the regulation of
ships' compasses, you will not fall, as some projectors have done,
into the error of screening off the magnetism of the ship by the
interposition of such substances.
If you wish to teach a class you must contrive that the effects which
you have thus far witnessed for yourself shall be witnessed by twenty
or thirty pupils. And here your private ingenuity must come into
play. You will attach bits of paper to your needles, so as to render
their movements visible at a distance, denoting the north and south
poles by different colours, say green and red. You may also improve
upon your darning-needle. Take a strip of sheet steel, heat it to
vivid redness and plunge it into cold water. It is thereby hardened;
rendered, in fact, almost as brittle as glass. Six inches of this,
magnetised in the manner of the darning-needle, will be better able to
carry your paper indexes. Having secured such a strip, you proceed
thus:
Magnetise a small sewing-needle and determine its poles; or, break
half an inch, or an inch, off your magnetised darning-needle and
suspend it by a fine silk fibre. The sewing-needle, or the fragment
of the darning needle, is now to be used as a test-needle, to examine
the distribution of the magnetism in your strip of steel. Hold the
strip upright in your left hand, and cause the test-needle to approach
the lower end of your strip; one end of the test-needle is attracted,
the other is repelled. Raise your needle along the strip; its
oscillations, which at first were quick, become slower; opposite the
middle of the strip they cease entirely; neither end of the needle is
attracted; above the middle the test-needle turns suddenly round, its
other end being now attracted. Go through the experiment thoroughly:
you thus learn that the entire lower half of the strip attracts one
end of the needle, while the entire upper half attracts the opposite
end. Supposing the north end of your little needle to be that
attracted below, you infer that the entire lower half of your
magnetised strip exhibits south magnetism, while the entire upper half
exhibits north magnetism. So far, then, you have determined the
distribution of magnetism in your strip of steel.
You look at this fact, you think of it; in its suggestiveness the
value of an experiment chiefly consists. The thought naturally
arises: 'What will occur if I break my strip of steel across in the
middle? Shall I obtain two magnets each possessing a single pole?'
Try the experiment; break your strip of steel, and test each half as
you tested the whole. The mere presentation of its two ends in
succession to your test-needle, suffices to show that you have _not_ a
magnet with a single pole--that each half possesses two poles with a
neutral point between them. And if you again break the half into two
other halves, you will find that each quarter of the original strip
exhibits precisely the same magnetic distribution as the whole strip.
You may continue the breaking process: no matter how small your
fragment may be, it still possesses two opposite poles and a neutral
point between them. Well, your hand ceases to break where breaking
becomes a mechanical impossibility; but does the mind stop there? No:
you follow the breaking process in idea when you can no longer realise
it in fact; your thoughts wander amid the very atoms of your steel,
and you conclude that each atom is a magnet, and that the force
exerted by the strip of steel is the mere summation, or resultant, of
the forces of its ultimate particles.