The Glaciers of the Alps Part 34

Let us now consider the case of the metal bis.m.u.th. If the molten metal be poured into a bullet-mould it will _expand_ on solidifying. I have myself filled a strong cast-iron bottle with the metal, and found its expansion on cooling sufficiently great to split the bottle from neck to bottom. Hence, in order to fuse the bis.m.u.th the substance must _contract_; and it is manifest that an external pressure which tends to squeeze the molecules more closely together here _a.s.sists_ the heat instead of opposing it. Hence, to fuse bis.m.u.th under great pressure, a less amount of heat will be required than when the pressure is removed; or, in other words, the fusing point of bis.m.u.th is _lowered_ by the pressure. Now, in pa.s.sing from the solid to the liquid state, _ice_, like bis.m.u.th, contracts, and if the contraction be promoted by external pressure, as shown by the Messrs. Thomson, a less amount of heat suffices to liquefy it.

[Sidenote: EXPERIMENTS.]

These remarks will enable us to understand a singular effect first obtained by myself at the close of 1856 or in January 1857, noticed at the time in the 'Proceedings of the Royal Society,' and afterwards fully described in a paper presented to the Society in December of that year.

A cylinder of clear ice two inches high and an inch in diameter was placed between two slabs of box-wood, and subjected to a gradual pressure. I watched the ice in a direction perpendicular to its length, and saw cloudy lines drawing themselves across it. As the pressure continued, these lines augmented in numbers, until finally the prism presented the appearance of a crystal of gypsum whose planes of cleavage had been forced out of optical contact. When looked at obliquely it was found that the lines were merely the sections of flat dim surfaces, which lay like laminae one over the other throughout the length of the prism. Fig. 50 represents the prism as it appeared when looked at in a direction perpendicular to its axis; Fig. 51 shows the appearance when viewed obliquely.[A]

[Ill.u.s.tration: Fig. 50, 51. Appearance of a prism of ice partially liquefied by Pressure.]

At first sight it might appear as if air had intruded itself between the separated surfaces of the ice, and to test this point I placed a cylinder two inches long and an inch wide upright in a copper vessel which was filled with ice-cold water. The ice cylinder rose about half an inch above the surface of the water. Placing the copper vessel on a slab of wood, and a second slab on the top of the cylinder of ice, the latter was subjected to the gradual action of a small hydraulic press.

When the hazy surfaces were well developed in the portion of the ice above the water, the cylinder was removed and examined: the planes of rupture extended throughout the entire length of the cylinder, just as if it had been squeezed in air. I subsequently placed the ice in a stout vessel of gla.s.s, and squeezed it, as in the last experiment: the surfaces of discontinuity were seen forming _under the liquid_ quite as distinctly as in air.

To prove that the surfaces were due to compression and not to any tearing asunder of the ma.s.s by tension, the following experiment was made:--A cylindrical piece of ice, one of whose ends, however, was not parallel to the other, was placed between the slabs of wood, and subjected to pressure. Fig. 52 shows the disposition of the experiment.

The effect upon the ice cylinder was that shown in Fig. 53, the surfaces being developed along that side which had suffered the pressure. On examining the surfaces by a pocket lens they resembled the effect produced upon a smooth cold surface by breathing on it.

[Ill.u.s.tration: Fig. 52, 53. Figures ill.u.s.trative of compression and liquefaction of ice.]

[Sidenote: LIQUID LAYERS PRODUCED BY PRESSURE.]

The surfaces were always dim; and had the s.p.a.ces been filled with air, or were they simply vacuous, the reflection of light from them would have been so copious as to render them much more brilliant than they were observed to be. To examine them more particularly I placed a concave mirror so as to throw the diffused daylight from a window full upon the cylinder. On applying the pressure dim spots were sometimes seen forming in the very middle of the ice, and these as they expanded laterally appeared to be in a state of intense motion, which followed closely the edge of each surface as it advanced through the solid ice.

Once or twice I observed the hazy surfaces pioneered through the ma.s.s by dim offshoots, apparently liquid, and const.i.tuting a kind of decrystallisation. From the closest examination to which I was able to subject them, the surfaces appeared to me to be due to internal liquefaction; indeed, when the melting point of ice, having already a temperature of 32, is lowered by pressure, its excess of heat must instantly be applied to produce this effect.

[Sidenote: APPLICATION TO THE VEINED STRUCTURE.]

I have already given a drawing (p. 386) showing the development of the veined structure at the base of the ice-cascade of the Rhone; and if we compare that diagram with Fig. 53 a striking similarity at once reveals itself. The ice of the glacier must undoubtedly be liquefied to some extent by the tremendous pressure to which it is here subjected.

Surfaces of discontinuity will in all probability be formed, which facilitate the escape of the imprisoned air. The small quant.i.ty of water produced will be partly imbibed by the adjacent porous ice, and will be refrozen when relieved from the pressure. This action, a.s.sociated with that ascribed to pressure in the last section, appears to me to furnish a complete physical explanation of the laminated structure of glacier-ice.

FOOTNOTES:

[A] This effect projected upon a screen is a most striking and instructive cla.s.s experiment.

WHITE ICE-SEAMS IN THE GLACIER DU GeANT.

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[Sidenote: GENERAL APPEARANCE OF WHITE ICE-SEAMS.]

On the 28th of July, 1857, while engaged upon the Glacier du Geant, my attention was often attracted by protuberant ridges of what at first appeared to be pure white snow, but which on examination I found to be compact ice filled with innumerable round air-cells; and which, in virtue of its greater power of resistance to wasting, often rose to a height of three or four feet above the general level of the ice. As I stood amongst these ridges, they appeared detached and without order of arrangement, but looked at from a distance they were seen to sweep across the proper Glacier du Geant in a direction concentric with its dirt-bands and its veined structure. In some cases the seams were admirable indications of the relative displacement of two adjacent portions of the glacier, which were divided from each other by a creva.s.se. Usually the sections of a seam exposed on the opposite sides of a fissure accurately faced each other, and the direction of the seam on both sides was continuous; but at other places they demonstrated the existence of lateral faults, being shifted asunder laterally through s.p.a.ces varying from a few inches to six or seven feet.

On the following day I was again upon the same glacier, and noticed in many cases the white ice-seams exquisitely honeycombed. The case was ill.u.s.trative of the great difference between the absorptive power of the ice itself and of the objects which lie upon its surface. Deep cylindrical cells were produced by spots of black dirt which had been scattered upon the surface of the white ice, and which sank to a depth of several inches into the ma.s.s. I examined several sections of the veins, and in general I found that their deeper portions blended gradually with the ice on either side of them. But higher up the glacier I found that the veins penetrated only to a limited depth, and did not therefore form an integrant portion of the glacier. Figs. 54 and 55 show the sections of two of the seams which were exposed on the wall of a creva.s.se at some distance below the great ice-fall of the Glacier du Geant.

[Sidenote: SECTIONS OF SEAMS.]

[Ill.u.s.tration: Fig. 54, 55. Sections of White Ice-seams.]

[Ill.u.s.tration: Fig. 56. Variations in the Dip of the Veined Structure.]

It was at the base of the Talefre cascade that the explanation of these curious seams presented itself to me. In one of my earliest visits to this portion of the glacier I was struck by a singular disposition of the blue veins on the vertical wall of a creva.s.se. Fig. 56 will ill.u.s.trate what I saw. The veins, within a short distance, dipped _backward_ and _forward_, like the junctions of stones used to turn an arch. In some cases I found this variation of the structure so great as to pa.s.s in a short distance from the vertical to the horizontal, as shown in Fig. 57.

[Sidenote: VARIATIONS IN "DIP" OF STRUCTURE.]

[Ill.u.s.tration: Fig. 57. Variations in the Dip of the Veined Structure.]

Further examination taught me that the glacier here is crumpled in a most singular manner; doubtless by the great pressure to which it is exposed. The following ill.u.s.tration will convey a notion of its aspect: Let one hand be laid flat upon a table, palm downwards, and let the fingers be bent until the s.p.a.ce between the first joint and the ends of the fingers is vertical; one of the crumples to which I refer will then be represented. The ice seems bent like the fingers, and the crumples of the glacier are cut by creva.s.ses, which are accurately typified by the s.p.a.ces between the fingers. Let the second hand now be placed upon the first, as the latter is upon the table, so that the tops of the bent fingers of the second hand shall rest upon the roots of the first: two crumples would thus be formed; a series of such protuberances, with steep fronts, follow each other from the base of the Talefre cascade for some distance downwards.

On Sat.u.r.day the 1st of August I ascended these rounded terraces in succession, and observed among them an extremely remarkable disposition of the structure. Fig. 58 is a section of a series of three of the crumples, on which the shading lines represent the direction of the blue veins. At the base of each protuberance I found a seam of white ice wedged firmly into the glacier, and _each of the seams marked a place of dislocation of the veins_. The white seams thinned off gradually, and finally vanished where the violent crumpling of the ice disappeared. In Fig. 59 I have sketched the wall of a creva.s.se, which represents what may be regarded as the incipient crumpling. The undulating line shows the contour of the surface, and the shading lines the veins. It will be observed that the direction of the veins yields in conformity with the undulation of the surface; and an augmentation of the effect would evidently result in the crumples shown in Fig. 58. The appearance of the white seams at those places where a dislocation occurred was, as far as I could observe, invariable; but in a few instances the seams were observed upon the platforms of the terraces, and also upon their slopes.

The width of a seam was very irregular, varying from a few inches at some places to three or four feet at others.

[Sidenote: CRUMPLES OF THE TALeFRE.]

[Ill.u.s.tration: Fig. 58. Section of three glacier Crumples.]

[Ill.u.s.tration: Fig. 59. Wall of a creva.s.se, with incipient crumpling.]

[Sidenote: MOULDS OF WHITE ICE-SEAMS.]

On the 3rd of August I was again at the base of the Talefre cascade, and observed a fact the significance of which had previously escaped me. The rills which ran down the ice-slopes collected at the base of each protuberance into a stream, which, at the time of my visit, had hollowed out for itself a deep channel in the ice. At some places the stream widened, at others its banks of ice approached each other, and rapids were produced; in fact, _the channels of such streams appeared to be the exact moulds of the seams of white ice_.

Instructed thus far, I ascended the Glacier du Geant on the 5th of August, and then observed on the wrinkles of this glacier the same leaning backwards and forwards of the blue veins as I had previously observed upon the Talefre. I also noticed on this day that a seam of white ice would sometimes open out into two branches, which, after remaining for some distance separate, would reunite and thus enclose a little glacier-island. At other places lateral branches were thrown off from the princ.i.p.al seam, thus suggesting the form of a glacier-rivulet which had been fed by tributary branches. On the 7th of August I hunted the seams still farther up the glacier; and found them at one place descending a steep ice-hill, being crossed by other similar bands, which however were far less white and compact. I followed these new bands to their origin, and found it to be a system of creva.s.ses formed at the summit of the hill, some of which were filled with snow. Lower down the creva.s.ses closed, and the snow thus jammed between their walls was converted into white ice. These seams, however, never attained the compactness and prominence of the larger ones which had their origin far higher up. I singled out one of the best of the latter, and traced it through all the dislocation and confusion of the ice, until I found it to terminate in a cavity filled with snow.

This was near the base of the _seracs_, and the streams here were abundant. Comparing the shapes of some of them with that of the ice-bands lower down the glacier, a striking resemblance was observed.

Fig. 60 is the plan of a deep-cut channel through which a stream flowed on the day to which I now refer. Fig. 61 is the plan of a seam of white ice sketched on the same day, low down upon the glacier. Instances of this kind might be multiplied; and the result, I think, renders it certain that the white ice-seams referred to are due to the filling up of the channels of glacier-streams by snow during winter, and the subsequent compression of the ma.s.s to ice during the descent of the glacier. I have found such seams at the bases of all cascades that I have visited; and in all cases they appear to be due to the same cause.

The depth to which they penetrate the glacier must be profound, or the _ablation_ of the ice must be less than what is generally supposed; for the seams formed so high up on the Glacier du Geant may be traced low down upon the trunk-stream of the Mer de Glace.[A]

[Sidenote: STREAMS AND SEAMS.]

[Ill.u.s.tration: Fig. 60. Plan of a Stream on the Glacier du Geant.]

[Ill.u.s.tration: Fig. 61. Plan of a Seam of White Ice on the Glacier du Geant.]

[Sidenote: SCALING OFF BY PRESSURE.]

These observations on the white ice-seams enable us to add an important supplement to what has been stated regarding the origin of the dirt-bands of the Mer de Glace; The protuberances at the base of the cascade are due not only to the toning down of the ridges produced by the transverse fracture of the glacier at the summit of the fall, but they undergo modifications by the pressure locally exerted at its base.

The state of things represented in Fig. 57 is plainly due to the partial pushing of one crumple over that next in advance of it. There seems to be a differential motion of the parts of the glacier in the same longitudinal line; showing that upon the general motion of the glacier smaller local motions are superposed. The occurrence of the seams upon the faces of the slopes seems also to prove that the pressure is competent, in some cases, to cause the bases of the protuberances to swell, so that what was once the base of a crumple may subsequently form a portion of its slope. Another interesting fact is also observed where the pressure is violent: the crumples _scale off_, bows of ice being thus formed which usually span the crumples over their most violently compressed portions. I have found this scaling off at the bases of all the cascades which I have visited, and it is plainly due to the pressure exerted at such places upon the ice.

FOOTNOTES:

[A] The more permanent seams may possibly be due to the filling of the profound creva.s.ses of the cascade.

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