layers of finer rock, through which and across the gritty layer pass
the planes of lamination. The coarse layer is found bent by the
pressure into sinuosities like a contorted ribbon. Mr. Sorby has
described a striking case of this kind. This crumpling can be
experimentally imitated; the amount of compression might, moreover, be
roughly estimated by supposing the contorted bed to be stretched out,
its length measured and compared with the shorter distance into which
it has been squeezed. We find in this way that the yielding of the
mass has been considerable.
Let me now direct your attention to another proof of pressure; you see
the varying colours which indicate the bedding on this mass of slate.
The dark portion is gritty, being composed of comparatively coarse
particles, which, owing to their size, shape and gravity, sink first
and constitute the bottom of each layer. Gradually, from bottom to
top the coarseness diminishes, and near the upper surface we have a
layer of exceedingly fine grain. It is the fine mud thus consolidated
from which are derived the German razor-stones, so much prized for the
sharpening of surgical instruments.
When a bed is thin, the fine-grain slate is permitted to rest upon a
slab of the coarse slate in contact with it; when the fine bed is
thick, it is cut into slices which are cemented to pieces of ordinary
slate, and thus rendered stronger. The mud thus deposited is, as
might be expected, often rolled up into nodular masses, carried
forward, and deposited among coarser material by the rivers from which
the slate-mud has subsided. Here are such nodules enclosed in
sandstone. Everybody, moreover, who has ciphered upon a school-slate
must remember the whitish-green spots which sometimes dotted the
surface of the slate, and over which the pencil usually slid as if the
spots were greasy. Now these spots are composed of the finer mud, and
they could not, on account of their fineness, bite the pencil like the
surrounding gritty portions of the slate. Here is a beautiful example
of these spots: you observe them, on the cleavage surface, in broad
round patches. But turn the slate edgeways and the section of each
nodule is seen to be a sharp oval with its longer axis parallel to the
cleavage. This instructive fact has been adduced by Mr. Sorby. I
have made excursions to the quarries of Wales and Cumberland, and to
many of the slate yards of London, and found the fact general. Thus
we elevate a common experience of our boyhood into evidence of the
highest significance as regards a most important geological problem.
From the magnetic deportment of these slates, I was led to infer that
these spots contain a less amount of iron than the surrounding dark
slate. An analysis was made for me by Mr. Hambly in the laboratory of
Dr. Percy at the School of Mines with the following result:
ANALYSIS OF SLATE.
Dark Slate, two analyses.
1. Percentage of iron 5.85
2. Percentage of iron 6.13
Mean 5.99
Whitish Green Slate.
1. Percentage of iron 3.24
2. Percentage of iron 3.12
Mean 3.18
According to these analyses the quantity of iron in the dark slate
immediately adjacent to the greenish spot is nearly double the
quantity contained in the spot itself. This is about the proportion
which the magnetic experiments suggested.
Let me now remind you that the facts brought before you are
typical--each is the representative of a class. We have seen shells
crushed; the trilobites squeezed, beds contorted, nodules of greenish
marl flattened; and all these sources of independent testimony point
to one and the same conclusion, namely, that slate-rocks have been
subjected to enormous pressure in a direction at right angles to the
Planes of cleavage.
In reference to Mr. Sorby's contorted bed, I have said that by
supposing it to be stretched out and its length measured, it would
give us an idea of the amount of yielding of the mass above and below
the bed. Such a measurement, however, would not give the exact amount
of yielding. I hold in my hand a specimen of slate with its bedding
marked upon it; the lower portions of each layer being composed of a
comparatively coarse gritty material something like what you may
suppose the contorted bed to be composed of. Now in crossing these
gritty portions, the cleavage turns, as if tending to cross the
bedding at another angle. When the pressure began to act, the
intermediate bed, which is not entirely unyielding, suffered
longitudinal pressure; as it bent, the pressure became gradually more
transverse, and the direction of its cleavage is exactly such as you
would infer from an action of this kind--it is neither quite across
the bed, nor yet in the same direction as the cleavage of the slate
above and below it, but intermediate between both. Supposing the
cleavage to be at right angles to the pressure, this is the direction
which it ought to take across these more unyielding strata.
Thus we have established the concurrence of the phenomena of cleavage
and pressure--that they accompany each other; but the question still
remains, Is the pressure sufficient to account for the cleavage? A
single geologist, as far as I am aware, answers boldly in the
affirmative. This geologist is Sorby, who has attacked the question
in the true spirit of a physical investigator. Call to mind the
cleavage of the flags of Halifax and Over Darwen, which is caused by
the interposition of layers of mica between the gritty strata. Mr.
Sorby finds plates of mica to be also a constituent of slate-rock. He
asks himself, what will be the effect of pressure upon a mass
containing such plates confusedly mixed up in it? It will be, he
argues, and he argues rightly, to place the plates with their flat
surfaces more or less perpendicular to the direction in which the
pressure is exerted. He takes scales of the oxide of iron, mixes them
with a fine powder, and on squeezing the mass finds that the tendency
of the scales is to set themselves at right angles to the line of
pressure. Along the planes of weakness produced by the scales the
mass cleaves.
By tests of a different character from those applied by Mr. Sorby, it
might be shown how true his conclusion is--that the effect of pressure
on elongated particles, or plates, will be such as he describes it.
But while the scales must be regarded as a true cause, I should not
ascribe to them a large share in the production of the cleavage. I
believe that even if the plates of mica were wholly absent, the
cleavage of slate-rocks would be much the same as it is at present.
Here is a mass of pure white wax: it contains no mica particles, no
scales of iron, or anything analogous to them. Here is the selfsame
substance submitted to pressure. I would invite the attention of the
eminent geologists now before me to the structure of this wax. No
slate ever exhibited so clean a cleavage; it splits into laminae of
surpassing tenuity, and proves at a single stroke that pressure is
sufficient to produce cleavage, and that this cleavage is independent
of intermixed plates or scales. I have purposely mixed this wax with
elongated particles, and am unable to say at the present moment that
the cleavage is sensibly affected by their presence--if anything, I
should say they rather impair its fineness and clearness than promote
it.
The finer the slate is the more perfect will be the resemblance of its
cleavage to that of the wax. Compare the surface of the wax with the
surface of this slate from Borrodale in Cumberland. You have
precisely the same features in both: you see flakes clinging to the
surfaces of each, which have been partially torn away in cleaving. Let
any close observer compare these two effects, he will, I am persuaded,
be led to the conclusion that they are the product of a common cause.
[Footnote: I have usually softened the wax by warming it, kneaded it
with the fingers, and pressed it between thick plates of glass
previously wetted. At the ordinary summer temperature the pressed wax
is soft, and tears rather than cleaves; on this account I cool my
compressed specimens in a mixture of pounded ice and salt, and when
thus cooled they split cleanly.]
But you will ask me how, according to my view, does pressure produce
this remarkable result? This may be stated in a very few words.
There is no such thing in nature as a body of perfectly homogeneous
structure. I break this clay which seems so uniform, and find that
the fracture presents to my eyes innumerable surfaces along which it
has given way, and it has yielded along those surfaces because in them
the cohesion of the mass is less than elsewhere. I break this marble,
and even this wax, and observe the same result; look at the mud at the
bottom of a dried pond; look at some of the ungravelled walks in
Kensington Gardens on drying after rain,--they are cracked and split,
and other circumstances being equal, they crack and split where the
cohesion is a minimum. Take then a mass of partially consolidated
mud. Such a mass is divided and subdivided by interior surfaces along
which the cohesion is comparatively small. Penetrate the mass in
idea, and you will see it composed of numberless irregular polyhedra
bounded by surfaces of weak cohesion. Imagine such a mass subjected
to pressure,--it yields and spreads out in the direction of least
resistance; the little polyhedra become converted into laminae,
separated from each other by surfaces of weak cohesion, and the
infallible result will be a tendency to cleave at right angles to the
line of pressure. [Footnote: It is scarcely necessary to say that if the
mass were squeezed equally in all directions no laminated structure
could be produced; it must have room to yield in a lateral direction.
Mr. Warren De la Rue informs me that he once wished to obtain
white-lead in a fine granular state, and to accomplish this he first
compressed it. The mould was conical, and permitted the lead to
spread out a little laterally. The lamination was as perfect as that
of slate, and it quite defeated him in his effort to obtain a granular
powder.]
Further, a mass of dried mud is full of cavities and fissures. If you
break dried pipe-clay you see them in great numbers, and there are
multitudes of them so small that you cannot see them. A flattening of
these cavities must take place in squeezed mud, and this must to some
extent facilitate the cleavage of the mass in the direction indicated.
Although the time at my disposal has not permitted me duly to develope
these thoughts, yet for the last twelve months the subject has
presented itself to me almost daily under one aspect or another. I
have never eaten a biscuit during this period without remarking the
cleavage developed by the rolling-pin. You have only to break a
biscuit across, and to look at the fracture, to see the laminated
structure. We have here the means of pushing the analogy further. I
invite you to compare the structure of this slate, which was subjected
to a high temperature during the conflagration of Mr. Scott Russell's
premises, with that of a biscuit. Air or vapour within the slate has
caused it to swell, and the mechanical structure it reveals is
precisely that of a biscuit. During these enquiries I have received
much instruction in the manufacture of puff-paste. Here is some such
paste baked under my own superintendence. The cleavage of our hills
is accidental cleavage, but this is cleavage with intention. The
volition of the pastrycook has entered into its formation. It has
been his aim to preserve a series of surfaces of structural weakness,
along which the dough divides into layers. Puff-paste in preparation
must not be handled too much; it ought, moreover, to be rolled on a
cold slab, to prevent the butter from melting, and diffusing itself,
thus rendering the paste more homogeneous and less liable to split.
Puff-paste is, then, simply an exaggerated case of slaty cleavage.
The principle here enunciated is so simple as to be almost trivial;
nevertheless, it embraces not only the cases mentioned, but, if time
permitted, it might be shown you that the principle has a much wider
range of application. When iron is taken from the puddling furnace it
is more or less spongy, an aggregate in fact of small nodules: it is
at a welding heat, and at this temperature is submitted to the process
of rolling. Bright smooth bars are the result. But notwithstanding
the high heat the nodules do not perfectly blend together. The
process of rolling draws them into fibres. Here is a mass acted upon
by dilute sulphuric acid, which exhibits in a striking manner this
fibrous structure. The experiment was made by my friend Dr. Percy,
without any reference to the question of cleavage.
Break a piece of ordinary iron and you have a granular fracture; heat
the iron, you elongate these granules, and finally render the mass
fibrous. Here are pieces of rails along which the wheels of
locomotives have slid-den; the granules have yielded and become
plates. They exfoliate or come off in leaves; all these effects
belong, I believe, to the great class of phenomena of which slaty
cleavage forms the most prominent example. [Footnote: For some further
observations on this subject by Mr. Sorby and myself, see
Philosophical Magazine for August, 1856.]
We have now reached the termination of our task. You have witnessed
the phenomena of crystallisation, and have had placed before you the
facts which are found associated with the cleavage of slate rocks.
Such facts, as expressed by Helmholtz, are so many telescopes to our
spiritual vision, by which we can see backward through the night of
antiquity, and discern the forces which have been in operation upon
the earth's surface
Ere the lion roared,
Or the eagle soared.
From evidence of the most independent and trustworthy character, we
come to the conclusion that these slaty masses have been subjected to
enormous pressure, and by the sure method of experiment we have
shown--and this is the only really new point which has been brought
before you--how the pressure is sufficient to produce the cleavage.
Expanding our field of view, we find the self-same law, whose
footsteps we trace amid the crags of Wales and Cumberland, extending
into the domain of the pastrycook and ironfounder; nay, a wheel cannot
roll over the half-dried mud of our streets without revealing to us
more or less of the features of this law. Let me say, in conclusion,
that the spirit in which this problem has been attacked by geologists,
indicates the dawning of a new day for their science. The great
intellects who have laboured at geology, and who have raised it to its
present pitch of grandeur, were compelled to deal with the subject in
mass; they had no time to look after details. But the desire for more
exact knowledge is increasing; facts are flowing in which, while they
leave untouched the intrinsic wonders of geology, are gradually
supplanting by solid truths the uncertain speculations which beset the
subject in its infancy. Geologists now aim to imitate, as far as
possible, the conditions of nature, and to produce her results; they
are approaching more and more to the domain of physics, and I trust
the day will soon come when we shall interlace our friendly arms
across the common boundary of our sciences, and pursue our respective
tasks in a spirit of mutual helpfulness, encouragement and goodwill.
[I would now lay more stress on the lateral yielding, referred to in
the footnote concerning Mr. Warren De la Rue's attempt to produce
finely granular white-lead, accompanied as it is by tangential
sliding, than I was prepared to do when this lecture was given. This
sliding is, I think, the principal cause of the planes of weakness,
both in pressed wax and slate rock. J. T. 1871.]
********************
XIII. ON PARAMAGNETIC AND DIAMAGNETIC FORCES
[Footnote: Abstract of a discourse delivered in the Royal Institution,
February 1, 1856.]
THE notion of an attractive force, which draws bodies towards the
centre of the earth, was entertained by Anaxagoras and his pupils, by
Democritus, Pythagoras, and Epicurus; and the conjectures of these
ancients were renewed by Galileo, Huyghens, and others, who stated
that bodies attract each other as a magnet attracts iron. Kepler
applied the notion to bodies beyond the surface of the earth, and
affirmed the extension of this force to the most distant stars. Thus
it would appear, that in the attraction of iron by a magnet originated
the conception of the force of gravitation. Nevertheless, if we look
closely at the matter, it will be seen that the magnetic force
possesses characters strikingly distinct from those of the force which
holds the universe together. The theory of gravitation is, that every
particle of matter attracts every other particle; in magnetism also we
have attraction, but we have always, at the same time, repulsion, the
final effect being due to the difference of these two forces. A body
may be intensely acted on by a magnet, and still no motion of
translation will follow, if the repulsion be equal to the attraction.
Previous to magnetization, a dipping needle, when its centre of
gravity is supported, stands accurately level; but, after
magnetization, one end of it, in our latitude, is pulled towards the
north pole of the earth. The needle, however, being suspended from
the arm of a fine balance, its weight is found unaltered by its
magnetization. In like manner, when the needle is permitted to float
upon a liquid, and thus to follow the attraction of the north magnetic
pole of the earth, there is no motion of the mass towards that pole.
The reason is known to be, that although the marked end of the needle
is attracted by the north pole, the unmarked end is repelled by an
equal force, the two equal and opposite forces neutralizing each
other.
When the pole of an ordinary magnet is brought to act upon the
swimming needle, the latter is attracted,--the reason being that the
attracted end of the needle being nearer to the pole of the magnet
than the repelled end, the force of attraction is the more powerful of
the two. In the case of the earth, its pole is so distant that the
length of the needle is practically zero. In like manner, when a
piece of iron is presented to a magnet, the nearer parts are
attracted, while the more distant parts are repelled; and because the
attracted portions are nearer to the magnet than the repelled ones, we
have a balance in favour of attraction. Here then is the special
characteristic of the magnetic force, which distinguishes it from that
of gravitation. The latter is a simple unpolar force, while the
former is duplex or polar. Were gravitation like magnetism, a stone
would no more fall to the ground than a piece of iron towards the
north magnetic pole: and thus, however rich in consequences the
supposition of Kepler and others may have been, it is clear that a
force like that of magnetism would not be able to transact the
business of the universe.
The object of this discourse is to enquire whether the force of
diamagnetism, which manifests itself as a repulsion of certain bodies
by the poles of a magnet, is to be ranged as a polar force, beside
that of magnetism; or as an unpolar force, beside that of gravitation.
When a cylinder of soft iron is placed within a wire helix, and
surrounded by an electric current, the antithesis of its two ends, or,
in other words, its polar excitation, is at once manifested by its
action upon a magnetic needle; and it may be asked why a cylinder of
bismuth may not be substituted for the cylinder of iron, and its state
similarly examined. The reason is, that the excitement of the bismuth
is so feeble, that it would be quite masked by that of the helix in
which it is enclosed; and the problem that now meets us is, so to
excite a diamagnetic body that the pure action of the body upon a
magnetic needle may be observed, unmixed with the action of the body
used to excite the diamagnetic.
How this has been effected may be illustrated in the following
manner:
When through an upright helix of covered copper wire, a voltaic
current is sent, the top of the helix attracts, while its bottom
repels, the same pole of a magnetic needle; its central point, on the
contrary, is neutral, and exhibits neither attraction nor repulsion.
Such a helix is caused to stand between the two poles N'S' of an
astatic system. [Footnote: The reversal of the poles of the two
magnets, which were of the same strength, completely annulled the
action of the earth as a magnet.] The two magnets S N' and S'N are
united by a rigid cross piece at their centres, and are suspended from
the point a, so that both magnets swing in the same horizontal plane.
It is so arranged that the poles N' s' are opposite to the central or
neutral point of the helix, so that when a current is sent through the
latter, the magnets, as before explained, are unaffected. Here then
we have an excited helix which itself has no action upon the magnets,
and we are thus enabled to examine the action of a body placed within
the helix and excited by it, undisturbed by the influence of the
latter. The helix being 12 inches high, a cylinder of soft iron 6
inches long, suspended from a string and passing over a pulley, can be
raised or lowered within the helix. When it is so far sunk that its
lower end rests upon the table, the upper end finds itself between the
poles N´S´ of the astatic system. The iron cylinder is thus converted
into a strong magnet, attracting one of the poles, and repelling the
other, and consequently deflecting the entire astatic system. When
the cylinder is raised so that the upper end is at the level of the
top of the helix, its lower end comes between the poles N´S´; and a
deflection opposed in direction to the former one is the immediate
consequence. To render these deflections more easily visible, a
mirror m is attached to the system of magnets; a beam of light thrown
upon the mirror being reflected and projected as a bright disk against
the wall. The distance of this image from the mirror being
considerable, and its angular motion double that of the latter, a very
slight motion of the magnet is sufficient to produce a displacement of
the image through several yards.
This then is the principle of the beautiful apparatus [Footnote:
Devised by Prof. W. Weber, and constructed by M. Leyser, of Leipzig.]
by which the investigation was conducted. It is manifest that if a
second helix be placed between the poles SN with a cylinder within
it, the action upon the astatic magnet may be exalted. This was the
arrangement made use of in the actual enquiry. Thus to intensify the
feeble action, which it is here our object to seek, we have in the
first place neutralized the action of the earth upon the magnets, by
placing them astatically. Secondly, by making use of two cylinders,
and permitting them to act simultaneously on the four poles of the
magnets, we have rendered the deflecting force four times what it
would be, if only a single pole were used. Finally, the whole
apparatus was enclosed in a suitable case which protected the magnets
from air-currents, and the deflections were read off through a glass
plate in the case, by means of a telescope and scale placed at a
considerable distance from the instrument.