When the temperature of water rises, the surface molecules first become liquid, then gaseous; being placed beyond the coercitive action of the surrounding particles, they are easily set free; transported, on the contrary, into the centre of the mass, they are brought absolutely under the influence of this action, which induces a new solidification,—or, to use the scientific term, a regelation. In this way it becomes easy to understand how very various forms can be communicated by simple pressure to a fragment of ice. If the observer successively places a straight bar in moulds of increasing curvature, he may easily compel it to assume the shape of a ring or even of a knot. In each mould, it is true, the ice breaks; but if the pressure is kept up, the surfaces of the fragments are brought into contact, and adhere so as to re-establish a condition of continuity. A snowball may thus be converted into a sphere of ice, and the sphere, by constant pressure, into a cup or a statue.
Professor Tyndall refers to a remarkable instance of regelation which he observed one day in early spring. A layer of snow, not quite two inches thick, had fallen on the glass roof of a small conservatory, and the internal air, warming the panes, had melted the snow so far as it was in immediate contact with them. The entire layer had slipped down the pane, and projected beyond the edge of the roof, without falling, and had bent and curved as required, just like a flexible body.
MOULDING ICE.
The snow-fields which overspread the upper part of every glacier, whether in the Arctic Regions or elsewhere, are composed of crystallized snow, whose fragile, delicate, and fairy-like architecture endures so long as it remains dry, but undergoes a great transformation when the sun, melting the upper stratum, allows the water to interpenetrate its substance. The fluid, congealing anew during the night, transforms the snow into the condition technically known as névé; a term given by the Swiss physicists to a granular mass composed of small rounded icicles, disaggregated, but more adhesive than snow-flakes, and of a density intermediate between that of snow and that of ice. Under the pressure of new layers, and as a result of infiltrations of water, the névé unites, and solders into ice of constantly increasing compactness.
But glacier-ice presents some other curious peculiarities. Every abundant snow-fall on the summit of the mountains forms a layer easily distinguishable from preceding layers—which, in most cases, have already passed into the névé condition. This stratification becomes more apparent when the whiteness of the surface has been sullied by dirt or dust wafted on “the wings of the wind.” It is perceptible also in ice; but here we must not confound it with another phenomenon of which the cause is different, the veined structure.
In places where glaciers have been accidentally cut down in an almost vertical direction, the section is found to exhibit a series of parallel veins, formed by a beautiful and very transparent azure ice in the midst of the general mass, which is of a whitish colour, and slightly opaque.
In different glaciers, and in different parts of the same glacier, these blue veins will vary in number and intensity of colouring. They are specially beautiful in crevasses of recent formation, and on the sides of channels excavated in the ice by tiny rills resulting from superficial fusion. Not a few glaciers exhibit this remarkable veined structure throughout their entire extent. When a vertical cutting exposes the delicate azure network to atmospheric influences, the softer ice melts prior to the fusion of the blue ice which then remains in their detached leaflets. On examining these attentively, we cannot fail to remark the absence, or, at all events, the extreme rarity, of air-bubbles, though they are so plentiful in the coarser ice.
Professor Tyndall’s explanation of this phenomenon is as interesting as it is ingenious. While on a visit of inspection to the slate-quarries of Wales, he had occasion to study the cleavage of the rocks which compose them; in other words, their faculty of dividing naturally, a property inherent in all crystals. The schistous slate separates easily into sheets, and in traversing different quarries one sees that all the planes of cleavage are parallel in each. From this circumstance our men of science were at first induced to look upon slates as the products of the stratification of different deposits. Such an explanation, however, could not be accepted by Tyndall, when he observed that the minute fossils embedded in them were constantly misshapen and flattened in the direction of the plane of cleavage, because the great modification they had undergone could not have taken place in superimposed strata at the bottom of the primeval sea. He concluded that these schists, therefore, must have been subjected to a considerable pressure; and further, that this pressure must have been exercised at right angles with the plane of separation of the different layers.
A long series of experiments proved that many bodies, when forcibly compressed, exhibit in their structure a very distinctly marked lamination, and frequently veins of very great beauty.