Chapter XVI: Part II: Chiefly Scientific (5)

The perusal of the conclusion of Professor Forbes's last volume shows me that a thought similar to that expressed above occurred to Mr. Bakewell also. Speaking of a shallow glacier which moved when the alleged temperature was so enormously below the freezing point that Professor Forbes regards the observation as open to question (in which I agree with him), Mr. Bakewell asks, "Is it possible that infiltrated water can have any action whatever under such circumstances?" The reply of Professor Forbes contains these words:--"I have nowhere affirmed the presence of liquid water to be a _sine qua non_ to the plastic motion of glaciers." This statement, I confess, took me by surprise, which was not diminished by further reading. Speaking of the influence of temperature on the motion of the Mer de Glace, Professor Forbes says, the glacier "took no real start until the frost had given way, and the tumultuous course of the Arveiron showed that its veins were again filled with the circulating medium to which the glacier, like the organic frame, owes its moving energy."[B] And again:--"It is this fragility precisely which, yielding to the hydrostatic pressure of the unfrozen water contained in the countless capillaries of the glacier, produces the crushing action which shoves the ice over its neighbour particles."[C]

[Sidenote: HUXLEY'S OBSERVATIONS.]

After the perusal of the foregoing paragraphs the reader will probably be less interested in the question as to whether the assumed capillaries exist at all in the glacier. According to Mr. Huxley's observations, they do not.[D] During the summer of 1857 he carefully experimented with coloured liquids on the Mer de Glace and its tributaries, and in no case was he able to discover these fissures in the sound unweathered ice. I have myself seen the red liquid resting in an auger-hole, where it had lain for an hour without diffusing itself in any sensible degree. This cavity intersected both the white ice and the blue veins of the glacier; and Mr. Huxley, in my presence, cut away the ice until the walls of the cavity became extremely thin, still no trace of liquid passed through them. Experiments were also made upon the higher portions of the Mer de Glace, and also on the Glacier du Geant, with the same result. Thus the very existence of these capillaries is rendered so questionable, that no theory of glacier-motion which invokes their aid could be considered satisfactory.

FOOTNOTES:

[A] 'Occ. Pap.,' p. 224.

[B] 'Phil. Trans.,' 1846, p. 137, and 'Occ. Pap.,' p. 138.

[C] 'Occ. Pap.,' p. 47.

[D] 'Phil. Mag.,' 1857, vol. xiv., p. 241.

THOMSON'S THEORY.

(21.)

In the 'Transactions' of the Royal Society of Edinburgh for 1849 is published a very interesting paper by Prof. James Thomson of Queen's College, Belfast, wherein he deduces, as a consequence of a principle announced by the French philosopher Carnot, that water, when subjected to pressure, requires a greater cold to freeze it than when the pressure is removed. He inferred that the lowering of the freezing point for every atmosphere of pressure amounted to .0075 of a degree Centigrade. This deduction was afterwards submitted to the test of experiment by his distinguished brother Prof. Wm. Thomson, and proved correct. On the fact thus established is founded Mr. James Thomson's theory of the "Plasticity of Ice as manifested in Glaciers."

[Sidenote: STATEMENT OF THEORY.]

The theory is this:--Certain portions of the glacier are supposed first to be subjected to pressure. This pressure liquefies the ice, the water thus produced being squeezed through the glacier in the direction in which it can most easily escape. But cold has been evolved by the act of liquefaction, and, when the water has been relieved from the pressure, it freezes in a new position. The pressure being thus abolished at the place where it was first applied, new portions of the ice are subjected to the force; these in their turn liquefy, the water is dispersed as before, and re-frozen in some other place. To the succession of processes here assumed Mr. Thomson ascribes the changes of form observed in glaciers.

This theory was first communicated to the Royal Society through the author's brother, Prof. William Thomson, and is printed in the 'Proceedings' of the Society for May, 1857. It was afterwards communicated to the British Association in Dublin, in whose 'Reports' it is further published; and again it was communicated to the Belfast Literary and Philosophical Society, in whose 'Proceedings' it also finds a place.

On the 24th of November, 1859, Mr. James Thomson communicated to the Royal Society, through his brother, a second paper, in which he again draws attention to his theory. He offers it in substitution for my views as the best argument that he can adduce against them; he also controverts the explanations of regelation propounded by Prof. James D. Forbes and Prof. Faraday, believing that his own theory explains all the facts so well as to leave room for no other.

[Sidenote: DIFFICULTIES OF THEORY.]

But the passage in this paper which demands my chief attention is the following:--"Prof. Tyndall (writes Mr. Thomson), in papers and lectures subsequent to the publication of this theory, appears to adopt it to some extent, and to endeavour to make its principles co-operate with the views he had previously founded on Mr. Faraday's fact of regelation." I may say that Mr. Thomson's main thought was familiar to me long before his first communication on the plasticity of ice appeared; but it had little influence upon my convictions. Were the above passage correct, I should deserve censure for neglecting to express my obligations far more explicitly than I have hitherto done; but I confess that even now I do not understand the essential point of Mr. Thomson's theory,--that is to say, its application to the phenomena of glacier motion. Indeed, it was the obscurity in my mind in connexion with this point, and the hope that time might enable me to seize more clearly upon his meaning, which prevented me from giving that prominence to the theory of Mr. Thomson which, for aught I know, it may well deserve. I will here briefly state one or two of my difficulties, and shall feel very grateful to have them removed.

[Sidenote: IMPROBABLE DEDUCTION.]

Let us fix our attention on a vertical slice of ice transverse to the glacier, and to which the pressure is applied perpendicular to its surfaces. The ice liquefies, and, supposing the means of escape offered to the compressed water to be equal all round, it is plain that there will be as great a tendency to squeeze the water upwards as downwards; for the mere tendency to flow down by its own gravity becomes, in comparison to the forces here acting on the water, a vanishing quantity. But the fact is, that the ice above the slice is more permeable than that below it; for, as we descend a glacier, the ice becomes more compact. Hence the greater part of the dispersed water will be refrozen on that side of the slice which is turned towards the origin of the glacier; and the consequence is, that, according to Mr. Thomson's principle, the glacier ought to move up hill instead of down.

I would invite Mr. Thomson to imagine himself and me together upon the ice, desirous of examining this question in a philosophic spirit; and that we have taken our places beside a stake driven into the ice, and descending with the glacier. We watch the ice surrounding the stake, and find that every speck of dirt upon it retains its position; there is no liquefaction of the ice that bears the dirt, and consequently it rests on the glacier undisturbed. After twelve hours we find the stake fifteen inches distant from its first position: I would ask Mr. Thomson how did it get there? Or let us fix our attention on those six stakes which M. Agassiz drove into the glacier of the Aar in 1841, and found erect in 1842 at some hundreds of feet from their first position:--how did they get there? How, in fine, does the end of a glacier become its end? Has it been liquefied and re-frozen? If not, it must have been _pushed_ down by the very forces which Mr. Thomson invokes to produce his liquefaction. Both the liquefaction, as far as it exists, and the motion, are products of the same cause. In short, this theory, as it presents itself to my mind, is so powerless to account for the simplest fact of glacier-motion, that I feel disposed to continue to doubt my own competence to understand it rather than ascribe to Mr. Thomson an hypothesis apparently so irrelevant to the facts which it professes to explain.

Another difficulty is the following:--Mr. Thomson will have seen that I have recorded certain winter measurements made on the Mer de Glace, and that these measurements show not only that the ice moves at that period of the year, but that it exhibits those characteristics of motion from which its plasticity has been inferred; the velocity of the central portions of the glacier being in round numbers double the velocity of those near the sides. Had there been any necessity for it, this ratio might have been augmented by placing the side-stakes closer to the walls of the glacier. Considering the extreme coldness of the weather which preceded these measurements, it is a moderate estimate to set down the temperature of the ice in which my stakes were fixed at 5 deg. Cent. below zero.

[Sidenote: REQUISITE PRESSURE CALCULATED.]

Let us now endeavour to estimate the pressure existing at the portion of the glacier where these measurements were made. The height of the Montanvert above the sea-level is, according to Prof. Forbes, 6300 feet; that of the Col du Geant, which is the summit of the principal tributary of the Mer de Glace, is 11,146 feet: deducting the former from the latter, we find the height of the Col du Geant above the Montanvert to be 4846 feet.

Now, according to Mr. Thomson's theory and his brother's experiments, the melting point of ice is lowered .0075 deg. Centigrade for every atmosphere of pressure; and one atmosphere being equivalent to the pressure of about thirty-three feet of water, we shall not be over the truth if we take the height of an equivalent column of glacier-ice, of a compactness the mean of those which it exhibits upon the Col du Geant and at the Montanvert respectively, at forty feet. The compactness of glacier ice is, of course, affected by the air-bubbles contained within it.

[Sidenote: ACTUAL PRESSURE INSUFFICIENT.]

If, then, the pressure of forty feet of ice lower the melting point .0075 deg. Centigrade, it follows that the pressure of a column 4846 feet high will lower it nine-tenths of a degree Centigrade. Supposing, then, the _unimpeded thrust of the whole glacier, from the Col du Geant downwards_, to be exerted on the ice at the Montanvert; or, in other words, supposing the bed of the glacier to be absolutely smooth and every trace of friction abolished, the utmost the pressure thus obtained could perform would be to lower the melting point of the Montanvert ice by the quantity above mentioned. Taking into account the actual state of things, the friction of the glacier against its sides and bed, the opposition which the three tributaries encounter in the neck of the valley at Trelaporte, the resistance encountered in the sinuous valley through which it passes; and finally, bearing in mind the comparatively short length of the glacier, which has to bear the thrust, and oppose the latter by its own friction merely;--I think it will appear evident that the ice at the Montanvert cannot possibly have its melting point lowered by pressure more than a small fraction of a degree.

The ice in which my stakes were fixed being -5 deg. Centigrade, according to Mr. Thomson's calculation and his brother's experiments, it would require 667 atmospheres of pressure to liquefy it; in other words, it would require the unimpeded pressure of a column of glacier-ice 26,680 feet high. Did Mont Blanc rise to two and a half times its present height above the Montanvert, and were the latter place connected with the summit of the mountain by a continuous glacier with its bed absolutely smooth, the pressure at the Montanvert would be rather under that necessary to liquefy the ice on which my winter observations were made.

[Sidenote: MEASUREMENTS APPLY TO SURFACE.]

If it be urged that, though the temperature near the surface may be several degrees below the freezing point, the great body of the glacier does not share this temperature, but is, in all probability, near to 32 deg., my reply is simple. I did not measure the motion of the ice in the body of the glacier; nobody ever did; my measurements refer to the ice at and near the surface, and it is this ice which showed the plastic deportment which the measurements reveal.

Such, then, are some of the considerations which prevent me from accepting the theory of Mr. Thomson, and I trust they will acquit me of all desire, to make his theory co-operate with my views. I am, however, far from considering his deduction the less important because of its failing to account for the phenomena of glacier motion.

THE PRESSURE-THEORY OF GLACIER-MOTION.

(22.)

[Sidenote: POSSIBLE MOULDING OF ICE.]

Broadly considered, two classes of facts are presented to the glacier-observer; the one suggestive of viscosity, and the other of the reverse. The former are seen where _pressure_ comes into play, the latter where _tension_ is operative. By pressure ice can be moulded to any shape, while the same ice snaps sharply asunder if subjected to tension. Were the result worth the labour, ice might be moulded into vases or statuettes, bent into spiral bars, and, I doubt not, by the proper application of pressure, a _rope_ of ice might be formed and coiled into a _knot_. But not one of these experiments, though they might be a thousandfold more striking than any ever made upon a glacier, would in the least demonstrate that ice is really a viscous body.

I have here stated what I believe to be feasible. Let me now refer to the experiments which have been actually made in illustration of this point. Two pieces of seasoned box-wood had corresponding cavities hollowed in them, so that, when one was placed upon the other, a lenticular space was enclosed. A and B, Fig. 30, represent the pieces of box-wood with the cavities in plan: C represents their section when they are placed upon each other.

[Sidenote: ACTUAL MOULDING OF ICE.]

A _sphere_ of ice rather more than sufficient to fill the lenticular space was placed between the pieces of wood and subjected to the action of a small hydraulic press. The ice was crushed, but the crushed fragments soon reattached themselves, and, in a few seconds, a lens of compact ice was taken from the mould.

This lens was placed in a cylindrical cavity hollowed out in another piece of box-wood, and represented at C, Fig. 31; and a flat piece of the wood was placed over the lens as a cover, as at D. On subjecting the whole to pressure, the lens broke, as the sphere had done, but the crushed mass soon re-established its continuity, and in less than half a minute a compact cake of ice was taken from the mould.

In the following experiment the ice was subjected to a still severer test:--A hemispherical cavity was formed in one block of box-wood, and upon a second block a hemispherical protuberance was turned, smaller than the cavity, so that, when the latter was placed in the former, a space of a quarter of an inch existed between the two. Fig. 32 represents a section of the two pieces of box-wood; the brass pins _a_, _b_, fixed in the slab G H, and entering suitable apertures in the mould I K, being intended to keep the two surfaces concentric. A lump of ice being placed in the cavity, the protuberance was brought down upon it, and the mould subjected to hydraulic pressure: after a short interval the ice was taken from the mould as a smooth compact _cup_, its crushed particles having reunited, and established their continuity.

[Sidenote: ICE MOULDED TO CUPS AND RINGS.]

To make these results more applicable to the bending of glacier-ice, the following experiments were made:--A block of box-wood, M, Fig. 33, 4 inches long, 3 wide, and 3 deep, had its upper surface slightly curved, and a groove an inch wide, and about an inch deep, worked into it. A corresponding plate was prepared, having its under surface part of a convex cylinder, of the same curvature as the concave surface of the former piece. When the one slab was placed upon the other, they presented the appearance represented in section at N. A straight prism of ice 4 inches long, an inch wide, and a little more than an inch in depth, was placed in the groove; the upper slab was placed upon it, and the whole was subjected to the hydraulic press. The prism broke, but, the quantity of ice being rather more than sufficient to fill the groove, the pressure soon brought the fragments together and re-established the continuity of the ice. After a few seconds it was taken from the mould a bent bar of ice. This bar was afterwards passed through three other moulds of gradually augmenting curvature, and was taken from the last of them a _semi-ring_ of compact ice.

The ice, in changing its form from that of one mould to that of another, was in every instance broken and crushed by the pressure; but suppose that instead of three moulds three thousand had been used; or, better still, suppose the curvature of a single mould to change by extremely slow degrees; the ice would then so gradually change its form that no rude rupture would be apparent. Practically the ice would behave as a _plastic_ substance; and indeed this plasticity has been contended for by M. Agassiz, in opposition to the idea of viscosity. As already stated, the ice, bruised, and flattened, and bent in the above experiments, was incapable of being sensibly stretched; it was plastic to pressure but not to tension.

A quantity of water was always squeezed out of the crushed ice in the above experiments, and the bruised fragments were intermixed with this and with air. Minute quantities of both remained in the moulded ice, and thus rendered it in some degree turbid. Its character, however, as to continuity may be inferred from the fact that the ice-cup, moulded as described, held water without the slightest visible leakage.

[Sidenote: SOFTNESS OF ICE DEFINED.]

[Sidenote: PRESSURE AND TENSION.]

Ice at 32 deg. may, as already stated, be crushed with extreme facility, and glacier-ice with still more readiness than lake-ice: it may also be scraped with a knife with even greater facility than some kinds of chalk. In comparison with ice at 100 deg. below the freezing point, it might be popularly called _soft_. But its softness is not that of paste, or wax, or treacle, or lava, or honey, or tar. It is the softness of calcareous spar in comparison with that of rock-crystal; and although the latter is incomparably harder than the former, I think it will be conceded that the term viscous would be equally inapplicable to both. My object here is clearly to define terms, and not permit physical error to lurk beneath them. How far this ice, with a softness thus defined, when subjected to the gradual pressures exerted in a glacier, is bruised and broken, and how far the motion of its parts may approach to that of a truly viscous body under pressure, I do not know. The critical point here is that the ice changes its form, and preserves its continuity, during its motion, in virtue of _external_ force. It remains continuous whilst it moves, because its particles are kept in juxtaposition by pressure, and when this external prop is removed, and the ice, subjected to tension, has to depend solely upon the mobility of its own particles to preserve its continuity, the analogy with a viscous body instantly breaks down.[A]

FOOTNOTES:

[A] "Imagine," writes Professor Forbes, "a long narrow trough or canal, stopped at both ends and filled to a considerable depth with treacle, honey, tar, or any such viscid fluid. Imagine one end of the trough to give way, the bottom still remaining horizontal: if the friction of the fluid against the bottom be greater than the friction against its own particles, the upper strata will roll over the lower ones, and protrude in a convex slope, which will be propagated backwards towards the other or closed end of the trough. Had the matter been quite fluid the whole would have run out, and spread itself on a level: as it is, it assumes precisely the conditions which we suppose to exist in a glacier." This is perfectly definite, and my equally definite opinion is that no glacier ever exhibited the mechanical effects implied by this experiment.

REGELATION.

(23.)

[Sidenote: FARADAY'S FIRST EXPERIMENT.]

I was led to the foregoing results by reflecting on an experiment performed by Mr. Faraday, at a Friday evening meeting of the Royal Institution, on the 7th of June, 1850, and described in the 'Athenaeum' and 'Literary Gazette' for the same month. Mr. Faraday then showed that when two pieces of ice, with moistened surfaces, were placed in contact, they became cemented together by the freezing of the film of water between them, while, when the ice was below 32 deg. Fahr., and therefore _dry_, no effect of the kind could be produced. The freezing was also found to take place under water; and indeed it occurs even when the water in which the ice is plunged is as hot as the hand can bear.

A generalisation from this interesting fact led me to conclude that a bruised mass of ice, if closely confined, must re-cement itself when its particles are brought into contact by pressure; in fact, the whole of the experiments above recorded immediately suggested themselves to my mind as natural deductions from the principle established by Faraday. A rough preliminary experiment assured me that the deductions would stand testing; and the construction of the box-wood moulds was the consequence. We could doubtless mould many solid substances to any extent by suitable pressure, breaking the attachment of their particles, and re-establishing a certain continuity by the mere force of cohesion. With such substances, to which we should never think of applying the term viscous, we might also imitate the changes of form to which glaciers are subject: but, superadded to the mere cohesion which here comes into play, we have, in the case of ice, the actual regelation of the severed surfaces, and consequently a more perfect solid. In the Introduction to this book I have referred to the production of slaty cleavage by pressure; and at a future page I hope to show that the lamination of the ice of glaciers is due to the same cause; but, as justly observed by Mr. John Ball, there is no tendency to cleave in the _sound_ ice of glaciers; in fact, this tendency is obliterated by the perfect regelation of the severed surfaces.

[Sidenote: RECENT EXPERIMENTS OF FARADAY.]

Mr. Faraday has recently placed pieces of ice, in water, under the strain of forces tending to pull them apart. When two such pieces touch at a single point they adhere and move together as a rigid piece; but a little lateral force carefully applied breaks up this union with a crackling noise, and a new adhesion occurs which holds the pieces together in opposition to the force which tends to divide them. Mr. James Thomson had referred regelation to the cold produced by the liquefaction of the pressed ice; but in the above experiment all pressure is not only taken away, but is replaced by tension. Mr. Thomson also conceives that, when pieces of ice are simply placed together without intentional pressure, the capillary attraction brings the pressure of the atmosphere into play; but Mr. Faraday finds that regelation takes place _in vacuo_. A true viscidity on the part of ice Mr. Faraday never has observed, and he considers that his recent experiments support the view originally propounded by himself, namely, that a particle of water on a surface of ice becomes solid when placed between two surfaces, because of the increased influence due to their joint action.

CRYSTALLIZATION AND INTERNAL LIQUEFACTION.

(24.)

[Sidenote: HOW CRYSTALS ARE "NURSED."]

In the Introduction to this book I have briefly referred to the force of crystallization. To permit this force to exercise its full influence, it must have free and unimpeded action; a crystal, for instance, to be properly built, ought to be suspended in the middle of the crystallizing solution, so that the little architects can work all round it; or if placed upon the bottom of a vessel, it ought to be frequently turned, so that all its facets may be successively subjected to the building process. In this way crystals can be _nursed_ to an enormous size. But where other forces mingle with that of crystallization, this harmony of action is destroyed; the figures, for example, that we see upon a glass window, on a frosty morning, are due to an action compounded of the pure crystalline force and the cohesion of the liquid to the window-pane. A more regular effect is obtained when the freezing particles are suspended in still air, and here they build themselves into those wonderful figures which Dr. Scoresby has observed in the Polar Regions, Mr. Glaisher at Greenwich, and I myself on the summit of Monte Rosa and elsewhere.

Not only however in air, but in water also, figures of great beauty are sometimes formed. Harrison's excellent machine for the production of artificial ice is, I suppose, now well known; the freezing being effected by carrying brine, which had been cooled by the evaporation of ether, round a series of flat tin vessels containing water. The latter gradually freezes, and, on watching those vessels while the action was proceeding very slowly, I have seen little six-rayed stars of thin ice forming, and rising to the surface of the liquid. I believe the fact was never before observed, but it would be interesting to follow it up, and to develop experimentally this most interesting case of crystallization.

[Sidenote: DISSECTION OF ICE BY SUNBEAM.]

The surface of a freezing lake presents to the eye of the observer nothing which could lead him to suppose that a similar molecular architecture is going on there. Still the particles are undoubtedly related to each other in this way; they are arranged together on this starry type. And not only is this the case at the surface, but the largest blocks of ice which reach us from Norway and the Wenham Lake are wholly built up in this way. We can reveal the internal constitution of these masses by a reverse process to that which formed them; we can send an agent into the interior of a mass of ice which shall take down the atoms which the crystallizing forces had set up. This agent is a solar beam; with which it first occurred to me to make this simple experiment in the autumn of 1857. I placed a large converging lens in the sunbeams passing through a room, and observed the place where the rays were brought to a focus behind the lens; then shading the lens, I placed a clear cube of ice so that the point of convergence of the rays might fall within it. On removing the screen from the lens, a cone of sunlight went through the cube, and along the course of the cone the ice became studded with lustrous spots, evidently formed by the beam, as if minute reflectors had been suddenly established within the mass, from which the light flashed when it met them. On examining the cube afterwards I found that each of these spots was surrounded by a liquid flower of six petals; such flowers were distributed in hundreds through the ice, being usually clear and detached from each other, but sometimes crowded together into liquid bouquets, through which, however, the six-starred element could be plainly traced. At first the edges of the leaves were unbroken curves, but when the flowers expanded under a long-continued action, the edges became serrated. When the ice was held at a suitable angle to the solar beams, these liquid blossoms, with their central spots shining more intensely than burnished silver, presented an exhibition of beauty not easily described. I have given a sketch of their appearance in Fig. 34.

[Sidenote: LIQUID FLOWERS IN ICE.]

I have here to direct attention to an extremely curious fact. On sending the sunbeam through the transparent ice, I often noticed that the appearance of the lustrous spots was accompanied by an audible clink, as if the ice were ruptured inwardly. But there is no ground for assuming such rupture, and on the closest examination no flaw is exhibited by the ice. What then can be the cause of the noise? I believe the following considerations will answer the question:--

Water always holds a quantity of air in solution, the diffusion of which through the liquid, as proved by M. Donny, has an immense effect in weakening the cohesion of its particles; recent experiments of my own show that this is also the case in an eminent degree with many volatile liquids. M. Donny has proved that, if water be thoroughly purged of its air, a long glass tube filled with this liquid may be inverted, while the tenacity with which the water clings to the tube, and with which its particles cling to each other, is so great that it will remain securely suspended, though no external hindrance be offered to its descent. Owing to the same cause, water deprived of its air will not boil at 212 deg. Fahr., and may be raised to a temperature of nearly 300 deg. without boiling; but when this occurs the particles break their cohesion suddenly, and ebullition is converted into explosion.

Now, when ice is formed, every trace of the air which the water contained is squeezed out of it; the particles in crystallizing reject all extraneous matter, so that in ice we have a substance quite free from the air, which is never absent in the case of water; it therefore follows that if we could preserve the water derived from the melting of ice from contact with the atmosphere, we should have a liquid eminently calculated to show the effects described by M. Donny. Mr. Faraday has proved by actual experiment that this is the case.

[Sidenote: WATER DEPRIVED OF AIR SNAPS ASUNDER.]

Let us apply these facts to the explanation of the clink heard in my experiments. On sending a sunbeam through ice, liquid cavities are suddenly formed at various points within the mass, and these cavities are completely cut off from atmospheric contact. But the water formed by the melting ice is less in volume than the ice which produces it; the water of a cavity is not able to fill it, hence a vacuous space must be formed in the cell. I have no doubt that, for a time, the strong cohesion between the walls of the cell and the drop within it augments the volume of the latter a little, so as to compel it to fill the cell; but as the quantity of liquid becomes greater the shrinking force augments, until finally the particles snap asunder like a broken spring. At the same moment a lustrous spot appears, which is a vacuum, and simultaneously with the appearance of this vacuum the clink was always heard. Multitudes of such little explosions must be heard upon a glacier when the strong summer sun shines upon it, the aggregate of which must, I think, contribute to produce the "crepitation" noticed by M. Agassiz, and to which I have already referred.

[Sidenote: FIGURES IN ICE; VACUOUS SPOTS.]

In Plate VI. of the Atlas which accompanies the 'Systeme Glaciaire' of M. Agassiz, I notice drawings of figures like those I have described, which he has observed in glacier-ice, and which were doubtless produced by direct solar radiation. I have often myself observed figures of exquisite beauty formed in the ice on the surface of glacier-pools by the morning sun. In some cases the spaces between the leaves of the liquid flowers melt partially away, and leave the central spot surrounded by a crimped border; sometimes these spaces wholly disappear, and the entire space bounded by the lines drawn from point to point of the leaves becomes liquid, thus forming perfect hexagons. The crimped borders exhibit different degrees of serration, from the full leaves themselves to a gentle undulating line, which latter sometimes merges into a perfect circle. In the ice of glaciers, I have seen the internal liquefaction ramify itself like sprigs of myrtle; in the same ice, and particularly towards the extremities of the glacier, disks innumerable are also formed, consisting of flat round liquid spaces, a bright spot being usually associated with each. These spots have been hitherto mistaken for air-bubbles; but both they and the lustrous disks at the centres of the flowers are vacuous. I proved them to be so by plunging the ice containing them into hot water, and watching what occurred when the walls of the cells were dissolved, and a liquid connexion established between them and the atmosphere. In all cases they totally collapsed, and no trace of air rose to the surface of the warm water.

No matter in what direction a solar beam is sent through lake-ice, the liquid flowers are all formed parallel to the surface of freezing. The beam may be sent parallel, perpendicular, or oblique to this surface; the flowers are always formed in the same planes. Every line perpendicular to the surface of a frozen lake is in fact an axis of symmetry, round which the molecules so arrange themselves, that, when taken down by the delicate fingers of the sunbeam, the six-leaved liquid flowers are the result.

In the ice of glaciers we have no definite planes of freezing. It is first snow, which has been disturbed by winds while falling, and whirled and tossed about by the same agency after it has fallen, being often melted, saturated with its own water, and refrozen: it is cast in shattered fragments down cascades, and reconsolidated by pressure at the bottom. In ice so formed and subjected to such mutations, definite planes of freezing are, of course, out of the question.

[Sidenote: CONSTITUTION OF GLACIER-ICE.]

The flat round disks and vacuous spots to which I have referred come here to our aid, and furnish us with an entirely new means of analysing the internal constitution of a glacier. When we examine a mass of glacier-ice which contains these disks, we find them lying in all imaginable planes; not confusedly, however--closer examination shows us that the disks are arranged in groups, the members of each group being parallel to a common plane, but the parallelism ceases when different groups are compared. The effect is exactly what would be observed, supposing ordinary lake-ice to be broken up, shaken together, and the confused fragments regelated to a compact continuous mass. In such a jumble the original planes of freezing would lie in various directions; but no matter how compact or how transparent ice thus constituted might appear, a solar beam would at once reveal its internal constitution by developing the flowers parallel to the planes of freezing of the respective fragments. A sunbeam sent through glacier-ice always reveals the flowers in the planes of the disks, so that the latter alone at once informs us of its crystalline constitution.

[Sidenote: VACUOUS CELLS MISTAKEN FOR AIR-CELLS.]

Hitherto, as I have said, these disks have been mistaken for bubbles containing air, and their flattening has been ascribed to the pressure to which they have been subjected. M. Agassiz thus refers to them:--"The air-bubbles undergo no less curious modifications. In the neighbourhood of the _neve_, where they are most numerous, those which one sees on the surface are all spherical or ovoid, but by degrees they begin to be flattened, and near the end of the glacier there are some that are so flat _that they might be taken for fissures when seen in profile_. The drawing represents a piece of ice detached from the gallery of infiltration. All the bubbles are greatly flattened. But what is most extraordinary is, that, far from being uniform, _the flattening is different in each fragment_; so that the bubbles, according to the face which they offer, appear either very broad or very thin." This description of glacier-ice is correct: it agrees with the statements of all other observers. But there are two assumptions in the description which must henceforth be given up; first, the bubbles seen like fissures in profile are not air-bubbles at all, but vacuous spots, which the very constitution of ice renders a necessary concomitant of its inward melting; secondly, the assumption that the bubbles have been _flattened_ by pressure must be abandoned; for they are found, and may be developed at will, in lake-ice on which no pressure has been exerted.

[Sidenote: CELLS OF AIR AND WATER.]

But these remarks dispose only of a certain class of cells contained in glacier-ice. Besides the liquid disks and vacuous spots, there are innumerable true bubbles entangled in the mass. These have also been observed and described by M. Agassiz; and Mr. Huxley has also given us an accurate account of them. M. Agassiz frequently found air and water associated in the same cell. Mr. Huxley found no exception to the rule: in each case the bubble of air was enclosed in a cell which was also partially filled with water. He supposes that the water may be that of the originally-melted snow which has been carried down from the _neve_ unfrozen. This hypothesis is worthy of a great deal more consideration than I have had time to give to it, and I state it here in the hope that it will be duly examined.

My own experience of these associated air and water cells is derived almost exclusively from lake-ice, in which I have often observed them in considerable numbers. In examining whether the liquid contents had ever been frozen or not, I was guided by the following considerations. If the air be that originally entangled in the solid, it will have the ordinary atmospheric density at least; but if it be due to the melting of the walls of the cell, then the water so formed being only eight-ninths of that of the ice which produced it, _the air of the bubble must be rarefied_. I suppose I have made a hundred different experiments upon these bubbles to determine whether the air was rarefied or not, and in every case found it so. Ice containing the bubbles was immersed in warm water, and always, when the rigid envelope surrounding a bubble was melted away, the air suddenly collapsed to a fraction of its original dimensions. I think I may safely affirm that, in some cases, the collapse reduced the bubbles to the thousandth part of their original volume. From these experiments I should undoubtedly infer, that in lake-ice at least, the liquid of the cells is produced by the melting of the ice surrounding the bubbles of air.

But I have not subjected the bubbles of glacier-ice to the same searching examination. I have tried whether the insertion of a pin would produce the collapse of the bubbles, but it did not appear to do so. I also made a few experiments at Rosenlaui, with warm water, but the result was not satisfactory. That ice melts internally at the surfaces of the bubbles is, I think, rendered certain by my experiments, but whether the water-cells of glacier-ice are entirely due to such melting, subsequent observers will no doubt determine.

[Sidenote: "LIQUID LIBERTY."]

I have found these composite bubbles at all parts of glaciers; in the ice of the moraines, over which a protective covering had been thrown; in the ice of sand-cones, after the removal of the superincumbent debris; also in ice taken from the roofs of caverns formed in the glacier, and which the direct sunlight could hardly by any possibility attain. That ice should liquefy at the surface of a cavity is, I think, in conformity with all we know concerning the physical nature of heat. Regarding it as a motion of the particles, it is easy to see that this motion is less restrained at the surface of a cavity than in the solid itself, where the oscillation of each atom is controlled by the particles which surround it; hence _liquid liberty_, if I may use the term, is first attained at the surface. Indeed I have proved by experiment that ice may be melted internally by heat which has been conducted through its external portions without melting them. These facts are the exact complements of those of "regelation;" for here, two moist surfaces of ice being brought into close contact, their liquid liberty is destroyed and the surfaces freeze together.

THE MOULINS.

(25.)

[Sidenote: MOULIN OF GRINDELWALD GLACIER.]

[Sidenote: DEPTH OF THE SHAFT.]

The first time I had an opportunity of seeing these remarkable glacier-chimneys was in the summer of 1856, upon the lower glacier of Grindelwald. Mr. Huxley was my companion at the time, and on crossing the so-called Eismeer we heard a sound resembling the rumble of distant thunder, which proceeded from a perpendicular shaft formed in the ice, and into which a resounding cataract discharged itself. The tube in fact resembled a vast organ-pipe, whose thunder-notes were awakened by the concussion of the falling water, instead of by the gentle flow of a current of air. Beside the shaft our guide hewed steps, on which we stood in succession, and looked into the tremendous hole. Near the first shaft was a second and smaller one, the significance of which I did not then understand; it was not more than 20 feet deep, but seemed filled with a liquid of exquisite blue, the colour being really due to the magical shimmer from the walls of the moulin, which was quite empty. As far as we could see, the large shaft was vertical, but on dropping a stone into it a shock was soon heard, and after a succession of bumps, which occupied in all seven seconds, we heard the stone no more. The depth of the moulin could not be thus ascertained, but we soon found a second and still larger one which gave us better data. A stone dropped into this descended without interruption for four seconds, when a concussion was heard; and three seconds afterwards the final shock was audible: there was thus but a single interruption in the descent. Supposing all the acquired velocity to have been destroyed by the shock, by adding the space passed over by the stone in four and in three seconds respectively, and making allowance for the time required by the sound to ascend from the bottom, we find the depth of the shaft to be about 345 feet. There is, however, no reason to suppose that this measures the depth of the glacier at the place referred to. These shafts are to be found in almost all great glaciers; they are very numerous in the Unteraar Glacier, numbers of them however being empty. On the Mer de Glace they are always to be found in the region of Trelaporte, one of the shafts there being, _par excellence_, called the Grand Moulin. Many of them also occur on the Glacier de Lechaud.

As truly observed by M. Agassiz, these moulins occur only at those parts of the glacier which are not much rent by fissures, for only at such portions can the little rills produced by superficial melting collect to form streams of any magnitude. The valley of unbroken ice formed in the Mer de Glace near Trelaporte is peculiarly favourable for the collection of such streams; we see the little rills commencing, and enlarging by the contributions of others, the trunk-rill pouring its contents into a little stream which stretches out a hundred similar arms over the surface of the glacier. Several such streams join, and finally a considerable brook, which receives the superficial drainage of a large area, cuts its way through the ice.

[Sidenote: MOULINS EXPLAINED.]

But although this portion of the glacier is free from those long-continued and permanent strains which, having once rent the ice, tend subsequently to widen the rent and produce yawning crevasses, it is not free from local strains sufficient to produce _cracks_ which penetrate the glacier to a great depth. Imagine such a crack intersecting such a glacier-rivulet as we have described. The water rushes down it, and soon scoops a funnel large enough to engulf the entire stream. The moulin is thus formed, and, as the ice moves downward, the sides of the crack are squeezed together and regelated, the seam which marks the line of junction being in most cases distinctly visible. But as the motion continues, other portions of the glacier come into the same state of strain as that which produced the first crack; a second one is formed across the stream, the old shaft is forsaken, and a new one is hollowed out, in which for a season the cataract plays the thunderer. I have in some cases counted the forsaken shafts of six old moulins in advance of an active one. Not far from the Grand Moulin of the Mer de Glace in 1857 there was a second empty shaft, which evidently communicated by a subglacial duct with that into which the torrent was precipitated. Out of the old orifice issued a strong cold blast, the air being manifestly impelled through the duct by the falling water of the adjacent moulin.

These shafts are always found in the same locality; the portion of the Mer de Glace to which I have referred is never without them. Some of the guides affirm that they are motionless; and a statement of Prof. Forbes has led to the belief that this was also his opinion.[A] M. Agassiz, however, observed the motion of some of these shafts upon the glacier of the Aar; and when on the spot in 1857, I was anxious to decide the point by accurate measurements with the theodolite.

My friend Mr. Hirst took charge of the instrument, and on the 28th of July I fixed a single stake beside the Grand Moulin, in a straight line between a station at Trelaporte and a well-defined mark on the rock at the opposite side of the valley. On the 31st, the displacement of the stake amounted to 50 inches, and on the 1st of August it had moved 74-1/2 inches--the moulin, to all appearance, occupying throughout the same position with regard to the stake. To render this certain, moreover we subsequently drove two additional stakes into the ice, thus enclosing the mouth of the shaft in a triangle. On the 8th of August the displacements were measured and gave the following results:--

Total Motion.
First (old) stake 198 inches.
Second (new) do. 123 "
Third 124 "

[Sidenote: MOTION OF THE MOULINS.]

The old stake had been fixed for 11 days, and its daily motion--_which was also that of the moulin_--averaged 18 inches a day. Hence the moulins share the general motion of the glacier, and their apparent permanence is not, as has been alleged, a proof of the semi-fluidity of the glacier, but is due to the breaking of the ice as it passes the place of local strain.

[Sidenote: DEPTH OF "GRAND MOULIN" SOUGHT.]

Wishing to obtain some estimate as to the depth of the ice, Mr. Hirst undertook the sounding of some of the moulins upon the Glacier de Lechaud, making use of a tin vessel filled with lumps of lead and iron as a weight. The cord gave way and he lost his plummet. To measure the depth of the Grand Moulin, we obtained fresh cord from Chamouni, to which we attached a four-pound weight. Into a cavity at the bottom of the weight we stuffed a quantity of butter, to indicate the nature of the bottom against which the weight might strike. The weight was dropped into the shaft, and the cord paid out until its slackening informed us that the weight had come to rest; by shaking the string, however, and walking round the edge of the shaft, the weight was liberated, and sank some distance further. The cord partially slackened a second time, but the strain still remaining was sufficient to render it doubtful whether it was the weight or the action of the falling water which produced it. We accordingly paid out the cord to the end, but, on withdrawing it, found that the greater part of it had been coiled and knotted up by the falling water. We uncoiled, and sounded again. At a depth of 132 feet the weight reached a ledge or protuberance of ice, and by shaking and lifting it, it was caused to descend 31 feet more. A depth of 163 feet was the utmost we could attain to. We sounded the old moulin to a depth of 90 feet; while a third little shaft, beside the large one, measured only 18 feet in depth. We could see the water escape from it through a lateral canal at its bottom, and doubtless the water of the Grand Moulin found a similar exit. There was no trace of dirt upon the butter, which might have indicated that we had reached the bed of the glacier.

FOOTNOTES:

[A] "Every year, and year after year, the watercourses follow the same lines of direction--their streams are precipitated into the heart of the glacier by vertical funnels, called 'moulins,' at the very same points."--Forbes's Fourth Letter upon Glaciers: 'Occ. Pap.,' p. 29.

DIRT-BANDS OF THE MER DE GLACE.

(26.)

[Sidenote: DIRT-BANDS FROM THE FLEGERE.]

These bands were first noticed by Prof. Forbes on the 24th of July, 1842, and were described by him in the following words:--"My eye was caught by a very peculiar appearance of the surface of the ice, which I was certain that I now saw for the first time. It consisted of nearly hyperbolic brownish bands on the glacier, the curves pointing downwards, and the two branches mingling indiscriminately with the moraines, presenting an appearance of a succession of waves some hundred feet apart."[A] From no single point of view hitherto attained can all the Dirt-Bands of the Mer de Glace be seen at once. To see those on the terminal portion of the glacier, a station ought to be chosen on the opposite range of the Brevent, a few hundred yards beyond the Croix de la Flegere, where we stand exactly in front of the glacier as it issues into the valley of Chamouni. The appearance of the bands upon the portion here seen is represented in Fig. 35.

It will be seen that the bands are confined to one side of the glacier, and either do not exist, or are obliterated by the debris, upon the other side. The cause of the accumulation of dirt on the right side of the glacier is, that no less than five moraines are crowded together at this side. In the upper portions of the Mer de Glace these moraines are distinct from each other; but in descending, the successive engulfments and disgorgings of the blocks and dirt have broken up the moraines; and at the place now before us the materials which composed them are strewn confusedly on the right side of the glacier. The portion of the ice on which the dirt-bands appear is derived from the Col du Geant. They do not quite extend to the end of the glacier, being obliterated by the dislocation of the ice upon the frozen cascade of Des Bois.

[Sidenote: DIRT-BANDS FROM LES CHARMOZ.]

Let us now proceed across the valley of Chamouni to the Montanvert; where, climbing the adjacent heights to an elevation of six or eight hundred feet above the hotel, we command a view of the Mer de Glace, from Trelaporte almost to the commencement of the Glacier des Bois. It was from this position that Professor Forbes first observed the bands. Fifteen, sixteen, and seventeen years later I observed them from the same position. The number of bands which Professor Forbes counted from this position was eighteen, with which my observations agree. The entire series of bands which I observed, with the exception of one or two, must have been the _successors_ of those observed by Professor Forbes; and my finding the same number after an interval of so many years proves that the bands must be due to some regularly recurrent cause. Fig. 36 represents the bands as seen from the heights adjacent to the Montanvert.