Translation: Edmund Atkinson
Introductory Note
Hermann Ludwig Ferdinand von Helmholtz was born at Potsdam, near Berlin, on August
31, 1821. His father was a man of high culture, a teacher in the gymnasium, whose
influence ensured to his son the foundations of a broad general education. His mother was
a descendant from William Penn, the English Quaker.
Helmholtz early showed mathematical ability, and wished to devote his life to the
study of physics; but practical considerations led him to take up medicine, and he became
a surgeon in the Prussian army. He began the publication of original contributions to
science in 1842, and for fifty-two years, till his death in 1894, he continued to produce
in an unbroken stream. He held a succession of academic positions, teaching physiology at
Konigsberg, Bonn, and Heidelberg, and for the last twenty-three years of his life filling
the chair of physics at Berlin.
The titles of his professorships, however, give a very inadequate idea of his
range. His contributions to science cover medicine, physiology, optics, acoustics,
mathematics, mechanics, and electricity. His interests in science and art came together in
his work on esthetics, and he had a lively appreciation of painting, poetry, and music.
The practice of popular lecturing on scientific subjects was almost unknown in
Germany when Helmholtz began, and he did much to give it dignity and to set a standard.
His own lectures, as the reader of the following papers will perceive, are masterpieces of
their kind. "The matter," says a biographer, "is discussed by a master, who
brings to bear upon it all his wealth of learning and research, while there is the
ever-enduring interest that attaches to an exposition by one who is giving forth from his
own treasury." It is fortunate for the layman when a scientist and thinker of the
first order has the skill and the inclination to share with the outside world the rich
harvest of his brilliant and laborious research.
Ice And Glaciers: Part I
A Lecture Delivered At Frankfort-On-Main, And At Heidelberg, In February, 1865
The world of ice and of eternal snow, as unfolded to us on the summits of the
neighbouring Alpine chain, so stern, so solitary, so dangerous, it may be, has yet its own
peculiar charm. Not only does it enchain the attention of the natural philosopher, who
finds in it the most wonderful disclosures as to the present and past history of the
globe, but every summer it entices thousands of travellers of all conditions, who find
there mental and bodily recreation. While some content themselves with admiring from afar
the dazzling adornment which the pure, luminous masses of snowy peaks, interposed between
the deeper blue of the sky and the succulent green of the meadows, lend to the landscape,
others more boldly penetrate into the strange world, willingly subjecting themselves to
the most extreme degrees of exertion and danger, if only they may fill themselves with the
aspect of its sublimity.
I will not attempt what has so often been attempted in vain - to depict in words the
beauty and magnificence of nature, whose aspect delights the Alpine traveller. I may well
presume that it is known to most of you from your own observation; or, it is to be hoped,
will be so. But I imagine that the delight and interest in the magnificence of those
scenes will make you the more inclined to lend a willing ear to the remarkable results of
modern investigations on the more prominent phenomena of the glacial world. There we see
that minute peculiarities of ice, the mere mention of which might at other times be
regarded as a scientific subtlety, are the causes of the most important changes in
glaciers; shapeless masses of rock begin to relate their histories to the attentive
observer, histories which often stretch far beyond the past of the human race into the
obscurity of the primeval world; a peaceful, uniform, and beneficent sway of enormous
natural forces, where at first sight only desert wastes are seen, either extended
indefinitely in cheerless, desolate solitudes, or full of wild, threatening confusion - an
arena of destructive forces. And thus I think I may promise that the study of the
connection of those phenomena of which I can now only give you a very short outline, will
not only afford you some prosaic instruction, but will make your pleasure in the
magnificent scenes of the high mountains more vivid, your interest deeper, and your
admiration more exalted.
Let me first of all recall to your remembrance the chief features of the external
appearance of the snow fields and of the glaciers; and let me mention the accurate
measurements which have contributed to supplement observation, before I pass to discuss
the casual connection of those processes.
The higher we ascend the mountains the colder it becomes. Our atmosphere is like a warm
covering spread over the earth; it is well-nigh entirely transparent for the luminous
darting rays of the sun, and allows them to pass almost without appreciable change. But it
is not equally penetrable by obscure heat rays, which, proceeding from heated terrestrial
bodies, struggle to diffuse themselves into space. These are absorbed by atmospheric air,
especially when it is moist; the mass of air is itself heated thereby, and only radiates
slowly into space the heat which has been gained. The expenditure of heat is thus retarded
as compared with the supply, and a certain store of heat is retained along the whole
surface of the earth. But on high mountains the protective coating of the atmosphere is
far thinner - the radiated heat of the ground can escape thence more freely into space;
there, accordingly, the accumulated store of heat and the temperature are far smaller than
at lower levels.
To this must be added another property of air which acts in the same direction. In a
mass of air which expands, part of its store of heat disappears; it becomes cooler, if it
cannot acquire fresh heat from without. Conversely, by renewed compression of the air, the
same quantity of heat is reproduced which had disappeared during expansion. Thus, if for
instance, south winds drive the warm air of the Mediterranean towards the north, and
compel it to ascend along the great mountain wall of the Alps, where the air, in
consequence of the diminished pressure, expands by about half its volume, it thereby
becomes very greatly cooled - for a mean height of 11,000 feet, by from 18 degrees to
30C., according as it is moist or dry - and it thereby deposits the greater part of its
moisture as rain or snow. If the same wind, passing over to the north side of the
mountains as Fohn-wind, reaches the valley and plains, it again becomes condensed, and is
again heated. Thus the same current of air which is warm in the plains, both on this side
of the chain and on the other, is bitterly cold on the heights, and can there deposit
snow, while in the plain we find it insupportably hot.
The lower temperature at greater heights, which is due to both these causes, is, as we
know, very marked on the lower mountain chains of our neighbourhood. In central Europe it
amounts to about 1C. for an ascent of 480 feet; in winter it is less - 1 degree for about
720 feet of ascent. In the Alps the differences of temperature at great heights are
accordingly far more considerable, so that upon the higher parts of their peaks and slopes
the snow which has fallen in winter no longer melts in summer. This line, above which snow
covers the ground throughout the entire year, is well known as the snow line; on the
northern side of the Alps it is about 8,000 feet high, on the southern side about 8,800
feet. Above the snow line it may on sunny days be very warm; the unrestrained radiation of
the sun, increased by the light reflected from the snow, often becomes utterly unbearable,
so that the tourist of sedentary habits, apart from the dazzling of his eyes, which he
must protect by dark spectacles or by a veil, usually gets severely sunburnt in the face
and hands, the result of which is an inflammatory swelling of the skin and great blisters
on the surface. More pleasant testimonies to the power of the sunshine are the vivid
colours and the powerful odour of the small Alpine flowers which bloom in the sheltered
rocky clefts among the snow fields. Notwithstanding the powerful radiation of the sun the
temperature of the air above the snow fields only rises to 5 degrees, or at most 8
degrees; this, however, is sufficient to melt a tolerable amount of the superficial layers
of snow. But the warm hours and days are too short to overpower the great masses of snow
which have fallen during colder times. Hence the height of the snow line does not depend
merely on the temperature of the mountain slope, but also essentially on the amount of the
yearly snowfall. It is lower, for instance, on the moist and warm south slope of the
Himalayas than on the far colder but also far drier north slope of the same mountain.
Corresponding to the moist climate of western Europe, the snowfall upon the Alps is very
great, and hence the number and extent of their glaciers are comparatively considerable,
so that few mountains of the earth can be compared with them in this respect. Such a
development of the glacial world is, as far as we know, met with only on the Himalayas,
favoured by the greater height; in Greenland and in Northern Norway, owing to the colder
climate; in a few islands in Iceland; and in New Zealand, from the more abundant moisture.
Places above the snow line are thus characterized by the fact that the snow which in
the course of the year falls on its surface does not quite melt away in summer, but
remains to some extent. This snow, which one summer has left, is protected from the
further action of the sun's heat by the fresh quantities that fall upon it during the next
autumn, winter, and spring. Of this new snow also next summer leaves some remains, and
thus year by year fresh layers of snow are accumulated one above the other. In those
places where such an accumulation of snow ends in a steep precipice, and its inner
structure is thereby exposed, the regularly stratified yearly layers are easily
recognised.
But it is clear that this accumulation of layer upon layer cannot go on indefinitely,
for otherwise the height of the snow peak would continually increase year by year. But the
more the snow is accumulated the steeper are the slopes, and the greater the weight which
presses upon the lower and older layers and tries to displace them. Ultimately a state
must be reached in which the snow slopes are too steep to allow fresh snow to rest upon
them, and in which the burden which presses the lower layers downwards is so great that
these can no longer retain their position on the sides of the mountain. Thus, part of the
snow which had originally fallen on the higher regions of the mountain above the snow
line, and had there been protected from melting, is compelled to leave its original
position and seek a new one, which it of course finds only below the snow line on the
lower slopes of the mountain, and especially in the valleys, where, however, being exposed
to the influence of a warmer air, it ultimately melts and flows away as water. The descent
of masses of snow from their original positions sometimes happens suddenly in avalanches,
but it is usually very gradual in the form of glaciers.
Thus we must discriminate between two distinct parts of the ice fields; that is, first,
the snow which originally fell - called firn in Switzerland above the snow line, covering
the slopes of the peaks as far as it can hang on to them, and filling up the upper wide
kettle-shaped ends of the valleys forming widely extending fields of snow or firnmeere.
Secondly, the glaciers, called in the Tyrol firner, which as prolongations of the snow
fields often extend to a distance of from 4,000 to 5,000 feet below the snow line, and in
which the loose snow of the snow fields is again found changed into transparent solid ice.
Hence the name glacier, which is derived from the Latin, glacies; French, glace, glacier.
The outward appearance of glaciers is very characteristically described by comparing
them, with Goethe, to currents of ice. They generally stretch from the snow fields along
the depth of the valleys, filling them throughout their entire breadth, and often to a
considerable height. They thus follow all the curvatures, windings, contractions, and
enlargements of the valley. Two glaciers frequently meet the valleys of which unite. The
two glacial current then join in one common principal current, filling up the valley
common to them both. In some places these ice currents present a tolerably level and
coherent surface, but they are usually traversed by crevasses, and both over the surface
and through the crevasses countless small and large water rills ripple, which carry off
the water formed by the melting of the ice. United, and forming a stream, they burst,
through a vaulted and clear blue gateway of ice, out at the lower end of the larger
glacier.
On the surface of the ice there is a large quantity of blocks of stone, and of rocky
debris, which at the lower end of the glacier are heaped up and form immense walls; these
are called the lateral and terminal moraine of the glacier. Other heaps of rock, the
central moraine, stretch along the surface of the glacier in the direction of its length,
forming long regular dark lines. These always start from the places where two glacier
streams coincide and unite. The central moraines are in such places to be regarded as the
continuations of the united lateral moraines of the two glaciers.
The formation of the central moraine is well represented in the view below given of the
Unteraar Glacier (Fig. 104). In the background are seen the two glacier currents emerging
from different valleys; on the right from the Shreckhorn, and on the left from the
Finsteraarhorn. From the place where they unite the rocky wall occupying the middle of the
picture descends, constituting the central moraine. On the left are seen individual large
masses of rock resting on pillars of ice, which are known as glacier tables.
To exemplify these circumstances still further, I lay before you in Fig. 105 a map of
the Mer de Glace of Chamouni, copied from that of Forbes.
The Mer de Glace in size is well known as the largest glacier in Switzerland, although
in length it is exceeded by the Aletsch Glacier. It is formed from the snow fields that
cover the heights directly north of Mont Blanc, several of which, as the Grande Jorasse,
the Aiguille Verte (a, Figs. 105 and 106), the Aiguille du Geant (b), Aiguille du Midi
(c), and the Aiguille du Dru (d), are only 2,000 to 3,000 feet below that king of the
European mountains. The snow fields which lie on the slopes and in the basins between
these mountains collect in three principal currents, the Glacier du Geant, Glacier de
Lechaud, the third, from the union of the last with the Glacier du Geant; and Glacier du
Talefre, which, ultimately, united as represented in the map, form the Mer de Glace; this
stretches as an ice current 2,600 to 3,000 feet in breadth down into the valley of
Chamouni, where a powerful stream, the Arveyron, bursts from its lower end at k, and
plunges into the Arve. The lowest precipice of the Mer de Glace, which is visible from the
valley of Chamouni, and forms a large cascade of ice, is commonly called Glacier des Bois,
from a small village which lies below.
Most of the visitors at Chamouni only set foot on the lowest part of the Mer de Glace
from the inn at the Montanvert, and when they are free from giddiness cross the glacier at
this place to the little house on the opposite side, the Chapeau (n). Although, as the map
shows, only a comparatively very small portion of the glacier is thus seen and crossed,
this way shows sufficiently the magnificent scenes, and also the difficulties of a glacier
excursion. Bolder wanderers march upwards along the glacier to the Jardin, a rocky cliff
clothed with some vegetation, which divides the glacial current of the Glacier du Talefre
into two branches; and bolder still they ascend yet higher, to the Col du Geant (11,000
feet above the sea), and down the Italian side to the valley of Aosta.
The surface of the Mer de Glace shows four of the rocky walls which we have designated
as medical moraines. The first, nearest the side of the glacier, is formed where the two
arms of the Glacier du Talefre unite at the lower end of the Jardin; the second proceeds
from the union of the glacier in question with the Glacier de Lechaud; the third, from the
union of the last with the Glacier du Geant; and the fourth, finally, from the top of the
rock ledge which stretches from the Aiguille du Geant towards the cascade (g) of the
Glacier du Geant.
To give you an idea of the slope and the fall of the glacier, I have given in Fig. 106
a longitudinal section of it according to the levels and measurements taken by Forbes,
with the view of the right bank of the glacier. The letters stand for the same objects as
in Fig. 105; p is the Aiguille de Lechaud, q the Aiguille Noire, r the Mont Tacul, f is
the Col du Geant, the lowest point in the high wall of rock that surrounds the upper end
of the snow fields which feed the Mer de Glace. The base line corresponds to a length of a
little more than nine miles: on the right the heights above the sea are given in feet. The
drawing shows very distinctly how small in most places is the fall of the glacier. Only an
approximate estimate could be made of the depth, for hitherto nothing certain has been
made out in reference to it. But that it is very deep is obvious from the following
individual and accidental observations.
At the end of a vertical rock wall of the Tacul, the edge of the Glacier du Geant is
pushed forth, forming an ice wall 140 feet in height. This would give the depth of one of
the upper arms of the glacier at the edge. In the middle and after the union of the three
glaciers the depth must be far greater. Somewhat below the junction Tyndall and Hirst
sounded a moulin, that is, a cavity through which the surface glacier waters escape, to a
depth of 160 feet; the guides alleged that they had sounded a similar aperture to a depth
of 350 feet, and had found no bottom. From the usually deep trough shaped or gorge - like
form of the bottom of the valleys, which is constructed solely of rock walls, it seems
improbable that for a breadth of 3,000 feet the mean depth should only be 350 feet;
moreover, from the manner in which ice moves, there must necessarily be a very thick
coherent mass beneath the crevassed part.
To render these magnitudes more intelligible by reference to more familiar objects,
imagine the valley of Heidelberg filled with ice up to the Molkencur, or higher, so that
the whole town, with all its steeples and the castle, is buried deeply beneath it; if,
further, you imagine this mass of ice, gradually extending in height, continued from the
mouth of the valley up to Neckargemund, that would about correspond to the lower united
ice current of the Mer de Glace. Or, instead of the Rhine and the Nahe at Bingen, suppose
two ice currents united which fill the Rhine valley to its upper border as far as we can
see from the river, and then the united currents stretching downwards to beyond
Asmannshausen and Burg Rheinstein; such a current would also about correspond to the size
of the Mer de Glace.
Fig. 107, which is a view of the magnificent Gorner Glacier seen from below, also gives
an idea of the size of the masses of ice of the larger glaciers.
The surface of most glaciers is dirty, from the numerous pebbles and sand which lie
upon it, and which are heaped together the more the ice under them and among them melts
away. The ice of the surface has been partially destroyed and rendered crumbly. In the
depths of the crevasses ice is seen of a purity and clearness with which nothing that we
are acquainted with on the plains can be compared. From its purity it shows a splendid
blue, like that of the sky, only with a greenish hue. Crevasses in which pure ice is
visible in the interior occur of all sizes; in the beginning they form slight cracks in
which a knife can scarcely be inserted; becoming gradually enlarged to chasms, hundreds or
even thousands, of feet in length, and twenty, fifty, and as much as a hundred feet in
breadth, while some of them are immeasurably deep. Their vertical dark blue walls of
crystal ice, glistening with moisture from the trickling water, form one of the most
splendid spectacles which nature can present to us; but, at the same time, a spectacle
strongly impregnated with the excitement of danger, and only enjoyable by the traveller
who feels perfectly free from the slightest tendency to giddiness. The tourist must know
how, with the aid of well-nailed shoes and a pointed Alpenstock, to stand even on slippery
ice, and at the edge of a vertical precipice the foot of which is lost in the darkness of
night, and at an unknown depth. Such crevasses cannot always be evaded in crossing the
glacier; at the lower part of the Mer de Glace, for instance, where it is usually crossed
by travellers, we are compelled to travel along some extent of precipitous banks of ice
which are occasionally only four to six feet in breadth, and on each side of which is such
a blue abyss. Many a traveller, who has crept along the steep rocky slopes without fear,
there feels his heart sink, and cannot turn his eyes from the yawning chasm, for he must
first carefully select every step for his feet. And yet these blue chasms, which lie open
and exposed in the daylignt, are by no means the worst dangers of the glacier; though,
indeed, we are so organised that a danger which we perceive, and which therefore we can
safely avoid, frightens us far more than one which we know to exist, but which is veiled
from our eyes. So also it is with glacier chasms. In the lower part of the glacier they
yawn before us, threatening death and destruction, and lead us, timidly collecting all our
presence of mind, to shrink from them; thus accidents seldom occur. On the upper part of
the glacier, on the contrary, the surface is covered with snow; this, when it falls
thickly, soon arches over the narrower crevasses of a breadth of from four to eight feet,
and forms bridges which quite conceal the crevasse, so that the traveller only sees a
beautiful plane snow surface before him. If the snow bridges are thick enough, they will
support a man; but they are not always so, and these are the places where men, and even
chamois, are so often lost. These dangers may readily be guarded against if two or three
men are roped together at intervals of ten or twelve feet. If then one of them falls into
a crevasse, the two others can hold him, and draw him out again.
In some places the crevasses may be entered, especially at the lower end of a glacier.
In the well-known glaciers of Grindelwald, Rosenlaui, and other places, this is
facilitated by cutting steps and arranging wooden planks. Then any one who does not fear
the perpetually trickling water may explore these crevasses, and admire the wonderfully
transparent and pure crystal walls of these caverns. The beautiful blue colour which they
exhibit is the natural colour of perfectly pure water; liquid water as well as ice is
blue, though to an extremely small extent, so that the colour is only visible in layers of
from ten to twelve feet in thickness. The water of the Lake of Geneva and of the Lago di
Garda exhibits the same splendid colour as ice.
The glaciers are not everywhere crevassed; in places where the ice meets with an
obstacle, and in the middle of great glacier currents the motion of which is uniform, the
surface is perfectly coherent.
Fig. 108 represents one of the more level parts of the Mer de Glace at the Montanvert,
the little house of which is seen in the background. The Gries Glacier, where it forms the
height of the pass from the Upper Rhone valley to the Tosa valley, may even be crossed on
horseback. We find the greatest disturbance of the surface of the glacier in those places
where it passes from a slightly inclined part of its bed to one where the slope is
steeper. The ice is there torn in all directions into a quantity of detached blocks, which
by melting are usually changed into wonderfully shaped sharp ridges and pyramids, and from
time to time fall into the interjacent crevasses with a loud rumbling noise. Seen from a
distance such a place appears like a wild frozen waterfall, and is therefore called a
cascade; such a cascade is seen in the Glacier du Talefre at 1, another is seen in the
Glacier du Geant at g. Fig. 110, while a third forms the lower end of the Mer de Glace.
The latter, already mentioned a the Glacier des Bois, which rises directly from the trough
of the valley at Chamouni to a height of 1,700 feet, the height of the Konigstuhl at
Heidelberg, affords at all times a chief object of admiration to the Chamouni tourist.
Fig. 109 represents a view of its fantastically rent blocks of ice.
We have hitherto compared the glacier with a current as regards its outer form and
appearance. This similarity, however, is not merely an external one: the ice of the
glacier does, indeed, move forwards like the water of a stream, only more slowly. That
this must be the case follows from the considerations by which I have endeavoured to
explain the origin of a glacier. For as the ice is being constantly diminished at the
lower end by melting, it would entirely disappear if fresh ice did not continually press
forward from above, which, again, is made up by the snowfalls on the mountain tops.
But by careful ocular observation we may convince ourselves that the glacier does
actually move. For the inhabitants of the valleys, who have the glaciers constantly before
their eyes, often cross them, and in so doing make use of the larger blocks of stone as
sign posts - detect this motion by the fact that their guide posts gradually descend in
the course of each year. And as the yearly displacement of the lower half of the Mer de
Glace at Chamouni amounts to no less than from 400 to 600 feet, you can readily conceive
that such displacements must ultimately be observed, notwithstanding the slow rate at
which they take place, and in spite of the chaotic confusion of crevasses and rocks which
the glacier exhibits.
Besides rocks and stones, other objects which have accidentally alighted upon the
glacier are dragged along. In 1788 the celebrated Genevese Saussure, together with his son
and a company of guides and porters, spent sixteen days on the Col du Geant. On descending
the rocks at the side of the cascade of the Glacier du Geant, they left behind them a
wooden ladder. This was at the foot of the Aiguille Noire, where the fourth band of the
Mer de Glace begins; this line thus marks at the same time the direction in which ice
travels from this point. In the year 1832, that is, forty-four years after, fragments of
this ladder were found by Forbes and other travellers not far below the junction of the
three glaciers of the Mer de Glace, in the same line (at s, Fig. 110), from which it
results that these parts of the glacier must on the average have each year descended 375
feet.
In the year 1827 Hugi had built a hut on the central moraine of the Unteraar Glacier
for the purpose of making observations; the exact position of this hut was determined by
himself and afterwards by Agassiz, and they found that each year it had moved downwards.
Fourteen years later, in the year 1841, it was 4,884 feet lower, so that every year it had
on the average moved through 349 feet. Agassiz afterwards found that his own hut, which he
had erected on the same glacier, had moved to a somewhat smaller extent. For these
observations a long time was necessary. But if the motion of the glacier be observed by
means of accurate measuring instruments, such as theodolites, it is not necessary to wait
for years to observe that ice moves - a single day is sufficient.
Such observations have in recent times been made by several observers, especially by
Forbes and by Tyndall. They show that in summer the middle of the Mer de Glace moves
through twenty inches a day, while towards the lower terminal cascade the motion amounts
to as much as thirty-five inches in a day. In winter the velocity is only about half as
great. At the edges and in the lower layers of the glacier, as in a flow of water, it is
considerably smaller than in the centre of the surface.
The upper sources of the Mer de Glace also have a slower motion, the Glacier du Geant
thirteen inches a day, and the Glacier du Lechaud nine inches and a half. In different
glaciers the velocity is in general very various, according to the size, the inclination,
the amount of snowfall, and other circumstances.
Such an enormous mass of ice thus gradually and gently moves on, imperceptibly to the
casual observer, about an inch an hour - the ice of the Col du Geant will take 120 years
before it reaches the lower end of the Mer de Glace - but it moves forward with
uncontrollable force, before which any obstacles that man could oppose to it yield like
straws, and the traces of which are distinctly seen even on the granite walls of the
valley. If, after a series of wet seasons, and an abundant fall of snow on the heights,
the base of a glacier advances, not merely does it crush dwelling houses, and break the
trunks of powerful trees, but the glacier pushes before it the boulder walls which form
its terminal moraine without seeming to experience any resistance. A truly magnificent
spectacle is this motion, so gentle and so continuous, and yet so powerful and so
irresistible.
I will mention here that from the way in which the glacier moves we can easily infer in
what places and in what directions crevasses will be formed. For as all layers of the
glacier do not advance with equal velocity, some points remain behind others; for
instance, the edges as compared with the middle. Thus if we observe the distance from a
given point at the edge to a given point of the middle, both of which were originally in
the same line, but the latter of which afterwards descended more rapidly, we shall find
that this distance continually increases; and since the ice cannot expand to an extent
corresponding to the increasing distance, it breaks up and forms crevasses, as seen along
the edge of the glacier in Fig. III, which represents the Gorner Glacier at Zermatt. It
would lead me too far if I were here to attempt to give a detailed explanation of the
formation of the more regular system of crevasses, as they occur in certain parts of all
glaciers; it may be sufficient to mention that the conclusions deducible from the
considerations above stated are fully borne out by observation.
I will only draw attention to one point - what extremely small displacements are
sufficient to cause ice to form hundreds of crevasses. The section of the Mer de Glace
(Fig.112, at g, c, h) shows places where a scarcely perceptible change in the inclination
of the surface of the ice occurs of from two to four degrees. This is sufficient to
produce a system of cross crevasses on the surface. Tyndall more especially has urged and
confirmed by observation and measurements, that the mass of ice of the glacier does not
give way in the smallest degree to extension, but when subjected to a pull is invariably
torn asunder.
The distribution of the boulders, too, on the surface of the glacier is readily
explained when we take their motion into account. These boulders are fragments of the
mountains between which the glacier flows. Detached partly by the weathering of the stone,
and partly by the freezing of water in its crevices, they fall, and for the most part on
the edge of the mass of ice. There they either remain lying on the surface, or if they
have originally burrowed in the snow, they ultimately reappear in consequence of the
melting of the superficial layers of ice and snow, and they accumulate especially at the
lower end of the glacier, where more of the ice between them has been melted. The blocks
which are gradually borne down to the lower end of the glacier are sometimes quite
colossal in size. Solid rocky masses of this kind are met with in the lateral and terminal
moraines, which are as large as a two - storied house.
The masses of stone move in lines which are always nearly parallel to each other and to
the longitudinal direction of the glacier. Those, therefore, that are already in the
middle remain in the middle, and those that lie on the edge remain at the edge. These
latter are the more numerous, for during the entire course of the glacier fresh boulders
are constantly falling on the edge, but cannot fall on the middle. Thus are formed on the
edge of the mass of ice the lateral moraines, the boulders of which partly move along with
the ice, partly glide over its surface, and partly rest on the solid rocky base near the
ice. But when two glacier streams unite, their coinciding lateral moraines come to lie
upon the centre of the united ice-stream, and then move forward as central moraines
parallel to each other and to the banks of the stream, and they show, as far as the lower
end, the boundary line of the ice which originally belonged to one or the other of the
arms of the glacier. They are very remarkable as displaying in what regular parallel bands
the adjacent parts of the ice-stream glide downwards. A glance at the map of the Mer de
Glace, and its four central moraines, exhibits this very distinctly.
On the Glacier du Geant and its continuation in the Mer de Glace, the stones on the
surface of the ice delineate, in alternately greyer and whiter bands, a kind of yearly
rings which were first observed by Forbes. For since in the cascade at g, Fig. 112, more
ice slides down in summer than in winter, the surface of the ice below the cascade forms a
series of terraces as seen in the drawing, and as those, slopes of the terraces which have
a northern aspect melt less than their upper plane surfaces, the former exhibit purer ice
than the latter. This, according to Tyndall, is the probable origin of these dirt bands.
At first they run pretty much across the glacier, but as afterwards their centre moves
somewhat more rapidly than the ends, they acquire farther down a curved shape, as
represented in the map, Fig. 110. By their curvature they thus show to the observer with
what varying velocity ice advances in the different parts of its course.
A very peculiar part is played by certain stones which are imbedded in the lower
surface of the mass of ice, and which have partly fallen there through crevasses, and may
partly have been detached from the bottom of the valley. For these stones are gradually
pushed with the ice along the base of the valley, and at the same time are pressed against
this base by the enormous weight of the superincumbent ice. Both the stones imbedded in
the ice as well as the rocky base are equally hard, but by their friction against each
other they are ground to powder with a power compared to which any human exertion of force
is infinitely small. The product of this friction is an extremely fine powder, which,
swept away by water, appears lower down in the glacier brook, imparting to it a whitish or
yellowish muddy appearance.
The rocks of the trough of the valley, on the contrary, on which the glacier exerts
year by year its grinding power, are polished as if in an enormous polishing machine. They
remain as rounded, smoothly polished masses, in which are occasional scratches produced by
individual harder stones. Thus we see them appear at the edge of existing glaciers, when
after a series of dry and hot seasons the glaciers have somewhat receded. But we find such
polished rocks as remains of gigantic ancient glaciers to a far greater extent in the
lower parts of many Alphine valleys. In the valley of the Aar more especially, as far down
as Meyringen, the rock-walls polished to a considerable height are very characteristic.
There also we find the celebrate polished plates, over which the way passes, and which are
so smooth that furrows have had to be hewn into them and rails erected to enable men and
animals to traverse them in safety.
The former enormous extent of glaciers is recognised by ancient moraine dykes and by
transported blocks of stone, as well as by these polished rocks. The blocks of stone which
have been carried away by the glacier are distinguished from those which water has rolled
down, by their enormous magnitude, by the perfect retention of all their edges which are
not at all rounded off, and finally by their being deposited on the glacier in exactly the
same order in which the rocks of which they formed part stand in the mountain ridge; while
the stones which currents of water carry along are completely mixed together.
From these indications, geologists have been able to prove that the glaciers of
Chamouni, of Monte Rosa, of the St. Gotthard, and the Bernese Alps, formerly penetrated
through the valley of the Arve, the Rhone, the Aar, and the Rhine to the more level part
of Switzerland and the Jura, where they have deposited their boulders at a height of more
than a thousand feet above the present level of the lake of Neufchatel. Similar traces of
ancient glaciers are found upon the mountains of the British Islands, and upon the
Scandinavian Peninsula.
The drift ice too of the Arctic Sea is glacier ice; it is pushed down into the sea by
the glaciers of Greenland, becomes detached from the rest of the glacier, and floats away.
In Switzerland we find a similar formation of drift ice, though on a far smaller scale, in
the little Marjelen See, into which part of the ice of the great Aletsch Glacier pushes
down. Blocks of stone which lie in drift ice may make long voyages over the sea. The vast
number of blocks of granite which are scattered on the North German plains, and whose
granite belongs to the Scandinavian mountains, has been transported by drift ice at the
time when the European glaciers had such a enormous extent.
I must unfortunately content myself with these few references to the ancient history of
glaciers, and revert now to the processes at present at work in them.
From the facts which I have brought before you it results that the ice of a glacier
flows slowly like the current of a very viscous substance, such for instance as honey,
tar, or thick magma of clay. The mass of ice does not merely flow along the ground like a
solid which glides over a precipice, but it bends and twists in itself; and although even
while doing this it moves along the base of the valley, yet the parts which are in contact
with the bottom and the sides of the valley are perceptibly retarded by the powerful
friction; the middle of the surface of the glacier, which is most distant both from the
bottom and the sides, moving most rapidly. Rendu, a Savoyard priest, and the celebrated
natural philosopher Forbes, were the first to suggest the similarity of a glacier with a
current of a viscous substance.
Ice And Glaciers: Part II
Now you will perhaps inquire with astonishment how it is possible that ice, which is
the most brittle and fragile of substances, can flow in the glacier like a viscous mass;
and you may perhaps be disposed to regard this as one of the wildest and most improbable
statements that have ever been made by philosophers. I will at once admit that
philosophers themselves were not a little perplexed by these results of their
investigations. But the facts were there, and could not be got rid of. How this mode of
motion originated was for a long time quite enigmatical, the more so since the numerous
crevasses in glaciers were a sufficient indication of the well-known brittleness of ice;
and as Tyndall correctly remarked, this constituted an essential difference between a
stream of ice and the flow of lava, of tar, of honey, or of a current of mud.
The solution of this strange problem was found, as is so often the case in the natural
sciences, in apparently recondite investigations into the nature of heat, which form one
of the most important conquests of modern physics, and which constitute what is known as
the mechanical theory of heat. Among a great number of deductions as to the relations of
the diverse natural forces to each other, the principles of the mechanical theory of heat
lead to certain conclusions as to the dependence of the freezing point of water on the
pressure to which ice and water are exposed.
Every one knows that we determine that one fixed point of our thermometer scale which
we call the freezing point of zero by placing the thermometer in a mixture of pure water
and ice. Water, at any rate when in contact with ice, cannot be cooled below zero without
itself being converted into ice; ice cannot be heated above the freezing point without
melting. Ice and water can exist in each other's presence at only one temperature, the
temperature of zero.
Now, if we attempt to heat such a mixture by a flame beneath it, the ice melts, but the
temperature of the mixture is never raised above that of 0 degree so long as some of the
ice remains unmelted. The heat imparted changes ice at zero into water at zero, but the
thermometer indicates no increase of temperature. Hence physicists say that heat has
become latent, and that water contains a certain quantity of latent heat beyond that of
ice at the same temperature.
On the other hand, when we withdraw more heat from the mixture of ice and water, the
water gradually freezes; but as long as there is still liquid water, the temperature
remains at zero. Water at 0 degree has given up its latent heat, and has become changed
into ice at 0 degree.
Now a glacier is a mass of ice which is everywhere interpenetrated by water, and its
internal temperature is therefore everywhere that of the freezing point. The deeper
layers, even of the fields of neve, appear on the heights which occur in our Alpine chain
to have everywhere the same temperature. For, though the freshly fallen snow of these
heights is, for the most part, at a lower temperature than that of 0 degree, the first
hours of warm sunshine melt its surface and form water, which trickles into the deeper
colder layers, and there freezes, until it has throughout been brought to the temperature
of the freezing point. This temperature then remains unchanged. For, though by the sun's
rays the surface of the ice may be melted, it cannot be raised above zero, and the cold of
winter penetrates as little into the badly conducting masses of snow and ice as it does
into our cellars. Thus the interior of the masses of neve, as well as of the glacier,
remains permanently at the melting point.
But the temperature at which water freezes may be altered by strong pressure. This was
first deduced from the mechanical theory of heat by James Thomson of Belfast, and almost
simultaneously by Clausius of Zurich; and, indeed, the amount of this change may be
correctly predicted from the same reasoning. For each increase of a pressure of one
atmosphere the freezing point is lowered by the 1-115th part of a degree Centigrade. The
brother of the former, Sir W. Thomson, the celebrated Glasgow physicist, made an
experimental confirmation of this theoretical deduction by compressing in a suitable
vessel a mixture of ice and snow. This mixture became colder and colder as the pressure
was increased, and to the extent required by the mechanical theory.
Now, if a mixture of ice and water becomes colder when it is subjected to increased
pressure without the withdrawal of heat, this can only be effected by some free heat
becoming latent; that is, some ice in the mixture must melt and be converted into water.
In this is found the reason why mechanical pressure can influence the freezing point. You
know that ice occupies more space than the water from which it is formed. When water
freezes in closed vessels, it can burst not only glass vessels, but even iron shells.
Inasmuch, therefore, as in the compressed mixture of ice and water some of the ice melts
and is converted into water, the volume of the mass diminishes, and the mass can yield
more to the pressure upon it than it could have done without such an alteration of the
freezing point. Pressure furthers in this case, as is usual in the interaction of various
natural forces, the occurrence of a change, that is fusion, which is favourable to the
development of its own activity.
In Sir W. Thomson's experiments water and ice were confined in a closed vessel from
which nothing could escape. The case is somewhat different when, as with glaciers, the
water disseminated in the compressed ice can escape through fissures. The ice is then
compressed, but not the water which escapes. The compressed ice becomes colder in
conformity with the lowering of its freezing point by pressure; but the freezing point of
water which is not compressed is not lowered. Thus under these circumstances we have ice
colder than 0 degree in contact with water at 0 degree. The consequence is that around the
compressed ice water continually freezes and forms new ice, while on the other hand part
of the compressed ice melts.
This occurs, for instance, when only two pieces of ice are pressed against each other.
By the water which freezes at their surfaces of contact they are firmly joined into one
coherent piece of ice. With powerful pressure, and the chilling therefore great, this is
quickly effected; but even with a feeble pressure it takes place, if sufficient time be
given. Faraday, who discovered this property, called it the regelation of ice; the
explanation of this phenomenon has been much controverted; I have detailed to you that
which I consider most satisfactory.
This freezing together of two pieces of ice is very readily effected by pieces of any
shape, which must not, however, be at a lower temperature than 0 degree, and the
experiment succeeds best when the pieces are already in the act of melting.1 They need only be strongly pressed together for a few minutes to make them adhere. The
more plane are the surfaces in contact, the more complete is their union. But a very
slight pressure is sufficient if the two pieces are left in contact for some time.2
[Footnote 1: In the Lecture a series of small cylinders of ice, which had been prepared
by a method to be afterwards described, were pressed with their plane ends against each
other, and thus a cylindrical bar of ice produced.]
[Footnote 2: Vide the additions at the end of this Lecture.]
This property of melting ice is also utilised by boys in making snowballs and snowmen.
It is well known that this only succeeds either when the snow is already melting, or at
any rate is only so much lower than 0 degrees that we warmth of the hand is sufficient to
raise it to this temperature. Very cold snow is a dry loose powder which does not stick
together.
The process which children carry out on a small scale in making snowballs takes place
in glaciers on the very largest scale. The deeper layers of what was originally fine loose
neve are compressed by the huge masses resting on them, often amounting to several hundred
feet, and under this pressure they cohere with an ever firmer and closer structure. The
freshly fallen snow originally consisted of delicate microscopically fine ice spicules,
united and forming delicate six-rayed, feathery stars of extreme beauty. As often as the
upper layers of the snow fields are exposed to the sun's rays, some of the snow melts;
water permeates the mass, and on reaching the lower layers of still colder snow, it again
freezes; thus it is that the firn first becomes granular and acquires the temperature of
the freezing point. But as the weight of the superincumbent masses of snow continually
increases by the firmer adherence of its individual granules, it ultimately changes into a
dense and perfectly hard mass.
This transformation of snow into ice may be artificially effected by using a
corresponding pressure.
We have here (Fig. 113) a cylindrical cast iron vessel, A A; the base, B B, is held by
three screws, and can be detached, so as to remove the cylinder of ice which is formed.
After the vessel has lain for a while in ice water, so as to reduce it to the temperature
of 0 degree, it is packed full of snow, and then the cylindrical plug, C C, which fits the
inner aperture, but moves in it with gentle friction, is forced in with the aid of an
hydraulic press. The press used was such that the pressure to which the snow was exposed
could be increased to fifty atmosphere. Of course the looser snow contracts to a very
small volume under such a powerful pressure. The pressure is removed, the cylindrical plug
taken out, the hollow again filled up with snow, and the process repeated until the entire
form is filled with the mass of ice, which no longer gives way to pressure. The compressed
snow which I now take out, you will see, has been transformed into a hard, angular, and
translucent cylinder of ice; and how hard it is appears from the crash which ensues when I
throw it to the ground. Just as the loose snow in the glaciers is pressed together to
solid ice, so also in many places ready-formed irregular pieces of ice are joined and form
clear and compact ice. This is most remarkable at the base of the glacier cascades. These
are glacier falls where the upper part of the glacier ends at a steep rocky wall, and
blocks of ice shoot down as avalanches over the edge of this wall. The heap of shattered
blocks of ice which accumulate become joined at the foot of the rock-wall to a compact,
dense mass, which then continues its way downwards as glacier. More frequent than such
cascades, where the glacier-stream is quite dissevered, are places where the base of the
valley has a steeper slope, as, for instance, the places in the Mer de Glace (Fig. 105),
at g, of the Cascade of the Glacier du Geant, and at i and h of the great terminal cascade
of the Glacier des Bois. The ice splits there into thousands of banks and cliffs, which
then recombine towards the bottom of the steeper slope and form a coherent mass.
This also we may imitate in our ice mould. Instead of the snow I take irregular pieces
of ice, press them together; add new pieces of ice, press them again, and so on, until the
mould is full. When the mass is taken out it forms a compact coherent cylinder of
tolerably clear ice, which has a perfectly sharp edge, and is an accurate copy of the
mould.
This experiment, which was first made by Tyndall, shows that a block of ice may be
pressed into any mould just like a piece of wax. It might, perhaps, be thought that such a
block had, by the pressure in the interior, been first reduced to powder so fine that it
readily penetrated every crevice of the mould, and then that this powdered ice, like snow,
was again combined by freezing. This suggests itself the more readily, since while the
press is being worked a continual creaking and cracking is heard in the interior of the
mould. Yet the mere aspect of the cylinders pressed from blocks of ice shows us that the
it has not been formed in this manner; for they are generally clearer than ice which is
produced from snow, and the individual larger pieces of ice which have been used to
produce them are recognized, though they are somewhat changed and flattened. This is most
beautiful when clear pieces of ice are laid in the form and the rest of the space stuffed
full of snow. The cylinder is then seen to consist of alternate layers of clear and opaque
ice, the former arising from the pieces of ice, and the latter from the snow; but here
also the pieces of ice seem pressed into flat discs.
These observations teach, then, that ice need not be completely smashed to fit into the
prescribed mould, but that it may give way without losing its coherence. This can be still
more completely proved, and we can acquire a still better insight into the cause of the
pliability of ice, if we press the ice between two plane wooden boards, instead of in the
mould, into which we cannot see.
I place first an irregular cylindrical piece of natural ice, taken from the frozen
surface of the river, with its two plane terminal surfaces between the plates of the
press. If I begin to work, the block is broken by pressure; every crack which forms
extends through the entire mass of the block; this splits into a heap of larger fragments,
which again crack and are broken the more the press is worked. If the pressure is relaxed,
all these fragments are, indeed, reunited by freezing, but the aspect of the whole
indicates that the shape of the block has resulted less from pliability than from
fracture, and that the individual fragments have completely altered their mutual
positions.
The case is quite different when one of the cylinders which we have formed from snow or
ice is placed between the plates of the press. As the press is worked the creaking and
cracking is heard, but it does not break; it gradually changes its shape, becomes lower
and at the same time thicker; and only when it has been changed into a tolerably flat
circular disc does it begin to give way at the edges and form cracks, like crevasses on a
small scale. Fig. 114 shows the height and diameter of such a cylinder in its original
condition; Fig. 115 represents its appearance after the action of the press.
A still stronger proof of the pliability of ice is afforded when one of our cylinders
is forced through a narrow aperture. With this view I place a base on the previously
described mould, which has a conical perforation, the external aperture of which is only
two thirds the diameter of the cylindrical aperture of the form. Fig. 116 gives a section
of the whole. If now I insert into this one of the compressed cylinders of ice, and force
down the plug a, the ice is forced through the narrow aperture in the base. It at first
emerges as a solid cylinder of the same diameter as the aperture; but as the ice follows
more rapidly in the centre than at the edges, the free terminal surface of the cylinder
becomes curved, the end thickens, so that it could not be brought back through the
aperture, and it ultimately splits off. Fig. 117 exhibits a series of shapes which have
resulted in this manner.3
[Footnote 3: In this experiment the lower temperature of the compressed ice sometimes
extended so far through the iron form, that the water in the slit between the base plate
and the cylinder froze and formed a thin sheet of ice, although the pieces of ice as well
as the iron mould had previously laid in ice water, and could not be colder than 0
degree.]
Here also the cracks in the emerging cylinder of ice exhibit a surprising similarity
with the longitudinal rifts which divide a glacier current where it presses through a
narrow rocky pass into a wider valley.
In the cases which we have described we see the change in shape of the ice taking place
before our eyes, whereby the block of ice retains its coherence without breaking into
individual pieces. The brittle mass of ice seems rather to yield like a piece of wax.
A closer inspection of a clear cylinder of ice compressed from clear pieces of ice,
while the pressure is being applied, shows us what takes, place in the interior; for we
then see an innumerable quantity of extremely fine radiating cracks shoot through it like
a turbid cloud, which mostly disappear, though not completely, the moment the pressure is
suspended. Such a compressed block is distinctly more opaque immediately after the
experiment than it was before; and the turbidity arises, as may easily be observed by
means of a lens, from a great number of whitish capillary lines crossing the interior of
the mass of what is otherwise clear. These lines are the optical expression of extremely
fine cracks4 which interpenetrate the mass of the ice. Hence we may conclude
that the compressed block is traversed by a great number of fine cracks and fissures which
render it pliable; that its particles become a little dispersed, and are therefore
withdrawn from pressure, and that immediately afterwards the greater part of the fissures
disappear, owing to their sides freezing. Only in those places in which the surfaces of
the small displaced particles do not accurately fit to each other some fissured spaces
remain open, and are discovered as white lines and surfaces by the reflection of the
light.
[Footnote 4: These cracks are probably quite empty and free from air, for they are also
formed when perfectly clear and air-free pieces of ice are pressed in the form which has
been previously filled with water, and where, therefore, no air could gain access to the
pieces of ice. That such air-free crevices occur in glacier ice has been already
demonstrated by Tyndall. When the compressed ice afterwards melts, these crevices fill up
with water, no air being left. They are then, however, far less visible, and the whole
block is therefore clearer. And just for this reason they could not originally have been
filled with water.]
These cracks and laminae also become more perceptible when the ice which, as I before
mentioned, is below zero immediately after pressure has been applied - is again raised to
this temperature and begins to melt. The crevices then fill with water, and such ice then
consists of a quantity of minute from the size of a pin's head to that of a pea, which are
closely pushed into one another at the edges and projections, and in part have coalesced,
while the narrow fissures between them are full of water. A block of ice thus formed of
ice granules adheres firmly together; but if particles be detached from its corners they
are seen to consist of these angular granules. Glacier ice, when it begins to melt, is
seen to possess the same structure, except that the pieces of which it consists are mostly
larger than in artificial ice, attaining the size of a pigeon's egg.
Glacier ice and compressed ice are thus seen to be substances of a granular structure,
in opposition to regularly crystallised ice, such as is formed on the surface of still
water. We here meet with the same differences as between calcareous spar and marble, both
of which consist of carbonate of lime; but while the former is in large, regular crystals,
the latter is made up of irregularly agglomerated crystalline grains. In calcareous spar,
as well as in crystallised ice, the cracks produced by inserting the point of a knife
extend through the mass, while in granular ice a crack which arises in one of the bodies
where it must yield does not necessarily spread beyond the limits of the granule.
Ice which has been compressed from snow, and has thus from the outset consisted of
innumerable very fine crystalline needles, is seen to be particularly plastic. Yet in
appearance it materially differs from glacier ice, for it is very opaque, owing to the
great quantity of air which was originally inclosed in the flaky mass of snow, and which
remains there as extremely minute bubbles. It can be made clearer by pressing a cylinder
of such ice between wooden boards; the air bubbles appear then on the top of the cylinder
as a light foam. If the discs are again broken, placed in the mould, and pressed into a
cylinder, the air may gradually be more and more eliminated, and the ice be made clearer.
No doubt in glaciers the originally whitish mass of neve is thus gradually transformed
into the clear, transparent ice of the glacier.
Lastly, when streaked cylinders of ice formed from pieces of snow and ice are pressed
into discs, they become finely streaked, for both their clear and their opaque layers are
uniformly extended.
Ice thus striated occurs in numerous glaciers, and is no doubt caused, as Tyndall
maintains, by snow falling between the blocks of ice; this mixture of snow and clear ice
is again compressed in the subsequent path of the glacier, and gradually stretched by the
motion of the mass: a process quite analogous to the artificial one which we have
demonstrated.
Thus to the eye of the natural philosopher the glacier, with its wildly heaped ice
blocks, its desolate, stony, and muddy surface, and its threatening crevasses, has become
a majestic stream whose peaceful and regular flow has no parallel; which according to
fixed and definite laws, narrows, expands, is heaped up, or, broken and shattered, falls
down precipitous heights. If we trace it beyond its termination we see its waters, uniting
to a copious brook, burst through its icy gate and flow away. Such a brook, on emerging
from the glacier, seems dirty and turbid enough, for it carries away as powder the stone
which the glacier has ground. We are disenchanted at seeing the wondrously beautiful and
transparent ice converted into such muddy water. But the water of the glacier streams is
as pure and beautiful as the ice, though its beauty is for the moment concealed and
invisible. We must search for these waters after they have passed through a lake in which
they have deposited this powdered stone. The Lakes of Geneva, of Thun, of Lucerne, of
Constance, the Lago Maggiore, the Lake of Como, and the Lago di Garda are chiefly fed with
glacier waters; their clearness and their wonderfully beautiful blue or blue green colour
are the delight of all travellers.
Yet, leaving aside the beauty of these waters, and considering only their utility, we
shall have still more reason for admiration. The unsightly mud which the glacier streams
wash away forms a highly fertile soil in the places where it is deposited; for its state
of mechanical division is extremely fine, and it is moreover an utterly unexhausted virgin
soil, rich in the mineral food of plants. The fruitful layers of fine loam which extend
along the whole Rhine plain as far as Belgium, and are known as Loess, are nothing more
than the dust of ancient glaciers.
Then, again, the irrigation of a district, which is effected by the snow fields and
glaciers of the mountains, is distinguished from that of other places by its comparatively
greater abundancy, for the moist air which is driven over the cold mountain peaks deposits
there most of the water it contains in the form of snow. In the second place, the snow
melts most rapidly in summer, and thus the springs which flow from the snow fields are
most abundant in that season of the year in which they are most needed.
Thus we ultimately get to know the wild, dead ice wastes from another point of view.
From them trickles in thousands of rills, springs, and brooks the fructifying moisture
which enables the industrious dwellers of the Alps to procure succulent vegetation and
abundance of nourishment from the wild mountain slopes. On the comparatively small surface
of the Alpine chain they produce the mighty streams the Rhine, the Rhone, the Po, the
Adige, the Inn, which for hundreds of miles form broad, rich river valleys, extending
through Europe to the German Ocean, the Mediterranean, the Adriatic, and the Black Sea.
Let us call to mind how magnificently Goethe, in "Mahomet's Song," has depicted
the course of the rocky spring, from its origin beyond the clouds to its union with Father
Ocean. It would be presumptuous after him to give such a picture in other than his own
words:
And along, in triumph rolling, Names he gives to regions; cities Grow amain beneath his
feet.
On and ever on he rushes; Spire and turret fiery crested Marble palaces, the creatures
Of his wealth, he leaves behind.
Pine-built houses bears the Atlas On his giant shoulders. O'er his Head a thousand
pennons rustle, Floating far upon the breezes, Tokens of his majesty.
And so beareth he his brothers, And his treasures, and his children. To their primal
sire expectant, All his bosom throbbing, heaving, With a wild tumultuous joy.
Theodore Martin's Translation.
Additions
The theory of the regelation of ice has led to scientific discussions between Faraday
and Tyndall on the one hand, and James and Sir W. Thomson on the other. In the text I have
adopted the theory of the latter, and must now accordingly defend it.
Faraday's experiments show that a very slight pressure, not more than that produced by
the capillarity of the layer of water between two pieces of ice, is sufficient to freeze
them together. James Thomson observed that in Faraday's experiments pressure which could
freeze them together was not utterly wanting. I have satisfied myself by my own
experiments that only very slight pressure is necessary. It must, however, be remembered
that the smaller the pressure the longer will be the time required to freeze the two
pieces, and that then the junction will be very narrow and very fragile. Both these points
are readily explicable on Thomson's theory. For under a feeble pressure the difference in
temperature between ice and water will be very small, and the latent heat will only be
slowly abstracted from the layers of water in contact with the pressed parts of the ice,
so that a long time is necessary before they freeze. We must further take into account
that we cannot in general consider that the two surfaces are quite in contact; under a
feeble pressure which does not appreciably alter their shape, they will only touch in what
are practically three points. A feeble total pressure on the pieces of ice concentrated on
such narrow surfaces will always produce a tolerably great local pressure under the
influence of which some ice will melt, and the water thus formed will freeze. But the
bridge which joins them will never be otherwise than narrow.
Under stronger pressure, which may more completely alter the shape of the pieces of
ice, and fit them against each other, and which will melt more of the surfaces that are
first in contact, there will be a greater difference between the temperature of the ice
and water, and the bridges will be more rapidly formed, and be of greater extent.
In order to show the slow action of the small differences of temperature which here
come into play, I made the following experiments.
A glass flask with a drawn-out neck was half filled with water, which was boiled until
all the air in the flask was driven out. The neck of the flash was then hermetically
sealed. When cooled, the flask was void of air, and the water within it freed from the
pressure of the atmosphere. As the water thus prepared can be cooled considerably below
0C. before the first ice is formed, while when ice is in the flask it freezes at 0C., the
flask was in the first instance placed in a freezing mixture until the water changed into
ice. It was afterwards permitted to melt slowly in a place the temperature of which was
+2C., until the half of its was liquefied.
The flask thus half filled with water, having a disc of ice swimming upon it, was
placed in a mixture of ice and water, being quite surrounded by the mixture. After an
hour, the disc within the flask was frozen to the glass. By shaking the flask the disc was
liberated, but it froze again. This occurred as often as the shaking was repeated.
The flask was permitted to remain for eight days in the mixture, which was kept
throughout at a temperature of 0C. During this time a number of very regular and sharply
defined ice crystals were formed, and augmented very slowly in size. This is perhaps the
best method of obtaining beautifully formed crystals of ice.
While, therefore, the outer ice which had to support the pressure of the atmosphere
slowly melted, the water within the flask, whose freezing point, on account of a defect of
pressure, was 0.0075C. higher, deposited crystals of ice. The heat abstracted from the
water in this operation had, moreover, to pass through the glass of the flask, which,
together with the small difference of temperature, explains the slowness of the freezing
process.
Now as the pressure of one atmosphere on a square millimetre amounts to about ten
grammes, a piece of ice weighing ten grammes, which lies upon another and touches it in
three places, the total surface of which is a square millimetre, will produce on these
surfaces a pressure of an atmosphere. Ice will therefore be formed more rapidly in the
surrounding water than it was in the flask, where the side of the glass was interposed
between the ice and the water. Even with a much smaller weight the same result will follow
in the course of an hour. The broader the bridges become, owing to the freshly formed ice,
the greater will be the surfaces over which the pressure exerted by the upper piece of ice
is distributed, and the feebler it will become; so that with such feeble pressure the
bridges can only slowly increase, and therefore they will be readily broken when we try to
separate the pieces.
It cannot, moreover, be doubted that in Faraday's experiments, in which two perforated
discs of ice were placed in contact on a horizontal glass rod, so that gravity exerted no
pressure, capillary attraction is sufficient to produce a pressure of some grammes between
the plates, and the preceding discussions show that such a pressure, if adequate time be
given, can form bridges between the plates.
If, on the other hand, two of the above-described cylinders of ice are powerfully
pressed together by the hands, they adhere in a few minutes so firmly that they can only
be detached by the exertion of a considerable force, for which indeed that of the hands is
sometimes inadequate.
In my experiments I found that the force and rapidity with which the pieces of ice
united were so entirely proportional to the pressure that I cannot but assign this as the
actual and sufficient cause of their union.
In Faraday's explanation, according to which regelation is due to a contact action of
ice and water, I find a theoretical difficulty. By the water freezing, a considerable
quantity of latent heat must be set free, and it is not clear what becomes of this.
Finally, if ice in its change into water passes through an intermediate viscous
condition, a mixture of ice and water which was kept for days at a temperature of 0
degrees must ultimately assume this condition in its entire mass, provided its temperature
was uniform throughout; this however is never the case.
As regards what is called the plasticity of ice, James Thomson has given an explanation
of it in which the formation of cracks in the interior is not presupposed. No doubt when a
mass of ice in different parts of the interior is exposed to different pressures, a
portion of the more powerfully compressed ice will melt; and the latent heat necessary for
this will be supplied by the ice which is less strongly compressed, and by the water in
contact with it. Thus ice would melt at the compressed places, and water would freeze in
those which are not pressed: ice would thus be gradually transformed and yield to
pressure. It is also clear that, owing to the very small conductivity for heat which ice
possesses, a process of this kind must be extremely slow, if the compressed and colder
layers of ice, as in glaciers, are at considerable distances from the less compressed
ones, and from the water which furnishes the heat for melting.
To test this hypothesis, I placed in a cylindrical vessel, between two discs of ice of
three inches in diameter, a smaller cylindrical piece of an inch in diameter. On the
uppermost disc I placed a wooden disc, and this I loaded with a weight of twenty pounds.
The section of the narrow piece was thus exposed to a pressure of more than an atmosphere.
The whole vessel was packed between pieces of ice, and left for five days in a room the
temperature of which was a few degrees above the freezing point. Under these circumstances
the ice in the vessel, which was exposed to the pressure of the weight, should melt, and
it might be expected that the narrow cylinder on which the pressure was most powerful
should have been most melted. Some water was indeed formed in the vessel, but mostly at
the expense of the larger discs at the top and bottom, which being nearest the outside
mixture of ice and water, could acquire heat through the sides of the vessel. A small
welt, too, of ice, was formed round the surface of contact of the narrower with the lower
broad piece, which showed that the water, which had been formed in consequence of the
pressure, had again frozen in places in which the pressure ceased. Yet under these
circumstances there was not appreciable alteration in the shape of the middle piece which
was most compressed.
This experiment shows that although changes in the shape of the pieces of ice must take
place in the course of time in accordance with J. Thomson's explanation, by which the more
strongly compressed parts melt, and new ice is formed at the places which are freed from
pressure, these changes must be extremely slow when the thickness of the pieces of ice
through which the heat is conducted is at all considerable. Any marked change in shape by
melting in a medium the temperature of which is everywhere 0 degrees, could not occur
without access of external heat, or from the uncompressed ice and water; and with the
small differences in temperature which here come into play, and from the badly conducting
power of ice, it must be extremely slow.
That on the other hand, especially in granular ice, the formation of cracks, and the
displacement of the surfaces of those cracks, render such a change of form possible, is
shown by the above-described experiments on pressure; and that in glacier ice changes of
form thus occur, follows from the banded structure, and the granular aggregation which is
manifest on melting, and also from the manner in which the layers change their position
when moved, and so forth. Hence, I doubt not that Tyndall has discovered the essential and
principal cause of the motion of glaciers, in referring it to the formation of cracks and
to regelation.
I would at the same time observe that a quantity of heat, which is far from
inconsiderable, must be produced by friction in the larger glaciers. It may be easily
shown by calculation that when a mass of firn moves from the Col du Geant to the source of
the Arveyron, the heat due to the mechanical work would be sufficient to melt a fourteenth
part of the mass. And as the friction must be greatest in those places that are most
compressed, it will at any rate be sufficient to remove just those parts of the ice which
offer most resistance to motion.
I will add, in conclusion, that the above-described granular structure of ice is
beautifully shown in polarised light. If a small clear piece is pressed in the iron mould,
so as to form a disc of about five inches in thickness, this is sufficiently transparent
for investigation. Viewed in the polarising apparatus, a great number of variously
coloured small bands and rings are seen in the interior; and by the arrangement of their
colours it is easy to recognize the limits of the ice granules, which, heaped on one
another in irregular order of their optical axes, constitute the plate. The appearance is
essentially the same when the plate has just been taken out of the press, and the cracks
appear in it as whitish lines, as afterwards when these crevices have been filled up in
consequence of the ice beginning to melt.
In order to explain the continued coherence of the piece of ice during its change of
form, it is to be observed that in general the cracks in the granular ice are only
superficial, and do not extend throughout its entire mass. This is directly seen during
the pressing of the ice. The crevices form and extend in different directions, like cracks
produced by a heated wire in a glass tube. Ice possesses a certain degree of elasticity,
as may be seen in a thin flexible plate. A fissured block of ice of this kind will be able
to undergo a displacement at the two sides which form the crack, even when these continue
to adhere in the unfissured part of the block. If then part of the fissure at first formed
is closed by regelation, the fissure can extend in the opposite direction without the
continuity of the block being at any time disturbed. It seems to me doubtful, too, whether
in compressed ice and in glacier ice, which apparently consists of interlaced polyhedral
granules, these granules, before any attempt is made to separate them, are completely
detached from each other, and are not rather connected by ice bridges which readily give
way; and whether these latter do not produce the comparatively firm coherence of the
apparent heap of granules.
The properties of ice here described are interesting from a physical point of view, for
they enable us to follow so closely the transition from a crystalline body to a granular
one; and they give the causes of the alteration of its properties better than in any other
well-known example. Most natural substances show no regular crystalline structure; our
theoretical ideas refer almost exclusively to crystallised and perfectly elastic bodies.
It is precisely in this relationship that the transition from fragile and elastic
crystalline ice into plastic granular ice is so very instructive.
Source:
Scientific papers: physics, chemistry, astronomy, geology, with introductions,
notes and illustrations. New York, P. F. Collier & son [c1910] Harvard classics ;
no.XXX.
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© Paul Halsall, August 1998
