Page:Popular Science Monthly Volume 5.djvu/423

Rh molecule to molecule, particle to particle, mass to mass, and a ball or other figure of solid ice is the result. If there be no moisture, there can be no refreezing until moisture is produced or applied. Thus pieces of ice below the freezing temperature will not adhere because the surfaces are dry. The same is true of dry, granular snow in very cold weather—only by long-continued moulding and pressure in the hands can it be compacted. But in this case some liquefaction has been produced, and then the surfaces in contact freeze together. Snow, in the upper Alps, often covers gorges in the glaciers, and if moist can be trodden into bridges sufficiently compact to pass safely over.

This property of ice and snow Prof. Tyndall calls regelation. It was discovered by Faraday in 1850, who found that moist surfaces of ice adhered if brought together. This occurs under water as well as in the air; at summer heat, and beneath water so hot as to be painful to the hands. The phenomenon may be explained in this way: If we hold in our hands two cubes of ice, their outer surfaces are exposed to the atmosphere, and, if it be warm enough, some liquefaction at the surfaces takes place, and they become moist. Now, if the cubes be brought together, two of the outer surfaces become inner ones, and the moisture, chilled to the temperature of the ice, freezes, and the two cubes become one mass. It is because the molecules of ice may be continually crowded into new positions that the mass may be changed in form without its continuity being broken. A slab of ice placed in a suitable position will bend by its own weight. In this case the molecules throughout undergo gradually a change of position; but, if the stress be too rapidly applied, fracture occurs.

All the properties and phenomena of ice which we have considered, and in a marked degree that of regelation, are shown in the growth and movement of glaciers. In these we see the development of ice on its grandest scale. Equally in structure and form in molecular and molar motion they are an expression of energies that are irresistible and sublime. "To produce from aqueous vapor," observes Prof. Tyndall, "the little mass of snow which a child can carry, demands an exertion of energy competent to gather up the blocks of the largest stone avalanche I have ever seen, and pitch them to twice the height from which they fell." Who, then, shall estimate the potential energy of the great ice-rivers of the Alps, or of the glaciers of the Arctic Zone?

The motion of the Mer de Glace, and of other glaciers, is so slow as to be ascertained only by persistent observation, or by careful measurement. In 1827 a hut was erected by Huji on the glacier of the Unteraar for purposes of observation, but the hut was found to move down the valley. In fourteen years it was nearly a mile below its first position. In 1820 three mountain-guides were plunged by an avalanche into a gorge of a glacier on the side of Mont Blanc. After a burial of forty years in the ice, they were found several miles below the spot