Scientific Papers.
The Harvard Classics. 1909–14.
Hermann von Helmholtz; Translated by Edmund Atkinson
Ice and Glaciers
A Lecture Delivered at Frankfort-on-the-Main, and at Heidelberg, in February, 1865T
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° to 30° C., 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 1° C. for an ascent of 480 feet; in winter it is less—1° 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°, or at most 8°; 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 débris, 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 (F
To exemplify these circumstances still further, I lay before you in F
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 Géant (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 Géant, Glacier de Léchaud, and Glacier du Talèfre, 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 Talèfre into two branches; and bolder still they ascend yet higher, to the Col du Géant (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 Talèfre unite at the lower end of the Jardin; the second proceeds from the union of the glacier in question with the Glacier de Léchaud; the third, from the union of the last with the Glacier du Géant; and the fourth, finally, from the top of the rock ledge which stretches from the Aiguille du Géant towards the cascade (g) of the Glacier du Géant.
To give you an idea of the slope and the fall of the glacier, I have given in F
At the end of a vertical rock wall of the Tacul, the edge of the Glacier du Géant 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 Neckargemünd, 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.
F
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 daylight, 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.
F
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 Géant. On descending the rocks at the side of the cascade of the Glacier du Géant, 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, F
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 Géant thirteen inches a day, and the Glacier du Léchaud 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 Géant 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 F
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 (F
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 Géant 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, F
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.
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° 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° has given up its latent heat, and has become changed into ice at 0°.
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 névé, 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°, 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 névé, 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 Zürich; 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° in contact with water at 0°. 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°, and the experiment succeeds best when the pieces are already in the act of melting. 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.
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° 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 névé 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 (F
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. F
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. F
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 cracks 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.
These cracks and laminæ 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 névé 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:—