Ice, the solid crystalline form of water (q.v.) which that substance assumes at a low temperature, originates in many different ways in nature. Precipitated from the air as hoar-frost (q.v.), hail (q.v.), or snow (q.v.), accumulating in this latter form above precipices to fall as the avalanche (q.v.), or, on more gradual slopes, to glide as the glacier (q.v.) until, perhaps reaching the sea, it breaks off in the iceberg (q.v.), forming in deep or elevated and sunless caverns, on the open surface of fresh water, on the sea itself, or, as ground-ice, at the bottom of the water, it naturally presents many varied characteristics. The temperature at which it forms, under ordinary conditions of pressure, is the zero (0°) of the Centigrade and Reaumur thermometer-scales, and is 32° Fahrenheit. By melting out single negative crystals with a beam of electric light it is shown that sheet-ice is equally with snow made up of crystals of hexagonal type. Water expands in freezing, its maximum density being attained at 39° F. or 4° C.; and though ice, so long as it remains ice, behaves like most solids in contracting in cooling and expanding when heated, in melting it contracts. Its specific heat is about half that of water - i.e. the heat required to raise 1 lb. of ice through 1° C. is only sufficient to raise half a pound of water through 1°. In the melting of ice there is no rise of temperature, but 79-25 heat-units are rendered latent in the process, or are employed merely in changing the condition of the solid into that of a liquid, without any increase of temperature - i.e. it takes as much heat to convert 1 lb. of ice at 0° C. into water at 0° C. as would raise 1 lb. of water from 0° C. to 79-25° C.
When two blocks of ice at 0° C. are pressed together, melting takes place, heat is absorbed, the pressure is momentarily relieved, and the resulting water re-freezes, so that the two blocks become welded ill one. This process, which is known as regelation, explains the "binding" of a snow-ball and, to a considerable extent, the flow of a glacier.
Fresh-water, when cooled by contact with cold air down to 4° C, sinks, and not until the whole mass is reduced to that temperature, so that further convection currents are impossible, does ice form at the surface. Sea-water, when stagnant, freezes at about - 2° C. or 29° F., and in so freezing precipitates most of its salt, much as dirty water throws down most of the mud it contains. Water, whether fresh or salt, resting on an uneven bed, such as that of the Baltic, the Gulf of St. Lawrence, the Upper Thames, and the Christchurch Avon, is apt to form what is known eis ground-ice, bottom-iee, or anchor-ice, cakes of ice forming from radiation in contact with large stones, or even anchors, at the bottom, especially in relatively stagnant holes, and often floating them to the surface. Along the sea-coast or on the banks of tidal rivers a ledge of ice is often formed, by a similar process of radiation, adherent to the bank at the highest tide-level, which is known as ice-foot. It may be subsequently floated off, or lifted by a fresh layer formed below it and reattached still higher, thus forming a thick shelf on which debris due to frost-action may accumulate. Masses of ice-foot when detached form small flat bergs. When, in polar regions, the surface of the sea freezes it is known as floe (meadow) ice. Wave-action commonly breaks this up and heaps it in an irregular manner that much impedes the transit of Arctic explorers. When the floe-ice breaks up in summer, "canals" forming throughout it, it is known as pack-ice, and it is the violent collisions of masses of this pack that so frequently "nip" and destroy vessels.