ICE is the solid formed when water freezes. It is a colourless substance crystallizing in the hexagonal system ( vide infra) ; the crystals display a marked tendency to "twinning," and this is what gives rise to the flower-like patterns so frequently noticed on windows. Hoar-frost, snow and hail result from the freezing of atmospheric moisture.
The temperature of melting ice is adopted as a standard in thermometric scales, being zero for both the Reaumur and Centi grade scales, and 3 2 ° for the Fahrenheit.
In the act of freezing, water undergoes a remarkable expansion although its temperature remains unchanged ; consequently, ice at o°C. has a density only o•9175 of that of water at the same temperature, and therefore floats on cold water. If the ice were denser than the water, it would sink and ponds would then freeze from the bottom upwards, thus becoming solid much more rapidly than they do ; this low density of ice is thus of great significance to pond life and therefore indirectly to all life.
The process of freezing in a pond presents features of interest. If the air is at or below o°C. and the temperature of the water is, say, Io°C., the upper layers of water become colder and sink into the main bulk, since the cold water is denser than the rest ; this process continues until the whole of the water is chilled to 4°C., but at this temperature it attains its maximum density (see WATER), and thereafter the surface layers, having a lower tem perature than 4°C., are relatively lighter, remain on the sur face and begin to freeze. The freezing is facilitated by the stillness of the water, resulting from the cessation of convection currents below 4°C. Sea water does not freeze until it is cooled to about — 2 ° C., even in the most favourable circumstances, and the re sulting ice is found to be practically free from the salt present in the water.
On being cooled, ice behaves like most other solids and con tracts; its specific heat is about half that of water. In order to melt ice at o°C. into water at the same temperature, it is neces sary to supply a definite quantity of heat, viz., 8o calories per gram ; this is called the "latent heat of fusion," since its absorp tion causes no rise of temperature. Conversely, the same amount of heat has to be dissipated during the freezing of water. In James Thomson showed that, since water expands on freezing, according to the laws of thermodynamics its freezing point must be lowered by increase of pressure, and he calculated this lowering to be 0.007 5 ° per atmosphere, an estimate subsequently fied by his brother, Lord Kelvin. When two blocks of ice at o°C. are pressed gether, the ice melts at the point of tact, the flow of resulting water relieves the pressure, and the water freezes again ; these processes continue until the blocks are firmly united. A similar explanation accounts for the movements of a glacier past restrictions in its course—the pressure causes local melting and flowing, and the ice gradually adjusts itself to the tion and refreezes on the farther side.
In India water is made to freeze on cold clear nights by leaving it in a porous vessel, (Chatti), the evaporation from the pores absorbing sufficient heat from it.
Ordinary block-ice is rendered opaque by minute bubbles of air which are produced in any but very slowly cooled water, owing to the decreased solubility of the air.
By subjecting ice to enormous pressures, P. W. Bridgman and G. Tammann have produced four other forms of ice (five accord ing to the latter) differing in crystalline form and in density, all being denser than water. Thus,- ordinary ice (Ice-I) at — Io°C. is converted under 4,40o atmos. to Ice-V, and at 6,30o atmos. to Ice-VI; similarly, at 2,200 atmos. pressure converts Ice-I to Ice-III, and 3,00o atmos. converts it to Ice-II, and higher pressures then produce Ice-V and Ice-VI.
The crystal structure of ice has been investigated by several workers. That deduced by Sir W. H. Bragg (Proc. Physical Soc., 1922, 34, p. 98) is shown in the annexed diagram, from which it will be seen that, when the structure is extended indefinitely in all directions, each oxygen atom is situated at the centre of gravity of four equidistant oxygen atoms, from each of which it is sepa rated by a hydrogen atom. Thus, each hydrogen atom touches two oxygen atoms and each oxygen touches four hydrogens. The whole structure agrees with the hexagonal habit characteristic of the crystals, and its open nature accounts for the low density of ice, which is exactly that calculated from the dimensions given. (For the manufacture of ice, see REFRIGERATION.) (A. D. M.)