Part 34 (1/2)

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.

(32.)

[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 s.h.i.+fted 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.]