strange that this idea once expressed did not at once receive
recognition and development. But in those early days explorers were
few, and the "Picturesque Journey" probably but little known, so
that the notion of plasticity lay dormant for more than half a
century. But Bordier was at length succeeded by a man of far greater
scientific grasp and insight than himself. This was Rendu, a Catholic
priest and canon when he wrote, and afterwards Bishop of Annecy. In
1841 Rendu laid before the Royal Academy of Sciences of Savoy his
"Theory of the Glaciers of Savoy," a contribution for ever memorable
in relation to this subject.[G]
[G] "Memoirs of the Academy," vol. x.
398. Rendu seized the idea of glacier plasticity with great power and
clearness, and followed it resolutely to its consequences. It is not
known that he had ever seen the work of Bordier; probably not, as he
never mentions it. Let me quote for you some of Rendu's expressions,
which, however, fail to give an adequate idea of his insight and
precision of thought:--"Between the Mer de Glace and a river there
is a resemblance so complete that it is impossible to find in the
glacier a circumstance which does not exist in the river. In currents
of water the motion is not uniform either throughout their width or
throughout their depth. The friction of the bottom and of the sides,
with the action of local hindrances, causes the motion to vary, and
only towards the middle of the surface do we obtain the full motion."
399. This reads like a prediction of what has since been established
by measurement. Looking at the glacier of Mont Dolent, which
resembles a sheaf in form, wide at both ends and narrow in the
middle, and reflecting that the upper wide part had become narrow,
and the narrow middle part again wide, Rendu observes, "There is a
multitude of facts which seem to necessitate the belief that glacier
ice enjoys a kind of ductility which enables it to mould itself to
its locality, to thin out, to swell, and to contract as if it were a
soft paste."
400. To fully test his conclusions, Rendu required the accurate
measurement of glacier motion. Had he added to his other endowments
the practical skill of a land-surveyor, he would now be regarded as
the prince of glacialists. As it was he was obliged to be content
with imperfect measurements. In one of his excursions he examined the
guides regarding the successive positions of a vast rock which he
found upon the ice close to the side of the glacier. The mean of five
years gave him a motion for this block of 40 feet a year.
401. Another block, the transport of which he subsequently measured
more accurately, gave him a velocity of 400 feet a year. Note his
explanation of this discrepancy:--"The enormous difference of these
two observations arises from the fact that one block stood near the
centre of the glacier, which moves most rapidly, while the other
stood near the side, where the ice is held back by friction." So
clear and definite were Rendu's ideas of the plastic motion of
glaciers, that had the question of curvature occurred to him, I
entertain no doubt that he would have enunciated beforehand the
shifting of the point of maximum motion from side to side across the
axis of the glacier (§ 25).
402. It is right that you should know that scientific men do not
always agree in their estimates of the comparative value of facts
and ideas; and it is especially right that you should know that your
present tutor attaches a very high value to ideas when they spring
from the profound and persistent pondering of superior minds, and
are not, as is too often the case, thrown out without the warrant
of either deep thought or natural capacity. It is because I believe
Rendu's labours fulfil this condition, that I ascribe to them so
high a value. But when you become older and better informed, you may
differ from me; and I write these words lest you should too readily
accept my opinion of Rendu. Judge me, if you care to do so, when your
knowledge is matured. I certainly shall not fear your verdict.
403. But, much as I prize the prompting idea, and thoroughly as
I believe that often in it the force of genius mainly lies, it
would, in my opinion, be an error of omission of the gravest kind,
and which, if habitual, would ensure the ultimate decay of natural
knowledge, to neglect verifying our ideas, and giving them outward
reality and substance when the means of doing so are at hand. In
science thought, as far as possible, ought to be wedded to fact.
This was attempted by Rendu, and in part accomplished by Agassiz and
Forbes.
§ 60. _Viscous Theory._
404. Here indeed the merits of the distinguished glacialist last
named rise conspicuously to view. From the able and earnest advocacy
of Professor Forbes, the public knowledge of this doctrine of glacial
plasticity is almost wholly derived. He gave the doctrine a more
distinctive form; he first applied the term viscous to glacier ice,
and sought to found upon precise measurements a "Viscous Theory" of
glacier motion.
405. I am here obliged to state facts in their historic sequence.
Professor Forbes when he began his investigations was acquainted with
the labours of Rendu. In his earliest work upon the Alps he refers
to those labours in terms of flattering recognition. But though as
a matter of fact Rendu's ideas were there to prompt him, it would
be too much to say that he needed their inspiration. Had Rendu
not preceded him, he might none the less have grasped the idea of
viscosity, executing his measurements and applying his knowledge to
maintain it. Be that as it may, the appearance of Professor Forbes on
the Unteraar glacier in 1841, and on the Mer de Glace in 1842, and
his labours then and subsequently, have given him a name not to be
forgotten in the scientific history of glaciers.
406. The theory advocated by Professor Forbes was enunciated by
himself in these words:--"A glacier is an imperfect fluid, or viscous
body, which is urged down slopes of certain inclination by the
natural pressure of its parts." In 1773 Bordier wrote thus:--"As the
glaciers always advance upon the plain, and never disappear, it is
absolutely essential that new ice shall perpetually take the place
of that which is melted: it must therefore be pressed forward from
above. One can hardly refuse then to accept the astonishing truth,
that this vast extent of hard and solid ice moves as a single piece
downwards." In the passage already quoted he speaks of the ice being
pressed as a fluid from above. These constitute, I believe, Bordier's
contributions to this subject. The quotations show his sagacity at an
early date; but, in point of completeness, his views are not to be
compared with those of Rendu and Forbes.
407. I must not omit to state here that though the idea of viscosity
has not been espoused by M. Agassiz, his measurements, and maps of
measurements, on the Unteraar glacier have been recently cited as the
most clear and conclusive illustrations of a quality which, at all
events, closely resembles viscosity.
408. But why, with proofs before him more copious and characteristic
than those of any other observer, does M. Agassiz hesitate to accept
the idea of viscosity as applied to ice? Doubtless because he
believes the notion to be contradicted by our every-day experience of
the substance.
409. Take a mass of ice ten or even fifteen cubic feet in volume;
draw a saw across it to a depth of half an inch or an inch; and
strike a pointed pricker, not thicker than a very small round file,
into the groove; the substance will split from top to bottom with a
clean crystalline fracture. How is this brittleness to be reconciled
with the notion of viscosity?
410. We have, moreover, been upon the glacier and have witnessed the
birth of crevasses. We have seen them beginning as narrow cracks
suddenly formed, days being required to open them a single inch. In
many glaciers fissures may be traced narrow and profound for hundreds
of yards through the ice. What does this prove? Did the ice possess
even a very small modicum of that power of stretching, which is
characteristic of a viscous substance, such crevasses could not be
formed.
411. Still it is undoubted that the glacier moves like a viscous
body. The centre flows past the sides, the top flows over the
bottom, and the motion through a curved valley corresponds to fluid
motion. Mr. Mathews, Mr. Froude, and above all Signor Bianconi,
have, moreover, recently made experiments on ice which strikingly
illustrate the flexibility of the substance. These experiments merit,
and will doubtless receive, full attention at a future time.
§ 61. _Regelation Theory._
412. I will now describe to you an attempt that has been made of
late years to reconcile the brittleness of ice with, its motion in
glaciers. It is founded on the observation, made by Mr. Faraday in
1850, that when two pieces of thawing ice are placed together they
freeze together at the place of contact.
413. This fact may not surprise you; still it surprised Mr. Faraday
and others, and men of very great distinction in science have
differed in their interpretation of the fact. The difficulty is to
explain where, or how, in ice already thawing the cold is to be
found requisite to freeze the film of water between the two touching
surfaces.
414. The word _Regelation_ was proposed by Dr. Hooker to express the
freezing together of two pieces of thawing ice observed by Faraday;
and the memoir in which the term was first used was published by Mr.
Huxley and Mr. Tyndall in the Philosophical Transactions for 1857.
415. The _fact_ of regelation, and its application irrespective of
the _cause_ of regelation, may be thus illustrated:--Saw two slabs
from a block of ice, and bring their flat surfaces into contact; they
immediately freeze together. Two plates of ice, laid one upon the
other, with flannel round them overnight, are sometimes so firmly
frozen in the morning that they will rather break elsewhere than
along their surface of junction. If you enter one of the dripping
ice-caves of Switzerland, you have only to press for a moment a slab
of ice against the roof of the cave to cause it to freeze there and
stick to the roof.
416. Place a number of fragments of ice in a basin of water, and
cause them to touch each other; they freeze together where they
touch. You can form a chain of such fragments; and then, by taking
hold of one end of the chain, you can draw the whole series after it.
Chains of icebergs are sometimes formed in this way in the Arctic
seas.
417. Consider what follows from these observations. Snow consists of
small particles of ice. Now if by pressure we squeeze out the air
entangled in thawing snow, and bring the little ice-granules into
close contact, they may be expected to freeze together; and if the
expulsion of the air be complete, the squeezed snow may be expected
to assume the appearance of compact ice.
418. We arrive at this conclusion by reasoning; let us now test it by
experiment, employing a suitable hydraulic press, and a mould to hold
the snow. In exact accordance with our expectation, we convert by
pressure the snow into ice.[H]
[H] A similar experiment was made by the Messrs. Schlagintweit prior
to the discovery which explains it, and which therefore remained
unsolved.
419. Place a compact mass of ice in a proper mould, and subject
it to pressure. It breaks in pieces: squeeze the pieces forcibly
together; they re-unite by regelation, and a compact piece of ice,
totally different in shape from the first one, is taken from the
press. To produce this effect the ice must be in a thawing condition.
When its temperature is much below the melting point it is crushed
by pressure, not into a pellucid mass of another shape, but into a
white powder.
420. By means of suitable moulds you may in this way change the shape
of ice to any extent, turning out spheres, and cups, and rings, and
twisted ropes of the substance; the change of form in these cases
being effected through rude fracture and regelation.
421. By applying the pressure carefully, rude fracture may be
avoided, and the ice compelled slowly to change its form as if it
were a plastic body.
422. Now our first experiment illustrates the consolidation of the
snows of the higher Alpine regions. The deeper layers of the névé
have to bear the weight of all above them, and are thereby converted
into more or less perfect ice. And our last experiment illustrates
the changes of form observed upon the glacier, where, by the slow and
constant application of pressure, the ice gradually moulds itself to
the valley, which it fills.
423. In glaciers, however, we have also ample illustrations of
rude fracture and regelation. The opening and closing of crevasses
illustrate this. The glacier is broken on the cascades and mended
at their bases. When two branch glaciers lay their sides together,
the regelation is so firm that they begin immediately to flow in
the trunk glacier as a single stream. The medial moraine gives no
indication by its slowness of motion that it is derived from the
sluggish ice of the sides of the branch glaciers.
424. The gist of the Regelation Theory is that the ice of glaciers
changes its form and preserves its continuity under _pressure_ which
keeps its particles together. But when subjected to _tension_, sooner
than stretch it _breaks_, and behaves no longer as a viscous body.
§ 62. _Cause of Regelation._
425. Here the fact of regelation is applied to explain the plasticity
of glacier ice, no attempt being made to assign the cause of
regelation itself. They are two entirely distinct questions. But a
little time will be well spent in looking more closely into the cause
of regelation. You may feel some surprise that eminent men should
devote their attention to so small a point, but we must not forget
that in nature nothing is small. Laws and principles interest the
scientific student most, and these may be as well illustrated by
small things as by large ones.
426. The question of regelation immediately connects itself with that
of "latent heat," already referred to (383), but which we must now
subject to further examination. To melt ice, as already stated, a
large amount of heat is necessary, and in the case of the glaciers
this heat is furnished by the sun. Neither the ice so melted nor
the water which results from its liquefaction can fall below 32°
Fahrenheit. The freezing point of water and the melting point of ice
touch each other, as it were, at this temperature. A hair's-breadth
lower water freezes; a hair's-breadth higher ice melts.
427. But if the ice could be caused to melt without this supply of
solar heat, a temperature lower than that of ordinary thawing ice
would result. When snow and salt, or pounded ice and salt, are mixed
together, the salt causes the ice to melt, and in this way a cold of
20 or 30 degrees below the freezing point may be produced. Here, in
fact, the ice consumes _its own warmth_ in the work of liquefaction.
Such a mixture of ice and salt is called "a freezing mixture."
428. And if by any other means ice at the temperature of 32°
Fahrenheit could be liquefied without access of heat from without,
the water produced would be colder than the ice. Now Professor James
Thomson has proved that ice may be liquefied by mere _pressure_, and
his brother, Sir William Thomson, has also shown that water under
pressure requires a lower temperature to freeze it than when the
pressure is removed. Professor Mousson subsequently liquefied large
masses of ice by a hydraulic press; and by a beautiful experiment
Professor Helmholtz has proved that water in a vessel from which
the air has been removed, and which is therefore relieved from the
pressure of the atmosphere, freezes and forms ice-crystals when
surrounded by melting ice. All these facts are summed up in the
brief statement _that the freezing point of water is lowered by
pressure_.[I]
[I] Professor James Thomson and Professor Clausius proved this
independently and almost contemporaneously.
429. For our own instruction we may produce the liquefaction of ice
by pressure in the following way:--You remember the beautiful flowers
obtained when a sunbeam is sent through lake ice (§ 11), and you have
not forgotten that the flowers always form parallel to the surface of
freezing. Let us cut a prism, or small column of ice with the planes
of freezing running across it at right angles; we place that prism
between two slabs of wood, and bring carefully to bear upon it the
squeezing force of a small hydraulic press.
430. It is well to converge by means of a concave mirror a good light
upon the ice, and to view it through a magnifying lens. You already
see the result. Hazy surfaces are formed in the very body of the
ice, which gradually expand as the pressure is slowly augmented.
Here and there you notice something resembling crystallisation;
fern-shaped figures run with considerable rapidity through the ice,
and when you look carefully at their points and edges you find them
in visible motion. These hazy surfaces are spaces of liquefaction,
and the motion you see is that of the ice falling to water under the
pressure. That water is colder than the ice was before the pressure
was applied, and if the pressure be relieved, not only does the
liquefaction cease, but the water re-freezes. The cold produced by
its liquefaction under pressure is sufficient to re-congeal it when
the pressure is removed.
431. If instead of diffusing the pressure over surfaces of
considerable extent, we concentrate it on a small surface, the
liquefaction will of course be more rapid, and this is what Mr.
Bottomley has recently done in an experiment of singular beauty and
interest. Let us support on blocks of wood the two ends of a bar of
ice 10 inches long, 4 inches deep, and 3 wide, and let us loop over
its middle a copper wire one-twentieth, or even one-tenth, of an
inch in thickness. Connecting the two ends of the wire together, and
suspending from it a weight of 12 or 14 pounds, the whole pressure
of this weight is concentrated on the ice which supports the wire.
What is the consequence? The ice underneath the wire liquefies; the
water of liquefaction escapes round the wire, but the moment it is
relieved from the pressure it freezes, and round about the wire, even
before it has entered the ice, you have a frozen casing. The wire
continues to sink in the ice; the water incessantly escapes, freezing
as it does so behind the wire. In half an hour the weight falls; the
wire has gone clean through the ice. You can plainly see where it
has passed, but the two severed pieces of ice are so firmly frozen
together that they will break elsewhere as soon as along the surface
of regelation.
432. Another beautiful experiment bearing upon this point has
recently been made by M. Boussingault. He filled a hollow steel
cylinder with water and chilled it. In passing to ice, water, as you
know, expands (§ 45); in fact, room for expansion is a necessary
condition of solidification. But in the present case the strong
steel resisted the expansion, the water in consequence remaining
liquid at a temperature of more than 30° Fahr. below the ordinary
freezing point. A bullet within the cylinder rattled about at this
temperature, showing that the water was still liquid. On opening the
tap the liquid, relieved of the pressure, was instantly converted
into ice.
433. It is only substances which _expand_ on solidifying that behave
in this manner. The metal bismuth, as we know, is an example similar
to water; while lead, wax, or sulphur, all of which contract on
solidifying, have their point of fusion _heightened_ by pressure.
434. And now you are prepared to understand Professor James Thomson's
theory of regelation. When two pieces of ice are pressed together
liquefaction, he contends, results. The water spreads out around the
points of pressure, and when released re-freezes, thus forming a kind
of cement between the pieces of ice.
§ 63. _Faraday's View of Regelation._
435. Faraday's view of regelation is not so easily expressed, still
I will try to give you some notion of it, dealing in the first place
with admitted facts. Water, even in open vessels, may be lowered many
degrees below its freezing temperature, and still remain liquid;
it may also be raised to a temperature far higher than its boiling
point, and still resist boiling. This is due to the mutual cohesion
of the water particles, which resists the change of the liquid
either into the solid or the vaporous condition.
436. But if into the over-chilled water you throw a particle of ice,
the cohesion is ruptured, and congelation immediately sets in. And if
into the superheated water you introduce a bubble of air or of steam,
cohesion is likewise ruptured, and ebullition immediately commences.
437. Faraday concluded that _in the interior_ of any body, whether
solid or liquid, where every particle is grasped so to speak by the
surrounding particles, and grasps them in turn, the bond of cohesion
is so strong as to require a higher temperature to change the state
of aggregation than is necessary _at the surface_. At the surface
of a piece of ice, for example, the molecules are free on one side
from the control of other molecules; and they therefore yield to
heat more readily than in the interior. The bubble of air or steam
in overheated water also frees the molecules on one side; hence the
ebullition consequent upon its introduction. Practically speaking,
then, the point of liquefaction of the interior ice is higher than
that of the superficial ice. Faraday also refers to the special
solidifying power which bodies exert upon their own molecules.
Camphor in a glass bottle fills the bottle with an atmosphere of
camphor. In such an atmosphere large crystals of the substance may
grow by the incessant deposition of camphor molecules upon camphor,
at a temperature too high to permit of the slightest deposit _upon
the adjacent glass_. A similar remark applies to sulphur, phosphorus,
and the metals in a state of fusion. They are deposited upon solid
portions of their own substance at temperatures not low enough to
cause them to solidify against other substances.
438. Water furnishes an eminent example of this special solidifying
power. It may be cooled ten degrees and more below its freezing
point without freezing. But this is not possible if the smallest
fragment of ice be floating in the water. It then freezes accurately
at 32° Fahr., depositing itself, however, not upon the sides of the
containing vessel, but _upon the ice_. Faraday observed in a freezing
apparatus thin crystals of ice growing in ice-cold water to a length
of six, eight, or ten inches, at a temperature incompetent to produce
their deposition upon the sides of the containing vessel.
439. And now we are prepared for Faraday's view of regelation.
When the surfaces of two pieces of ice, covered with a film of
the water of liquefaction, are brought together, the covering
film is transferred from the surface to the centre of the ice,
where the point of liquefaction, as before shown, is higher than
at the surface. The special solidifying power of ice upon water
is now brought into play _on both sides of the film_. Under these
circumstances, Faraday held that the film would congeal, and freeze
the two surfaces together.
440. The lowering of the freezing point by pressure amounts to
no more than one-seventieth of a degree Fahrenheit for a whole
atmosphere. Considering the infinitesimal fraction of this pressure
which is brought into play in some cases of regelation, Faraday
thought its effect insensible. He suspended pieces of ice, and
brought them into contact without sensible pressure, still they
froze together. Professor James Thomson, however, considered that
even the capillary attraction exerted between two such masses would
be sufficient to produce regelation. You may make the following
experiments, in further illustration of this subject:--
441. Place a small piece of ice in water, and press it underneath the
surface by a second piece. The submerged piece may be so small as to
render the pressure infinitesimal; still it will freeze to the under
surface of the superior piece.