ICE. ■124 ICE. the shape of six-raved stars of exquisite lieauty. (See article Snow.) The freezinppoint of pure water is marked 0° on the Centi;;rade and Roaumur scales, and 32° on the Fahrenheit scale. The influence of changes of pressure oii the freezing-point is so slight that for all ordi- nary atmospheric pressures the freezing-point of pure water may he considered a constant quan- tity. Great pressures, however, have the elfect of lowering the freezing-point very considerably, and by raising the pressure to nuiny thou.sand pounds per square inch ice has been caused to melt at 0° F. By cooling pure and clear water cautiously, it may be "undercooled,' i.e. it may, under ordinary pressure, be obtained in the liquid state at temperatures several degrees be- low its normal freezing-point. But this is quite dilfcrent from the ell'ect of high pressures; the slate of 'undercooled' water is just as unstable as the state of a "supersaturated' solution, and a slight disturbance may cause the whole mass to freeze very rapidly, just as it may cause rapid crystallization to take place in a supersaturated solution. The lowering of the freezing-point by pressure has furnished one of the theories ex- plaining the motion of glaciers. This "regclation theory* is illustrated by the following phe- nomenon: Tf a wire holding heavy weights at its ends is thrown over a block of ice. it gradu- ally cuts its way through the block, and yet the latter remains entire. Along the line where the wire presses upon the block, the melting-point is lowered, and hence the ice melts and allows the wire to descend through a minute distance; the water immediately above the wire then freezes, because it is freed from pressure, and at the same time the ice immediately below the wire melts, allowing the wire to descend a little farther. Thus melting and almost immediate regclation proceed until the wire has cut through the block. The freezing-point of water, as that of any other liquid, is considerably lowered by dissolving in it substances of any kind, and sea- water, on account of its salt, is found to freeze at — 2. .5° to — .3.0° C. The solid separating out, under ordinary circumstances, from freezing sea- water, is not a mi.xture of ice and salt, but pure ice. and this fact has been utilized in two ways: (1) for concentrating brine in the manufacture of salt; (2) for obtaining sweet drinking-water from the salt water of the sea. l"nlike most other substances, water, on pass- ing from the liquid to the solid state, does not contract, but expands. Therefore ice. having a lower specific gravity than water, floats on its surface. The sjiecific gravity of ice at its normal melting-point is 0.918. The specific heat of ice, i.e. the amount of heat required to mise its temperature 1° C. is much less than that of water in the liquid state; within .30° C. below the freezing-point it is very near one-half, and at lower tempcratires it is even somewhat less than one-half, that of water. In the process of melt- ing, ice absorbs more heat than any other solid (see FREEZixn-MiXTiRES). more than 80 calories being required to melt one gram of ice; the same amount of heat would raise the temperature of the gram of melted water to 80° C. (176° F.). Ice-Pack. or Pack-Ipe. A name given to the large sheets or pans of ice (flors) which have united to form a pack, and which occupy the open surface of the sea. The iee is almost wholly the result of direct freezing of the superficial stratum of the ocean, continued through a suc- cession of years to a possible thickness of 10 to 2U feet, and of snow accunuilations on this sur- face. Through 'shearing' and over-riding the ice-pack is frequently of greater thickness than would be brought about by freezing alone, and it is not rare to mei't with 'hununocks' on the borders of the pack 30 to 40 feet in height, and more. The individual pans, sheds, or Hoes that help to build up the p:uk are of varying dimen- sions, ranging from small cakes to .solid sheets many miles across, and a pack it.self may meas- ure a hundred miles, or even considerably more. Fven where virtually unbroken, they niaj' con- tain here and there jiockets or 'lakes' of inclosed water, and penetrating water-ways {leads), con- necting these lakes or wholly independent of them, are of common occurrence. The central ]'art of the Arctic Basin is frequently assumed, l>ut ]icrhaps without sullicient reason, to be oceui)ied by a stationary pack ; along its south- ern border it is freely moving, sending down those vast sheets of ice which constitute the 'pack-drift.' The remarkable drift of the From has demonstrated that the northern pack was still moving close to the 8Gth parallel of north latitude, or within less than 300 miles of the Pole. The strong southerly drift of the pack- ice in the region about Greenland, GrinncU, and (Irant Lands, jamming between the opposing land-masses, is that which has so largely baffled exploration along what is known as tlie 'Ameri- can route' to the Pole. The great centrally lo- cated ice-mass which in Davis Strait and Baffin's Bay separates Greenland on the east from the disconnected lands of .rctic and sub-Arctic America on the West, is known by whalers and others as the 'Middle Pack.' The Antarctic pack is less thoroughly known than the Arctic, although its e.tent appears to be very much greater, and during many years its front edge or that of the pack-drift has passed far beyond (northward of) the Antarctic Circle. Various names have been given by navi- gators to particular conditions of ice. Tlie bor- der of sea-ice which clings to the land, and is not aflTeeted by the movements of tides, is called the ire-foot. Ice that has been crushed into frag- ments by the impact of moving floes is called rubble. A piece of ice that floats with the upper surface just awash is called a ffroulcr. Anchor- ice or firottnd-ire is fresh-water ice that has frozen to the bottom of a lake or river. Toung ice is ice new-formed, in distinction from ice of previous seasons. Palwocrjixtie ice is ice that has been formed during a number of seasons by Hoes overlaid one upon another, has been in- creased year by year by snowfall, and has been welded by sun and rain into a single mass. See Arctic Region ; Antarctic Region. Iceberg. Under this name is understood a detached fragment or section of a glacier, whose terminal foot has reached a body of water, whether fresh or salt, and there broken (calved) off. Fresh-water bergs are those of lakes, and are of comparatively rare occurrence, and never of large size. The oceanic bergs, on the other hand, are frequently of very great size; they may measure even miles in length, as those of parts of the Antarctic waters and even of Mel- ville Bay. off the west coast of Greenland: and they not infrequently rise 250 or even 300 feet above the surface of the sea. In the case of the
