Page:Popular Science Monthly Volume 5.djvu/418

402 been solidified, the tendency to freeze being held in check by pressure, but occurred the instant the pressure diminished.

The expansion of water at freezing is vastly important in its relations to life. Without this property, water in high latitudes would become permanently solid, and the aspect of Nature be one of lifeless desolation. Water, in cooling from a high temperature, contracts in volume, and the cooled particles sink until the mass is reduced throughout to a temperature about seven degrees above the freezing-point, when an important change takes place. Contraction of volume ceases, and expansion begins. The chilled particles remain at the surface from their lightness, and there solidify, while the water beneath, in its deeper portions, may be 7° warmer than the point of freezing. By this means a temperature of water in lakes is maintained adequate to the wants of life.

The structure of ice is crystalline, and the fundamental pattern of the crystals is six-rayed stars. But it is only in entire freedom of molecular motion that crystals attain perfection of symmetry. They form upon the surface of water, when the cold is severe, with great rapidity; but are modified in their arrangement or aggregation the instant the first crust is produced. The additions to the thickness of the ice are always at its underside, and the result is a prismatic form.



the prisms growing downward. These prisms are hexagonal in shape, and are so joined at their sides as to present an apparently homogeneous structure. That such is not the case, however, will be seen when we speak of the decay of ice. That the internal structure of ice is upon the stellate type is shown when a small cube of it is dissected by a beam of light. By the heat rays of the beam, the ice is decrystallized—its molecular architecture is taken down, and the result appears in stellate figures of exquisite beauty upon the screen. These figures are areas in the ice which have been liquefied by the beam, which thus throws on the screen an "image of its own work." In Fig. 5 we have magnified pictures of the crystalline structure of ice. They seem shadows of living objects, rivaling fern-leaf and blossom in